ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-16-2703-2016Transport pathways from the Asian monsoon anticyclone to the stratosphereGarnyHellahella.garny@dlr.deRandelWilliam J.https://orcid.org/0000-0002-5999-7162Deutsches Zentrum für Luft- und Raumfahrt, Institut für Physik der Atmosphäre, Oberpfaffenhofen, GermanyNational Center for Atmospheric Research, Boulder, CO, USAHella Garny (hella.garny@dlr.de)3March2016164270327185August201525September20152January201617February2016This 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/16/2703/2016/acp-16-2703-2016.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/16/2703/2016/acp-16-2703-2016.pdf
Transport pathways of air originating in the upper-tropospheric Asian monsoon
anticyclone are investigated based on three-dimensional trajectories. The
Asian monsoon anticyclone emerges in response to persistent deep convection
over India and southeast Asia in northern summer, and this convection is
associated with rapid transport from the surface to the upper troposphere and possibly into the stratosphere. Here, we investigate the fate of air that
originates within the upper-tropospheric anticyclone from the outflow of deep
convection, using trajectories driven by ERA-interim reanalysis data.
Calculations include isentropic estimates, plus fully three-dimensional
results based on kinematic and diabatic transport calculations. Isentropic
calculations show that air parcels are typically confined within the
anticyclone for 10–20 days and spread over the tropical belt within
a month of their initialization. However, only few parcels (3 % at
360 K, 8 % at 380 K) reach the extratropical
stratosphere by isentropic transport. When considering vertical transport we
find that 31 % or 48 % of the trajectories reach the
stratosphere within 60 days when using vertical velocities or
diabatic heating rates to calculate vertical transport, respectively. In both
cases, most parcels that reach the stratosphere are transported upward within
the anticyclone and enter the stratosphere in the tropics, typically
10–20 days after their initialization at 360 K. This
suggests that trace gases, including pollutants, that are transported into
the stratosphere via the Asian monsoon system are in a position to enter the
tropical pipe and thus be transported into the deep stratosphere. Sensitivity
calculations with respect to the initial altitude of the trajectories showed
that air needs to be transported to levels of 360 K or above by deep
convection to likely (≧ 50 %) reach the stratosphere through
transport by the large-scale circulation.
Left panel: mean JJA 2006 divergence over 20–120∘ E
overlaid with mean zonal winds (light gray: negative; dark gray: positive;
contour interval at 10 m s-1). Right panel: mean profile of divergence
in theta coordinates averaged over 20–120∘ E and 0–30∘ N
(thick solid black line) and daily profiles (thin gray
lines).
Introduction
The atmospheric circulation associated with the Asian summer monsoon leads to
efficient vertical transport from the surface to the upper troposphere. The
upper-tropospheric monsoon circulation consists of a large anticyclone of a size similar to the northern stratospheric winter polar vortex. Distinct
tracer anomalies in the Asian monsoon anticyclone provide a signature of
strong upward transport from the surface to the upper troposphere
.
suggested that the Asian monsoon system provides a potentially efficient
pathway to the tropical stratosphere, based on satellite observations of
hydrogen cyanide (HCN). The seasonality of the Asian summer monsoon and thus
of transport along this pathway likely contributes to an HCN maximum in the
tropical lower stratosphere during Northern Hemisphere (NH) summer, which then propagates
vertically with the Brewer–Dobson circulation, creating a “tape
recorder”-like signal of HCN in the tropical stratosphere
. A number of studies have used backward
trajectories to study transport through the tropical tropopause layer from
heights of convective outflow to the lower stratosphere .
Consistent with , they
conclude that, in northern summer, the Asian monsoon anticyclone is the
dominant source region of young air in the lower stratosphere. A similar
conclusion has recently been drawn by in a systematic
analysis of air mass origins in the tropical lower stratosphere.
On the other hand, the anticyclone is strongly variable in its extent,
location and strength, and frequent eddy shedding indicates substantial flux
out of the anticyclonic circulation hereafter GR13. suggested that interaction with
extratropical Rossby waves could mix air from the anticyclone into the
extratropics. Recently, reported on a case of transport of
air from the anticyclone to the northern extratropics by eddy shedding, as
observed during an aircraft measurement campaign. Recent work has also
highlighted that the monsoon circulation provides effective mixing between
the extratropics and tropics (on the eastern and western flanks of the
anticyclone), which influences the tropical seasonal cycle of ozone and other
constituents . However, it is
unclear how much air from the interior of the anticyclone, or air recently
transported in deep convection, participates in this mixing to the extratropics.
The main goal of this study is to investigate the transport pathways and
destinations of air originating within the upper-tropospheric Asian monsoon
anticyclone. In particular, we study the efficiency, timescales and
preferred pathways of transport from the anticyclone into the stratosphere.
Possible pathways include quasi-horizontal mixing across the edge of the
anticyclone and into the extratropical stratosphere and vertical transport
into the tropical stratosphere, from where air can be transported into the
deep stratosphere by the Brewer–Dobson circulation. We calculate
trajectories on isentropic surfaces to study the confinement of the
anticyclone and quasi-horizontal mixing and additionally investigate the
full three-dimensional transport.
Previous studies have used back trajectories to evaluate the source regions
of air in the tropical lower stratosphere or boundary layer source regions for the upper-tropospheric anticyclone . In contrast, we utilize forward
trajectories initialized within the upper-tropospheric anticyclone. In
effect, we assume that air has been transported into the upper troposphere,
either by convection and/or large-scale advection, and only regard the
further transport. We focus on trajectories released at the 360 K
level, which is close to the maxima in the wind jets comprising the
anticyclone and close to the level of maximum divergence in meteorological
analyses (see Fig. ). The levels of convective injection are
not well known from observations: derived the heights of
convective outflow in the deep tropics from observed profiles of temperature
and water vapor and find strongest convective mass flux divergence around
340–365 K (10–17 km), peaking at 347 K.
