It is well established that variable wintertime planetary wave forcing in the stratosphere controls the variability of Arctic stratospheric ozone through changes in the strength of the polar vortex and the residual circulation. While previous studies focused on the variations in upward wave flux entering the lower stratosphere, here the impact of downward planetary wave reflection on ozone is investigated for the first time. Utilizing the MERRA2 reanalysis and a fully coupled chemistry–climate simulation with the Community Earth System Model (CESM1(WACCM)) of the National Center for Atmospheric Research (NCAR), we find two downward wave reflection effects on ozone: (1) the direct effect in which the residual circulation is weakened during winter, reducing the typical increase of ozone due to upward planetary wave events and (2) the indirect effect in which the modification of polar temperature during winter affects the amount of ozone destruction in spring.
Winter seasons dominated by downward wave reflection events (i.e., reflective winters) are characterized by lower Arctic ozone concentration, while seasons dominated by increased upward wave events (i.e., absorptive winters) are characterized by relatively higher ozone concentration. This behavior is consistent with the cumulative effects of downward and upward planetary wave events on polar stratospheric ozone via the residual circulation and the polar temperature in winter. The results establish a new perspective on dynamical processes controlling stratospheric ozone variability in the Arctic by highlighting the key role of wave reflection.
The dynamical linkage between the stratosphere and troposphere is dominated
by planetary waves, which are generated in the troposphere by orographic
and/or non-orographic forcing (e.g.,
The vertical coupling of planetary-scale waves between the stratosphere and
troposphere can be directly examined via the meridional eddy heat flux since
it is proportional to the vertical group velocity of the planetary waves
(e.g.,
It is well established that planetary waves play an important role in shaping
the ozone hole through their impact on the polar vortex and residual
circulation (e.g.,
Weaker planetary wave driving in the stratosphere, which affects ozone
through both dynamical and chemical processes, could arise from an enhanced
number of extreme negative wave-1 heat flux events (i.e., DWC events) or
from anomalously low positive heat flux values. Many studies have shown that
an increased number of major SSW events that are associated with enhanced
upward wave propagation into the stratosphere have led to significant
increases in total column ozone and polar temperature in the winter and
subsequently less springtime ozone destruction (e.g.,
The goal of this study is to investigate the impact of DWC on polar
stratospheric ozone in the NH using both Modern-Era Retrospective analysis
for Research and Applications, version 2 (MERRA2,
The paper is structured as follows. Section 2 describes the data and methods. This includes the description of data, model simulation, and methods. In Sect. 3, the direct effects of DWC on the mean residual transport and temperature and ozone concentrations are analyzed, based on the MERRA2 reanalysis product and a 100-year transient simulation from the fully coupled chemistry–climate model CESM1(WACCM). Section 4 discusses the seasonal impacts of DWC events on the Arctic column ozone during seasons that are dominated by enhanced DWC events (reflective winters) and by enhanced upward wave events (absorptive winters). Finally, the results are summarized and discussed in Sect. 5.
The new MERRA2 daily ozone product from 1980 to 2013
In addition, daily three-dimensional geopotential height, wind, and
temperature fields from MERRA2 with the same period as the ozone (1980 to
2013) were also employed in this study. We note that the nature of downward
wave pulses and the associated wave geometry in MERRA2 were found to be in
good agreement with the results from the European Centre for Medium-Range
Weather Forecasts (ECMWF) Re-Analysis
The model simulation used in this study was performed with the NCAR's
CESM version 1.0.2. CESM is a state-of-the-art
coupled model system that includes an ocean, land, sea ice, and atmosphere
components
In this study one 100-year simulation (1955–2054) is free run with fixed
surface emissions of greenhouse gases (GHGs) and ozone-depleting substances (ODSs)
at 1960s levels, allowing us to study the ozone variability unmasked
from any anthropogenic influence. The simulation is run with interactive
ocean and sea ice components. To represent a more realistic interaction
between the tropics and extratropical dynamics, the quasi-biennial oscillation (QBO) is nudged by
relaxing tropical stratospheric zonal-mean winds towards observations
following
The influence of eddies on ozone transport is quantified from the transformed
Eulerian mean (TEM) continuity equation for zonal mean tracer concentration
(
We use a similar definition of a DWC event as in
Applying our identification algorithm leads to a total number of 19 potential
DWC events in MERRA2 reanalysis (see Table 1) and 58 in CESM1(WACCM) (see
Table S1 in the Supplement). The frequency of DWC events in MERRA and
CESM1(WACCM) is similar, about six events per decade. We also note that the
number of events found here and the evolution of the downward and upward wave
pulses are in good agreement with the results of
In the following sections we examine the effect of DWC events on polar stratospheric ozone in the observations and then in the model.
