ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-16-9983-2016Satellite observations of middle atmosphere gravity wave
absolute momentum flux and of its vertical
gradient during recent stratospheric warmingsErnManfredm.ern@fz-juelich.dehttps://orcid.org/0000-0002-8565-2125TrinhQuang Thaihttps://orcid.org/0000-0001-5588-2184KaufmannMartinhttps://orcid.org/0000-0002-1761-6325KrischIsabellhttps://orcid.org/0000-0001-6646-7277PreussePeterUngermannJörnhttps://orcid.org/0000-0001-9095-8332ZhuYajunhttps://orcid.org/0000-0002-8884-0885GilleJohn C.MlynczakMartin G.Russell IIIJames M.https://orcid.org/0000-0002-4835-7696SchwartzMichael J.https://orcid.org/0000-0001-6169-5094RieseMartinhttps://orcid.org/0000-0001-6398-6493Institut für Energie- und Klimaforschung, Stratosphäre (IEK–7),
Forschungszentrum Jülich GmbH, 52425 Jülich, GermanyCenter for Limb Atmospheric Sounding, University of Colorado at Boulder, Boulder, Colorado, USANational Center for Atmospheric Research, Boulder, Colorado, USANASA Langley Research Center, Hampton, Virginia, USACenter for Atmospheric Sciences, Hampton University, Hampton, Virginia, USAJet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USAManfred Ern (m.ern@fz-juelich.de)9August2016161599831001930March201625April201618July201620July2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://acp.copernicus.org/articles/16/9983/2016/acp-16-9983-2016.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/16/9983/2016/acp-16-9983-2016.pdf
Sudden stratospheric warmings (SSWs) are circulation anomalies in the polar
region during winter. They mostly occur in the Northern Hemisphere and affect
also surface weather and climate. Both planetary waves and gravity waves
contribute to the onset and evolution of SSWs. While the role of planetary
waves for SSW evolution has been recognized, the effect of gravity waves is
still not fully understood, and has not been comprehensively analyzed based
on global observations. In particular, information on the gravity wave
driving of the background winds during SSWs is still missing.
We investigate the boreal winters from 2001/2002 until 2013/2014. Absolute gravity
wave momentum fluxes and gravity wave dissipation (potential drag) are
estimated from temperature observations of the satellite instruments HIRDLS
and SABER. In agreement with previous work, we find that sometimes gravity
wave activity is enhanced before or around the central date of major SSWs,
particularly during vortex-split events. Often, SSWs are associated with
polar-night jet oscillation (PJO) events. For these events, we find that
gravity wave activity is strongly suppressed when the wind has reversed from
eastward to westward (usually after the central date of a major SSW). In
addition, gravity wave potential drag at the bottom of the newly forming
eastward-directed jet is remarkably weak, while considerable potential drag
at the top of the jet likely contributes to the downward propagation of both
the jet and the new elevated stratopause. During PJO events, we also find
some indication for poleward propagation of gravity waves. Another striking
finding is that obviously localized gravity wave sources, likely mountain
waves and jet-generated gravity waves, play an important role during the
evolution of SSWs and potentially contribute to the triggering of SSWs by
preconditioning the shape of the polar vortex. The distribution of these hot
spots is highly variable and strongly depends on the zonal and meridional
shape of the background wind field, indicating that a pure zonal average view
sometimes is a too strong simplification for the strongly perturbed
conditions during the evolution of SSWs.
Introduction
The unperturbed arctic winter stratosphere is characterized by a strong
eastward-directed zonal wind jet (polar vortex). Occasionally, however,
forcing by upward propagating planetary Rossby waves can lead to strong
deceleration and even reversals of this polar jet. These events are
associated with strong warming of the polar stratosphere, and they are
therefore called sudden stratospheric warmings (SSWs). Such events were first
reported by , and the importance of upward propagating
planetary waves for the driving of SSWs was first pointed out by
. A climatology and characterization of SSW events into
different categories can be found, for example, in
. Often, the following classification is used:
during a “minor warming”, the temperature gradient in the stratosphere
between 60∘ latitude and pole, on zonal average, becomes
positive over a certain altitude range at or below the 10 hPa level
(about 32 km altitude). During a “major warming”, additionally, the
stratospheric zonal wind at 60∘ latitude reverses from eastward
to westward at or below the 10 hPa level
e.g.,.
SSWs are dynamical processes that involve strong vertical coupling through
atmospheric waves. Much of the dynamics during SSWs can be understood by
upward propagation of planetary waves from the troposphere, amplification of
their amplitudes, followed by wave dissipation and heat flux convergence.
During this process considerable wave drag is exerted on the zonal mean
background flow. Particularly during major warmings, stationary planetary
waves will encounter critical levels when the zonal jet reverses from
eastward to westward. The waves dissipate and can no longer propagate to
higher altitudes. However, as was shown by , accurate
representation of major SSWs also requires the inclusion of gravity wave drag
in models.
Relevance of SSWs for global modeling
Simulating realistic SSWs in general circulation models and chemistry climate
models (GCMs/CCMs) and including the physical processes that are involved in
the onset and evolution of SSWs is important for several reasons:
Effects of SSWs are not limited to the polar region. SSWs influence the
global meridional residual circulation, and meridional coupling between
different latitudes is observed. For example, SSWs have influence on
mesospheric temperatures in the tropics e.g.,,
and they likely also have an effect on the opposite hemisphere
e.g.,.
There are teleconnections between high latitudes and the tropics. For
example, it is known that the frequency of SSWs is influenced by the
direction of the stratospheric quasi-biennial oscillation (QBO) of the zonal
wind in the tropics e.g.,: SSWs are more frequently
observed if the zonal wind in the tropics at 50 hPa is westward.
However, there are also indications for influences in the opposite direction:
SSWs can influence the tropospheric temperature and convection in the tropics
e.g.,, and thereby also tropical clouds and moisture
. In particular, SSWs have strong influence on the
composition of the stratosphere by modulating the tropical ascent of trace
species into the stratosphere . These
teleconnections between tropics and high latitudes are still not fully
captured by GCMs/CCMs e.g.,.
The forecast skill of weather prediction is related to the occurrence of
SSWs e.g.,, and this can be
utilized for an improvement of weather forecasts.
SSWs have effect on the average temperature of the stratosphere at high
northern latitudes. Changes in the frequency of SSWs will therefore result in
temperature trends . In addition, the warming of the
lower stratosphere induced by SSWs can lead to temperatures too warm for
polar stratospheric clouds to form, which has a strong influence on ozone
depletion in the Arctic.
On the other hand, changes in the frequency of SSWs will also affect the
meridional circulation and tracer transport from lower latitudes, possibly
resulting in changes in ozone depletion of opposite sign
e.g.,.
Particularly the timing of SSWs is of importance, with later SSWs favoring
stronger ozone depletion
e.g.,. The timing of SSWs,
however, may change in response to climate change. If SSWs are shifted to
later in winter, as expected by , stronger ozone
depletion would be expected in the Arctic. Therefore, realistic
representation of SSWs and their evolution could be important for obtaining
more realistic ozone projections for a changing climate.
It has been found that the circulation anomalies of SSWs have influence
on surface weather and climate
e.g.,, as well as on sea
surface temperature . Near-surface effects of SSWs
are not limited to the polar region, but can also extend to low latitudes
e.g.,.
A recent review on the mechanism of downward coupling during SSWs is given,
for example, by . Because of this downward influence,
representation of SSWs is required for accurate simulation of Northern
Hemisphere climate both on global and regional scales e.g.,and
references therein.
All these effects and interlinks show the importance of understanding all
processes that are relevant for the formation and evolution of SSWs, and of
including them in GCMs/CCMs. As mentioned before, one important process for
the onset and evolution of SSWs is driving by atmospheric waves, particularly
by planetary waves. The effect of gravity waves is still not well known.
However, there are indications that they significantly contribute.
Effects of gravity waves during SSWs
The effects of gravity waves during SSWs are manifold. For example, in model
simulations, the frequency of SSWs does not depend on planetary waves alone,
but also on parameterized gravity wave drag e.g.,.
Further, the effect of SSWs is not limited to the stratosphere. Also the
mesosphere
e.g.,,
and even the thermosphere/ionosphere are affected
e.g.,. The
selective filtering of gravity waves by the anomalous winds during (major)
SSWs is an important mechanism in these vertical influences.
SSWs are associated with mesospheric coolings
e.g., that are likely
driven by dissipation of eastward propagating gravity waves
e.g.,. As a consequence, the zonal wind
in the mesosphere/lower thermosphere (MLT), that is usually directed westward
during winter, can change its sign to eastward e.g.,. A
recent review about coupling between stratosphere and mesosphere during SSWs
is given by .
One particular subset of SSWs are polar-night jet oscillation events (PJO
events; e.g., ). These PJO events often are
related to strong major SSWs, but sometimes they can be related also to minor
warmings. Polar-night jet oscillation events can be characterized as follows:
after the peak of the warming, the stratopause altitude drops rapidly within
a few days, and the stratopause disappears e.g.,.
This rapid drop of the stratopause is driven by downwelling induced by
breaking planetary waves. Then, after a short period of nearly isothermal
conditions in the whole middle atmosphere, a new stratopause forms at
altitudes around 75 km, propagates gradually downward with time, and
reaches its nominal (climatological) altitude of around 50 km after
1–2 months. Similarly, an eastward-directed polar jet re-establishes at
elevated altitudes and propagates downward together with the elevated
stratopause
e.g.,.
Likely both planetary waves e.g., and gravity
waves contribute to the formation of the new elevated stratopause. For
example, model simulations show that the formation of the new stratopause is
sensitive to nonorographic gravity wave drag
e.g.,. Further, model simulations
that explicitly resolve a considerable part of the gravity wave spectrum
reveal that during PJO events the dissipation altitude of westward
propagating gravity waves is higher after the SSW than before. Consequently,
the residual circulation induced by those gravity waves that is responsible
for the formation of the stratopause is also raised, which explains the
formation of the new stratopause at an elevated level
.
During PJO events, gravity waves are also important for the recovery of the
eastward-directed polar jet after a period of anomalous westward winds. In
particular, it has been suggested in a modeling study by
that in cases when a band of anomalous westward
winds (induced by the SSW) is located below the eastward jet, westward
propagating gravity waves are filtered out more effectively, and less
westward momentum of gravity waves is available to slow down the eastward jet
above, and it can therefore reach considerable strength. During this recovery
phase of polar jet and stratopause, both the evolution of stratopause height
and the descent of tracers from the mesosphere are sensitive to the settings
of the gravity wave drag schemes used in model simulations
e.g.,, and the importance of both orographic and
nonorographic gravity wave drag during the recovery of the polar jet has been
pointed out e.g.,.
Gravity waves could also play an important role for the onset and triggering
of SSWs. As has been pointed out by , before the
onset of a SSW, gravity wave drag in the stratosphere is non-negligible.
Therefore, gravity waves may contribute to the preconditioning of the polar
vortex and its shape such that resonant amplification of planetary wave
amplitudes occurs and a SSW takes place. For details see
and references therein.
Still, the role of gravity waves in SSWs is not fully understood, as has been
shown in several recent studies
e.g.,.
One of the main reasons is that gravity waves have short horizontal scales
(tens of kilometers to around thousand kilometers at high latitudes).
Therefore their scales are too small to be resolved in most GCMs/CCMs, and
their effect on the background flow has to be parameterized. These
parameterization schemes are very simplified, and they contain a number of
tunable parameters. The improvement of those parameterization schemes by
comparison with global observations is a still ongoing issue
e.g.,. However, also
the part of the gravity wave spectrum that is resolved by high resolution
models is not necessarily fully realistic, and needs to be validated
e.g.,.
Given the sensitivity of simulated SSWs to uncertain representation of
gravity waves in global models, observations of gravity waves (both ground
and satellite based) provide a vital tool for the clarification of the role
of gravity waves in the evolution of observed SSWs and to evaluate the
adequacy of their representation in models.
Gravity wave observations during SSWs
Much work about gravity waves during SSWs has been done using ground-based
observations, for example lidar data e.g., and radar
data e.g.,. One of the main
findings is that during SSWs gravity wave activity in the upper stratosphere
and in the mesosphere is related to the background winds, and selective
filtering of gravity waves by the background winds is an important effect
e.g.,. Recent work has
derived gravity wave momentum fluxes from radar observations in the upper
mesosphere e.g.,, and the gravity
wave drag on the background flow was estimated during the major SSW in 2013
.
Although ground-based stations have already provided a wealth of information
during SSWs, these observations do not provide a global or zonal average
view. In particular, gravity wave activity strongly depends on the exact
location of the vortex edge. The shape of the polar vortex, however, will
vary considerably during a SSW due to changes in planetary wave amplitudes
and phases.
In recent years, also considerable work has been done utilizing satellite
observations of gravity waves during SSWs. For example, it has been shown by
that sometimes gravity wave amplitudes are
positively correlated with warming peaks of SSWs and that the selective
filtering by the background wind is important for the propagation of gravity
waves into the stratosphere. Also the study by indicates
that the background wind has a strong influence on gravity wave activity in the
stratosphere. Other studies revealed that sometimes after major SSWs, gravity
wave activity in the stratosphere is strongly suppressed
e.g.,. On the other hand,
before the major SSW in winter 2005/2006, an enhancement of gravity wave
momentum flux was observed in the lower mesosphere .
Gravity waves may also contribute to the observed tracer descent from the
mesosphere after (major) SSWs . Further, the study
by suggests that meridional propagation of gravity
waves could be important during major warmings.
Most of these studies are based on either gravity wave amplitudes, variances
or potential energies
.
Studies based on gravity wave momentum fluxes, on the other hand, allow a
more direct comparison with gravity wave parameterization schemes. However,
so far only few studies of SSWs based on satellite observations used gravity
wave momentum fluxes , and these studies
are focused mainly on the stratosphere. Momentum flux observations at higher
altitudes, as well as direct estimation of gravity wave dissipation from
momentum flux vertical gradients (gravity wave potential drag) derived from
satellite observations have not been exploited in SSW studies. In addition,
previous work using gravity wave momentum flux observations from space was
focused only on single major SSW events, and a more comprehensive comparison
of different Arctic winters and different major SSW events is still an open
issue.
In our work, we derive absolute gravity wave momentum fluxes and gravity wave
potential drag from Sounding of the Atmosphere using Broadband Emission
Radiometry (SABER) satellite observations for the Arctic winters 2001/2002
until 2013/2014, and from High Resolution Dynamics Limb Sounder (HIRDLS)
satellite observations for the winters 2004/2005 until 2007/2008. This
gravity wave activity is compared to atmospheric background conditions
provided by SABER, the Microwave Limb Sounder (MLS) on the Aura satellite,
and the ERA-Interim reanalysis of the European Centre for Medium-Range
Weather Forecasts (ECMWF).
The manuscript is organized as follows: in Sect. the
different data sets used in our study are briefly introduced. For the boreal
winters 2001/2002 until 2013/2014 the zonal average temporal evolution of
atmospheric background temperature and zonal winds averaged over
60–80∘ N is discussed in
Sect. . In Sect. , the altitude–time
distribution of gravity wave squared amplitudes, momentum fluxes and gravity
wave potential drag, averaged over the latitude band
60–80∘ N, is studied for all considered
Arctic winters. In addition, in Sects.
and the horizontal distribution of gravity waves, as well
as zonal average cross sections of gravity wave squared amplitudes, momentum
flux and gravity wave potential drag are investigated before, during, and
after the major SSWs of the years 2009 (Sect. ) and 2006
(Sect. ). Based on the zonal average cross sections
particularly the role of meridional propagation of gravity waves is
discussed. Finally, Sect. summarizes our main findings.
Data sets
Our work is based on data of the satellite instruments MLS, SABER, and
HIRDLS, as well as on the ERA-Interim reanalysis. In the following, some
information on ERA-Interim and the different satellite instruments is given.
Further, we describe how atmospheric background fields are obtained, and how
gravity wave amplitudes, momentum fluxes and potential drag are derived from
HIRDLS and SABER temperatures.
ERA–Interim
For parts of our study we use meteorological fields (temperature and winds)
of the ECMWF ERA–Interim reanalysis e.g.,. Our
ERA-Interim data are interpolated on a longitude/latitude grid of
1∘× 1∘ resolution. The altitude resolution is
about 1.4 km. Global fields are available at 00:00, 06:00, 12:00, and
18:00 UT. Because numerous observations are assimilated in ERA-Interim
, ERA-Interim meteorological fields are considered to be
quite reliable in the troposphere and lower and middle stratosphere. At
higher altitudes, however, due to a lack of observations, reanalyses become
more and more unreliable, particularly during the complicated dynamical
situation of SSWs e.g.,. It should also be noted
that ERA-Interim simulates the effect of nonorographic gravity waves by
Rayleigh friction. A parameterization of nonorographic gravity waves was only
introduced in later ECMWF model versions in order to provide more realistic
results particularly at higher altitudes .
The MLS Aura instrument
The Microwave Limb Sounder (MLS) instrument on NASA's Aura satellite is a
microwave radiometer that observes atmospheric microwave emissions using the
limb sounding method e.g.,. MLS
observes atmospheric temperature and numerous trace species. In our study, we
use MLS version 3.3 temperatures, as well as geopotential height. Useful
altitude range is between the 316 and 0.001 hPa pressure levels
(between about the middle troposphere and somewhat above the mesopause).
Vertical resolution is about 4 km in the stratosphere, and about
14 km in the mesopause region. For more details on the
temperature/pressure retrieval and the altitude resolution defined by the
averaging kernels see . Latitude coverage is between
82∘ S and 82∘ N. Measurements are available
starting from 8 August 2004 and are still ongoing at the time of writing.
The HIRDLS instrument
Like MLS, the HIRDLS instrument (e.g., Gille et al., 2003) was launched on NASA's Aura satellite. From
January 2005 until March 2008 HIRDLS observed atmospheric infrared radiances
in limb-viewing geometry. From these infrared emissions in limb-viewing
geometry, temperature-pressure profiles were derived at altitudes between the
tropopause region and well above 70 km. Latitudes between about
63∘ S and 80∘ N are covered by HIRDLS
observations. In our study we use version V006 HIRDLS temperatures see
also. Detailed information about the HIRDLS temperature
retrieval can be found in . The vertical resolution of
observed temperature profiles is about 1 kme.g.,. Along-track
sampling distance is about 90 km. Dense along-track sampling,
together with good altitude resolution, allows to estimate absolute gravity
wave momentum fluxes from observed HIRDLS temperature fluctuations
e.g.,.