, and estimate
cloud top heights from brightness temperatures combined with
precipitation in the case of and infer levels of convective
injection in northern summer peaking as high as 360 to 370 K. In
Sect. , we examine the sensitivity of our results to
a broad range of injection heights. These calculations provide an estimate of
the convective injection height necessary for air to be transported further
upward and into the stratosphere.
One common problem of three-dimensional Lagrangian trajectory modeling is the
uncertainty and excessive noisiness that exists in vertical motion fields.
Two methods are commonly used: the kinematic approach uses vertical
velocities as provided by the reanalysis products, while the diabatic
approach uses diabatic heating rates as vertical velocities in a coordinate
system with potential temperature as vertical coordinate. The noisy character
of vertical velocities as provided by (re-)analysis data sets usually results
in strong dispersion of kinematic trajectories , resulting in unrealistic transport
characteristics such as excessive age of air in the stratosphere
. In our study, we use both kinematic and
diabatic trajectory calculations to test sensitivities and estimate the
uncertainties associated with the vertical velocities.
Methods
Trajectories are calculated using a simple parcel trajectory model that was
implemented for the purpose of this study. The model is a standard fourth-order
Runge–Kutta trajectory calculation with a time step of 0.5 h, driven
by winds and heating rates from reanalysis products. We calculate
two-dimensional trajectories at isentropic levels as well as full
three-dimensional trajectories. The three-dimensional trajectories are
calculated using both vertical velocities as given by the reanalysis fields
(“kinematic” trajectories) and using heating rates as vertical velocities
(“diabatic” trajectories).
The reanalysis used for both isentropic and three-dimensional trajectories is
the ERA-Interim data set from the European Centre for Medium-range Weather Forecasts (ECMWF) . We use the
ERA-Interim wind fields with a horizontal resolution of
1.5∘× 1.5∘ latitude and longitude and 6-hourly temporal resolution. For
kinematic calculations, we use data on 37 pressure levels from the surface to
1 hPa. For diabatic calculations, winds and heating rates are
interpolated to 42 levels of constant potential temperature ranging from
250 to 2500 K, with 10 K resolution between 250 and
450 K. Heating rates are provided as ERA-Interim forecast data, as
described in detail in . We follow their approach of
obtaining heating rates from the 6 and 12 h forecasts and use total
diabatic heating rates (including all-sky radiation, latent heat release and
diffusive and turbulent heat transport).
Trajectory calculations focus on the Asian summer monsoon season
(June–August) for the year 2006. While using only 1 year for the analysis
might limit the conclusions of our study, it was found in GR13 that 2006 is
no outlier in terms of strength and variability of the monsoon anticyclone
(see their Fig. 5). Furthermore, no major El Niño or La Niña event that
might influence the anticyclone system occurred around this year. Therefore, we expect similar results for other years without unusual conditions.
The vertical distribution of deep convective outflow in the Asian monsoon
region is not well known. While convective up- and downdrafts are not resolved
in the wind field, their mean effect is reflected in the large-scale winds:
the anticyclone region is characterized by mean upward transport that
maximizes around 330–340 K (or 500 to 300 hPa).
Consequently, mean divergence is found above this region, representing mean
convective outflow (note, however, that the detailed vertical profile of
divergence is likely to be strongly influenced by the convective
parametrization used in the reanalysis forecast model).
Figure (left panel) shows the structure of seasonal mean
divergence in the monsoon region (together with zonal winds and isentropes),
and Fig. (right panel) shows daily variations of divergence
throughout summer 2006. GR13 showed that potential vorticity (PV) tendencies
in the monsoon region are closely linked to the analyzed divergence fields,
and it is reasonable to assume that the outflow of constituents is likewise
tied to mass divergence in the upper troposphere. In Fig. ,
maximum divergence occurs over 0–25∘ N for
altitudes 11–15 km, centered on isentropes 350–360 K (but
extending over a somewhat deeper layer 345–380 K). This behavior is
consistent with estimates of injection heights inferred from cloud statistics
in , and .
Based on this structure, we focus on studying isentropic transport at the
360 and 380 K levels and a vertical distribution of outflows over 340–380 K.
Isentropic trajectories are initialized every day between 1 June and
31 August of the year 2006 within the center of the anticyclone at 360 and at
380 K. Following GR13, the center of the anticyclone is defined as
the grid points between 15–45∘ N and 45–120∘ E where the
PV is less or equal to 0.3 PVU at the 360 K level and less or equal
to 3 PVU at the 380 K level. This procedure results in about 1000 to
2000 trajectories initialized each day at both levels, depending on the
amount of low PV on each day. Trajectories are run forward for 30 days.
Three-dimensional trajectories are initialized at the 360 K level
within the anticyclone in regions of low PV as for the isentropic
calculations. Trajectories are initialized each day between 1 June and
31 August and are run forward 60 days. In Sect. ,
calculations are repeated with initial locations of trajectories within
regions of most intense deep convection. In this case we use outgoing
longwave radiation (OLR) as a proxy for deep convection, based on the daily
gridded OLR data set from the National Oceanic and Atmospheric Administration–Cooperative
Institute for Research in Environmental Sciences (NOAA-CIRES) (; at
http://www.esrl.noaa.gov/psd/data/). Trajectories are
initialized in regions of low OLR (OLR less or equal 160 Wm-2)
within 0–110∘ E and 0–45∘ N at different potential
temperature levels between 340 to 380 K.