Central dates of potential DWC events at 50 hPa and the minimum total
5-day smoothed wave-1 heat flux (K m s
Evolution of DWC events as a function of
time from days
To examine the effects of DWC events on polar stratospheric ozone we first
examine the connection between DWC, residual circulation, and Arctic
temperatures during the composite life cycle of DWC in MERRA2 reanalysis. Figure 1a–d
show the composite life cycle of downward planetary wave events as a
function of pressure and time from
From days
Next, we analyze the implication of transient changes in residual circulation
induced by DWC on polar stratospheric ozone in MERRA2. Figure 2 shows the
corresponding transient evolution of the zonal-mean ozone tendencies averaged
between 60 to 90
Evolution of the ozone tendencies for the composite of DWC event as
a function of time and pressure, averaged from 60 to 90
To investigate the source of the transient changes in polar stratospheric
ozone during DWC events, we decompose the total ozone tendency (Fig. 2a) into
contributions of dynamics and chemistry-plus-analysis terms. It is shown that
the evolution of total ozone tendency in the mid-lower and mid-upper
stratosphere (between 100 and 1 hPa) is dominated by the dynamical term
(Fig. 2b). The ozone tendency due to dynamics in the mid-lower stratosphere
(100 and 10 hPa) is mainly attributed to the ozone transport via vertical
(advection) residual circulation (Fig. 2d), while the tendency in the
mid-upper stratosphere (above 5 hPa) is mainly attributed to effects of eddy
transport (Fig. 2f). Therefore, the dominance of the dynamical term on the
total ozone tendency in the mid-lower stratosphere during the composite
life cycle is consistent with the transient changes in residual mean transport
(Fig. 1c). Conversely, the contribution of the chemistry to the total
ozone tendency (Fig. 2c) is significant above 10 hPa prior to the mature
stage of DWC event (from days
Two-dimensional joint probability density distribution of
The same conclusion can be drawn by assessing the instantaneous correlation
between the two extreme stratospheric wave-1 heat flux events. Figure 3a
shows a two-dimensional histogram of total ozone tendency versus residual
vertical wind anomaly (
The time evolution of residual mass–stream function anomaly,
temperature, and ozone tendencies for the composite of the DWC event (blue lines)
and upward wave event (red lines).
Our results so far indicate that the life cycle of DWC involves a transient reversal of poleward to equatorward residual circulation anomaly and subsequent changes in potential temperature tendency and ozone from being positive to negative values. Therefore, it is worth checking whether the effects on residual circulation, temperature tendency, and ozone over the life cycle of the wave could integrate to zero, indicating that the impacts would be reversible. To this end, we first calculated the time integration of residual circulation anomaly and temperature tendency averaged over the levels where their effects are significant (i.e., between 100 and 1 hPa) (Fig. 4a and b). The results show that both time-integrated residual circulation and temperature tendency over the composite of DWC life cycle are nearly zero, indicating that the impacts are reversible. The reversibility implies that the effect of DWC on the residual circulation and temperature is canceled out over the life cycle of the wave, as indicated by the time tendencies that change from being positive to negative (Fig. 4a and b). We also compared the results from the DWC life cycle to the upward wave events. The upward wave event is defined in a similar manner as the DWC event but for the meridional heat flux values above the 95th percentile of the JFM distribution. It is shown that the time integration of both quantities over the life cycle of upward wave events is irreversible over the life cycle (i.e., positive value), with the time tendencies not reversing during the life cycle (Fig. 1c and d and see also Fig. S1 in the Supplement). This means that increased upward wave events result in stronger residual circulation and warmer polar vortex in winter. Thus, the overall effect of having more DWC events in winter is to have weaker residual circulation and colder polar vortex (i.e., DWC weakens the typical increase in residual circulation and temperature induced by upward wave events in winter).