The SABER instrument
The SABER instrument was launched on the Thermosphere-Ionosphere-Mesosphere
Energetics and Dynamics (TIMED) satellite. Like for HIRDLS, atmospheric
temperatures are derived from atmospheric infrared emissions observed in
limb-viewing geometry. SABER temperatures are available from the tropopause
region to well above 100 km in the lower thermosphere. SABER switches
between southward-viewing and northward-viewing geometries every about
60 days for about 60 days. The latitude coverage is either
about 82∘ S–50∘ N (southward view) or about
50∘ S–82∘ N (northward view). This is
particularly relevant for our study, because SABER does not continuously
observe high northern latitudes. SABER temperature profiles have an altitude
resolution of about 2 km, and every second pair of consecutive
altitude profiles has an along track sampling of better than 300 km,
allowing to derive gravity wave absolute momentum fluxes
e.g.,. In our study, we use SABER version
v2.00 temperatures. More information about the SABER instrument is available,
for example, in , or in . Details
about the SABER temperature retrieval can be found, for example, in
.
Satellite observations of gravity wave amplitudes, momentum fluxes, and potential drag
The determination of gravity wave absolute momentum fluxes from satellite
data is a procedure that requires several steps. First, the zonal average
background is removed from every altitude profile of observed temperatures.
Further, global-scale waves with zonal wavenumbers 1–6 are removed from each
altitude profile by reconstructing the contribution of global-scale waves at
the exact location and time of each observation. For this purpose,
longitude-time spectra are estimated for a fixed set of altitudes and
latitudes. This procedure is described in detail in , and
it accounts also for fast traveling planetary waves, such as short-period
Kelvin waves e.g., or quasi
2-day waves e.g.,. Tides, however, are removed
separately, as described in . The result of this procedure
are altitude profiles of temperature perturbations that can be attributed to
small-scale gravity waves.
For each altitude profile of temperature perturbations, the dominant wave
structures are identified by a two-step procedure called Maximum Entropy
Method/Harmonic Analysis (MEM/HA), as described in detail by
. For these waves, vertical wavelengths and amplitudes
are estimated in sliding 10 km vertical windows, and altitude
profiles of vertical wavelengths and amplitudes are obtained.
HIRDLS and SABER are limb sounders that observe the atmosphere with only a
single viewing direction. At a given altitude, the satellite soundings are
arranged in a measurement track in parallel to the ground track of the
satellite.
We assume that the same gravity wave is observed in two consecutive altitude
profiles of a satellite measurement track, if for a given altitude the
vertical wavelengths in these two profiles do not differ by more than 40 %.
If these pairs of altitude profiles are observed at almost the same location
(horizontal separation of less than 300 km) and at the same time
(within 1 min or less), the projection of the horizontal wavelength of
the observed gravity wave on the satellite measurement track can be estimated
from the vertical phase shift of the wave structures seen in both profiles
. This estimate of the horizontal wavelength, however, is in most cases an upper estimate of the “true” horizontal wavelength of the
gravity wave .
Absolute values of gravity wave momentum fluxes Fph can be calculated in
the following way :
Fph=12ϱ0λzλhgN2T^T02.
In this equation, ϱ0 and T0 are the atmospheric background
density and temperature, g is the gravity acceleration, N the buoyancy
frequency, λh and λz are the horizontal and the vertical
wavelength, respectively, and T^ the temperature amplitude of the
wave.
For a limb-viewing instrument with only a single viewing direction, such as
HIRDLS or SABER, the uncertainty of these momentum fluxes is large, at least
a factor of 2. Two of the main shortcomings are biases in the determination
of the gravity wave horizontal wavelength, as well as the sensitivity of the
instrument to observe gravity waves of a given horizontal and vertical
wavelength (observational filter).
For a detailed error discussion see . In particular, for
instruments with only a single measurement track the direction of momentum
flux is not known.
Estimation of momentum fluxes in this way is possible for about 3000 altitude
profile pairs per day for HIRDLS, and for about 350 altitude profile pairs
per day for SABER. Of course, satellite instruments are sensitive only to
part of the whole spectrum of gravity waves see
also. In our study, we cover horizontal wavelengths
longer than about 100–200 kme.g.,, and
vertical wavelengths in the range 2–25 km for HIRDLS, and
4–25 km for SABER see also. For a detailed
discussion of the observational filter of limb observations from satellite
see .
Absolute (total) values of gravity wave drag XY can be estimated from
vertical gradients of absolute momentum fluxes:
XY=-1ϱ0∂Fph∂z.
As is the case with absolute momentum fluxes, the direction of this drag is
not known without additional information. For this reason, we call these
values gravity wave “potential drag”. Like for absolute momentum fluxes,
uncertainties of potential drag are at least a factor of 2. Net gravity
wave drag could be even zero, while gravity wave potential drag is non-zero,
and, of course, net vectors of momentum flux would be needed in order to
estimate the net drag of gravity waves on the background flow.
In situations when the gravity wave spectrum is filtered by strong background
winds, it can be assumed that the gravity wave momentum flux spectrum is
dominated by waves propagating opposite to the background wind
e.g.,. Therefore it can be assumed that at the top
of strong wind jets gravity wave potential drag can be used as a proxy for
net gravity wave drag, and the direction of the drag is opposite to the
vertical gradient of the wind at the top of the jet. In spite of the large
uncertainty of gravity wave potential drag, relative variations of this drag
have already led to meaningful results in several cases: for the mesospheric
zonal wind jets in the summer hemisphere , and for both
the QBO and the semiannual oscillation (SAO;
) of the zonal wind in the tropics. Therefore, it can be
expected that meaningful results can also be obtained in our current study
for the polar jets in boreal winter.
Atmospheric background conditions in the boreal winters
2001/2002–2013/2014
In order to characterize the zonal average
meteorological background conditions during SSWs, temperatures and winds in
both stratosphere and mesosphere are needed. In this section we focus on the
latitude range 60–80∘ N. In the stratosphere,
temperatures and winds of ERA-Interim have shown to be quite reliable. In
addition, SABER and MLS provide temperatures in the whole stratosphere and
mesosphere. Further, we derive quasi-geostrophic winds from SABER and MLS
pressure and geopotential fields using the method by
that has been applied to SABER data recently
.
Because MLS data are not available before August 2004, and SABER observes
high northern latitudes for only about 60 days in the months December until
March, we compose merged wind fields covering the whole altitude range
20–90 km. Below 40 km we utilize winds from ERA-Interim, at
altitudes between 40 and 60 km winds are relaxed with a linear
transition toward the SABER or MLS winds, where available, and at altitudes
above 60 km only winds from MLS or SABER are used. Because of their
better altitude resolution, we use SABER geostrophic winds, where available,
otherwise MLS geostrophic winds are used. Differences between SABER and MLS
usually are small. ERA-Interim zonal winds are daily averages, while MLS and
SABER geostrophic winds are 3-day averages calculated with a time step of
3 days and interpolated to obtain daily values.
Altitude–time cross-sections of daily
60–80∘ N zonal average zonal wind in
ms-1 from ERA-Interim, MLS Aura and SABER combined. The
time scale on the x axis is given in “days of the year” (doy) with 1 January
00:00 UT as doy = 0. Overplotted contour lines have a contour increment of
20 ms-1. Westward (eastward) winds are indicated by
dashed (solid) contour lines. The zero wind line is bold solid. Times and
altitudes where no data are available are marked in gray.
Altitude–time cross-sections of the merged zonal average zonal winds in
ms-1, averaged over the latitude band
60–80∘ N, are given in Fig.
for the different boreal winters covering the period 2001/2002 to 2013/2014.
Temporal coverage is always from 1 December until 31 March. Also given on the
x axis is a time scale in “days of the year” (doy) with 1 January 00:00 UT
as doy = 0. Areas where no data are available are indicated in gray. Zonal wind
contour lines overplotted in Fig. are also given in all
other figures showing altitude–time cross sections.
Figure shows altitude–time cross sections of daily zonal
average temperatures in the stratosphere and mesosphere for the boreal
winters 2001/2002 until 2013/2014.
Figure d–m show temperatures
observed by MLS. Different from this, because MLS was not yet in orbit in the
first three winters considered,
Fig. a–c show temperatures observed
by SABER, where available, and ERA-Interim temperatures otherwise (but only
at altitudes below 60 km).
Altitude–time cross-sections of daily
60–80∘ N zonal average temperatures from MLS
Aura for the winters 2004/2005–2013/2014, and from ERA-Interim and SABER
combined for the winters 2001/2002–2003/2004. The time scale on the x axis
is given in “days of the year” (doy) with 1 January 00:00 UT as doy = 0. Like
in Fig. , overplotted contour lines indicate zonal average
zonal winds at 60–80∘ N in steps of
20 ms-1.
Central dates of major SSWs in the time period 2001/2002–2013/2014.
Vortex displacement events are indicated by “D”, and vortex split events by
“S”. Events with an elevated stratopause forming after the SSW are
additionally marked by “ES”.
In the time period considered, a number of major SSWs took place. These major
SSW events can be identified in Fig. by sudden
enhancements of the temperatures in the stratosphere. The “central date” of
these major SSWs is the day when the polar jet reverses from eastward to
anomalously westward wind at 10 hPa (about 32 km altitude)
and 60∘ N. A compilation of major SSW central dates is given,
for example, by for the years 1958 until 2002, or
by for the years 1958 until 2010. A compilation of
recent central dates is listed in Table for the time
period considered in our study. This compilation is mainly based on the
references and . In addition,
the type of the major SSW is classified by either “D” for “displacement”
or “S” for “split”, depending on whether the polar vortex was either
displaced by a strong planetary wave number one, or split into two parts by a
strong planetary wave number two see also.
Further, it is noted by “ES” if an elevated new stratopause forms after the
warming. A classification of SSWs can also be found in
, including their type. Of course, the classification
of SSWs into major and minor SSWs somewhat depends on the data, as well as on
the criterion used e.g.,.
Major SSWs that were associated with a PJO event took place in the winters
2003/2004, 2005/2006, 2008/2009, 2009/2010, and 2012/2013. During these
warmings, the stratopause altitude drops, and a new elevated stratopause
forms around 75 km altitude. After the central date a new eastward-directed polar jet re-establishes at higher altitude and gradually propagates
downward in altitude, while the anomalous westward winds associated with the
major SSW persist for a longer time (for one month and longer) in an altitude
range below the newly formed eastward polar jet.
In the winter 2011/2012 another PJO event took place. However, because the
winds only reversed above the 10 hPa level at 60∘ N the
criterion for a major warming is not matched see, for
example,, and the SSW in January 2012 is classified only
as a minor SSW, even though after the warming a new elevated stratopause is
formed in the mesosphere . If an average over a wider
latitude range of 60–80∘ N is considered,
like in Fig. k, the wind reversal in the winter 2011/2012
reaches as low as about 30 km. Therefore, averaged over this latitude
band, the response of gravity waves to this change in the global circulation
may be similar to the conditions of a major warming.
There are also several major SSWs that were not associated with a PJO event,
for example in the winters 2001/2002, 2002/2003, and in late winter
2006/2007. In addition, there was a pronounced oscillation of the polar jet
during the winter 2007/2008: three minor warmings with anomalous westward
winds only in the upper stratosphere were followed by a weak major warming on
22 February. The only winters that were only slightly perturbed and had no
major SSW or PJO event are 2004/2005, 2010/2011, and 2013/2014.
Time series of gravity wave activity and gravity wave potential drag
In this section, the zonal average gravity wave activity
(gravity wave squared amplitudes and absolute momentum fluxes), as well as
gravity wave potential drag derived from vertical gradients of absolute
momentum fluxes are investigated. Figure shows
gravity wave squared amplitudes in the latitude band
60–80∘ N for the winters
2001/2002–2013/2014. Values are given in K2 on a logarithmic
scale. For the same years, Fig. shows absolute
gravity wave momentum fluxes in mPa on a logarithmic scale, and
Fig. gravity wave potential drag in
ms-1day-1, also on a logarithmic scale.
Please note that, as detailed below, gravity wave parameters are only
available from HIRDLS and SABER observations in limited time and altitude
ranges. Therefore, compared to Figs.
and , in Figs. ,
, and there are much
larger areas indicated in gray where no gravity wave data are available.
In the winters 2001/2002 until 2004/2005 and the winters 2008/2009 until
2013/2014 only SABER gravity wave observations are available. SABER gravity
wave data are most reliable at altitudes above 30 km. Therefore in
Figs. , ,
and SABER data are shown only above 30 km.
In the winters 2005/2006, 2006/2007, and 2007/2008, the values in
Figs. , ,
and are a combination of HIRDLS observations at
altitudes below 55 km, and of SABER observations at altitudes above.
HIRDLS observations are daily averages, while SABER observations are 3-day
averages calculated with a time step of 1 day. During time periods when
observations from both instruments are available, in the altitude range
50–55 km a smooth transition is made between both data sets.
Altitude–time cross-sections of
a combination of 1-day zonal average HIRDLS and
3-day zonal average SABER gravity wave
squared amplitudes in the latitude band
60–80∘ N
during the winters 2001/2002–2013/2014,
derived in 10 km vertical windows.
Time step is 1 day for both HIRDLS and SABER.
The time scale on the x axis is given in “days of the year” (doy)
with 1 January 00:00 UT as doy = 0.
Squared amplitudes are given in K2 on a logarithmic color scale.
Like in Fig. , overplotted contour lines indicate
zonal average zonal winds at
60–80∘ N in steps of
20 ms-1.
Gravity wave squared amplitudes in K2
for the winters 2001/2002–2013/2014
averaged over the latitude band
60–80∘ N
and the altitude range 30–40km.
Values for vertical averaging were taken from
Fig. .
The different winters are grouped into
major SSW with split vortex and PJO event (red solid),
major SSW with split vortex but without PJO event (red dotted),
major SSW with displaced vortex and PJO event (blue solid),
minor SSW with displaced vortex and PJO event (blue dashed),
major SSW with displaced vortex but without PJO event (blue dotted),
and winters without major SSW or PJO event (black solid).
For winters without major SSW or PJO event, the
time scale on the x axis is given in “days of the year”
with 1 January 00:00 UT as time = 0.
For the winter 2011/2012 time = 0 refers to 15 January 2012.
For all other winters, time = 0 refers to the time of the central date of
the major SSW.
For winters with PJO event and for winters without major SSW or PJO event,
the curves are labeled with the respective years.
Overplotted contour lines in Figs. ,
, and indicate the zonal
average zonal wind, determined as described in Sect. .
Contour interval is 20 ms-1, eastward (westward) wind
is indicated by solid (dashed) contour lines, the zero wind line is indicated
in bold solid.
Atmospheric background winds have strong influence on the global distribution
of gravity wave activity. Because zonal winds usually are much stronger than
meridional winds, their influence on the gravity wave distribution is also
much stronger, and we will focus on the effect of the zonal average zonal
wind u‾ on zonal average gravity wave distributions.
Same as in Fig. ,
but for gravity wave absolute momentum fluxes
at 60–80∘ N.
Momentum fluxes are given in mPa on a logarithmic color scale.
Like in Fig. , overplotted contour lines indicate
zonal average zonal winds at
60–80∘ N in steps of
20 ms-1.
There are two main processes related to the background wind that shape the
gravity wave distribution. The first process is critical level filtering:
during wave propagation it can happen that background wind u‾ and
ground-based phase speed cφ of a gravity wave become equal. In
this case, the intrinsic phase speed of the wave
c^φ=cφ-u‾
becomes zero, the wave cannot propagate further, and it dissipates completely.
The second process is wave saturation. If a gravity wave propagates
conservatively upward (without dissipation), the wave amplitude will grow
exponentially according to the decrease in background density. At some point,
the wave amplitude cannot grow further. The wave amplitude reaches its
saturation limit, and the wave breaks. This can happen without a critical
level being reached. The saturation temperature amplitude
(T^sat) is proportional to the intrinsic phase speed, i.e., to
the difference between ground-based phase speed and background wind:
T^sat=T0gN|cφ-u‾|.
The saturation momentum flux of a gravity wave is proportional to the
intrinsic phase speed to the power of 3. See also ,
Eq. (10) in , and the discussion in .
Discussion of gravity wave squared amplitudes
In the upper mesosphere gravity wave squared amplitudes are usually quite
high (30 K2 and higher), and there is little interannual
variation. Main differences are found in the stratosphere and in the lower
mesosphere. These differences will be discussed in the following for
different vortex conditions.
Strong polar jets
Situations of strong unperturbed (i.e., continuously eastward-directed)
stratospheric polar jets are found, for example, in the winters 2004/2005,
2010/2011, and during December 2006 until mid February 2007, during December
2007 until mid January 2008, as well as during the first half of January
2013/2014.
During these unperturbed periods, on zonal average there is no wind reversal
in the stratosphere and lower mesosphere. Under these conditions, gravity
waves with westward or zero ground-based phase speeds can propagate in this
whole-altitude range without encountering critical levels. This means that
those gravity waves can attain large amplitudes already in the stratosphere
and lower mesosphere, because their intrinsic phase speeds and thus their
saturation temperature amplitudes are high. This effect is clearly seen in
Fig. : during the mentioned periods of strong
unperturbed polar jets gravity wave squared amplitudes are quite high with
values of about 5 K2 around 30 km altitude, and of about
20 K2 around 50 km altitude.
Weak polar jets
Compared to the situation of strong polar vortices, during weak vortex
conditions (zonal mean zonal wind weaker than about
20 ms-1), gravity wave squared amplitudes in the mid
stratosphere around 30 km altitude are somewhat reduced (about
3 K2). These conditions are found, for example, during parts of
the winter 2001/2002, during much of the winter 2002/2003
(Fig. b), and during winter 2013/2014 in the
second half of January and during February. A likely reason for this reduced
gravity wave activity are reduced gravity wave saturation amplitudes that are
not enhanced by strong favorable background winds.
PJO events
In Fig. , the highest values of gravity wave
squared amplitudes (about 10 K2 and more) in the mid stratosphere
(at ∼30 km altitude) are found before or around the central date
of strong major SSWs with PJO event. This is the case, for example, for
2008/2009 around 24 January (Fig. h), or for
2012/2013 around 7 January (Fig. l). For the
2009/2010 PJO event, gravity wave squared amplitudes are enhanced around
25 January, somewhat before the central date 9 February
(Fig. i). On the other hand, around the central
date 21 January of the 2005/2006 PJO event
(Fig. e)
and for the minor SSW in January 2012 that is associated with a PJO event
(Fig. k),
gravity wave squared amplitudes are not so much enhanced. Since zonal average
wind speeds are not necessarily stronger than during other situations,
enhancements of squared amplitudes close to the SSW central dates are
probably not only caused by favorable propagation conditions, but also by
stronger activity of gravity wave sources.