Example of isentropic trajectories on 360 K, released on 10 June.
Black dots highlight trajectory locations on 10, 12, 14 and 16 June, together
with PV (red: > 2 PVU; blue: < 0.3 PVU; yellow:
intermediate).
Mean JJA distribution of isentropic trajectories (in percentage of total
number of trajectories) at the 380 K (top panels) and 360 K (bottom panels)
level on day 0, 10, 20 and 30 after release, overlaid with mean JJA PV
contours (at ±3, 4, 6, 8 and 12 PVU at 380 K and ±0.7, 1, 2, 4 and 8 PVU
at 360 K). Panels on the right show the zonal mean distribution on day
0 (black) and 30 (red).
Confinement of the anticyclone on isentropic levels
As a first step, we study the confinement of air within the anticyclone at isentropic levels. An example of trajectories in the first 6 days
after their initialization within the anticyclone is shown in
Fig. . Trajectories largely follow regions of low PV.
A few trajectories are advected out of the anticyclone following the PV
streamer that is shed from the anticyclone to the east. This shedding event
from the anticyclone was found to be associated with elevated CO
concentrations colocated with the PV streamer (see Fig. 10 of GR13). Thus,
observed tracer anomalies, low PV as well as the trajectory calculations show
a consistent picture of air being shed from the anticyclone. The trajectory
calculations enable us to investigate the further transport of air that was
shed from the anticyclone and the role of such shedding events over the season.
The mean horizontal distributions of trajectories after 10, 20 and
30 days of their initialization are shown in Fig.
(bottom panels). The mean distributions are obtained by combining the distributions
of all trajectories initialized each day between 1 June and 31 August of the
year 2006. The distribution is given in percentage of the total number of
trajectories, and in the following, percentage numbers mentioned in the text
always refer to a fraction of the total number of trajectories.
Within the first 10 days, trajectories mostly remain within the
anticyclone both at 360 and 380 K. After
20 days, the likelihood of trajectories to be shed from the
anticyclone to the east or west and also to the south increases. While the
maximum of the distribution remains within the anticyclone after
30 days, trajectories are spread over the tropical belt, and a second
maximum in the distribution is found in the Southern Hemisphere near
20∘ S (see Fig. , bottom right panel). No trajectories
reach the southern extratropics (likely because the subtropical jet acts as
a transport barrier), but a few (3 %) reach the northern
extratropics after 30 days.
At higher altitudes (380 K), the anticyclone becomes more confined:
the mean distributions of trajectories initialized within the anticyclone at the 380 K level after 10, 20 and 30 days are shown in
Fig. (top panels). After 30 days, trajectories are spread
to the east and west of the anticyclone, but few trajectories reach the
Southern Hemisphere. Thus, a single maximum in the latitudinal distribution
is found at 380 K after 30 days (see Fig. ,
top right panel). However, more trajectories reach the northern extratropics
(8 % of all trajectories) at 380 K compared to 360 K.
The upper-tropospheric anticyclone has a distinct signature in tracers like
CO, which has near-surface sources (from combustion) and is transported
upward by deep convection associated with the monsoon. While tracers are
confined by the anticyclone, shedding events as shown in
Fig. cause transport out of the anticyclone. The mean
distributions of trajectories 20–30 days after their initialization
in the anticyclone closely resemble the time average CO distribution both at
360 and 380 K (Fig. ). Both CO and the trajectory
distribution show maxima between 20 and 110∘ E that are elongated in
the longitudinal direction. Furthermore, at 360 K higher
probabilities of trajectories spreading to the south of the anticyclone are
colocated with elevated CO concentrations there. This suggests that the
isentropic outflow of high-CO air from the anticyclone might contribute to
the distribution of CO in the tropical upper troposphere.
Mean JJA distribution of parcel locations averaged over day 20–30
(white: 0.02 to 0.07 %; black: 0.08 to 0.25 %) overlaid on mean JJA CO
concentrations as measure by Microwave Limb Sounder (MLS) at 380 K (top panel) and 360 K (bottom
panel).
Trajectories released within the anticyclone at 360 K on 10 June
after 20, 40 and 60 days for kinematic (top panels) and diabatic (bottom
panels) calculation of vertical transport. Green dots mark the initial
location. The blue line is the mean thermal tropopause at 20–120∘ E,
the red line the mean 360 K isentrope and the gray lines the 340 and 380 K
isentropes.
Transport from the anticyclone to the stratosphere
Full three-dimensional transport of air parcels that originate in the upper-tropospheric anticyclone is investigated in the following. Trajectories are
initialized at the 360 K level in the same fashion as the isentropic
trajectories above (i.e., in regions of low PV). Trajectories are initialized
each day from 1 June to 31 August and are run forward for 60 days.
An example of the temporal development of the latitude–height distribution of
trajectories over 60 days is shown in Fig.
for trajectories initialized on 10 June. The top row shows kinematic
trajectories, the bottom row trajectories with identical initial positions
but transported diabatically in the vertical. After the first
20 days, most trajectories are still confined to the Northern
Hemisphere tropics but are spread in the vertical. A large fraction of the
trajectories has moved upward to higher potential temperatures, and some
trajectories have crossed the tropopause. The trajectories spread over the
tropics and in the vertical as time progresses, including some systematic
downward transport. The distributions are similar overall for kinematic and
the diabatic calculations, but the trajectories are more dispersed vertically
in the kinematic calculations.
Probability distribution functions and transport timescales
The mean probability distribution functions (PDFs) averaged over all
trajectories released each day between 1 June to 31 August are shown in
Fig. for kinematic and for diabatic calculation of
vertical transport. After 20 days, the maximum in the PDF lies above
the mean 360 K level, and trajectories spread slightly over the
tropics. A considerable fraction of air parcels move downward, and for
diabatic transport a second maximum is found around 400–500 hPa.