Same as Fig. 1, but from a 100-year CESM1(WACCM) simulation.
Likewise, we also calculated the time-integrated evolution of ozone tendency
at 50 hPa and TCO tendency averaged between 60 and
90
A complementary analysis of wave-2 heat flux events is shown in the
Supplement (Figs. S4 and S5). Overall, the evolution of the
wave-2 events involve weaker heat flux values and weaker EP flux divergence
(Fig. S4a and b, consistent with
Determining the connection between DWC, stratospheric residual circulation, and polar temperature is one of the keys to improving our understanding of the link between stratospheric dynamics and ozone variability, both in the real atmosphere and in the stratosphere-resolving chemistry–climate models. In this section, we attempt to test if the linkages between DWC and transient changes in polar stratospheric ozone can be reproduced in the current chemistry–climate model CESM1(WACCM). The larger sample of events in the model also allows us to underpin the findings from the reanalysis dynamics using robust, statistically significant results.
Figure 5 shows the composite life cycle of downward planetary wave events as
a function of pressure and time from
Same as Fig. 2, but from a 100-year CESM1(WACCM) simulation.
From days
Next we examine the effect on ozone tendencies, as was done in Fig. 2. Figure 6
shows the time evolution of total ozone tendencies during the composite
life cycle. Consistent with MERRA2, the negative ozone tendency in the model
occurs during the time of strongest DWC events (days
By decomposing the total ozone tendency into dynamical and chemical terms, it is shown that transient changes in ozone dynamics dominate the total ozone tendency during the composite life cycle (Fig. 6b and c). In the mid-lower stratosphere, the ozone tendency due to dynamics is mainly attributed to the vertical advection process (Fig. 6d), while in the upper stratosphere both vertical advection and eddy transport effects become equally important (Fig. 6d and f). In particular, a strong positive total ozone tendency in the upper stratosphere during the time of maximum DWC events is attributed to both eddy transport effect and vertical advection process through the residual circulation (Fig. 6f). The magnitude of these two quantities in the model is relatively higher compared to MERRA2 and therefore leads to the large differences in ozone tendency in the upper stratosphere (Fig. 2d and f vs. Fig. 6d and f). The detailed analysis of why these model terms are biased is beyond the scope of the current study. Conversely, the ozone tendency due to chemistry in the upper stratosphere is somewhat weaker and in the opposite sign of the ozone tendency due to dynamics (Fig. 6c). The overall results support our observationally based analysis that the transient changes in polar stratospheric ozone during a DWC event are mainly attributed to changes in the dynamical ozone transport.
As with MERRA2, this general relationship between DWC and polar stratospheric
ozone in the model simulation can also be assessed through the instantaneous
correlation between the two extreme stratospheric wave-1 heat flux events.
Figure 7 shows a similar diagnostic as in Fig. 3, comparing the
two-dimensional distribution of the polar-cap-averaged ozone tendencies and
residual circulation during extreme wave-1 heat flux events. Consistent with
MERRA2, the days with positive and negative extremes (red and blue dots,
respectively) are systematically skewed compared to the background
distribution, so that stronger DWC events correspond to stronger negative
ozone tendency and weaker residual circulation (Fig. 7a, (
Same as Fig. 3, but from a 100-year CESM1(WACCM) simulation.