In particular, the three SSWs that had enhanced gravity wave squared
amplitudes were all vortex split events (the events in the winters 2008/2009,
2009/2010, and 2012/2013). In Sects.
and we will see that split vortex events may cover a
larger longitude range than vortex displacements. On the one hand, this would
generally improve propagation conditions for gravity waves propagating
opposite to the background wind. On the other hand, this may result in
stronger activity of gravity wave sources. For example, there would be more
opportunities for the excitation of mountain waves, resulting in enhanced
gravity wave squared amplitudes on zonal average. In addition, during vortex
split events jet-related gravity wave sources may be more active than during
vortex displacements: there could be more jet exit regions, and other
jet-related gravity wave source mechanisms could be enhanced, too. The
importance of the vortex shape will be discussed in more detail in
Sects. and where gravity wave
horizontal distributions in the polar regions are presented.
Regarding gravity waves sources, one important question is at which source
altitude the gravity waves were excited that are being observed. However,
from our observations alone, it is difficult to obtain this information. In
particular, for all gravity waves observed in the stratosphere and
mesosphere, the source could be at altitudes much lower than the observation
altitude, i.e., in the troposphere, in the tropopause region, or in the
lowermost stratosphere. Further information could be obtained, for example,
by comparison with modeling studies, or by gravity wave ray tracing
simulations. This, however, is beyond the scope of our study. In the
following, we will therefore just suggest possible explanations for our
observations.
After the onset of SSWs associated with PJO events (not multiple SSWs
associated with a single PJO), gravity wave activity in the stratosphere is
reduced for two reasons. First, zonal winds are much weaker, not resulting in
favorable enhancements of gravity wave saturation amplitudes. Second, due to
anomalous westward winds there are wind reversals in the troposphere and/or
in the stratosphere, such that gravity waves with zero ground-based phase
speed (e.g., mountain waves) or with slow westward-directed phase speeds will
encounter critical levels. Particularly during the phases of PJO events when
weak anomalous westward winds persist for 1–2 months below the newly forming
eastward jet; i.e., well after the SSW central date, gravity wave squared
amplitudes are quite low in the whole stratosphere (as low as
1–2 K2). This is the case for all PJO events in the time period
considered – even for the PJO event in winter 2011/2012 that is associated
with only a minor SSW, although the zonal wind in the mid and lower
stratosphere only weakens after the SSW, and does not reverse on zonal
average in the latitude range considered.
The time evolution of gravity wave activity in terms of gravity wave squared
amplitudes, averaged over the altitude range 30–40 km, is
illustrated in more detail in Fig. . Values for this
averaging were taken from Fig . In
Fig. , the different winters were grouped into PJO
events with split vortex (red solid lines) and PJO events with displaced
vortex (blue solid and blue dashed lines). Winters with no major SSW or PJO
event are given as a reference (black lines), and other winters are indicated
just by dotted lines. For the winters with major SSW, time = 0 days
refers to the central date of the major SSW, and for the winter 2011/2012 to
15 January 2012, while for all other winters time = 0 days refers to 1
January of the respective year.
As can be seen from Fig. , the three PJO events with
split vortex display enhanced gravity wave squared amplitudes before or
around the central date, while the PJO events with vortex displacement show
no particular enhancement with respect to winters without major SSW or PJO
event (black lines). Further, Fig. shows that for the
PJO events it takes about 40–60 days after the central date for
gravity squared amplitudes to recover to values comparable to years without
major SSW or PJO event. (Please note that for the individual PJO events,
according to their central date, the time axis is shifted to the left by
between 4 and 39 days.) The time range of around 40–60 days
for recovery is about comparable to the simulations by
who obtained values of about
40–50 days.
In the six PJO events considered, a very strong eastward polar jet
re-establishes in the upper mesosphere and slowly descends into the
stratosphere within 1–2 months. In the lower part of this re-established
polar jet, gravity waves over a wide range of eastward phase speeds will
encounter critical levels, dissipate, and it could be expected that they
might significantly contribute to the formation and maintenance of this jet.
However, as will be shown later in Sect. , this is likely
not the case.
High westward phase speed gravity waves are not filtered out by the
comparably weak anomalous westward winds in the troposphere and lower
stratosphere. In the strong eastward jets, their critical amplitudes can
become very large (cf. Eq. ). This is one possible
explanation why very high gravity-wave squared amplitudes of well above
30 K2; i.e., values similar to those in the other years, are found
in the upper part of the re-established eastward jets and above (at altitudes
above about 70 to 80 km). Another explanation for these high squared
amplitudes could be meridional propagation of gravity waves from lower
latitudes, as suggested by .
Other SSWs
After other SSWs, the zonal wind in the stratosphere is usually much weaker
than before. Consequently, there is no favorable enhancement of gravity wave
saturation amplitudes like in the strong polar jets of unperturbed winters,
or like often before SSWs. In addition, during periods of anomalous westward
winds, gravity waves with zero or slow westward-directed phase speeds will
encounter critical levels. The result are reduced gravity wave squared
amplitudes in the stratosphere and sometimes the lower mesosphere. This is
seen, for example, in winter 2001/2002 after mid February, in late winter/early spring 2006/2007 after mid February, and after the major SSW in 2008
after mid February.
Around the central dates of other major SSWs or just before minor SSWs, in
the latitude range considered, there are no clear enhancements of gravity
wave squared amplitudes in the mid stratosphere. However, there are four
pulses of enhanced gravity wave variances in the lower stratosphere (below
30 km altitude) in January and February 2008
(Fig. g) that are related to enhanced planetary
wave amplitudes during three minor SSWs and one major SSW. This effect has
been discussed in detail by for the winter
2007/2008.
Together with the findings from the PJO events, this shows that there is no
clear relationship between onset of stratospheric warming (central date) and
strength of gravity wave squared amplitudes. This indicates that the shape
and location of the polar vortex is important in determining the strength of
observed gravity wave activity: depending on the shape of the vortex,
enhancements of jet-related gravity wave source processes could be expected.
In particular, the strongest enhancement of gravity wave squared amplitudes
is found for the central dates of the 2008/2009 and 2012/2013 major SSWs,
which were both vortex split events.
Same as in Fig. ,
but for gravity wave potential drag
at 60–80∘ N.
Potential drag is given in ms-1day-1
on a logarithmic color scale.
Like in Fig. , overplotted contour lines indicate
zonal average zonal winds at
60–80∘ N in steps of
20 ms-1.
This is qualitatively in good agreement with the findings by
who found enhanced orographic gravity wave drag for
split vortex events in the ERA-Interim reanalysis and in the Japanese
Meteorological Agency and Central Research Institute of Electrical Power
Industry 25 year Reanalysis (JRA-25).
One possible reason for enhanced gravity wave activity during vortex-split
events could be the strong jet curvature and the existence of two jet exit
regions that will lead to enhanced jet-related gravity wave sources.
But also the location of the polar vortex should be important. For example,
stronger gravity wave activity would be expected if the polar jet crosses
mountain ranges, resulting in stronger excitation of mountain waves. For this
reason, the importance of the particular conditions of the polar vortex will
be discussed later in more detail in Sects.
and for the 2008/2009 vortex split event and for the
2005/2006 vortex displacement event.
Discussion of gravity wave momentum fluxes
Much of the discussion of gravity wave squared amplitudes in
Sect. is also valid for gravity wave momentum fluxes
and will therefore not be repeated in detail. The main difference is that
gravity wave amplitudes usually grow with altitude. If a gravity wave
propagates conservatively in a constant wind, this amplitude growth is
exponential, compensating the exponential decrease of atmospheric background
density with altitude.
Different from this, gravity wave pseudomomentum flux is conserved, i.e.,
remains constant, if a wave propagates conservatively. In all panels of
Fig. , however, we find that gravity wave momentum
flux gradually decreases with altitude, indicating an overall dissipation of
gravity waves while the waves are propagating upward.
There are several further findings. First, in situations of strong polar
jets, an increased amount of gravity wave momentum flux enters the
stratosphere. Like for gravity wave variances, this is the case during the
periods of strong polar jets in the winters 2004/2005, 2010/2011, 2013/2014
(see Fig. d, k, m), and during December 2006 until January 2007 (see
Fig. f) as well as in December 2007 (see
Fig. g). Second, sometimes zonal average gravity
wave momentum fluxes are enhanced before or around the central dates of major
SSWs, for example for the major SSW in the winter 2008/2009. However, these
enhancements are less pronounced than for gravity wave squared amplitudes.
And, third, gravity wave momentum fluxes are reduced in the stratosphere,
when zonal winds are weak.
For the PJO events, during the 1–2 month phases of anomalous stratospheric
westward winds persisting after the SSW, we find that gravity wave momentum
fluxes are strongly reduced in both stratosphere and mesosphere. This is the
case particularly when the polar jet starts to re-establish at elevated
altitudes. During this period momentum flux vertical gradients are sometimes
close to zero, while negative gradients would be expected. This again might
indicate that meridional propagation of gravity waves could play an important
role and additional momentum flux is transported from lower latitudes into
the latitude band 60–80∘ N.
Discussion of gravity wave potential drag
While gravity wave squared amplitudes and gravity wave momentum fluxes have
already indicated that there are strong interactions between gravity waves
and the background winds, calculation of gravity wave potential drag can
serve as a metric whether the variations seen could have significant effect
on the background flow, or not. Considering Fig. ,
there are a number of noteworthy findings.
In the stratosphere, usually there seems to be no particular enhancement
of gravity wave potential drag that would contribute to the onset of SSWs
directly in the latitude range considered (i.e.,
60–80∘ N).
In the case of strong, unperturbed stratospheric polar jets (see also
Sect. ) gravity wave potential drag in the range
1–3 ms-1day-1 is found in the
stratosphere. This suggests that wave driving by gravity waves somewhat
contributes to the zonal momentum budget for unperturbed conditions. These
values are similar to the drag due to planetary waves during unperturbed
conditions. This supports the findings by that
stratospheric gravity wave drag before SSWs is non-negligible, and gravity
waves could therefore be important for preconditioning the polar vortex such
that a SSW is triggered. For perturbed conditions, however, the drag due to
planetary waves can be up to around
30 ms-1day-1 in the stratosphere
see, for example,, i.e., considerably
stronger.
When background winds are weak (see also Sect. )
only little gravity wave potential drag is found in the stratosphere
(1 ms-1day-1 and below). The likely
reason is that only a reduced amount of gravity wave momentum flux can enter
the stratosphere: due to the weak background winds, there is no favorable
enhancement of gravity wave saturation amplitudes.
Momentum fluxes are even more reduced in the case of wind reversals in
the troposphere and lower stratosphere, as is the case during PJO events when
anomalous westward winds are persisting in the stratosphere after the SSW.
Consequently, during these periods, gravity wave potential drag in the
stratosphere is also much weaker. This is the case for all PJO events: the
major SSWs in the winters 2003/2004, 2005/2006, 2008/2009, 2009/2010, and
2012/2013, as well as for the minor warming in the winter 2011/2012.
In the cases when during a PJO event a strong polar jet re-establishes
after the SSW, one might expect that enhanced gravity wave potential drag
would be seen in the lower part of the newly formed polar jet, because
gravity waves with eastward phase speeds will encounter critical levels for a
wide range of ground-based phase speeds (in some cases even about
10–80 ms-1). In Fig. c,
e, h,
i, k,
and l, however, only very weak gravity wave
potential drag is seen in the lower part of the re-established eastward jets.
One possible explanation for this finding is that, for these situations,
background winds in an altitude range below the re-established polar jet are
quite weak. At the beginning of the jet recovery, weak winds are found in the
whole stratosphere, while in the later part of the jet recovery this altitude
range covers only the lower stratosphere. As will be seen later in
Sects. and , during phases of jet
recovery, the zonal wind in the lower stratosphere usually is quite weak at
all longitudes. Due to the generally weak winds, also gravity wave saturation
amplitudes will be quite low in this altitude range.
This considerably reduces the amount of gravity wave momentum flux that is
available for interacting with the background winds in the lower part of the
eastward jets. Another possible explanation could be that gravity wave
activity in boreal winters may be dominated by gravity waves with slow ground-based phase speeds, for example mountain waves, as indicated in previous
model-measurement comparisons e.g.,.
On the one hand, these waves would encounter critical levels in the
troposphere or lower stratosphere due to the wind reversals caused by
anomalous westward winds in the lower stratosphere. And, on the other hand,
having only low ground-based phase speeds, these waves are too slow to
contribute to the formation of wind jets with zonal wind speeds as high as
80 ms-1.
Obviously, the momentum flux of high eastward phase speed gravity waves is
too small to produce significant driving of the eastward jets in their lower
parts. Consequently, the re-formation of the eastward polar jets after major
SSWs is basically an effect induced by an anomalous residual circulation,
i.e., by changes in the poleward flow and vertical motion inducing dynamical
warming and thermal wind changes, and not directly driven by gravity waves or
planetary waves.
This finding is in good agreement with modeling studies. For example,
point out that the situation of the
re-established polar jets during PJO events is very different from
wave-driven circulations like the QBO. For the QBO enhanced wave drag is seen
both on top and at the bottom of an eastward or westward-directed wind jet,
i.e., for both positive and negative vertical shear of the zonal wind
see also. For the re-established eastward-directed
polar jets, however, gravity wave drag is only enhanced at the top of the
jet. As suggested by , at the top of the jet
wave saturation effects should be more important than critical level
filtering.
The absence of strong gravity wave drag on the lower flank of the jet is in
good agreement with the theoretical picture that the residual circulation
drives the thermal structure of the mesosphere and the stratopause in the
polar region, and the new polar jet is forming in response to these changes
in the residual circulation: in the mesosphere, the gravity-wave-driven
branch of the residual circulation, which is directed poleward and downward
in the polar region, enforces the warm winter stratopause
e.g.,. During SSWs, anomalous breaking of
planetary waves changes the circulation in the stratosphere, and, as a
consequence, the net forcing by gravity waves changes its sign, which leads
to an anomalous residual circulation resulting in a cooling of the (upper)
stratosphere and mesosphere (i.e., at altitudes above about 50 km).
Later, during the jet recovery, the sign of net gravity wave forcing changes
again, and the stratopause is rebuilt
e.g.,.
The theoretical picture of the mesospheric gravity-wave-driven branch of the
residual circulation being responsible for changes in the residual vertical
motion and related dynamical warming is well supported by the fact that the
strongest gravity wave potential drag is usually observed above the
temperature maximum of the stratopause (cf. Figs.
and ).
The importance of the residual circulation for the formation of the new
elevated stratopause is also confirmed by several studies that observe
enhanced transport of trace species from the mesosphere downward, induced by
an enhanced poleward and downward-directed residual circulation
e.g.,.
In addition to the observed tracer transport, the long time required until
the newly formed stratopause reaches the climatological stratopause altitude
(see Sect. ) indicates that the anomalous residual
circulation accompanying a PJO event persists for a longer time after the
central date of the SSW.
While gravity wave driving is not observed in the lower part of the
re-established polar jets during PJO events, considerable gravity wave
potential drag of well above 10 ms-1day-1
is observed in the upper part of these jets. From meteor radar observations,
even stronger gravity wave drag of about
100 ms-1day-1 is reported
. However, these particular measurements are from a
location that is known for enhanced activity of mountain waves and may
therefore not be representative for zonal averages. Further, the quite high
gravity-wave momentum fluxes seen by meteor radars are currently under debate
e.g.,.
During PJO events high values of potential drag in the upper mesosphere are
observed even though momentum fluxes in the lower stratosphere are much
reduced after the SSW. These high values of potential drag are comparable to
those in the upper mesosphere during most other periods considered in our
study (see Fig. ). As already indicated by the
quite strong gravity wave amplitudes in the altitude range 70–80 km
(see Sect. ), likely a mixture of gravity waves that
have propagated from lower latitudes, as well as vertically propagating
gravity waves with westward-directed ground-based phase speeds act to
decelerate the re-established polar jets in their upper part.
Overall, this suggests that gravity waves contribute to the wind reversal of
the re-established polar jets at their top, and, consequently, to the
downward propagation of the newly formed stratopause to its nominal altitude
around 50 km. The issue of meridional propagation of gravity waves
will be addressed again in Sects. and .
From Fig. , it can also be seen that for the
PJO events around the central date of the major SSWs, or, in the case of the
PJO event during winter 2011/2012, around the onset of the minor SSW, gravity
wave potential drag in the upper mesosphere usually is not reduced. This
might indicate that gravity wave drag could be involved in the formation of
the new elevated stratopause around 75 km altitude.
This finding indicates a difference between observations and the model
simulations by . Around the SSW central date and
during the phase of anomalous westward winds over a large altitude range for
a few days directly after the central date, the model results by
indicate a strong decrease in gravity wave
momentum flux over almost the entire vertical column in the stratosphere and
the mesosphere, see their Fig. 5c. Different from this, during these phases,
observed momentum fluxes are still quite strong in the mesosphere. This is
the case for all PJO events (see Fig. e,
h,
i, k,
and l).
Possible reasons could be either a less effective filtering of gravity waves
in the real atmosphere than in the models, or effects of non-vertical
propagation, or a combination of both. With this not much reduced amount of
gravity wave momentum flux still available, the reversed winds after the
central date will not lead to a reduction of potential drag in the upper
mesosphere.
Finally, it should be mentioned that, in all winters considered, enhanced
values of gravity wave potential drag are preferentially found in the upper
stratosphere and in the mesosphere (i.e., at altitudes above about
40 km). In many cases, these enhanced values are related to vertical
shear of the background wind. This is not only the case for the upper part of
eastward-directed polar jets, but also for some occasions of positive
vertical shear of anomalous westward winds at altitudes above 40 km,
for example in the second half of January 2006
(Fig. e), in mid January 2012
(Fig. k), and in the first half of January 2013
(Fig. l).
Gravity waves during different phases of the major SSW in 2009
In this section we will address the effect of gravity waves during different
phases of the major SSW in boreal winter 2008/2009. There are several reasons
for choosing this major SSW. First, this SSW is associated with a PJO event.
This means that after the SSW anomalous westward winds persist for a longer
time in the stratosphere (from about 21 January until end of February), a new
elevated stratopause is formed, and a new strong eastward-directed polar jet
is re-established after the SSW. Second, during this SSW strong activity of
the quasi-stationary planetary wave with zonal wavenumber 2 is observed,
leading to a split vortex. Third, SABER is observing high northern latitudes
already somewhat before the central date of the SSW.
Horizontal distributions during the 2009 major SSW
Figure illustrates the temporal evolution
of the polar vortex and of gravity wave activity during the different phases
of the 2009 major SSW/PJO event at 30 km altitude. Shown are
horizontal distributions of ERA-Interim temperatures (left column), zonal
wind (second column), absolute horizontal wind (third column), SABER gravity
wave squared amplitudes on a logarithmic scale (fourth column), and SABER
gravity wave momentum fluxes on a linear scale (right column). For
comparison, the first row shows an average over an unperturbed vortex period
during 13–28 February 2011, while the other rows show different phases
before, around and after the central date (24 January) of the 2009 major SSW.
