Climate change will be felt primarily through changes in extreme weather: intense storms, precipitation events, and temperature anomalies.  Extreme events in the stratosphere, namely Sudden Stratospheric Warmings (SSWs), are known to impact surface weather extremes, driving an equatorward shift of the storm tracks and associated jet streams.  Efforts to quantify potential changes in SSWs in response to anthropogenic forcing, both their frequency and their surface impact, however, have been hampered by the large uncertainty in the observational record.  The problem becomes more acute for the most extreme SSWs, which are known to have a stronger surface impact. A once-in-a-century event takes, on average, 100 years of observations or simulation time to appear just once.  This is far beyond the typical integration length of our most accurate weather models, which provide the best representation of stratosphere-troposphere coupling, so the task is often left to cheaper, but less accurate, low-resolution or statistical models.  One reduces the sampling error (aleatoric uncertainty) at the expense of increased model error (epistemic uncertainty).

In this work, we propose methods to extract climatological information from subseasonal forecast ensembles.  Despite being short in duration, weather forecast ensembles are produced multiple times a week, collectively, adding up to thousands of years of data.  Using ensemble hindcasts produced by the European Center for Medium-range Weather Forecasting (ECMWF) archived in the subseasonal-to-seasonal (S2S) database, we compute multi-centennial return times of extreme SSW events. Consistent results are found between alternative methods, including basic counting strategies and Markov state modeling. By combining different trajectories together in a statistically rigorous way, we obtain estimates of SSW frequencies and their seasonal distributions that are consistent with reanalysis-derived estimates for moderately rare events, but can be extended to events of unprecedented severity that have not yet been observed historically. The same methods hold potential for assessing extreme events throughout the climate system, beyond the example of stratospheric extremes presented here, and could be adopted in the context of climate change integrations to quantify the impact of anthropogenic forcing on extreme weather.

How to cite: Gerber, E., Finkel, J., Abbot, D., and Weare, J.: Revealing the statistics of extreme stratospheric sudden warming events hidden in short weather forecast data, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6234, https://doi.org/10.5194/egusphere-egu23-6234, 2023.

The proposed drivers of wintertime North American cold spells are multifarious, including modes of climate variability, planetary wave patterns and regional-to-continental-scale weather regimes. The Arctic stratospheric polar vortex has also been reported as a potential remote driver. One proposed coupling mechanism between the stratospheric polar vortex and the troposphere is upward-propagating planetary waves being reflected downward by the polar vortex. Here, we present a physically interpretable regional stratospheric wave reflection detection metric and identify the tropospheric circulation anomalies over North America associated with prolonged periods of wave reflection. The stratospheric reflection events show a systematic evolution from a Pacific Trough regime – associated on average with positive temperature anomalies and a near-complete absence of anomalously cold temperatures in North America – to an Alaskan Ridge regime, which favours low temperatures over much of the continent. The most striking feature of the stratospheric reflection events is thus a systematic shift in the tropospheric circulation in North America, associated with a rapid, continental-scale decrease in temperatures. These emerge as continental-scale cold spells by the end of the reflection events. Stratospheric reflection events are thus highly relevant in a tropospheric predictability perspective.

How to cite: Messori, G., Kretschmer, M., H. Lee, S., and Wendt, V.: Stratospheric wave reflection modulates North American cold spells, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1904, https://doi.org/10.5194/egusphere-egu23-1904, 2023.

The Energy Exascale Earth System Model version 2 (E3SMv2) was publicly released by the Department of Energy (DoE) in September 2021. An important component of the model validation were historical climate simulations that followed the protocols of the Coupled Model Intercomparison Project Phase 6 (CMIP6). In particular, five historical ensemble members were recently released, and 16 additional ensemble members will soon become available as part of an E3SMv2 Large Ensemble (LE). The paper sheds light on the characteristics of the E3SMv2 CMIP6 stratospheric circulation which has not yet been documented in the literature. Particular attention is paid to the tropical stratosphere which includes the so-called water vapor tape recorder and the Quasi-Biennial Oscillation. In addition, the general circulation and its variability are briefly described to reveal E3SMv2’s strengths and weaknesses. We compare E3SMv2’s circulation to observations and ERA5 reanalysis data. Furthermore, selected comparisons to the predecessor version E3SMv1 as well as other CMIP6 models are provided to put the results into context. The analysis informs the DoE Sandia National Laboratories project CLDERA which uses the Mt. Pinatubo volcanic eruption for climate attribution studies.