After 60 days, the maximum in the PDF is found above the tropical
tropopause. The maximum is stronger and more confined for diabatic transport,
while trajectories are more widely spread over the tropical troposphere and
lower stratosphere for kinematic calculations. This behavior can be seen more
clearly in the PDF of trajectories with respect to potential temperature
(Fig. ): diabatic trajectories show two distinct maxima
after 60 days; one is formed by trajectories that traveled upward (at
390–400 K) and one by trajectories that traveled downward (at
320–330 K). Kinematic trajectories, on the other hand, show
a broader distribution that maximizes in the lower troposphere at
320–330 K. Overall, the diabatic calculations result in more
trajectories traveling to higher levels that lie well above the tropopause.
To understand the causes for the vertical spread in the trajectories,
Fig. shows the mean vertical velocities (top panel) and diabatic
heating rates dΘ/dt (bottom panel) at the 360 K
level (i.e., the level where trajectories are initialized). Strong positive
heating rates are found in regions of low OLR, namely over the Bay of Bengal
and the western Pacific Ocean north of Indonesia. The upward vertical
velocities are located further away from the Equator and are overall
patchier, even in the monthly mean. While the distributions differ in many
respects, both vertical velocities and diabatic heating rates show mean
upward transport in the eastern part of the anticyclone but downward
transport in the western part. This explains why trajectories released in the
anticyclone travel both upward and downward.
The transit time distributions for parcels traveling upward to the 380 and
the 400 K levels is shown in Fig. . The most likely
time it takes an air parcel from its initial level at 360 to 380 K is
around 15 to 20 days. To reach the 400 K level, an air
parcels travels 30 to 40 days in the mean. These transit times are
faster compared to those estimated by of around
15–20 days/10 K, but this difference may be explained by
their focus on the entire tropics compared to our focus on the anticyclone.
At both 380 and 400 K, the transit time distributions peak earlier
for kinematic transport, but larger fractions of trajectories reach those
levels for diabatic transport. This further illustrates the dispersive nature
of kinematic calculations. Overall, it can be concluded that diabatic upward
transport is stronger and more persistent, while kinematic results are more
diffusive and weaker in the mean.
Transport pathways
To quantify the destination of parcels originating from the monsoon
anticyclone, we subdivide the atmosphere into nine regions, and the fraction of
trajectories located in each of those regions after 30 and 60 days of
integration is given in Table . The tropopause is defined
here as the thermal tropopause (according to the WMO definition), and daily
data of tropopause height are used to assign each particle at each time step
to the stratospheric or tropospheric region. With this definition of
cross-tropopause transport, short-term fluctuations across the tropopause are
included, but tests including residence time criteria showed that this does
not affect the conclusions of this study. The locations of tropopause
crossings (shown in Fig. ) are, however, limited to
crossings where the trajectory remains 5 days in the troposphere prior to the
crossing and 5 days in the stratosphere after the crossing. This limitation
sharpens the geographical distribution but does not significantly differ from
the pattern of the distribution of all crossings. The latitudinal boundaries
of the defined regions (see Table ) are chosen to
approximately match the locations of the transport barriers, i.e., jets (see, e.g., Figs. , and ).
The time series for the most important regions are shown in
Fig. . Within the first 10 days, few trajectories enter
the stratosphere, but after 30 days 21 % (35 %) of
the trajectories entered the lower stratosphere for kinematic (diabatic)
calculations, and after 60 days 31 % (48 %) are
located in the stratosphere. Most of the trajectories that entered the
stratosphere are located within the tropics (21 %/32 % of
all trajectories after 60 days) but only about a third of those in
the region of the anticyclone (see Table ). Eight percent (15 %) of all trajectories are located in the northern extratropical
stratosphere after 60 days, mostly entering this region after
30 days (see green curve in Fig. ). The transit
time distribution to the stratosphere (lower panel of
Fig. ) maximizes at transit times of around
10–15 days. As shown in Fig. , the most likely
geographical entry point of trajectories to the stratosphere is located over
the Bay of Bengal and to the east of the anticyclone, for both kinematic and
diabatic calculations. This is consistent with upward transport velocities in
this region (Fig. ).
Fraction of trajectories located in different regions of the
atmosphere after 30 and 60 days for kinematic/diabatic transport.
30 days60 daysDefinitionAC UT19 %/11 %4 %/1 %0–45∘ N, 45–180∘ E, 250 hPa to tropopauseAC LS7 %/13 %6 %/8 %0–45∘ N, 45–180∘ E, above tropopauseTrop UT28 %/17 %15 %/6 %30∘ S–45∘ N but not AC, 250 hPa to tropopauseTrop LS11 %/18 %15 %/24 %30∘ S–45∘ N but not AC, above tropopauseExTR NH UT3 %/3 %3 %/3 %45–90∘ N, 250 hPa to tropopauseExTR SH UT1 %/0 %2 %/1 %90–30∘ S, 250 hPa to tropopauseExTR NH LS3 %/4 %8 %/15 %45–90∘ N, above tropopauseExTR SH LS0 %/0 %1 %/3 %90–30∘ S, above tropopause> 250 hPa28 %/34 %46 %/39 %below 250 hPa
Mean distribution of three-dimensional kinematic trajectories (top panels) and
three-dimensional diabatic trajectories (bottom panels) released within the anticyclone at
360 K on day 20, 40 and 60 after their release given as fraction of total
number of trajectories (dark (5 × 10-3) to light yellow (5 × 10-4)). The black contour indicates the initial distribution
of trajectories, and the dark blue line is the mean thermal tropopause between
20–120∘ E. The red line is the mean location of the 360 K level in
45–120∘ E, and the gray lines are the 340 and 380 K
isentropes.