As a last step, the direct cumulative effects of DWC on residual circulation,
temperature tendency, and ozone were analyzed (Fig. 8). We first analyzed the
time integration of the residual circulation anomaly and polar temperature
tendency during the composite life cycle of the wave (Fig. 8a and b). It is shown
that the impacts on both residual circulation and potential temperature
tendency over the life cycle of DWC events are reversible, consistent with
MERRA2. We note, however, that there are differences in the values of
potential temperature tendency between the model and MERRA2. The relatively
larger values of the potential temperature tendency in the model are likely
associated with the modeled temperature bias, which is a common problem in
chemistry–climate models (CCMs)
Same as Fig. 4, but from a 100-year CESM1(WACCM) simulation.
Furthermore, the direct cumulative effects of DWC events on ozone in the
model were examined in the same way as in Fig. 8a and b. The time evolution of
TCO tendency and ozone tendency at 50 hPa averaged
between 60 to 90
The reflective and absorptive years defined based on the vertical
reflecting surface (
Time series of seasonal (JFM) mean of
The former analysis shows that an individual DWC event has a statistically significant impact on the polar stratospheric ozone. While the impact of an individual event occurs on a short timescale, several events in an individual JFM season may produce an impact on a longer timescale. In this section, we briefly examine the cumulative impacts of DWC on ozone during seasons that are dominated by DWC events.
In order to analyze the seasonal impact of DWC on polar stratospheric ozone,
we classify winters as reflective and absorptive based on the basic state of
the stratosphere characterized by the formation of reflecting surface and the
strength of the polar vortex, whereas previous study used cumulative wave-1
heat flux definition
Composites of the zonal-mean zonal wind, wave-1 geopotential height,
and temperature mean differences for reflective (REF) and absorptive (ABS)
winters in JFM from MERRA2 and CESM1(WACCM).
Reflective winters (with dominant DWC events) are defined when
Figure 10 shows composites of the zonal-mean wind, wave-1 geopotential
height, and temperature difference in JFM for the composite of reflective and
absorptive winters in MERRA2 and in CESM1(WACCM). During reflective winters,
the maximum zonal-mean zonal wind resides in the mid-stratosphere, and
consequently the region of vertical reflecting surface extends down to 3 hPa
(Fig. 10a and b). There is also a clear meridional waveguide below 10 hPa
between 50 and 80
Composite-mean difference (REF minus ABS) of the
In contrast, during absorptive winters, waves can propagate all the way up
through the stratosphere because the reflecting surface vanishes, meaning the
vortex tends to be weaker due to more wave deceleration (Fig. 10g and h). The
meridional waveguide occurs below 30 hPa between 50 and 80
In order to estimate the seasonal impacts of DWC events on ozone, we analyzed ozone differences between reflective and absorptive seasons for midwinter (January–February) and late winter–early spring (March–April) in MERRA2 and CESM1(WACCM) (Fig. 11). Here, we recall the reflective seasons (i.e., winters dominated by DWC events) as REF and the absorptive seasons (i.e., winters dominated by upward wave events) as ABS.