Horizontal distributions at 30 km altitude
of ERA-Interim temperatures (left column), zonal wind (second column),
absolute horizontal wind (third column), and SABER gravity wave
squared amplitudes in K2 on a logarithmic scale (fourth column),
as well as
SABER gravity wave momentum fluxes in mPa on a linear scale (right column).
The first row represents an unperturbed vortex situation,
averaged over 13–28 February 2011,
while the other rows represent different periods before, during, and after
the central date (24 January) of the 2009 major SSW.
Unperturbed vortex during 2011
For an unperturbed vortex situation, there is a temperature minimum centered
at the pole (Fig. a1), and the polar jet is
strong and axisymmetric around the pole
(Fig. a2
and a3).
Enhanced gravity wave activity is usually found in regions of high wind speed
(Fig. a4
and a5).
This enhancement may be a consequence of enhanced saturation amplitudes of
gravity waves having phase speeds opposite to the background wind. Still,
there are some regions where momentum fluxes are more enhanced. This could be
an indication of localized gravity wave sources, for example jet-related
sources or orography. Of course, the period 13–28 February 2011 represents
conditions of a very stable and axisymmetric polar vortex. Usually, even for
widely unperturbed conditions, there will be some displacement (activity of
planetary wave number 1) and/or elongation of the vortex (activity of
planetary wave number 2).
Well before the central date of SSW 2009
The second row in Fig. shows horizontal
distributions for the period 12–16 January 2009, i.e., well before the
central date of the 2009 major SSW. The temperature at the pole is still
quite low (Fig. b1), but the polar vortex is
already somewhat perturbed. As a consequence of activity of planetary wave
number 2, it is elongated towards North America and northern Asia
(Fig. b2
and b3).
Further, there seem to be two jet exit regions (i.e., regions of strong jet
deceleration), one over North America, and another one close to Scandinavia.
In the vicinity of those jet exit regions we find strongly enhanced gravity
wave momentum fluxes (Fig. b5). These
gravity waves are likely a mixture of jet-generated waves and orographically
induced waves via the Rocky Mountains and the Norwegian Alps. Although the jet
exit regions are seen at 30 km altitude, we expect that they are a feature
persistent over a larger altitude range, and the sources of the jet-generated
gravity waves could therefore be well below 30 km.
A review of jet-related gravity wave source processes is given, for example,
by . Somewhat enhanced momentum fluxes are also
found over other mountainous regions, such as northeastern North America or the Ural
Mountains. For an overview of regions that are known as sources for
orographically generated gravity waves see, for example,
, or .
The period 12–16 January 2009 almost coincides with the period 11–15
January 2009 investigated in . The distribution of
gravity wave momentum fluxes in our Fig. b5
is in remarkable agreement with the distribution of orographic gravity wave
drag derived from the JRA-25 and ERA-Interim reanalyses, particularly over
North America seetheir Figs. 6d and 7d. This is
an important finding because state that, for the
conditions prior to the 2009 major SSW, gravity wave forcing, particularly in
the longitude range between 60 and 160∘ W,
could trigger the evolution from an elongated to a peanut-shaped vortex,
finally leading to the SSW and vortex split to happen.
Of course, there are some differences between the distributions of orographic
gravity wave drag by and the gravity wave momentum
fluxes in Fig. b5. For example, in
Fig. b5 there is a strong enhancement of
gravity wave momentum fluxes over Europe that is not seen in
their Figs. 6d and 7d. Such differences may be due
to the fact that the satellite observations contain not only mountain waves,
but also gravity waves from jet-related sources that are not covered by the
analysis of .
Zonal average cross-sections of
MLS temperatures (upper),
SABER gravity wave
squared amplitudes in K2 (second row),
momentum fluxes in mPa (third row),
and potential drag in ms-1day-1 (lower)
for the latitudes
20–90∘ N
during different phases of the strong major SSW in winter 2008/2009.
For comparison, the left column shows an unperturbed vortex situation
during February 2011.
For the lower three rows logarithmic color scales are used.
Overplotted contour lines indicate the zonal average of MLS geostrophic
zonal winds,
averaged over the respective time periods shown.
Contour interval is 20 ms-1.
Dashed contour lines indicate westward wind.
Shortly before the central date of SSW 2009
The third row in Fig. coincides with the
maximum of gravity wave squared amplitudes and momentum fluxes shortly before
the central date of the 2009 SSW (see also
Figs. h
and h).
During this period (17–21 January), there are two distinct temperature
minima off the pole (Fig. c1), and the
vortex is extremely elongated with the polar jet extending even to the Gulf
of Mexico and far into Central Asia
(Fig. c2
and c3).
While one of the jet exit regions is still located above North America, the
other one has somewhat shifted toward Asia. Accordingly, we find hot spots of
gravity wave momentum fluxes in the vicinity of these jet exit regions. One
hot spot is located over central North America, and the other over Central
Asia. Again, enhanced momentum fluxes are found over mountainous regions,
like northwestern North America, the Ural Mountains or Scandinavia. Compared to the
period of 12–16 January, however, the momentum fluxes over Scandinavia are
much reduced. One possible reason could be the northward shift of the polar
jet, another reason could be the shift of the jet exit region towards Asia.
It is also noteworthy that enhanced gravity wave activity is found even at
latitudes as low as 30∘ N. There are also two regions of weak
westward-directed wind, apparently some outflow of the polar vortex that
seems to be a first indication of vortex instability and breaking of the
planetary wave number 2. One region is located over the North Pacific, and
the other over the Mediterranean. Similar to the polar jet, these regions of
enhanced winds could provide favorable propagation conditions for gravity
waves. While no enhancement of gravity wave activity is found in the North Pacific region, indeed, enhanced gravity wave momentum fluxes are found over
the Mediterranean. Another enhancement of momentum fluxes over the North
Atlantic might also be related to this secondary circulation and the breaking
of the planetary wave number 2. This, however, is difficult to decide from
the gravity wave observations alone.
Around the central date of SSW 2009
The period of 22–26 January, which is centered around the central date of
the 2009 SSW (24 January), is shown in the fourth row of
Fig. . As seen in
Fig. d1, now there is a pronounced
temperature maximum close to the pole, as expected for a SSW. At the same
time, the polar vortex has weakened and split into two sub-vortices, and two
regions of anomalous westward winds are located close to the pole
(Fig. d2
and d3).
Additionally, the two wider regions of westward winds at lower latitudes over
the North Pacific and over the Mediterranean have strengthened. As mentioned
before, this may be an outflow of the polar vortex and related to rising
vortex instability and breaking of the planetary wave 2.
From Fig. d4
and d5 we find that gravity wave activity
has somewhat weakened, compared to the period directly before the central
date. Still, some gravity wave activity will be caused by localized
orographic sources. However, gravity wave momentum fluxes are similarly
enhanced over the whole area of the two vortices, suggesting that the strong
jet curvature leads to a wide distribution of jet-related
gravity-wave-generating processes. Further, there is a large area of enhanced
momentum fluxes over the North Pacific that coincides with an area of low
latitude anomalous westward winds in this region (see
Fig. d2), and thus this enhancement may be
related to the breaking of the planetary wave 2. At the same time, however,
gravity wave momentum flux is not much enhanced in the other region of low
latitude anomalous winds over the Mediterranean. This shows that breaking
planetary waves can act as gravity wave sources. The strong variation of
gravity wave activity during this process, however, indicates that these
source processes may be very intermittent.
Anomalous winds shortly after the central date of SSW 2009
The fifth row in Fig. shows average
horizontal distributions for the period 25–29 January 2009, i.e., shortly
after the central date of the SSW. Temperatures still show a zonal wave
number 2 structure with two temperature maxima in the latitude range
40–80∘ N
(Fig. e1). The two sub-vortices of the
vortex split event are still clearly visible, and the horizontal separation
between these two vortices has considerably grown
(Fig. e3). The zonal wind displays a zonal
wave number 2 pattern of alternating positive (= eastward) and negative
(= westward) winds at all latitudes north of 30∘ N. In the
latitude range 60–80∘ N negative winds are
much stronger, such that the zonal wind is negative (anomalously westward) on
zonal average. At latitudes 30–50∘ N positive
winds are stronger, but the regions of negative winds are more extended
resulting in close to zero winds on zonal average
(Fig. e2).
We still find considerable gravity wave activity related to the polar
vortices (Fig. e4
and e5).
Enhancements are found in regions of strong jet curvature (above North
America and, much weaker, over central Asia), as well as over eastern Europe,
possibly related to the jet exit region. The source altitude, however, could
be well below 30 km.
Of course, also mountain waves will play an important role. There is also
some remaining gravity wave activity over the North Pacific that seems to be
related to the anomalous westward winds in this region. It should also be
noted that, due to the off-pole displacement of the two vortices, winds are
much reduced north of 70∘ N. Further, there are two regions of
anomalous westward winds north of 60∘ N, which may increase the
probability of mountain waves to encounter wind reversals (i.e., critical
levels). Both vortex displacement and anomalous winds may therefore
contribute to the overall reduction of gravity wave activity at latitudes
north of 60∘ N (see also Figs. h
and h).
Extended phase of stratospheric anomalous winds
The sixth row in Fig. covers the time
period 8–23 February 2009, i.e., the extended period of stratospheric
anomalous westward winds in the latitude range
60–80∘ N (cf. Fig. h).
During this period, there is little structure in the temperature distribution
of the Northern Hemisphere, with a polar temperature minimum just starting to
form (Fig. f1). The polar vortices have
disappeared, and zonal wind is generally weak. There is an almost
axisymmetric band of weak anomalous westward winds at latitudes north of
about 50∘ N, while winds are prevalently eastward south of
about 50∘ N (Fig. f2
and f3).
Due to the very weak winds in the whole Northern Hemisphere, gravity wave
activity is also strongly reduced (Fig. f4
and f5).
Partly, this is the case because there are no favorable enhancements of
gravity wave saturation amplitudes by the background winds. And, particularly
at latitudes north of 50∘ N, due to the anomalous westward
winds there is an increased probability for mountain waves to encounter
critical levels before reaching the altitude level of 30 km displayed
in Fig. . The highest values of gravity wave
activity are found in the vicinity of Japan and the Korean Peninsula (see
Fig. f4).
Shortly after the period of anomalous winds
The bottom row of Fig. represents an
average over the period 5–15 March 2009. During this period, zonal average
zonal winds in the latitude range 60–80∘ N
are no longer anomalous and have turned to eastward again in the stratosphere
(cf. Fig. h). A new temperature minimum has formed at the
pole (Fig. g1), and the winds at
30 km altitude have started to strengthen again, particularly at
latitudes around 40∘ N (see
Fig. g2
and g3).
As a consequence, gravity wave activity has started to increase again,
particularly south of 50∘ N. Squared amplitudes and momentum
fluxes are maximum over northeast Asia, even reaching as far north as about
60∘ N. Compared to January 2009, however, momentum fluxes are
still relatively low, likely because of the still quite weak background winds
and thus comparably low gravity-wave saturation amplitudes.
Zonal average cross sections during the 2009 major SSW
Next, we will investigate zonal average cross sections of MLS temperatures,
SABER gravity wave squared amplitudes, absolute momentum fluxes, and gravity
wave potential drag during the major SSW in winter 2008/2009 for the same
periods as discussed in Sect. . The results are given in
Fig. for temperatures (upper row), gravity
wave squared amplitudes (second row), momentum fluxes (third row), and
gravity wave potential drag (bottom row) in the latitude range
20–90∘ N. Overplotted contour lines are zonal
average MLS geostrophic zonal winds with eastward (westward) winds indicated
by solid (dashed) contour lines. Zero zonal wind is indicated by a bold solid
contour line. Contour increment is 20 ms-1.
Unperturbed vortex during 2011
The left column of Fig. shows zonal average
temperatures, gravity wave squared amplitudes, momentum fluxes and potential
drag for the characteristic situation of an unperturbed polar jet during
13–28 February 2011. The temperature structure during this period displays
the typical wintertime split stratopause pattern
(Fig. a1) with the polar temperature
enhancement being an effect of adiabatic heating by the downwelling branch of
the Brewer Dobson circulation e.g.,. The polar
jet is tilted equatorward (with increasing altitude), displaying the
well-known funnel-like shape of an unperturbed polar vortex.
For unperturbed vortex conditions, stratospheric gravity wave squared
amplitudes and momentum fluxes are enhanced only at high latitudes where the
strong polar jet is located (Fig. b1
and c1).
At higher altitudes, the momentum flux maximum shifts from around
65∘ N in the stratosphere to about 45 in the
mesosphere, which might be an indication for gravity waves propagating from
higher to lower latitudes while propagating upward, following the tilted
polar jet. We also find enhanced gravity wave potential drag at latitudes
55–80∘ N in the altitude range
45–55 km where the polar jet significantly weakens. Obviously, this
gravity wave potential drag has a net decelerating effect at the top of the
polar jet. Another enhancement of gravity wave potential drag is found in the
upper mesosphere with particularly high values around the zero wind line at
the top of the polar jet equatorward of about 55∘ N. In the
lower part of the polar jet, where zonal wind vertical gradients are
positive, gravity wave potential drag is not as strong as in the upper part
of the jet.
Well before the central date of SSW 2009
In the period 12–16 January 2009, i.e., well before the central date of the
major SSW 2009, the zonal average temperature structure, as well as the shape
of the zonal wind jet (Fig. a2) is very
similar to the unperturbed situation in 2011 (distinct winter stratopause,
equatorward tilt of the polar jet). The main difference is that the polar
jet is somewhat stronger in the upper stratosphere and mesosphere. Further,
due to the elongation of the polar vortex (see
Fig. b3) zonal winds are somewhat stronger
even at lower latitudes.
Accordingly, gravity wave squared amplitudes and momentum fluxes
(Fig. b2
and c2) are somewhat stronger at low latitudes
in the stratosphere, and momentum fluxes are also somewhat more enhanced
around 60∘ N in the lower mesosphere. Again, high values of
gravity wave potential drag are found in the upper part of the polar jet
(around the 20 ms-1 contour line), where zonal wind
vertical gradients are negative. Another enhancement of gravity wave
potential drag is located in the upper mesosphere (above about 80 km
altitude), around the zero wind line. Again, potential drag is comparably
weak in the lower part of the polar jet.
Shortly before the central date of SSW 2009
During the period 17–21 January 2009, shortly before the central date of the
SSW, the thermal structure in the stratosphere is still close to unperturbed
conditions (Fig. a3). Only the altitude of the
polar stratopause is somewhat lower than before. At latitudes poleward of
about 40–50∘ N, however, already the well-known mesospheric cooling that is related to SSWs is observed, and the zero
wind line has started to descend from about 90 km altitude down to
about 60–70 km. On zonal average, eastward-directed zonal wind (u)
has significantly weakened in the whole altitude range. This zonally averaged
behavior, however, does not necessarily mean that the polar jet
(u2+v2) itself has weakened at all altitudes. As can be seen from
Fig. c2 and c3, the polar vortex at 30 km
altitude is still very strong. One reason is the strengthening of the
meridional wind component (v). Further, due to the extreme elongation of
the vortex, considering a fixed latitude circle the region of strong winds
now covers a narrower range of longitudes, resulting in reduced zonal average
winds. In addition, larger regions of weak westward winds, apparently
secondary circulations and outflows of the polar vortex, have formed at
midlatitudes. This indicates that for strongly perturbed vortex conditions a
zonal average view may be too simple.
Due to the extreme elongation of the polar vortex, high values of gravity
wave squared amplitudes and momentum fluxes are now spread over all latitudes
north of 30∘ N (Fig. b3
and c3).
The same is found for gravity wave potential drag
(Fig. d3). Still, enhanced values of potential
drag are found close to the zero wind line at the top of the jet around
85 km altitude in the latitude range
20–40∘ N, and poleward of 40∘ N
around 70 km altitude (i.e., somewhat above the zero wind line).
Around the central date of SSW 2009
In the period 22–26 January, which is centered around the central date of
the SSW, the polar stratopause has further descended, while zonal average
zonal winds have further weakened and are westward in the whole altitude
range 30–90 km north of 40∘ N
(Fig. a4).
Gravity wave squared amplitudes and momentum fluxes are still spread out over
a large latitude range, but have started to reduce due to the reduction of
wind speeds in the split vortex (see
Fig. d2
and d3),
and the increased probability of wind reversals at low levels. Also gravity
wave potential drag is still spread out over a large latitude range
(Fig. d4). Enhancements of potential drag are
found above 80 km altitude and, less pronounced, around 70 km
altitude.
Anomalous winds shortly after the central date of SSW 2009
Shortly after the central date, in the period 25–29 January 2009, the polar
stratopause has weakened and further descended to about 40 km
altitude (Fig. a5). At the same time, there
are first indications of a new elevated polar stratopause forming at
altitudes above 80 km. On zonal average, zonal wind is anomalously
westward in the whole Northern Hemisphere in the altitude range
30–50 km, while it is eastward in the altitude range of about
50–85 km equatorward of about 65∘ N. This, however, is
only the zonal average view of the zonal winds. As can be seen from
Fig. e2
and e3, in the stratosphere the longitudinal
structure of the horizontal winds is still quite complicated due to the split
vortex conditions and the secondary circulations (outflows) of the two polar
vortices.
Gravity wave squared amplitudes, momentum fluxes and potential drag
(Fig. b5, c5,
and d5) display a similar zonal average
structure as in the previous period (22–26 January), but have further
weakened at altitudes below about 60 km.
Extended phase of stratospheric anomalous winds
Later during the PJO event, in the period 8–23 February 2009, the old polar
stratopause has descended to an altitude of around 20 km, and the new
elevated polar stratopause is now well established and has descended to about
75 km (Fig. a6). A new strong eastward-directed polar jet has formed, which is tilted poleward (with increasing
altitude), while zonal winds are still anomalously westward below about
35 km altitude poleward of about 50∘ N.
Gravity wave squared amplitudes and momentum fluxes are now strongly reduced
in the whole Northern Hemisphere with the strongest reduction at latitudes
north of 60∘ N (Fig. b6
and c6).
Still, gravity wave squared amplitudes and momentum fluxes can attain
considerable values in the upper part of the newly formed polar jet. A
remarkable feature can be seen in the zonal average gravity wave momentum
fluxes (Fig. c6): in upper stratosphere and
lower mesosphere, a broad tongue of enhanced momentum fluxes has formed,
which extends from around 35∘ N at 40 km altitude to
around 50∘ N at 70 km. At the poleward side of this
tongue, momentum flux vertical gradients sometimes even reverse, and become
positive. This could be an indication for poleward propagation of midlatitude
gravity waves into the newly formed strong polar jet. Another explanation for
reversed vertical gradients could be gravity wave sources in this altitude
range. To our knowledge, however, there are no pronounced gravity wave
sources at mid latitudes and altitudes of 50–60 km. Further, it is
unlikely that these sources would only be active during one particular time
period of a PJO event. Therefore this explanation for reversed momentum flux
vertical gradients should be less likely.