Of those trajectories that do not travel upward and into the stratosphere,
most descend to levels below 250 hPa (46 and 39 % after
60 days). For both diabatic and kinematic transport, most of those
trajectories descend within the first 10 days.
Both the fraction of trajectories that travel to the stratosphere and those
that are transported downward are relatively constant after about
40–50 days (see Fig. ). Therefore, it can be
concluded that timescales of transport processes as examined here are well
captured by the 60-day duration of the trajectories.
The destinations of trajectories that originate from the anticyclone that
are discussed above give no information about the pathways the trajectories
take to the respective region. Trajectories do not travel necessarily
directly from the upper-tropospheric anticyclone to the destination region.
For example, while 15 % of all trajectories end up in the northern
extratropical stratosphere after 60 days, they are not necessarily
transported there directly from the upper-tropospheric anticyclone via
isentropic transport. Therefore, we discuss the pathways of trajectories from
the upper-tropospheric anticyclone to the three most important destinations
(tropical lower stratosphere (LS), northern extratropical LS and lower troposphere) in more detail
in the following. While we discuss pathways for diabatic transport in the
following, the relative importance of the different pathways for kinematic
transport are qualitatively the same.
Mean distribution of trajectories with respect to potential
temperature for kinematic (black) and diabatic (red) transport. The gray bar
denotes the initial location.
Monthly mean vertical velocity (top panel, in pascal per second; note the
flipped color bar) and dΘ/dt (bottom panel, in kelvin per day) at
360 K together with PV (black contours, for ±8, 4, 2, 1 and 0.7 PVU) and
OLR (white contours, at 190, 200 and 210 W m-2) for
July 2006.
Transit time distribution from 360 to 380 K (top panel) and to
400 K (bottom panel) for diabatic (red) and kinematic (black)
trajectories.
Fraction of trajectories located in the stratosphere (black), the
tropical stratosphere only (blue), the northern extratropical stratosphere
(green) and below 250 hPa (red) as a function of time for kinematic (top
panel) and diabatic (middle panel) transport. Bottom panel: transit time
distribution to the stratosphere.
Geographical distribution of locations of tropopause crossings (as
fraction of all trajectories; top panel: kinematic; bottom panel: diabatic)
and contours of thermal tropopause height (black; contours between 110 and
290 hPa with interval 20 hPa). In order to exclude short-term reversible
events, results here only include parcels remaining in the troposphere 5 days
before and in the stratosphere 5 days after crossing the
tropopause.
Transport to the tropical lower stratosphere
To highlight the pathways to and from the tropical lower stratosphere, total
air parcel fluxes over 60 days to and from this region are shown in
Fig. a. The numbers are obtained by counting all trajectories
that enter or leave the specific region within the 60 days via the
different pathways, and percentages are the fraction of the total number of
trajectories initially released in the anticyclone. The budget analysis shows
that the preferred pathway of trajectories is to travel from within the upper-tropospheric anticyclone region to the tropical lower stratosphere
(32 % of all trajectories). Another 14 % are first mixed
outside of the anticyclone into the tropical upper troposphere and are
subsequently transported upward into the tropical lower stratosphere. From
the tropical lower stratosphere, some trajectories (14 % of all
trajectories) are further transported to the extratropical stratosphere, the
majority (12 %) to the north. Thus, of the 46 % transported
into the tropical lower stratosphere, only 32 % remain here after 60 days.
Total air parcel fluxes over 60 days (in percentage of total number of
trajectories) into and out of (a) the tropical lower stratosphere
and (b) the northern extratropical lower stratosphere. The numbers
in the boxes are the fraction of trajectories that remain in the region after
60 days.
Transport to the northern extratropical stratosphere
A total of 15 % of all trajectories is located in the northern
extratropical lower stratosphere after 60 days. However, as revealed
by the budget for this region (Fig. b), most of those
trajectories do not travel directly from the upper-tropospheric anticyclone
to the northern extratropical stratosphere: the most common pathway is via
upward transport into the tropical stratosphere and subsequent transport to
the northern extratropics (12 % of all trajectories). Far fewer
trajectories (5 % of all trajectories) were transported there directly from the
tropical upper troposphere, likely by mixing of tropical and extratropical
air masses. With a total of 2 % of all trajectories leaving the
extratropical lower stratosphere to the troposphere below, 15 %
remain here after 60 days. Thus, the pathway via the tropics with
slow vertical motion dominates over the pathway by fast isentropic mixing for
air to be transported from the upper-tropospheric anticyclone to the northern
extratropical stratosphere. This is consistent with transport times of
30 days or more for most air parcels to reach the extratropical
stratosphere (see green curve in Fig. ).
Downward transport to the middle troposphere
The distribution of trajectories that are transported downward to below
250 hPa has a maximum over the Middle East (Fig. ),
likely caused by the downward velocities over this region (see
Fig. ). The downward transport is strong in particular during July, in line with strong negative heating rates over the western part
of the anticyclone during the same time period (not shown).
and discussed a local ozone maximum over the Middle East in
the middle troposphere (around 400 to 500 hPa), and they show that
this ozone anomaly is caused by a combination of local ozone production and
by transport of ozone from Asia . The distribution of
trajectories that are transported downward from the Asian monsoon anticyclone
resembles the ozone anomaly in the Middle East see Fig. 1
of and is similar to the contribution of ozone produced in the
upper troposphere over Asia see Fig. 9b of.
suggest that this upper-tropospheric production of ozone over Asia
is linked to the generation of NOx from lightning, which occurs in the
region of deep convection. Therefore, we speculate that the persistent
downward transport of air from within the anticyclone, and thus from the
region of convective outflow, might contribute to the buildup of ozone in
the middle troposphere over the Middle East.