The results show that the seasonal effects of DWC leads to a reduction of ozone concentration in the stratosphere during midwinter (Fig. 11a and b) and early spring (Fig. 11g and h). These results are in agreement with our analysis over the life cycle of DWC showing that (1) a reversible reduction of poleward residual circulation induced by a DWC event weakens the typical increase in ozone induced by an upward wave event during midwinter (the direct effect of DWC) and (2) the cold polar vortex in midwinter due to DWC events results in more ozone loss during early spring (the indirect effect of DWC). To distinguish between the two effects quantitatively, we examine the contribution of dynamics and chemistry terms to the mean differences between the two types of winters (i.e., REF minus ABS). We note that responses of ozone and ozone tendency in REF and ABS winters are symmetric with respect to the climatological mean, meaning that ABS and REF anomalies (relative to climatology) are similar but opposite to each other (see Figs. S8 and S9). The results showed that a reduction of ozone concentration during midwinter in REF is mainly maintained by negative ozone tendency due to dynamics, in both MERRA2 (Fig. 11a) and model simulation (Fig. 11b). This is consistent with the direct impact of DWC on ozone in midwinter, where the reversible impact of DWC on ozone over the life cycle leads to less ozone increase in the Arctic. Conversely, there is a positive chemically driven ozone anomaly in the upper stratosphere during midwinter (Fig. 11e). This positive anomaly is likely associated with less chemical ozone loss in the upper stratosphere due to decreased temperature (cold) in REF during midwinter. However, we found that this positive ozone tendency anomaly due to chemistry is relatively small compared to those from the dynamics, so that the total net effect of DWC on ozone is still negative. In summary, the results showed that the accumulative impact of DWC on ozone during midwinter leads to less ozone transport to the polar region (see also Figs. S9c and S10c).
During early spring, the lower ozone concentration in REF compared to ABS in
the upper stratosphere (between 3 and 1 hPa) and in the lower-to-middle
stratosphere (between 100 and 50 hPa) (Fig. 11g and h) is mainly maintained
by negative ozone tendency due to chemistry terms (Fig. 11k and l). This is
consistent with the indirect impact of DWC on ozone, which leaves the polar
vortex cold in midwinter (i.e., dampening the typical warming induced by
upward wave events in winter; Figs. 1f and 5f), and thus resulting in more
ozone destruction in spring due to more accumulation of ODS on polar
stratospheric clouds. This is also consistent with a strong polar vortex
associated with increased DWC events (Fig. 10a and b). Comparing the ozone
tendency due to chemistry in REF to the climatological mean values, we found
that the indirect accumulative impact of DWC in spring results in an increase
in
seasonally averaged chemical ozone loss (see Figs. S8k, l and S9k, l).
The aforementioned indirect effect of DWC should only affect ozone
concentration in spring when the polar stratosphere becomes sunlit. Moreover,
we also note that there is a positive dynamical ozone tendency anomaly in
late winter (Fig. 11i and j) resulting from the opposite responses of dynamical
ozone tendency in REF and ABS during late winter (Figs. S8i, j and S9i, j).
The increased ozone tendency due to dynamics in REF is likely
associated with the occurrence of early final warming events (Fig. S10),
allowing more waves to break into the stratosphere in late winter and thus
enhancing the dynamical ozone transport to the pole during this period.
However, since the contribution from chemistry is dominant during REF, the
total net effect of DWC on ozone is still negative (i.e., less ozone
concentration), which is expected during reflective winters. In contrast,
during ABS, the final warming is delayed until late spring, resulting in less
dynamical ozone transport to the pole during late winter (Fig. S10). This
behavior is consistent with the previous observational studies (e.g.,
The goal of this study was to investigate the impact of DWC on polar
stratospheric ozone in order to fully understand the mechanisms controlling
the variability of Arctic stratospheric ozone. The key results of this study
are as follows:
The impact of DWC on the residual circulation and on potential temperature
tendency is reversible over the life cycle, as indicated by the time
tendencies that change from being positive to negative. Thus, the overall
effect of having more DWC events in winter is to have a weaker residual
circulation and colder polar vortex (i.e., DWC weakens the typical increase
in residual circulation and temperature induced by upward wave events in winter). A direct effect of DWC events on ozone, via advection to the pole by the
induced residual circulation, is reversible, suggesting that DWC weakens the
typical increase in ozone due to upward planetary wave events in winter. This
is consistent with a stronger transport of ozone to the pole during upward
planetary wave events in the absence of DWC events. An indirect effect of DWC events, via a cooling of the winter polar vortex as
a result of the reversible impact of DWC on the polar temperature tendency
over the life cycle, leads to increased springtime ozone loss through
heterogeneous chemical processes. Winter seasons dominated by DWC events (i.e., reflective winters) are
characterized by a lower stratospheric ozone concentration in winter and
spring. This behavior is consistent with the cumulative effects of downward
planetary wave events on polar stratospheric ozone via the residual
circulation and the polar temperature in winter.