Gravity wave potential drag (Fig. d6) is
strongly enhanced at the top of the new polar jet where zonal wind vertical
gradients are strongly negative (= westward). This enhanced drag likely
contributes significantly to the deceleration of the jet. At the bottom of
the new polar jet, however, where zonal wind vertical gradients are strongly
positive, potential drag is quite weak. This is the case even though zonal
wind vertical gradients at the top and at the bottom of this jet are
similarly strong. As already mentioned in Sect. , this
finding is in good agreement with simulations of PJO events by
.
Like in most global models, in the simulations of
only purely vertical propagation of gravity
waves is assumed. From theoretical considerations, however, refraction of
gravity waves into strong wind jets is expected if full 3-D propagation of
gravity waves is taken into account
. Evidence for this effect from
observations has been found, for example, by ,
, or for gravity waves in the
summertime subtropics, and by for mountain waves over
South America. First indication of gravity wave meridional propagation for
the re-established polar jet during PJO events has been found by
. These findings can now be further confirmed by the
characteristic zonal average distribution of gravity momentum fluxes
resulting from our study. As has been pointed out by
, meridional propagation of gravity waves is usually
not considered in gravity wave parameterizations used in global models. For
GCMs/CCMs using gravity wave parameterizations that assume only vertical
propagation of gravity waves, the simulation of elevated stratopause events
and the re-formation of the polar jet after SSWs, as well as the downward
propagation of both elevated stratopause and the polar jet after a SSW, may
therefore not be fully realistic.
Shortly after the period of anomalous winds
Somewhat later during the PJO event, in the period 5–15 March 2009, the new
elevated polar stratopause has descended to about 65 km, and also the
core of the re-established polar jet has descended to about 60 km
(Fig. a7).
During this period, gravity wave squared amplitudes and momentum fluxes have
started to increase again, particularly at mid latitudes
(Fig. b7
and c7).
A likely reason is favorably increased gravity wave saturation amplitudes due
to strengthening winds. Further, the probability for gravity waves to
encounter wind reversals due to anomalous westward winds has strongly
decreased, and troposphere and lower stratosphere will be more permeable for
gravity waves than before. Consequently, the tongue of increased momentum
fluxes, that was seen during the previous period, is now much less pronounced
and broader.
Enhanced values of gravity wave potential drag at the top of the newly formed
polar jet are now found at somewhat lower altitudes because meanwhile the jet
has somewhat descended in altitude (Fig. d7).
Further, the absence of a pronounced tongue of enhanced momentum fluxes from
lower latitudes seems to indicate that in the later phase of the newly formed
polar jet the role of gravity waves propagating meridionally from lower
latitudes into the polar jet is less dominant, and the contribution of
vertically propagating gravity waves in decelerating the polar jet at its top
has increased.
Gravity waves during different phases of the major SSW in 2006
Now, as a second event, we will investigate the development during different
phases of the major SSW/PJO event in winter 2005/2006. We will use the same
diagnostics and structure as for the 2009 PJO event.
In two ways, this SSW is different from the one in 2009: first, while the SSW
2009 was a split vortex event, dominated by a strong quasi-stationary
planetary wave 2, the major SSW in 2006 is a displaced vortex event,
dominated by a strong quasi-stationary planetary wave 1; second, before the
central date of the SSW 2009 the zonal average wind in the latitude band
60–80∘ N was strongly eastward, and the wind
reversal to westward winds took place on the central date of the major SSW.
This is different for the SSW in 2006. On zonal average, the zonal wind in
the latitude band 60–80∘ N was oscillating
between eastward and westward in the stratosphere well before the central
date (21 January 2006). See also Figs. e
and e. Therefore also the evolution of the polar jet
before the major SSW 2006, as well as its effect on the global gravity wave
distribution, is of interest and will be investigated.
Horizontal distributions at 30 km altitude
of ERA-Interim temperatures (left column), zonal wind (second column),
absolute horizontal wind (third column), and HIRDLS gravity wave
squared amplitudes in K2 on a logarithmic scale (fourth column),
as well as
HIRDLS gravity wave momentum fluxes in mPa on a linear scale (right column).
The different rows represent different periods before, during, and after
the central date (21 January) of the 2006 major SSW.
Horizontal distributions during the 2006 major SSW
To illustrate the temporal evolution of the polar vortex in winter 2005/2006,
the different rows in Fig. show horizontal
distributions at 30 km altitude for different phases of the 2006 PJO
event before, during, and after the central date. Shown are ERA-Interim
temperatures (left column), ERA-Interim zonal wind (second column) and
absolute horizontal wind (third column), as well as HIRDLS gravity wave
squared amplitudes (fourth column) and gravity wave absolute momentum fluxes
(right column). Because HIRDLS offers a much better horizontal sampling than
SABER, horizontal maps of HIRDLS gravity wave activity can be calculated and
displayed with much better horizontal resolution.
Well before the major SSW 2006,
eastward winds around the stratopause
The first period considered is 3–7 January 2006, i.e., well before the major
SSW 2006. From Fig. a1, it can be seen that
already several weeks before the central date of the SSW, there is a
pronounced planetary wave 1 structure in the temperature at 30 km
altitude. The polar vortex is displaced
(Fig. a3), but the displacement is not
strong enough to result in westward winds on zonal average
(Fig. a2). Still, there are first
indications of vortex instability and breaking of the planetary wave 1: a
region of weak westward winds (apparently some flow out of the polar
vortex) is located over North America and the North Pacific, and a region of
weak eastward winds (apparently some flow into the polar vortex) extends
from around 30∘ N over the Atlantic Ocean to the Mediterranean.
The bulk of gravity wave activity is found over the North Atlantic, Europe, and North Asia, related to the southern part of the displaced vortex. Some
enhancement that seems to be related to the weak inflowing and outflowing
circulations of the polar vortex is also seen over the Mediterranean, as well
as over the North Pacific. Overall, the distribution of gravity wave squared
amplitudes and momentum fluxes is quite spotty, which is an indication of
strongly intermittent and localized gravity wave sources. Like for the SSW
2009, the observed gravity wave distribution will be a mixture of
orographically generated, and of jet-generated gravity waves. The individual
sources, however, cannot easily be attributed from the observations alone.
Well before the major SSW 2006,
westward winds around the stratopause
Somewhat later, during 8–12 January 2006, the zonal average wind in the
latitude range 60–80∘ N is westward (see
Fig. e). As can be seen from
Fig. b1, the phase of the planetary wave 1
has shifted somewhat to the west, and the polar vortex is displaced somewhat
more to the south (Fig. b3). Due to this
slight southward shift, zonal average zonal wind is now westward close to the
North Pole (Fig. b2).
Although the vortex has only slightly shifted, this has a strong effect on the
global distribution of gravity wave squared amplitudes and momentum fluxes
(Fig. b4
and b5). The strongest gravity wave
activity is no longer found over Northern Asia, but has shifted toward
northern Europe, and strongly increased momentum fluxes are also found over
the North Atlantic. The gravity wave activity related to the weak inflow and
outflow circulations is somewhat reduced, and there is a shift from the
central Mediterranean to the western Mediterranean and to the Canary Islands
west of Africa. Overall, these strong changes show the strongly intermittent
nature particularly of orographically generated gravity waves
e.g.,,
but obviously also jet-related gravity wave sources show strong day-to-day
variability, as could be the case over the North Atlantic. Of course, it is
difficult to provide reliable estimates of intermittency time scales because
the observations are limited by the daily sampling patterns of the satellite
instruments. However, the strong changes from one 5-day period to the
following suggest that time scales are much shorter than 5 days. This
is further supported by the strong changes in hemispheric gravity wave
momentum fluxes by a factor of 3 from one day to another as obtained from
the gravity waves resolved in ECMWF analyses .
Around the central date of major SSW 2006
The period 19–23 January is centered around the central date (21 January) of
the major SSW 2006. The planetary wave 1 is strongly displaced towards lower
latitudes, such that the warm phase of the planetary wave leads to the
stratospheric warming in the polar region
(Fig. c1). The polar vortex is strongly
displaced towards western Europe, and it has considerably weakened
(Fig. c3). Due to the strong displacement,
zonal winds are anomalously westward east of Greenland. Further, winds are
slightly westward over North America, Asia, and the North Pacific
(Fig. c2). Therefore, on zonal average,
zonal wind is slightly westward north of about 50∘ N.
Compared to the previous periods, gravity wave activity has considerably
weakened. Also compared to the period centered around the central date of the
major SSW 2009 gravity wave activity is much weaker. One of the reasons is
the much smaller size of the polar vortex during the SSW 2006.
This becomes obvious, in particular, when comparing gravity wave momentum
flux (5th column) and absolute wind velocities (3rd column) for the two cases
in Figs.
and .
Around the central date of the major SSW 2009 almost the entire Northern
Hemisphere was affected by the split vortex and its secondary inflow and
outflow circulations, resulting in large regions of enhanced gravity wave
activity.
Different from this, around the central date of the major SSW 2006 we find
only several hot spots of gravity wave activity in the vicinity of the much
smaller polar vortex (Fig. c4
and c5).
Two hot spots of strong gravity wave momentum fluxes are located over
Greenland and the Alps, and may therefore be caused by strong activity of
mountain waves. Another hot spot of momentum fluxes is located over the North
Atlantic around 25∘ W and 45∘ N, in a region
where strong deceleration of the polar jet is observed. This indicates that
these gravity waves could be excited by jet-related sources in the vicinity
of the jet exit region.
Another enhancement of momentum fluxes is found close to the North African
coast at the southern edge of the polar vortex. Because enhanced momentum
fluxes persistently show up in this region, this may be an indication of
orographic sources, for example the Atlas Mountains.
Zonal average cross-sections of
MLS temperatures (upper),
as well as a combination of HIRDLS and SABER gravity wave
squared amplitudes in K2 (second row),
momentum fluxes in mPa (third row),
and potential drag in ms-1day-1 (lower)
for the latitudes
20–90∘ N
during different phases of the strong major SSW in winter 2005/2006.
For the lower three rows logarithmic color scales are used.
Overplotted contour lines indicate the zonal average of MLS geostrophic
zonal winds,
averaged over the respective time periods shown.
Contour interval is 20 ms-1.
Dashed contour lines indicate westward wind.
Strong anomalous winds shortly after the central
date of major SSW 2006
The next period is from 22–26 January 2006. Somewhat overlapping with the
previous period, it covers the phase of strongest stratospheric anomalous
westward winds in the polar region
(Fig. d2
and d3). While temperatures in the polar
region are still strongly enhanced due to the displaced planetary wave 1
(Fig. d1), the polar vortex has further
weakened and has started to decay
(Fig. d3).
Gravity wave activity is now strongly reduced
(Fig. d4
and d5). Apart from some scattered gravity
wave activity, there are just three momentum flux maxima south of
60∘ N. The first maximum is located over the Alps, the second
over the Atlas Mountains, and the third over eastern Europe. Not much
momentum flux is left north of 60∘ N, possibly a consequence of
the anomalous westward winds causing wind reversals at lower altitudes.
Growing stage of new polar jet
The following period is from 1–10 February 2006. As can be seen from
Fig. e, this is a phase of anomalous westward winds that are
persisting in the polar stratosphere. These winds are situated below a polar
jet that is newly forming at higher altitudes. From
Fig. e1 we find that the planetary wave 1
has almost dissipated, and only little temperature variation is left in the
Northern Hemisphere at 30 km altitude. Also the polar vortex has
decayed, and winds at 30 km are anomalously westward in most of the
polar region (Fig. e2
and e3).
Enhanced gravity wave activity is found mainly south of 60∘ N
over western Asia and northern Africa, related to the final remnants of the
polar vortex (Fig. e4
and e5). Particularly the western Asian
region is very mountainous, and part of the momentum flux enhancement may be
due to mountain waves.
Mature stage of new polar jet
During 27 February until 3 March 2006, a new temperature minimum has started
to form close to the North Pole (Fig. f1).
Winds in the polar region are still weak, but at mid latitudes three regions
of enhanced eastward wind have emerged
(Fig. f2
and f3). The first region is located above
the coast of northeastern North America and extends over the Atlantic Ocean, the
second region is located over the Mediterranean, and the third region is
located over northeastern Asia.
Interestingly, strong gravity wave activity is found closely related to those
three regions of enhanced winds (Fig. f4
and f5). While some of the gravity wave
activity will be related to favorable enhancements of saturation amplitudes
or to sources directly related to those wind systems, also hot spots of
gravity wave momentum fluxes are located close to known sources of mountain
waves. For example, increased momentum fluxes are seen over North America in
the vicinity of the Appalachian Mountains, and along the eastern Canadian
coast. Also the gravity waves around the Mediterranean Sea may partly be
mountain waves. The strongest hot spot, however, is located over the coast
range of northeastern Asia. Some enhancements are also seen over Kamchatka and
over the mountains in southeastern Russia and northeastern China. Another hot spot
located at 40∘ N, 180∘ E might be caused by a
localized weather system.
First weakening of new polar jet
Finally, during the period 10–14 March 2006, the temperature minimum is
somewhat displaced from the North Pole
(Fig. g1). Over most of the Northern
Hemisphere, we find weak eastward-directed winds. Only over the North Pacific
the winds are quite weak and partly directed westward
(Fig. g2
and g3). The strongest winds are found over
North America, the Atlantic Ocean, and close to the North Pole, north of the
Bering Strait.
During this period, we find scattered gravity wave activity in most of the
Northern Hemisphere, even at latitudes north of 60∘ N. There is
only one major region of low gravity-wave activity over the North Pacific.
This region coincides with the above mentioned region of comparably weak
background winds. Another region of low gravity-wave activity is found over
the North Pole. However, compared with the previously discussed period 27
February until 3 March, its area is somewhat smaller, and its gravity wave
activity somewhat stronger (cf. Fig. f4
and g4).
Zonal average cross sections during the 2006 major SSW
In the following, we will investigate zonal average cross sections of MLS
temperatures, gravity wave squared amplitudes, absolute momentum fluxes, and
gravity wave potential drag during the major SSW in winter 2005/2006 for the
same periods as discussed in Sect. . The gravity wave
cross sections are a combination of HIRDLS below 50 km and SABER
above 55 km, with a smooth transition between HIRDLS and SABER in the
altitude range 50–55 km. The results are given in
Fig. for temperatures (upper row),
gravity wave squared amplitudes (second row), momentum fluxes (third row),
and gravity wave potential drag (bottom row) in the latitude range
20–90∘ N. Again, overplotted contour lines
indicate zonal average MLS geostrophic zonal winds.
Well before the major SSW 2006,
eastward winds around the stratopause
In the first period considered (3–7 January 2006) the polar stratopause is
not as pronounced as during unperturbed conditions
(cf. Figs. a1
and a1).
One of the reasons may be that the stratospheric polar vortex is already
somewhat displaced and perturbed
(cf. Fig. a3). For the same reason, in
Fig. a1 the zonal average wind in the
stratosphere is only slightly positive (= eastward). Also the larger region of
weak westward winds that (at 30 km altitude) is located over North
America and the North Pacific will contribute to the reduction on zonal
average (see Fig. a2). Different from this,
at 30∘ N in the mesosphere one part of the vortex is as strong
as about 40 ms-1 (i.e., similarly strong as for
unperturbed vortex conditions), and therefore does not seem to be affected
much. A similar situation when the polar vortex is shifted off-pole during a
non-SSW period is found during 22–26 January 2010. This suggests that the
case from early January 2006 is not a singular event, and that a zonal
average view of polar vortex dynamics may be too simple.
During 3–7 January 2006, gravity wave squared amplitudes and momentum fluxes
(Fig. b1
and c1) are still high with a
pronounced maximum around 60∘ N. As can be seen from
Fig. a3, a4, and a5, this maximum is caused by the high
gravity-wave activity in the southern part of the polar jet. Please note that
above 55 km no gravity wave data are available at latitudes north of
about 50∘ N because SABER is in southward-viewing geometry;
this is still the case for the time period 8–12 January, discussed in the
next subsection.
Some enhancement of gravity wave potential drag
(Fig. d1) is found already in the
stratosphere, similar to during unperturbed vortex conditions, or during the
period well before the major SSW 2009 (cf.
Fig. a5
and b5). Further, we find a strong enhancement
of potential drag close to the top of the mesospheric part of the polar jet.
Well before the major SSW 2006,
westward winds around the stratopause
During the second period (8–12 January 2006), we find a pronounced polar
stratopause (Fig. a2). Zonal average
zonal wind is negative (= westward) at latitudes north of
40–50∘ N and altitudes above about 30–40 km. Again,
this is an effect of the vortex displacement, and of the large region of weak
westward winds over North America and the North Pacific. Compared to the
previous period, this region has even grown in size (cf.
Fig. b2). Like before, the mesospheric part
of the polar vortex is strong with zonal average eastward-directed wind
speeds exceeding 40 ms-1 at latitudes south of about
30∘ N.
Gravity wave activity is enhanced only in the vicinity of the polar vortex
and not affected much by the extended region of weak westward winds.
Therefore zonal average distributions of squared amplitudes, momentum fluxes
and potential drag are very similar to those discussed in
Sect. . This shows that a zonal average view may be
too simple, if the distribution of zonal winds has a complicated longitudinal
structure. On the other hand, the zonal average distribution of gravity wave
activity and potential drag has not changed much, even though there is a
considerable change in the longitudinal distribution of gravity wave activity
(cf. Fig. a5
and b5).
Around the central date of major SSW 2006
In the time period 19–23 January, centered around the central date (21
January) of the 2006 major SSW, the polar stratopause has started to weaken
and to descend (Fig. a3). Related to
the oscillation of the polar vortex, there were already mesospheric coolings
during the periods 3–7 and 8–12 January (see also
Fig. e). The cooling around the central date is not more
pronounced, and therefore not much difference is seen when comparing
Fig. a3 with
Fig. a1
and a2. At altitudes above about
30 km the zonal average zonal wind is anomalously westward north of
about 50∘ N. South of 50∘ N the zonal wind is
eastward on average.
In the stratosphere, gravity wave squared amplitudes and momentum fluxes have
started to decrease (Fig. b3
and c3).
Still, a maximum is found at latitudes 40–70∘ N in the
stratosphere. This maximum is mainly caused by the gravity wave hot spots
seen in Fig. c4
and c5. Compared to the previous periods,
however, gravity wave potential drag is strongly reduced in the stratosphere
(Fig. d3). In the mesosphere, however,
potential drag is strongly enhanced at altitudes above 70 km, where
zonal average winds are weakening (or even reversing above 85 km
equatorward of 30∘ N).