Mean JJA distribution of diabatic trajectories (in percentage of total
number of trajectories) that are transported downward (to > 250 hPa)
averaged over day 20 to 40 after their release in the anticyclone at 360 K,
overlaid with mean JJA PV (contours at ±0.7, 1, 2, 4 and 8 PVU).
Summary of pathways
The different transport pathways from the upper-tropospheric anticyclone (at
360 K) to the stratosphere are summarized in Fig. .
The most likely pathway for trajectories initialized in the upper-tropospheric anticyclone is to travel upward within the anticyclone and enter
the stratosphere in the northern (sub-)tropics, in particular in the
southeastern part of the anticyclone. Another smaller fraction of
trajectories is first transported into the tropical upper troposphere and
transported upward from there. Fewer trajectories are transported from the
anticyclone to the extratropical lower stratosphere, and only a very small
number of those are transported directly from the anticyclone to the
extratropical lower stratosphere, likely by isentropic mixing. Trajectories
that are located in the extratropical stratosphere after 60 days more
likely traveled there via upward transport into the tropical stratosphere and
subsequent mixing to the northern extratropics.
Schematic of the most prominent transport pathways of air
originating in the upper-tropospheric anticyclone around 360 K (gray box).
Numbers indicate fraction of trajectories (in percentage) that are located in the
respective regions after 60 days for diabatic transport. The width of the
arrows reflects the importance of the respective pathway. Contours show the
zonal mean wind (black solid: positive; black dashed: negative), the
tropopause (blue) and the 340, 360 and 380 K isentrope (red) averaged over
20–120∘ E.
Trajectories with realistic sources and broad vertical distributions
So far, we have analyzed air parcels that were initialized in the upper-tropospheric anticyclone at 360 K. However, deep convection might not
lift air parcels as high as 360 K. The profile of mean divergence
suggests maximum outflow at levels around 340 to 370 K
(Fig. ). Furthermore, the geographical location of air
parcels that were lifted upward by deep convection is not equivalent to the
region of the anticyclone. Therefore, we investigate in the following the
distributions of air parcels that are located initially in regions of low OLR
(specifically, OLR < 160 Wm-2, as a proxy for deep convection) at
different altitudes between 340 to 380 K. We conduct these
sensitivities using diabatic transport. As will be discussed in
Sect. , there is little difference found between the
distribution of trajectories initialized at 360 K within the
anticyclone (regions of low PV), as investigated so far in this paper, and
the distribution of trajectories initialized at 360 K within regions of low OLR.
Left panel: distribution of diabatic trajectories in potential
temperature initiated between 340 to 380 K (x axis) in regions of low OLR
(≤ 160 W m-2) after 60 days. The thick black dashed line
indicates the level of initialization. Right: mean JJA 2006 profile of
dΘ/dt averaged over 60–120∘ E and 0–30∘ N (solid)
and over the entire tropical belt, 30∘ S–30∘ N
(dashed).
Figure compares the distribution of
trajectories with respect to potential temperature after 60 days for
different initial heights between 340 to 380 K. A clear separation of
air parcels is found after 60 days: while air parcels that were
initialized below 360 K most likely descend within the 2 months
after initialization to potential temperature levels around 300 to
330 K, air parcels that are initialized above 360 K most
likely rise to potential temperature levels of 380 K and above. The
360 K level appears to act as a boundary between rising and sinking
parcels. Consequently, only less than 15 % of the trajectories
initialized at 350 K or below reaches the stratosphere, while about
50 % of those initialized at 360 K and more than
90 % of those initialized at 370 K or above reach the
stratosphere. At all levels, the dominant pathway to the stratosphere is
upward advection to the tropical stratosphere, while isentropic transport to
the extratropical lower stratosphere is of minor importance.
The mean profile of diabatic heating in the anticyclone region suggests that
air is lifted in the mean throughout the atmosphere, and that lifting is
strongest at 330–340 K (see Fig. ,
right panel). This seems to contradict the finding that air parcels initialized
below 360 K are more likely descending to lower levels. However, in
order to be lifted by the positive vertical winds air parcels need to be
confined to the region of lifting, i.e., the anticyclone region. Heating rates
are close to or less than zero at levels below 355 K in the tropical
mean (see dashed line in Fig. , right panel). As
shown in Fig. the fraction of air
parcels that remains in the initial anticyclone region (here defined as
0–45∘ N, 0–180∘ E) quickly drops to below 20 %
after about 10 days for trajectories initialized below 360 K,
while those initialized above are more likely to remain in the anticyclone
region. As was shown in the last section, most trajectories initialized at
360 K are lifted upward and into the stratosphere within
10–20 days after their initialization, and the most likely pathway
is through the anticyclone region. Below 360 K the anticyclone
appears not to be strong enough to confine air in the region of ascent, and
thus it is transported away and descends elsewhere.
Fraction of diabatic trajectories that remain in the anticyclone
region as a function of time after initialization for different initial
potential temperature levels (see
legend).
Overall, these results suggest that air parcels need to be lifted by deep
convection to levels around 360 K or above in order to be likely
transported further upward and into the stratosphere. It is well known that
parcels need to be lifted to a certain height threshold (commonly thought of
as the level of zero radiative heating) to travel further upward into the
stratosphere e.g.,. Since diabatic
heating rates are positive essentially at all heights in the anticyclone
region according to ERA-Interim (see Fig. ,
right panel) one might assume that the injection height is no limitation for upward
transport here. However, it turns out that the horizontal confinement to the
anticyclone region is the limiting factor here: the confinement is given only
at or above 360 K; thus, the injection height necessary for upward
motion in the anticyclone turns out to be similar to the level of zero
(diabatic) heating in the entire tropics, albeit for different reasons. The
role of dynamical horizontal systems, in particular the anticyclone, in
confining parcels to regions of upward motion was already pointed out by
.