Schematic presentation of the DWC influence on polar stratospheric
ozone over the life cycle of DWC:
Our results establish a new perspective on dynamical processes controlling
Arctic ozone variability. Previous studies have shown that variations in
upward planetary waves entering the lower stratosphere in midwinter determine
the magnitude of ozone loss in the Arctic polar vortex (e.g.,
Recent studies showed that large chemical ozone loss in the spring of 2011
is one of the major reasons for the unprecedented low Arctic column
ozone (e.g.,
Our results also reveal that the amount of wave absorption directly
influences polar Arctic temperatures and therefore the amount of ozone
destruction in spring. Since wave absorption is minimal during reflective
winters, the winters tend to be cold with more ozone destruction. The process
of wave reflection and absorption is highly variable, and the amount and
location depends on the tropospheric source of the waves, the structure of
the vortex on which the waves propagate, and on non-conservative effects
A recent multi-model intercomparison of CCMs
concludes that the models do not produce a consistent prediction of the
evolution of Arctic temperatures and ozone loss in the 21st century,
mainly because of discrepancies in the model's dynamics (
Reanalysis data used in this paper are publicly available from the GES DISC for the MERRA and MERRA-2 products. The CESM1(WACCM) model data requests should be addressed to Katja Matthes (kmatthes@geomar.de).
The Aura Microwave Limb Sounder (MLS) ozone product
Figure A2 shows the differences between the daily total ozone and the ozone
anomaly time series in MERRA1 and MERRA2 relative to MLS from 2005 to 2013.
It is shown that a positive bias of total ozone mixing ratio in MERRA1 in the
lower-to-middle stratosphere (between 100 and 5 hPa) no longer exists in
MERRA2 (Fig. A2d and e). There is also good improvement in total ozone mixing
ratio in the upper stratosphere, as indicated by a significant reduction of
negative ozone bias in MERRA2 in comparison to MERRA1. Likewise, ozone
anomaly in MERRA2 is in a good agreement with the MLS dataset compared to MERRA1
(Fig. A2i and j). In summary, we find that the ozone dataset from MERRA2 is
significantly improved compared to MERRA1
Time series of total ozone (left column panels) and ozone anomaly
(right column panels) averaged between 60 and 90
Understanding how circulation is controlled by planetary waves is the key to
connecting the wave driving to polar temperatures and transport of trace
gases such as ozone. The transformed Eulerian mean formulation
Furthermore, in order to diagnose vertical reflecting surfaces and meridional
wave guides for stratospheric planetary wave propagation, the index of
refraction (
The statistical significance of the composite anomalies was evaluated using a
Monte Carlo test similar to
The authors declare that they have no conflict of interest.
This work is supported by the German-Israeli Foundation for Scientific Research and Development under grant GIF1151-83.8/2011. This work has also been partially performed within the Helmholtz University Young Investigators Group NATHAN funded by the Helmholtz Association through the Presidents Initiative and Networking Fund and the GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany. Nili Harnik also acknowledges the support of a Rossby Visiting Fellowship from the International Meteorological Institute (IMI) of Stockholm University, Sweden. We also thank Mijeong Park for providing daily AURA-MLS ozone datasets and NASA's Global Modeling and Assimilation Office for providing the MERRA-1 and MERRA2 datasets. The authors are grateful to Susann Tegtmeier and the two anonymous reviewers whose insightful comments led to significant improvements of the paper. The model simulations were performed at the German Climate Computing Centre (Deutsches Klimarechenzentrum, DKRZ) in Hamburg, Germany. The article processing charges for this open-access publication were covered by a Research Centre of the Helmholtz Association. Edited by: T. J. Dunkerton Reviewed by: four anonymous referees