Strong anomalous winds shortly after the central
date of major SSW 2006
Shortly after the central date of the major SSW 2006, during the time period
22–26 January, the polar stratopause has descended to about 40 km,
and there is still no indication for the forming of an elevated stratopause
(Fig. a4). The zonal average
distributions of the zonal winds, as well as of the gravity wave squared
amplitudes, momentum fluxes and potential drag are very similar to those in
the period 19–23 January containing the central date of the SSW. The only
differences are that squared amplitudes, momentum fluxes and potential drag
in the stratosphere have further decreased
(Fig. b4,
c4, and d4). In addition, the anomalous westward winds have somewhat descended in altitude, while eastward winds have extended to higher latitudes above about 75 km, thereby forming some
kind of transition stage towards the re-establishment of a new poleward
tilted eastward-directed polar jet.
Growing stage of new polar jet
During the period 1–10 February, the old stratopause has descended to below
30 km, and a new stratopause has formed around 75 km
(Fig. a5). A new eastward-directed
poleward tilted polar jet has formed, while zonal winds are quite weak and directed westward in the polar stratopause (poleward of 50∘ N
below about 40 km altitude).
As a consequence of these weak and westward-directed winds, gravity wave
squared amplitudes and momentum fluxes are strongly reduced in the polar
stratosphere (Fig. b5,
and c5). The momentum flux distribution
is somewhat tilted, following the upper part of the new polar jet, but a
pronounced tongue of enhanced momentum fluxes, like after the major SSW 2009
(cf. Fig. c6), does not show up. Still,
meridional propagation of gravity waves may play an important role. Enhanced
gravity wave potential drag is mainly found at the top of the new polar jet
(at altitudes above about 60 km) where vertical gradients of the
zonal wind are strong (Fig. d5). Some
weak enhancement of potential drag is also found in the lower part of the new
jet in regions of strong vertical gradients of the zonal wind.
Mature stage of new polar jet
In the period 27 February until 3 March 2006, the elevated polar stratopause
has descended to about 60 km
(Fig. a6). Compared to the period 1–10
February, the new polar jet has considerably strengthened and descended in
altitude. On zonal average, zonal wind in the lower stratosphere is weak and
eastward.
Gravity wave squared amplitudes and momentum fluxes are still weak in the
lower polar stratosphere (Fig. b6
and c6), but have started to increase
around 40 km, related to the increased winds in the polar jet. Still,
some poleward tilt is found in the momentum flux distribution at altitudes
above about 50 km.
There is some enhancement of gravity wave potential drag in the strong zonal
wind vertical gradients at the bottom of the new polar jet
(Fig. d6). These values are somewhat
stronger than those found for comparable conditions during the jet recovery
of the 2009 PJO event (Fig. d7), which
indicates that during the jet recovery of the 2006 PJO event the troposphere
and lower stratosphere are more permeable to gravity wave propagation from
below. This is confirmed by the horizontal distributions of squared
amplitudes and momentum fluxes during these periods (cf.
Figs. g4,
g5,
and f4,
f5). However, like for the PJO event in
2009, gravity wave potential drag related to the strong zonal wind vertical
gradients at the top of the new polar jet is much stronger and likely
contributes to the deceleration and reversal of the zonal winds at the top of
the jet.
First weakening of new polar jet
In the period 10–14 March 2006, the elevated polar stratopause has descended
to below 60 km (Fig. a7). Also
the new polar jet has further descended and has started to weaken.
In the stratosphere, gravity wave squared amplitudes and momentum fluxes have
further increased (Fig. b7
and c7), and there is some kind of
double peak structure with peaks at about 40 and
60∘ N. This double peak is caused by the distribution of the
spots of enhanced gravity wave activity, as can be seen from
Fig. g4,
and g5. There is only little indication for
a poleward tilt of the zonal average momentum flux distribution.
The zonal average distribution of gravity wave potential drag
(Fig. d7) is very similar to the
distribution during the previous period of the PJO event. However, potential
drag has somewhat decreased in the lower part of the new polar jet, and in
the stratosphere potential drag at the poleward side of the jet is now
stronger than at the equatorward side.
Summary and discussion
In our work, we investigate the effect of gravity waves during the boreal
winters 2001/2002 until 2013/2014 in the whole middle atmosphere
(20–90 km altitude) based on observations of the infrared limb
sounding instruments SABER and HIRDLS (depending on data availability).
Altitude–time cross sections illustrate the evolution of zonal average
temperatures (Fig. ) and zonal winds
(Fig. ) at latitudes
60–80∘ N. Temperatures were taken from SABER
and MLS observations, as well as from ERA-Interim, and also winds are a
composite of ERA-Interim winds and of geostrophic winds derived from MLS and
SABER observations. In the 13 winters considered, there are only a few
winters when the polar vortex was little perturbed (the winters 2004/2005,
2010/2011, and 2013/2014), while most of the other winters had at least one
major SSW. In six of the perturbed winters, a polar-night jet oscillation
(PJO) event took place (in the winters 2003/2004, 2005/2006, 2008/2009,
2009/2010, 2011/2012, and 2012/2013). During these events, the polar
stratopause rapidly drops in altitude, a new elevated stratopause forms after
the SSW at altitudes around 75 km, and a new polar jet is
re-established. Both, elevated stratopause and new polar jet gradually
descend in altitude with time over a period of about 1–2 months.
Altitude–time cross sections of observed gravity wave squared amplitudes,
momentum fluxes and potential drag (Figs. ,
, and ) give an overview
of the different effects of gravity waves during the different conditions of
the polar vortex. Particularly, the interaction between gravity waves and the
background winds has a strong influence on the gravity wave time series. For
example, enhanced values of gravity wave potential drag (i.e., strong
vertical gradients of absolute momentum fluxes) are often observed where
zonal wind vertical gradients are strong. This is mainly the case in the
mesosphere, and related to strong zonal wind vertical gradients at the top of
the polar jet, but sometimes enhanced potential drag is also seen together
with strong zonal wind vertical gradients related to anomalous westward winds
after a SSW.
For unperturbed or just somewhat perturbed vortex conditions, there is
notable gravity wave activity already in the stratosphere, because usually
there is no wind reversal that would filter out relevant parts of the gravity
wave spectrum excited by tropospheric sources. The distribution of gravity
wave potential drag indicates that dissipating gravity waves will contribute
to the zonal momentum budget in the stratosphere. Values are, however, more
enhanced in the upper part of the jet, related to negative (i.e., westward-directed) vertical gradients of the zonal wind. This suggests that
dissipating gravity waves contribute significantly to the deceleration and
reversal of the polar jets in the (upper) mesosphere.
Before or around the central date of major SSWs, sometimes enhanced
stratospheric gravity wave activity is found in the latitude range
60–80∘ N. However, this is not always the
case. Obviously, the particular shape and position of the polar vortex plays
an important role: enhancements of gravity wave activity seem to be more likely for vortex split
events (cf. Fig. ).
This is further confirmed by investigating the horizontal distributions of
gravity wave activity at 30 km altitude before, during, and after the
central dates of the major SSWs 2006 and 2009
(Figs.
and ). Both these SSWs are PJO events, but
they are very different in their temporal evolution.
The SSW 2009 is a vortex split event (i.e., strong activity of planetary wave
2). During the evolution of the vortex, we find strong gravity wave activity
in regions that are known as hot spots of mountain wave activity, for example
over the Rocky Mountains and Scandinavia. However, there is also a lot of
gravity wave activity due to jet-related source processes. For example,
enhanced gravity wave activity is found in regions of strong jet deceleration
(jet exit regions) and of strong curvature of the jet. Of course, the source
altitude of the gravity waves seen in these regions at 30 km could be
well below this altitude.
In addition, enhanced gravity wave activity is found coinciding with patterns
of horizontal winds that are caused by secondary circulations (vortex
outflows) that seem to be related to the breaking of the planetary wave 2.
The gravity wave distribution changes rapidly within only a few days .
Further, due to the strong elongation and later split of the vortex, shortly
before and around the central date of the SSW almost the entire Northern
Hemisphere north of about 30∘ N is covered with enhanced
gravity wave activity. This is one of the reasons why gravity wave activity
increases on zonal average shortly before the central date of the SSW.
The situation is very different for the SSW 2006. This SSW is a vortex
displacement event (i.e., strong activity of planetary wave 1). Compared to
the SSW 2009, the polar vortex covers a much smaller area, and it even does
not elongate much during the evolution of the SSW. Therefore also the area of
enhanced gravity wave activity is smaller than during the SSW 2009, and since
the vortex does not extend much, gravity wave activity does not increase much
on zonal average (at 60–80∘ N) around the
central date of the SSW 2006. Also during the evolution of the SSW 2006,
there seems to be gravity wave activity due to jet-related sources. However,
at 30 km altitude only little activity is found over the patterns of
horizontal winds that are apparently caused by secondary circulations (vortex
inflow and outflow) related to vortex instability and breaking of the
planetary wave 1. On the other hand, there are a lot of very localized hot
spots that could be caused by mountain waves, for example over northeastern
North America, Scandinavia, the Alps, the Atlas Mountains, and later, during the
jet recovery, also over western Asia and northeast Asia. The distribution of
these hot spots changes very rapidly, as a consequence of the rapid vortex
evolution, and also due to the strongly intermittent gravity wave source
processes.
It should also be mentioned that, for both SSWs considered, the gravity wave
distribution follows the absolute wind velocity and displays a strong
longitudinal structure. A zonal average view may therefore be too simple, at
least during some phases of the vortex evolution. Further, during phases of
vortex displacement or elongation, gravity wave activity may be strongly
enhanced over a large range of latitudes, which could have an important
effect on the overall residual meridional circulation, and thereby on the
evolution of the SSW.
In particular, our findings support the study by ,
and it is suggested that gravity waves may contribute to the triggering of
SSWs by preconditioning the shape of the polar vortex such that a SSW can
take place.
During PJO events, shortly after the central date of the SSW (or shortly
after the SSW in cases of PJO events related to a minor SSW) a new eastward-directed polar jet emerges around 75 km altitude. Different from the
“regular” polar jet, which is usually tilted equatorward, this newly formed
jet is tilted poleward. Below this new jet, zonal winds in the stratosphere
are usually very weak, and initially they are prevalently directed
anomalously westward. Due to these weak winds, there is no favorable
enhancement of gravity wave saturation amplitudes for any gravity wave
propagation direction. Furthermore, due to anomalously westward winds, there
is an increased probability for low horizontal phase speed gravity waves, for
example mountain waves, to encounter critical wind levels.
For these reasons, stratospheric gravity wave activity (amplitudes and
momentum fluxes) is very weak during the phase of jet recovery. Therefore,
although gravity waves with eastward-directed phase speeds in the wide range
of about 0–80 ms-1 encounter critical wind levels in
the lower part of the new polar jet, little gravity wave potential drag is
observed. Different from this, we find quite strong potential drag in the
wind shear at the top of the newly formed polar jet. In spite of the weak
stratospheric gravity wave activity, these values of potential drag are
comparable to those during unperturbed vortex conditions.
The weak potential drag on the lower flank of the new polar jet indicates
that the strong winds in the jet are not caused directly by wave driving of
the polar jet. Instead, the re-establishment of the polar jet is induced by
changes in the residual circulation.
Still, the descent of the shear zone on the upper flank of the new polar jet,
and thus also the formation and descent of the newly formed elevated
stratopause is likely dynamically driven by breaking gravity waves, as
indicated by the enhanced gravity wave potential drag. These findings are
qualitatively in good agreement with modeling studies by, for example,
, or .
It is also noteworthy that during the first phase of jet recovery after the
SSW 2009, a poleward tilt of the observed zonal average momentum flux
distribution indicates that meridional propagation of gravity waves from
lower latitudes may also be important for explaining the strong momentum
fluxes and potential drag at the top of the new polar jet. This confirms
first indications from observed gravity wave variances and a gravity wave ray
tracing study by . As has been pointed out by
, this effect is not included in gravity wave drag
parameterizations that assume only vertical propagation of gravity waves.
Later during the jet recovery, propagation conditions for gravity waves
improve, and vertically propagating gravity waves become more and more
important, as indicated by a less tilted momentum flux distribution. During
the jet recovery after the SSW 2006, the troposphere and stratosphere seem to
be more permeable to gravity waves, and meridional propagation seems to be
somewhat less important than during the recovery after the SSW 2009.
Of course, gravity wave observations by limb-viewing satellite instruments
such as HIRDLS and SABER have several limitations. First, there are
constraints by the observational filter. Only gravity waves with horizontal
wavelengths longer than about 100–200 km are visible for those
instruments. For details about the observation geometry and the resulting
sensitivity for gravity waves see, for example, or
. Second, no directional information is available, and
only absolute values of gravity wave momentum flux and potential drag can be
derived. Further, errors of observed momentum fluxes and potential drag are
quite large (at least a factor of 2).
However, uncertainties in modeling SSWs are sometimes even larger. For
example, in model simulations of PJO events values of gravity wave drag at
the top of the new polar jet after the SSW range from about
30 ms-1day-1e.g., to about
150 ms-1day-1e.g.,. This means that, in spite of their large
uncertainties, gravity wave observations by current satellite instruments
like HIRDLS and SABER provide an important confirmation of our physical
understanding of the dynamics of SSWs and PJO events. In addition, these
observations indicate that models should be improved: the pronounced
longitudinal structure and the strong day-to-day variation of the global
gravity wave distribution shows the need for global models to include
physical gravity wave sources that are as realistic as possible. Further,
also non-vertical propagation of gravity waves should be considered. A more
quantitative observational approach would be possible by the limb imaging
technique, i.e., an improvement of conventional limb measurement techniques
e.g.,. This
technique would provide directional information of gravity waves, and also
errors could be considerably reduced.
Data availability
The used ERA-Interim data
can be retrieved from ECMWF Web API:
https://software.ecmwf.int/wiki/display/WEBAPI/Access+ECMWF+Public+Datasets.
The satellite data used in our study are open access.
HIRDLS and MLS data are freely available from the
NASA Goddard Earth Sciences Data and Information Services Center (GES DISC)
at http://disc.sci.gsfc.nasa.gov/Aura.
SABER data are freely available from GATS Inc. at http://saber.gats-inc.com.
Acknowledgements
This work was partly supported by the Deutsche Forschungsgemeinschaft (DFG)
project PR 919/4–1 (MS–GWaves/SV)
which is part of the DFG researchers group MS–GWaves,
by the DFG project ER 474/3–1 (TigerUC)
which is part of the DFG priority program SPP1788 “Dynamic Earth”,
as well as by the
Bundesministerium für Bildung und Forschung
(BMBF) project no. 01LG1206C (ROMIC/GW–LCYCLE).
Work at the Jet Propulsion
Laboratory, California Institute of Technology, was done under contract
with NASA.
We thank NASA for providing access to the HIRDLS version V006 and to the
Aura-MLS version 3.3 level 2 data.
These data are freely available via the
NASA Goddard Earth Sciences Data and Information Services Center (GES DISC)
at http://disc.sci.gsfc.nasa.gov/Aura.
ERA-Interim data were obtained from ECMWF (http://www.ecmwf.int).
SABER data were provided by GATS Inc. and are
freely available at http://saber.gats-inc.com.
The authors would like to thank the teams of the HIRDLS, MLS and SABER
instruments for their effort in providing and continuously improving
the high-quality data sets used in this study.
Very helpful comments by three anonymous reviewers are
gratefully acknowledged.
The article processing charges for this open-access publication were covered by a Research Centre of the Helmholtz Association.
Edited by: F. Khosrawi
Reviewed by: three anonymous referees
ReferencesAlbers, J. R. and Birner, T.:
Vortex preconditioning due to planetary and gravity waves prior to sudden
stratospheric warmings,
J. Atmos. Sci., 71, 4028–4054, 10.1175/JAS-D-14-0026.1, 2014.Alexander, M. J., Gille, J., Cavanaugh, C., Coffey, M., Craig, C.,
Eden, T., Francis, G., Halvorson, C., Hannigan, J., Khosravi, R.,
Kinnison, D., Lee, H., Massie, S., Nardi, B., Barnett, J., Hepplewhite, C.,
Lambert, A., and Dean, V.:
Global estimates of gravity wave momentum flux from High Resolution
Dynamics Limb Sounder Observations,
J. Geophys. Res., 113, D15S18,
10.1029/2007JD008807, 2008.Alexander, M. J., Geller, M., McLandress, C., Polavarapu, S., Preusse, P.,
Sassi, F., Sato, K., Eckermann, S. D., Ern, M., Hertzog, A., Kawatani, Y.,
Pulido, M., Shaw, T., Sigmond, M., Vincent, R., and Watanabe, S.:
Recent developments in gravity-wave effects in climate models
and the global distribution of gravity-wave momentum flux from
observations and models,
Q. J. Roy. Meteor. Soc., 136,
1103–1124, 10.1002/qj.637, 2010.Angot, G., Keckhut, P., Hauchecorne, A., and Claud, C.:
Contribution of stratospheric warmings to temperature trends in the
middle atmosphere from the lidar series obtained at
Haute-Provence Observatory (44∘ N),
J. Geophys. Res., 117, D21102, 10.1029/2012JD017631, 2012.Bailey, S. M., Thurairajah, B., Randall, C. E.,
Holt, L., Siskind, D. E., Harvey, V. L., Venkataramani, K.,
Hervig, M. E., Rong, P., and Russell III, J. M.:
A multi tracer analysis of thermosphere to stratosphere
descent triggered by the 2013 Stratospheric Sudden Warming,
Geophys. Res. Lett., 41, 5216–5222,
10.1002/2014GL059860, 2014.
Baldwin, M. P. and Dunkerton, T. J.:
Stratospheric harbingers of anomalous weather regimes,
Science, 294, 581–584, 2001.Barnett, J. J., Hepplewhite, C. L., Osprey, S., Gille, J. C., and
Khosravi, R.:
Cross-validation of HIRDLS and COSMIC radio-occultation retrievals,
particularly in relation to fine vertical structure,
Proc. SPIE Int. Soc. Opt. Eng., 7082,
708216, 10.1117/12.800702, 2008.
Becker, E. and Fritts, D. C.:
Enhanced gravity-wave activity and interhemispheric coupling
during the MaCWAVE/MIDAS northern summer program 2002,
Ann. Geophys., 24,
1175–1188, 2006.Butler, A. H., Seidel, D. J., Hardiman, S. C., Butchart, N., Birner, T.,
and Match, A.:
Defining sudden stratospheric warmings,
B. Am. Meteorol. Soc.,
96, 1913–1928, 10.1175/BAMS-D-13-00173.1, 2015.Chandran, A., Collins, R. L., Garcia, R. R., and Marsh, D. R.:
A case study of an elevated stratopause generated in the
whole atmosphere community climate model,
Geophys. Res. Lett., 38, L08804, 10.1029/2010GL046566, 2011.Chandran, A., Garcia, R. R., Collins, R. L., and Chang, L. C.:
Secondary planetary waves in the middle and upper atmosphere following the
stratospheric sudden warming event of January 2012,
Geophys. Res. Lett., 40, 1861–1867, 10.1002/grl.50373, 2013Chandran, A., Collins, R. L., and Harvey, V. L.:
Stratosphere-mesosphere coupling during stratospheric sudden warming events,
Adv. Space Res., 53, 1265–1289,
10.1016/j.asr.2014.02.005, 2014.Charlton, A. J., and Polvani, L. M.:
A new look at stratospheric sudden warming.