Sources of uncertainties
Several sources of uncertainties affect the trajectory calculations presented
here. We performed sensitivity simulations to test the robustness of our
results. The sensitivity simulations are performed for a limited number of
initial days (10 days spaced evenly over the 3-month period June to
August). Results of the sensitivity calculations, described in detail below,
are shown in Fig. .
Fraction of trajectories located in the indicated regions (x axis)
averaged over day 50–60 for a subset of 10 initial starting days between
June and August. Shown are results from calculations with diabatic transport
(gray bars), kinematic transport (black dot), diabatic heating rates scaled
by 0.5 (blue x), number of trajectories increased by a factor of 10 (green
cross), initial positions in a box region (red star) and initial positions in
low OLR regions (magenta circle).
Trajectory setup
To test the sensitivity of our results to the exact setup of initial
trajectory positions, we performed additional calculations with trajectories
initialized in a box region from 15 to 45∘ N and 20 to 130∘ E.
Another sensitivity is given by the calculations with initial positions at
360 K in region of low OLR (see Sect. ). The mean
initial distribution of trajectories initiated in regions of low OLR lies
further southward (centered over India and the Indian ocean) compared to
those initiated in regions of low PV. Furthermore, a total of
1000–2000 trajectories released each day might be considered too small a number given
the length of the trajectories of 60 days. We increased the number of
trajectories by a factor of 10, distributing the additional trajectories
randomly in each latitude–longitude box that previously contained one
individual trajectory. As can be seen in Fig. , the final
(day 50–60) destinations of trajectories are very robust against the number of
trajectories released and initial positions. Within the first
10–20 days, some differences occur for the different setups of
initial positions. For example, trajectories initialized in regions of low
OLR are slightly more likely to rise within the first 20 days.
However, overall the conclusions of this study are unaffected by the exact
setup of trajectory initialization.
Vertical velocities
Vertical velocities that are crucial to three-dimensional trajectory
modeling underlie two sources of uncertainty: biases of the data set used
and uncertainties in the calculation of vertical advection
e.g.,. While the ERA-Interim reanalysis data that our
study is based on are generally improved compared to earlier reanalysis
products , uncertainties and biases remain. Diabatic heating
fields are significantly different among reanalyses. ERA-Interim exhibits
comparably strong diabatic heating rates in the tropics above 150 hPa
, in particular in the region of the
anticyclone, where convective processes strongly contribute to total diabatic
heating . Furthermore, tropical upwelling in the lower
stratosphere was found to be overestimated by 30–40 % in ERA-Interim in
previous studies . To evaluate the impact
of this bias on our results, we performed a sensitivity simulation in which
we simply scaled diabatic heating rates by a factor of 0.5. This crude
scaling does of course not evaluate the full three-dimensional biases in the data (which
are not known) but gives an idea of the impact of a bias towards too strong
heating rates on our results. As expected, fewer trajectories reach the
stratosphere and fewer are transported downward with these weaker heating
rates, and more remain in the upper troposphere (see Fig. ,
blue x). However, the fraction of trajectories transported to the
stratosphere is affected by this crude scaling of heating rates less strongly
than by the method of vertical velocity calculations (i.e., kinematic versus
diabatic). We used both reanalysis vertical velocities and total heating
rates throughout the paper to test the sensitivity of our results to
uncertainties in vertical transport. While we find shorter transit times from
360 K to isentropic levels above (380 and 400 K) for
kinematic transport in agreement with , overall diabatic
transport is found to be more persistent and thus a larger fraction of
trajectories reaches the stratosphere for diabatic compared to kinematic
transport. While exact numbers of trajectories transported to different
regions differ, overall our results regarding the relative role of different
transport pathways are not sensitive to the method of vertical transport calculations.
Discussion and summary
Transport pathways of air from within the upper-tropospheric Asian monsoon
anticyclone have been investigated using trajectory calculations driven by
ERA-Interim reanalysis winds and diabatic heating fields. Efficient transport
from the surface to the upper-tropospheric anticyclone is indicated by observations of tracer anomalies in this region, with maxima in tracer
concentrations of tropospheric origin (like CO) and minima in those of stratospheric origin (like ozone) . Much of the transport
from the surface to the upper troposphere is likely caused by convective
updrafts which are not resolved in current reanalysis products, and large
uncertainties arise for the transport of air from the surface to the upper
troposphere . Here, we focus on transport pathways of air
that have already reached the upper-tropospheric anticyclone. Isentropic
trajectory calculations showed that air parcels initialized within the
anticyclone at 360 K are relatively well confined in the anticyclone
for about 10–20 days. However, after 15–20 days, a considerable
fraction of air parcels is shed to the east and west of the anticyclone, and
also to the south across the Equator. After 30 days, trajectories are
widely spread across the tropical belt. The shedding of air is in agreement
with variability observed in PV fields and tracer distributions
GR2013. Despite strong PV streamer
activity associated with the exchange of air masses between tropics and
extratropics in the vicinity of the anticyclone region reported by
, we find that only a small fraction (3 %) of the
trajectories reaches the northern extratropics (poleward of 45∘ N)
within 30 days. Due to the different setup of the two studies, our
results are not directly comparable to those of . However,
most (80–95 %) of the air mass exchange across the tropical barrier
found by occurs equatorward, and trajectories that take part
in transport across the barrier are not located in the anticyclone core at
360 and 380 K (see Fig. 9a and b in ); thus, the small
fraction of trajectories transported poleward in our study does not
necessarily contradict . At higher levels (380 K),
the anticyclone is more confined compared to 360 K, and trajectories
are less likely to be spread across the tropical belt. This is consistent
with the finding by that a transport barrier for the
anticyclone can be defined at 380 K but not at 360 K.