Part I: Climatology and modeling benchmarks,
J. Climate, 20, 449–469, 10.1175/JCLI3996.1, 2007.Cohen, J. and Jones, J.:
Tropospheric precursors and stratospheric warmings,
J. Climate, 24, 6562–6572,
10.1175/2011JCLI4160.1, 2011.Cullens, C. Y., England, S. L., and Immel, T. J.:
Global responses of gravity waves to planetary waves during
stratospheric sudden warming observed by SABER,
J. Geophys. Res.-Atmos., 120, 12018–12026, 10.1002/2015JD023966, 2015.Dee, D. P., Uppala, S. M., Simmons, A. J., Berrisford, P., Poli, P.,
Kobayashi, S., Andrae, U., Balmaseda, M. A., Balsamo, G.,
Bauer, P., Bechtold, P., Beljaars, A. C. M., van de Berg, L., Bidlot, J.,
Bormann, N., Delsol, C., Dragani, R., Fuentes, M., Geer, A. J.,
Haimberger, L., Healy, S. B., Hersbach, H., Holm, E. V.,
Isaksen, L., Kallberg, P., Koehler, M., Matricardi, M., McNally, A. P.,
Monge-Sanz, B. M., Morcrette, J.-J., Park, B.-K., Peubey, C.,
de Rosnay, P., Tavolato, C., Thepaut, J.-N., and Vitart, F.:
The ERA-Interim reanalysis: configuration and performance of
the data assimilation system,
Q. J. Roy. Meteor. Soc., 137,
553–597, 10.1002/qj.828, 2011.deWit, R. J., Hibbins, R. E., Espy, P. J., Orsolini, Y. J., Limpasuvan, V.,
and Kinnison, D. E.:
Observations of gravity wave forcing of the mesopause region during the
January 2013 major Sudden Stratospheric Warming,
Geophys. Res. Lett., 41, 4745–4752,
10.1002/2014GL060501, 2014.deWit, R. J., Hibbins, R. E., Espy, P. J., and Hennum, E. A.:
Coupling in the middle atmosphere related to the 2013 major
sudden stratospheric warming,
Ann. Geophys., 33, 309–319,
10.5194/angeo-33-309-2015, 2015.Domeisen, D. I. V., Butler, A. H., Fröhlich, K., Bittner, M.,
Müller, W. A., and Baehr, J.:
Seasonal predictability over Europe arising from el Niño and stratospheric
variability in the MPI-ESM seasonal prediction system,
J. Climate, 28, 256–271,
10.1175/JCLI-D-14-00207.1, 2015.
Duck, T. J., Whiteway, J. A., and Carswell, A. I.:
Lidar observations of gravity wave activity and Arctic stratospheric
vortex core warming,
Geophys. Res. Lett., 25, 2813–2816, 1998.Eckermann, S. D. and Preusse, P.:
Global measurements of stratospheric mountain waves from space,
Science, 286, 1534–1537, 10.1126/science.286.5444.1534, 1999.
Eguchi, N., and Kodera, K.:
Impacts of stratospheric sudden warming on tropical clouds and moisture
fields in the TTL: A case study,
SOLA, 6, 137–140, 2010.Ern, M. and Preusse, P.: Wave fluxes of equatorial Kelvin waves and QBO zonal wind forcing derived from
SABER and ECMWF temperature space-time spectra, Atmos. Chem. Phys., 9, 3957–3986, 10.5194/acp-9-3957-2009, 2009.Ern, M. and Preusse, P.:
Gravity wave momentum flux spectra observed from satellite
in the summertime subtropics: Implications for global modeling,
Geophys. Res. Lett., 39, L15810,
10.1029/2012GL052659, 2012.Ern, M., Preusse, P., Alexander, M. J., and Warner, C. D.:
Absolute values of gravity wave momentum flux derived from satellite data,
J. Geophys. Res., 109, D20103,
10.1029/2004JD004752, 2004.Ern, M., Preusse, P., and Warner, C. D.: Some experimental constraints for spectral parameters used in the Warner and
McIntyre gravity wave parameterization scheme, Atmos. Chem. Phys., 6, 4361–4381, 10.5194/acp-6-4361-2006, 2006.Ern, M., Preusse, P., Krebsbach, M., Mlynczak, M. G., and Russell III, J. M.: Equatorial wave analysis
from SABER and ECMWF temperatures, Atmos. Chem. Phys., 8, 845–869, 10.5194/acp-8-845-2008, 2008.
Ern, M., Lehmann, C., Kaufmann, M., and Riese, M.:
Spectral wave analysis at the mesopause from SCIAMACHY airglow data
compared to SABER temperature spectra,
Ann. Geophys., 27, 407–416, 2009.Ern, M., Preusse, P., Gille, J. C., Hepplewhite, C. L., Mlynczak, M. G.,
Russell III, J. M., and Riese, M.:
Implications for atmospheric dynamics derived from global observations of
gravity wave momentum flux in stratosphere and mesosphere,
J. Geophys. Res., 116,
D19107, 10.1029/2011JD015821, 2011.Ern, M., Preusse, P., Kalisch, S., Kaufmann, M., and Riese, M.:
Role of gravity waves in the forcing of quasi two-day waves in the
mesosphere: An observational study,
J. Geophys. Res.-Atmos., 118, 3467–3485, 10.1029/2012JD018208, 2013.Ern, M., Ploeger, F., Preusse, P., Gille, J. C., Gray, L. J., Kalisch, S.,
Mlynczak, M. G., Russell III, J. M., and Riese, M.:
Interaction of gravity waves with the QBO: A satellite perspective,
J. Geophys. Res.-Atmos., 119, 2329–2355, 10.1002/2013JD020731, 2014.Ern, M., Preusse, P., and Riese, M.:
Driving of the SAO by gravity waves as observed from satellite,
Ann. Geophys., 33, 483–504, 10.5194/angeo-33-483-2015, 2015.France, J. A., Harvey, V. L., Alexander, M. J., Randall, C. E.,
and Gille, J. C.:
High resolution dynamics limb sounder observations of the gravity
wave-driven elevated stratopause in 2006,
J. Geophys. Res., 117, D20108, 10.1029/2012JD017958, 2012.Fuller–Rowell, T., Wu, F., Akmaev, R., Fang, T.–W.,
and Araujo–Pradere, E.:
A whole atmosphere model simulation of the impact of a sudden stratospheric
warming on thermosphere dynamics and electrodynamics,
J. Geophys. Res., 115, A00G08, 10.1029/2010JA015524, 2010.Geller, M. A., Alexander, M. J., Love, P. T., Bacmeister, J., Ern, M.,
Hertzog, A., Manzini, E., Preusse, P., Sato, K., Scaife, A. A.,
and Zhou, T.:
A comparison between gravity wave momentum fluxes in observations
and climate models,
J. Climate, 26, 6383–6405, 10.1175/JCLI-D-12-00545.1, 2013.Gerber, E. P., Butler, A., Calvo, N., Charlton-Perez, A., Giorgetta, M.,
Manzini, E., Perlwitz, J., Polvani, L. M., Sassi, F., Scaife, A. A.,
Shaw, T. A., Son, S.–W., and Watanabe, S.:
Assessing and understanding the impact of stratospheric dynamics and
variability on the Earth system,
B. Am. Meteorol. Soc., 93,
845–859, 10.1175/BAMS-D-11-00145.1, 2012.
Gille, J. C., Barnett, J. J., Whitney, J., Dials, M., Woodard, D.,
Rudolf, W., Lambert, A., and Mankin, W.:
The High Resolution Dynamics Limb Sounder (HIRDLS) experiment on Aura,
Proc. SPIE Int. Soc. Opt. Eng., 5152,
162–171, 2003.Gille, J. C., Barnett, J., Arter, P., Barker, M., Bernath, P., Boone, C.,
Cavanaugh, C., Chow, J., Coffey, M., Craft, J., Craig, C., Dials, M.,
Dean, V., Eden, T., Edwards, D. P., Francis, G., Halvorson, C., Harvey, L.,
Hepplewhite, C., Khosravi, R., Kinnison, D., Krinsky, C., Lambert, A.,
Lee, H., Lyjak, L., Loh, J., Mankin, W., Massie, S., McInerney, J.,
Moorhouse, J., Nardi, B., Packman, D., Randall, C., Reburn, J., Rudolf, W.,
Schwartz, M., Serafin, J., Stone, K., Torpy, B., Walker, K., Waterfall, A.,
Watkins, R., Whitney, J., Woodard, D., and Young, G.:
High Resolution Dynamics Limb Sounder: Experiment overview, recovery,
and validation of initial temperature data,
J. Geophys. Res., 113,
D16S43, 10.1029/2007JD008824, 2008.Gille, J. C., Gray, L. J., Cavanaugh, C., Choi, K. Y., Coffey, M.,
Craig, C., Karol, S., Hepplewhite, C., Khosravi, R., Kinnison, D.,
Massie, S., Nardi, B., Belmonte Rivas, M., Smith, L., Waterfall, A.,
and Wright, C.:
High Resolution Dynamics Limb Sounder Earth Observing System (EOS):
Data description and quality, Version 6,
available at:
http://archive-eos.acom.ucar.edu/hirdls/data/products/HIRDLS-DQD_V6-1.pdf
(last access: 15 July 2016), 2011.Goncharenko, L. and Zhang, S.–R.:
Ionospheric signatures of sudden stratospheric warming: Ion temperature
at middle latitude,
Geophys. Res. Lett., 35, L21103, 10.1029/2008GL035684, 2008.
Hertzog, A., Boccara, G., Vincent, R. A., Vial, F., and Coquerez, Ph.:
Estimation of gravity-wave momentum flux and phase speeds from long-duration
stratospheric balloon flights: 2. Results from the Vorcore campaign
in Antarctica,
J. Atmos. Sci., 65, 3056–3070, 2008.
Hertzog, A., Alexander, M. J., and Plougonven, R.:
On the intermittency of gravity wave momentum flux in the stratosphere,
J. Atmos. Sci., 69, 3433–3448, 2012.Hindley, N. P., Wright, C. J., Smith, N. D., and Mitchell, N. J.: The southern stratospheric gravity wave hot
spot: individual waves and their momentum fluxes measured by COSMIC GPS-RO, Atmos. Chem. Phys., 15, 7797–7818, 10.5194/acp-15-7797-2015, 2015.
Hitchcock, P. and Shepherd, T. G.:
Zonal-mean dynamics of extended recoveries from stratospheric sudden
warmings,
J. Atmos. Sci., 70, 688–707, 2013.Hitchcock, P. and Simpson, I. R.:
The downward influence of stratospheric sudden warmings,
J. Atmos. Sci., 71, 3856–3876, 10.1175/JAS-D-14-0012.1, 2014.
Hitchman, M. H., Gille, J. C., Rodgers, C. D., and Brasseur, G.:
The separated polar winter stratopause: A gravity wave driven
climatological feature,
J. Atmos. Sci., 46, 410–422, 1989.
Hoffmann, P., Singer, W., and Keuer, D.:
Variability of the mesospheric wind field at middle and Arctic latitudes
in winter and its relation to stratospheric circulation disturbances,
J. Atmos. Sol. Terr. Phys., 64, 1229–1240, 2002.
Hoffmann, P., Singer, W., Keuer, D., Hocking, W., Kunze, M.,
and Murayama, Y.:
Latitudinal and longitudinal variability of mesospheric winds and
temperatures during stratospheric warming events,
J. Atmos. Sol. Terr. Phys., 69, 2355–2366, 2007.Hoffmann, L., Xue, X., and Alexander, M. J.:
A global view of stratospheric gravity wave hotspots located with
Atmospheric Infrared Sounder observations,
J. Geophys. Res.-Atmos., 118, 416–434,
10.1029/2012JD018658, 2013.
Holton, J. R.:
The influence of gravity wave breaking on the general
circulation of the middle atmosphere,
J. Atmos. Sci., 40, 2497–2507, 1983.Holton, J. R. and Tan, H.–C.:
The influence of the equatorial quasi-biennial oscillation on the global
circulation at 50 mb,
J. Atmos. Sci., 37, 2200–2208, 1980.Jacobi, C., Kürschner, D., Muller, H. G., Pancheva, D., Mitchell, N. J.,
and Naujokat, B.:
Response of the mesopause region dynamics to the February 2001 stratospheric
warming,
J. Atmos. Sol. Terr. Phys., 65, 843–855,
10.1016/S1364-6826(03)00086-5, 2003.Jia, Y., Zhang, S. D., Yi, F., Huang, C. M., Huang, K. M., Gan, Q.,
and Gong, Y.:
Observations of gravity wave activity during stratospheric sudden warmings
in the Northern Hemisphere,
Sci. China Tech. Sci., 58, 951–960,
10.1007/s11431-015-5806-3, 2015.Jiang, J. H., Wu, D. L., and Eckermann, S. D.:
Upper Atmosphere Research Satellite (UARS) MLS observation of
mountain waves over the Andes,
J. Geophys. Res., 107, 8273,
10.1029/2002JD002091, 2002.Jiang, J. H., Eckermann, S. D., Wu, D. L., and Ma, J.:
A search for mountain waves in MLS stratospheric limb radiances from the
winter northern hemisphere: Data analysis and global mountain wave modeling,
J. Geophys. Res., 109, D03107, 10.1029/2003JD003974, 2004a.Jiang, J. H., Wang, B., Goya, K., Hocke, K., Eckermann, S. D., Ma, J.,
Wu, D. L., and Read, W. G.:
Geographical distribution and interseasonal variability of tropical deep
convection: UARS MLS observations and analyses,
J. Geophys. Res., 109, D03111, 10.1029/2003JD003756, 2004b.Kalisch, S., Preusse, P., Ern, M., Eckermann, S. D., and Riese, M.:
Differences in gravity wave drag between realistic oblique
and assumed vertical propagation,
J. Geophys. Res.-Atmos., 119, 10,081–10,099,
10.1002/2014JD021779, 2014.Kidston, J., Scaife, A. A., Hardiman, S. C., Mitchell, D. M., Butchart, N.,
Baldwin, M. P., and Gray, L. J.:
Stratospheric influence on tropospheric jet streams, storm tracks and
surface weather,
Nat. Geosci., 8, 433–440,
10.1038/NGEO2424, 2015.Kodera, K.:
Influence of stratospheric sudden warming on the equatorial troposphere,
Geophys. Res. Lett., 33, L06804,
10.1029/2005GL024510, 2006.Kodera, K., Mukougawa, H., Maury, P., Ueda, M., and Claud, C.:
Absorbing and reflecting sudden stratospheric warming events and their
relationship with tropospheric circulation,
J. Geophys. Res.-Atmos., 121, 80–94, 10.1002/2015JD023359, 2016.Konopka, P., Engel, A., Funke, B., Müller, R., Grooss, J.–U.,
Günther, G., Wetter, T., Stiller, G., von Clarmann, T.,
Glatthor, N., Oelhaf, H., Wetzel, G., Lopez-Puertas, M.,
Pirre, M., Huret, N., and Riese, M.:
Ozone loss driven by nitrogen oxides and triggered by stratospheric warmings
can outweigh the effect of halogens,
J. Geophys. Res., 112, D05105, 10.1029/2006JD007064, 2007.Kuttippurath, J. and Nikulin, G.: A comparative study of the major sudden
stratospheric warmings in the Arctic winters 2003/2004–2009/2010, Atmos.
Chem. Phys., 12, 8115–8129, 10.5194/acp-12-8115-2012, 2012.
Labitzke, K.:
Temperature changes in the mesosphere connected with circulation
changes in winter,
J. Atmos. Sci., 29, 756–766, 1972.Limpasuvan, V., Alexander, M. J., Orsolini, Y. J., Wu, D. L., Xue, M.,
Richter, J. H., and Yamashita, C.:
Mesoscale simulations of gravity waves during the 2008–2009
major stratospheric sudden warming,
J. Geophys. Res., 116, D17104, 10.1029/2010JD015190, 2011.Limpasuvan, V., Richter, J. H., Orsolini, Y. J., Stordal, F.,
and Kvissel, O.–K.:
The roles of planetary and gravity waves during a major stratospheric
sudden warming as characterized in WACCM,
J. Atmos. Sol. Terr. Phys., 78/79, 84–98,
10.1016/j.jastp.2011.03.004, 2012.
Livesey, N. J., Read, W. G., Froidevaux, L., Lambert, A., Manney, G. L.,
Pumphrey, H. C., Santee, M. L., Schwartz, M. J., Wang, S., Cofield, R. E.,
Cuddy, D. T., Fuller, R. A., Jarnot, R. F., Jiang, J. H., Knosp, B. W.,
Stek, P. C., Wagner, P. A., and Wu, D. L.:
Earth Observing System (EOS) Aura Microwave Limb Sounder (MLS)
Version 3.3 and 3.4 Level 2 data quality and
description document,
Technical Report JPL D-33509, Version 3.3x/3.4x-1.1,
Jet Propulsion Lab.,
California Institute of Technology, Pasadena, California 91109-8099, 2013.Manney, G. L., Krüger, K., Pawson, S., Minschwaner, K., Schwartz, M. J.,
Daffer, W. H., Livesey, N. J., Mlynczak, M. G., Remsberg, E. E.,
Russell III, J. M., and Waters, J. W.:
The evolution of the stratopause during the 2006 major warming:
Satellite data and assimilated meteorological analyses,
J. Geophys. Res., 113, D11115, 10.1029/2007JD009097, 2008.Manney, G. L., Schwartz, M. J., Krüger, K., Santee, M. L., Pawson, S.,
Lee, J. N., Daffer, W. H., Fuller, R. A., and Livesey, N. J.:
Aura Microwave Limb Sounder observations of dynamics and transport
during the record-breaking 2009 Arctic stratospheric major warming,
Geophys. Res. Lett., 36, L12815, 10.1029/2009GL038586, 2009a.Manney, G. L., Harwood, R. S., MacKenzie, I. A., Minschwaner, K., Allen, D.
R., Santee, M. L., Walker, K. A., Hegglin, M. I., Lambert, A., Pumphrey, H.
C., Bernath, P. F., Boone, C. D., Schwartz, M. J., Livesey, N. J., Daffer, W.
H., and Fuller, R. A.: Satellite observations and modeling of transport in
the upper troposphere through the lower mesosphere during the 2006 major
stratospheric sudden warming, Atmos. Chem. Phys., 9, 4775–4795,
10.5194/acp-9-4775-2009, 2009b.Matthias, V., Hoffmann, P., Rapp, M., and Baumgarten, G.:
Composite analysis of the temporal development of waves in the polar MLT
region during stratospheric warmings,
J. Atmos. Sol. Terr. Phys., 90/91, 86–96,
10.1016/j.jastp.2012.04.004, 2012.
Matsuno, T.:
A dynamical model of the stratospheric sudden warming,
J. Atmos. Sci., 28, 1479–1494, 1971.
McLandress, C., Scinocca, J. F., Shepherd, T. G., Reader, M. C.,
and Manney, G. L.:
Dynamical control of the mesosphere by orographic and nonorographic
gravity wave drag during the extended northern winters of 2006 and 2009,
J. Atmos. Sci., 70, 2152–2169, 2013.Miller, A., Schmidt, H., and Bunzel, F.:
Vertical coupling of the middle atmosphere during stratospheric warming
events,
J. Atmos. Sol. Terr. Phys., 97, 11–21,
10.1016/j.jastp.2013.02.008, 2013.
Mlynczak, M. G.:
Energetics of the mesosphere and lower thermosphere and the SABER instrument,
Adv. Space Res., 44, 1177–1183, 1997.Naoe, H. and Shibata, K.:
Future changes in the influence of the quasi-biennial oscillation
on the northern polar vortex simulated with an MRI chemistry climate model,
J. Geophys. Res., 117,
D03102, 10.1029/2011JD016255, 2012.Oberheide, J., Lehmacher, G. A., Offermann, D., Grossmann, K. U.,
Manson, A. H., Meek, C. E., Schmidlin, F. J., Singer, W., Hoffmann, P.,
and Vincent, R. A.:
Geostrophic wind fields in the stratosphere and mesosphere from satellite
data,
J. Geophys. Res., 107, 8175,
10.1029/2001JD000655, 2002.O'Callaghan, A., Joshi, M., Stevens, D., and Mitchell, D.:
The effects of different sudden stratospheric warming type on the ocean,
Geophys. Res. Lett., 41, 7739–7745,
10.1002/2014GL062179, 2014.Orr, A., Bechtold, P., Scinocca, J. F., Ern, M., and Janiskova, M.:
Improved middle atmosphere climate and forecasts in the ECMWF model
through a nonorographic gravity wave drag parameterization,
J. Climate, 23, 5905–5926, 10.1175/2010JCLI3490.1, 2010.Orsolini, Y. J., Urban, J., Murtagh, D. P., Lossow, S.,
and Limpasuvan, V.:
Descent from the polar mesosphere and anomalously high
stratopause observed in 8 years of water vapor and temperature satellite
observations by the Odin Sub-Millimeter Radiometer,
J. Geophys. Res., 115, D12305, 10.1029/2009JD013501, 2010.Placke, M., Hoffmann, P., Latteck, R., and Rapp, M.:
Gravity wave momentum fluxes from MF and meteor radar measurements in
the polar MLT region,
J. Geophys. Res.-Space, 120, 736–750, 10.1002/2014JA020460, 2015.Plougonven, R. and Zhang, F.:
Internal gravity waves from atmospheric jets and fronts,
Rev. Geophys., 52, 33–76, 10.1002/2012RG000419, 2014.Preusse, P., Dörnbrack, A., Eckermann, S. D., Riese, M., Schaeler, B.,
Bacmeister, J. T., Broutman, D., and Grossmann, K. U.:
Space-based measurements of stratospheric mountain waves by CRISTA,
1. Sensitivity, analysis method, and a case study,
J. Geophys. Res., 106,
8178, 10.1029/2001JD000699, 2002.Preusse, P., Ern, M., Eckermann, S. D., Warner, C. D., Picard, R. H.,
Knieling, P., Krebsbach, M., Russell III, J. M., Mlynczak, M. G.,
Mertens, C. J., and Riese, M.:
Tropopause to mesopause gravity waves in August: Measurement and modeling,
J. Atmos. Sol. Terr. Phys., 68, 1730–1751,
10.1016/j.jastp.2005.10.019, 2006.Preusse, P., Schroeder, S., Hoffmann, L., Ern, M., Friedl-Vallon, F.,
Ungermann, J., Oelhaf, H., Fischer, H., and Riese, M.: New perspectives on
gravity wave remote sensing by spaceborne infrared limb imaging, Atmos. Meas.
Tech., 2, 299–311, 10.5194/amt-2-299-2009, 2009a.Preusse, P., Eckermann, S. D., Ern, M., Oberheide, J., Picard, R. H.,
Roble, R. G., Riese, M., Russell III, J. M., and Mlynczak, M. G.:
Global ray tracing simulations of the SABER gravity wave climatology,
J. Geophys. Res., 114, D08126, 10.1029/2008JD011214, 2009b.Preusse, P., Ern, M., Bechtold, P., Eckermann, S. D., Kalisch, S., Trinh, Q. T., and Riese, M.: Characteristics of
gravity waves resolved by ECMWF, Atmos. Chem. Phys., 14, 10483–10508, 10.5194/acp-14-10483-2014, 2014.Remsberg, E. E., Gordley, L. L., Marshall, B. T., Thompson, R. E.,
Burton, J., Bhatt, P., Harvey, V. L., Lingenfelser, G.,
and Natarajan, M.:
The Nimbus 7 LIMS version 6 radiance conditioning and temperature
retrieval methods and results,
J. Quant. Spectrosc. Radiat. Transfer,
86, 395–424, 10.1016/j.jqsrt.2003.12.007, 2004.Remsberg, E. E., Marshall, B. T., Garcia-Comas, M., Krueger, D.,
Lingenfelser, G. S., Martin-Torres, J., Mlynczak, M. G., Russell III, J. M.,
Smith, A. K., Zhao, Y., Brown, C., Gordley, L. L., Lopez-Gonzalez, M. J.,
Lopez-Puertas, M., She, C.-Y., Taylor, M. J., and Thompson, R. E.:
Assessment of the quality of the Version 1.07 temperature-versus-pressure
profiles of the middle atmosphere from TIMED/SABER,
J. Geophys. Res., 113,
D17101, 10.1029/2008JD010013, 2008.Ren, S., Polavarapu, S., Beagley, S. R., Nezlin, Y., and Rochon, Y. J.:
The impact of gravity wave drag on mesospheric analyses of the 2006
stratospheric major warming,
J. Geophys. Res., 116, D19116, 10.1029/2011JD015943, 2011.
Richter, J. H., Sassi, F., and Garcia, R. R.:
Toward a physically based gravity wave source parameterization in
a general circulation model,
J. Atmos. Sci., 67, 136–156, 2010.Riese, M., Oelhaf, H., Preusse, P., Blank, J., Ern, M., Friedl-Vallon, F.,
Fischer, H., Guggenmoser, T., Höpfner, M., Hoor, P., Kaufmann, M.,
Orphal, J., Plöger, F., Spang, R., Suminska-Ebersoldt, O., Ungermann, J.,
Vogel, B., and Woiwode, W.: Gimballed Limb Observer for Radiance Imaging of
the Atmosphere (GLORIA) scientific objectives, Atmos. Meas. Tech., 7,
1915–1928, 10.5194/amt-7-1915-2014, 2014.Riggin, D. M., Tsuda, T., and Shinbori, A.:
Evaluation of momentum flux with radar,
J. Atmos. Sol. Terr. Phys., 142, 98–107,
10.1016/j.jastp.2016.01.013, 2016.
Russell III, J. M., Mlynczak, M. G., Gordley, L. L., Tansock, J., and
Esplin, R.:
An overview of the SABER experiment and preliminary calibration results,
Proc. SPIE Int. Soc. Opt. Eng., 3756,
277–288, 1999.Salmi, S.-M., Verronen, P. T., Thölix, L., Kyrölä, E., Backman,
L., Karpechko, A. Yu., and Seppälä, A.: Mesosphere-to-stratosphere
descent of odd nitrogen in February–March 2009 after sudden stratospheric
warming, Atmos. Chem. Phys., 11, 4645–4655, 10.5194/acp-11-4645-2011,
2011.Scaife, A. A., Athanassiadou, M., Andrews, M., Arribas, A., Baldwin, M.,
Dunstone, N., Knight, J., MacLachlan, C., Manzini, E., Müller, W. A.,
Pohlmann, H., Smith, D., Stockdale, T., and Williams, A.:
Predictability of the quasi-biennial oscillation and its northern winter
teleconnection on seasonal to decadal timescales,
Geophys. Res. Lett., 41, 1752–1758,
10.1002/2013GL059160, 2014.
Scherhag, R.:
Die explosionsartigen Stratosphärenerwärmungen des
Spätwinters 1951/52,
Berichte des Deutschen Wetterdienstes in der US Zone,
38, 51–63, 1952.Schroeder, S., Preusse, P., Ern, M., and Riese, M.:
Gravity waves resolved in ECMWF and measured by SABER,
Geophys. Res. Lett., 36,
L10805, 10.1029/2008GL037054, 2009.Schwartz, M. J., Lambert, A., Manney, G. L., Read, W. G., Livesey, N. J.,
Froidevaux, L., Ao, C. O., Bernath, P. F., Boone, C. D., Cofield, R. E.,
Daffer, W. H., Drouin, B. J., Fetzer, E. J., Fuller, R. A., Jarnot, R. F.,
Jiang, J. H., Jiang, Y. B., Knosp, B. W., Krüger, K., Li, J.-L. F.,
Mlynczak, M. G., Pawson, S., Russell III, J. M., Santee, M. L.,
Snyder, W. V., Stek, P. C., Thurstans, R. P., Tompkins, A. M., Wagner, P. A.,
Walker, K. A., Waters, J. W., and Wu, D. L.:
Validation of the Aura Microwave Limb Sounder temperature and
geopotential height measurements,
J. Geophys. Res., 113,
D15S11, 10.1029/2007JD008783, 2008.Shepherd, M. G., Wu, D. L., Fedulina, I. N., Gurubaran, S., Russell, J. M.,
Mlynczak, M. G., and Shepherd, G. G.:
Stratospheric warming effects on the tropical mesospheric temperature field,
J. Atmos. Sol. Terr. Phys., 69, 2309–2337,
10.1016/j.jastp.2007.04.009, 2007.Sigmond, M., Scinocca, J. F., Kharin, V. V., and Shepherd, T. G.:
Enhanced seasonal forecast skill following stratospheric sudden warmings,
Nat. Geosci., 6, 98–102,
10.1038/NGEO1698, 2013.Siskind, D. E., Coy, L., and Espy, P.:
Observations of stratospheric warmings and mesospheric coolings
by the TIMED SABER instrument,
Geophys. Res. Lett., 32,
L09804, 10.1029/2005GL022399, 2005.Siskind, D. E., Eckermann, S. D., McCormack, J. P., Coy, L., Hoppel, K. W.,
and Baker, N. L.:
Case studies of the mesospheric response to recent minor, major
and extended stratospheric warmings,
J. Geophys. Res., 114, D00N03, 10.1029/2010JD014114, 2010.Tao, M., Konopka, P., Ploeger, F., Grooß, J.-U., Müller, R., Volk, C. M., Walker, K. A., and
Riese, M.: Impact of the 2009 major sudden stratospheric warming on the composition of the stratosphere,
Atmos. Chem. Phys., 15, 8695–8715, 10.5194/acp-15-8695-2015, 2015a.Tao, M., Konopka, P., Ploeger, F., Riese, M., Müller, R.,
and Volk, C. M.:
Impact of stratospheric major warmings and the quasi-biennial
oscillation on the variability of stratospheric water vapor,
Geophys. Res. Lett., 42, 10.1002/2015GL064443, 2015b.Thurairajah, B., Collins, R. L., Harvey, V. L., Lieberman, R. S.,
Gerding, M., Mizutani, K., and Livingston, J. M.:
Gravity wave activity in the Arctic stratosphere and mesosphere during the
2007–2008 and 2008–2009 stratospheric sudden warming events,
J. Geophys. Res., 115, D00N06, 10.1029/2010JD014125, 2010.Thurairajah, B., Bailey, S. M., Cullens, C. Y., Hervig, M. E.,
and Russell III, J. M.:
Gravity wave activity during recent stratospheric sudden warming events
from SOFIE temperature measurements,
J. Geophys. Res.-Atmos., 119, 8091–8103,
10.1002/2014JD021763, 2014.Tomikawa, Y., Sato, K., Watanabe, S., Kawatani, Y., Miyazaki, K.,
and Takahashi, M.:
Growth of planetary waves and the formation of an elevated stratopause
after a major stratospheric sudden warming in a T213L256 GCM,
J. Geophys. Res., 117, D16101, 10.1029/2011JD017243, 2012.Trinh, Q. T., Kalisch, S., Preusse, P., Chun, H.-Y., Eckermann, S. D., Ern,
M., and Riese, M.: A comprehensive observational filter for satellite
infrared limb sounding of gravity waves, Atmos. Meas. Tech., 8, 1491–1517,
10.5194/amt-8-1491-2015, 2015.von Hobe, M., Bekki, S., Borrmann, S., Cairo, F., D'Amato, F., Di Donfrancesco, G., Dörnbrack, A., Ebersoldt, A.,
Ebert, M., Emde, C., Engel, I., Ern, M., Frey, W., Genco, S., Griessbach, S., Grooß, J.-U., Gulde, T., Günther, G., Hösen, E.,
Hoffmann, L., Homonnai, V., Hoyle, C. R., Isaksen, I. S. A., Jackson, D. R., Jánosi, I. M., Jones, R. L.,
Kandler, K., Kalicinsky, C., Keil, A., Khaykin, S. M., Khosrawi, F., Kivi, R., Kuttippurath, J., Laube, J. C., Lefèvre, F., Lehmann, R.,
Ludmann, S., Luo, B. P., Marchand, M., Meyer, J., Mitev, V., Molleker, S., Müller, R., Oelhaf, H., Olschewski, F., Orsolini, Y.,
Peter, T., Pfeilsticker, K., Piesch, C., Pitts, M. C., Poole, L. R., Pope, F. D., Ravegnani, F., Rex, M., Riese, M.,
Röckmann, T., Rognerud, B., Roiger, A., Rolf, C., Santee, M. L., Scheibe, M., Schiller, C., Schlager, H., Siciliani de Cumis, M.,
Sitnikov, N., Søvde, O. A., Spang, R., Spelten, N., Stordal, F., Suminska-Ebersoldt, O., Ulanovski, A., Ungermann, J.,
Viciani, S., Volk, C. M., vom Scheidt, M., von der Gathen, P., Walker, K., Wegner, T., Weigel, R., Weinbruch, S.,
Wetzel, G., Wienhold, F. G., Wohltmann, I., Woiwode, W., Young, I. A. K., Yushkov, V., Zobrist, B., and Stroh, F.:
Reconciliation of essential process parameters for an enhanced predictability of Arctic stratospheric ozone loss and
its climate interactions (RECONCILE): activities and results, Atmos. Chem. Phys., 13, 9233–9268, 10.5194/acp-13-9233-2013, 2013.Wang, L. and Alexander, M. J.:
Gravity wave activity during stratospheric sudden warmings in the
2007–2008 Northern Hemisphere winter,
J. Geophys. Res., 114, D18108, 10.1029/2009JD011867, 2009.
Warner, C. D., Scaife, A. A., and Butchart, N.:
Filtering of parameterized nonorographic gravity waves in the
Met Office unified model,
J. Atmos. Sci., 62, 1831–1848, 2005.Waters, J. W., Froidevaux, L., Harwood, R. S., Jarnot, R. F., Pickett, H. M.,
Read, W. G., Siegel, P. H., Cofield, R. E., Filipiak, M. J., Flower, D. A.,
Holden, J. R., Lau, G. K., Livesey, N. J., Manney, G. L., Pumphrey, H. C.,
Santee, M. L., Wu, D. L., Cuddy, D. T., Lay, R. R., Loo, M. S.,
Perun, V. S., Schwartz, M. J., Stek, P. C., Thurstans, R. P., Boyles, M. A.,
Chandra, K. M., Chavez, M. C., Chen, G.-S., Chudasama, B. V., Dodge, R.,
Fuller, R. A., Girard, M. A., Jiang, J. H., Jiang, Y., Knosp, B. W.,
LaBelle, R. C., Lam, J. C., Lee, K. A., Miller, D., Oswald, J. E.,
Patel, N. C., Pukala, D. M., Quintero, O., Scaff, D. M., Van Snyder, W.,
Tope, M. C., Wagner, P. A., and Walch, M. J.:
The Earth Observing System Microwave Limb Sounder (EOS MLS) on the Aura
Satellite,
IEEE Transactions on Geoscience and Remote Sensing, 44,
1075–1092, 10.1109/TGRS.2006.873771, 2006.Wright, C. J., Osprey, S. M., Barnett, J. J., Gray, L. J.,
and Gille, J. C.:
High Resolution Dynamics Limb Sounder measurements of gravity wave activity
in the 2006 Arctic stratosphere,
J. Geophys. Res., 115, D02105,
10.1029/2009JD011858, 2010.
Wright, C. J., Rivas, M. B., and Gille, J. C.: Intercomparisons of HIRDLS,
COSMIC and SABER for the detection of stratospheric gravity waves, Atmos.
Meas. Tech., 4, 1581–1591, 10.5194/amt-4-1581-2011, 2011.Wright, C. J., Osprey, S. M., and Gille, J. C.:
Global observations of gravity wave intermittency and its impact
on the observed momentum flux morphology,
J. Geophys. Res.-Atmos., 118, 10,980–10,993,
10.1002/jgrd.50869, 2013.Yamashita, C., Liu, H.–L., and Chu, X.:
Gravity wave variations during the 2009 stratospheric sudden warming
as revealed by ECMWF T799 and observations,
Geophys. Res. Lett., 37, L22806, 10.1029/2010GL045437, 2010a.Yamashita, C., Liu, H.-L., and Chu, X.:
Responses of mesosphere and lower thermosphere temperatures to
gravity wave forcing during stratospheric sudden warming,
Geophys. Res. Lett., 37, L09803, 10.1029/2009GL042351, 2010b.Yamashita, C., England, S. L., Immel, T. J., and Chang, L. C.:
Gravity wave variations during elevated stratopause events using
SABER observations,
J. Geophys. Res.-Atmos., 118, 5297–5306,
10.1002/jgrd.50474, 2013.Yigit, E., Medvedev, A. S., England, S. L., and Immel, T. J.:
Simulated variability of the high-latitude thermosphere induced by
small-scale gravity waves during a sudden stratospheric warming,
J. Geophys. Res.-Space, 119, 357–365,
10.1002/2013JA019283, 2014.Zülicke, C. and Becker, E.:
The structure of the mesosphere during sudden stratospheric warmings
in a global circulation model,
J. Geophys. Res.-Atmos., 118, 2255–2271,
10.1002/jgrd.50219, 2013.