However, more trajectories reach the extratropics after 30 days at the 380 K level (8 %) compared to 360 K (3 %).
Using three-dimensional trajectory calculations, we found that a considerable
fraction of air parcels initially located within the upper-tropospheric
anticyclone (at 360 K) reach the stratosphere within 60 days
(31 % for kinematic and 48 % for diabatic trajectory
calculations). The horizontal confinement over 10–20 days is
sufficiently long to efficiently transport air upward and into the
stratosphere: typical transit times from the anticyclone at 360 K to
the tropopause are 10–15 days.
The most likely pathway from the upper-tropospheric anticyclone into the
stratosphere is ascent within the anticyclone region. In the mean, air
masses move upward and across the (sub-)tropical tropopause, as shown in
Fig. . A very similar pattern of transport of an
idealized tracer through the monsoon anticyclone was reported by
(their Fig. 4), based on tracer experiments in a global
chemistry–climate model. A small fraction of air is transported from the
upper-tropospheric anticyclone to the northern extratropical lower
stratosphere, likely by isentropic mixing. report on recent
observations of this pathway, but in our study this pathway is found to be of
minor importance. Rather, vertical advection is more efficient at
transporting air from the upper-tropospheric anticyclone into the
stratosphere as compared to isentropic mixing into the extratropical
stratosphere. Thus, pollutants that are transported by convection into the
upper troposphere can be transported into the tropical pipe and further into the deep stratosphere.
The results presented here on the role of different transport pathways of air
from the upper-tropospheric anticyclone to the stratosphere were found to be
robust against the details of trajectory initialization. While results differ
when using kinematic versus diabatic vertical velocities, the overall
conclusions are valid for both methods. It is known that heating rates are
likely overestimated in the upper troposphere in ERA-Interim
. However, even when reducing heating rates to
half their value, a considerable fraction of trajectories reaches the
stratosphere after 60 days, and the relative role of transport pathways
remains the same.
Outflow from tropical deep convection is estimated by previous studies to be
found around levels of 340 to 370 K ,
consistent with the patterns of large-scale
divergence in reanalyses (Fig. ). Calculation of transport
initialized over levels 340–380 K shows that air is far more likely
to be advected to the stratosphere by large-scale (resolved) winds when
injected at 360 K or above (Fig. ).
Air parcels injected below 360 K only reach the stratosphere with
a probability of less than 20 %, despite strong positive heating
rates between 340 to 350 K in the monsoon region. Our results suggest
that the lack of confinement of air parcels to the region of upward transport
below 360 K results in the low probability of rising air parcels.
Only at 360 K and above, where the anticyclone confines air to the
region of lifting, can large-scale winds effectively transport air upward.
Thus, our calculations suggest that deep convection needs to transport air to
levels around 360 K in order to be advected further upward by the
large-scale circulation. This results is in line with recent findings by
, who show that air at 380 K with convective sources in
the Asian monsoon region, originates from cloud tops of about 355 to
365 K (see their Fig. 3). report transit times
from cloud tops to 380 K of about 20–30 days for the Asian monsoon
region, approximately in line with transit times of 15–20 days from 360 to
380 K found here.
The calculations and results shown here are relevant to the observations of
transport of volcanic gases and aerosols in the monsoon region associated
with the eruption of Mt. Nabro in June 2011 . Briefly,
the Nabro eruption injected gases and aerosols into the upper troposphere and
lower stratosphere, resulting in the formation of an enhanced aerosol layer in
the lower-stratosphere monsoon region approximately 2 to 4 weeks later.
The time lag is consistent with conversion of volcanic sulfur dioxide to
sulfate aerosol , with details that can depend on
altitude (see and references therein). While there is some
debate regarding the altitude of the plume injection heights
, suggest that the
main injection occurred over altitudes of 15–17 km consistent, with observed plume trajectories, as shown in . The
stratospheric aerosol layer occurred at somewhat higher altitudes
above 18 km;, consistent with
slow upward transport in the monsoon circulation.
furthermore show that the stratospheric aerosol plume continued to move
upward during the summer at a rate of about 10 K month-1,
reasonably consistent with the trajectory calculations in this work. Overall
our calculations of large-scale upward transport within the monsoon
circulation, including transport into the lower stratosphere, are consistent
with the evolution of the unique Nabro event.
Acknowledgements
We thank S. Brinkop, M. Nützel and J. Bergman for comments on the
manuscript and P. Jöckel for assistance with the ECMWF data. Three
anonymous reviewers provided constructive comments which significantly
improved the manuscript. We thank the ECMWF for providing the ERA-Interim
data. This study was partially funded by the Deutsche
Forschungsgemeinschaft (DFG) through the DFG research group SHARP
(Stratospheric Change and its Role for Climate Prediction) and by the
Helmholtz Association under grant number VH-NG-1014
(Helmholtz-Hochschul-Nachwuchsforschergruppe MACClim). This work was
partially supported by the NASA Aura Science Program. The National Center
for Atmospheric Research is operated by the University Corporation for
Atmospheric Research, under sponsorship of the National Science Foundation.
The article processing charges for this open-access publication
were covered by a Research Centre of the Helmholtz Association.
Edited by: G. Stiller
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