How to cite: Jablonowski, C., Nguyen, L., Golaz, J.-C., Rosenbloom, N., and Meehl, G. A.: Characteristics of the Stratospheric Tropical Circulation of the Energy Exascale Earth System Model E3SMv2, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15173, https://doi.org/10.5194/egusphere-egu23-15173, 2023.

The Quasi-Biennial Oscillation (QBO) is known to influence the boreal winter surface circulation. The QBO affects extratropical surface circulation and temperature over Asia as well as the North Atlantic and Europe. While these surface responses are undoubtedly associated with the QBO, it is less clear whether they arise solely due to modulations in the frequency of extreme stratospheric events such as sudden stratospheric warmings (SSWs) or due to other pathways of the QBO. SSWs rapidly evolve in the stratosphere but can initiate persistent hemispheric scale surface temperature and circulation responses and hence, may account for some of the surface response to the QBO. While not reproduced in climate models, reanalysis tends to show that the frequency of SSW is higher during easterly QBO (EQBO), which could suggest the SSW do in fact account for some of the EQBO surface response. However, the QBO has multiple pathways to influence boreal winter surface conditions and hence may affect surface conditions independently of SSWs.
Here, we study the teleconnections and extratropical surface responses to EQBO by prescribing EQBO in an ensemble of branched simulations derived from a control simulation run without a QBO. The EQBO branched simulations, which run with repeating annual cycles of sea surface temperatures and sea ice, reproduce many of the observed EQBO teleconnections and surface impacts. After subsampling the branched runs for members
with and without SSWs, it becomes clear that SSWs are necessary to fully realize both the polar and subtropical routes of EQBO influence. However, there are regions such as east Asia where the surface circulation is able to be modulated by the EQBO even in the absence of SSWs.

How to cite: Butler, A., Elsbury, D., Peings, Y., and Magnusdottir, G.: Sensitivity of EQBO’s boreal winter teleconnections and surface impacts to SSWs, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3526, https://doi.org/10.5194/egusphere-egu23-3526, 2023.

In addition to the well-known warming at high latitudes, sudden stratospheric warmings (SSWs) produce cooling and reduced static stability in the tropical lower stratosphere.  Based on 40 years of ERA5 reanalysis data and MJO amplitude data compiled at the U.S. National Oceanic and Atmospheric Administration, if these events occur in early winter (prior to ~ mid-January), the reduced static stability is sufficient to produce a statistically significant, lagged strengthening of the MJO peaking about 25 days after the SSW central date. Late winter SSWs produce no detectable strengthening.  This may be due to the timing of the SSW in early winter when tropical lower stratospheric temperatures and static stabilities are approaching their climatological minima.  This produces lower static stabilities than can occur following a late winter SSW when lower stratospheric temperatures are already higher and rising.  Positive feedbacks from MJO convection-induced temperature anomalies, cloud-radiative effects, and increased tropospheric Rossby wave amplitudes acting to further increase tropical upwelling rates, may further enhance MJO amplitudes.

The lagged strengthening of the MJO following early winter SSWs is also found in at least one climate model simulation in the CMIP6 archive (MRI-ESM-2.0).  We have so far analyzed in detail three full ensemble members (453 model years) of the 4xCO₂ forcing version, which has relatively low climatological static stabilities in the tropical lower stratosphere.  Because of the large number of model years analyzed, the lagged strengthening is statistically robust but is weaker and occurs at a shorter time lag of 10-15 days than is estimated from the available observations.  Using the large number of available early winter SSWs, it is found that those SSWs that produce the largest reductions in static stability in the tropical lower stratosphere (70 to 100 hPa) also produce the largest lagged strengthenings of the MJO.  This supports a top-down static stability mechanism for producing the strengthening. Analyses of data from other climate models in the CMIP6 archive are in progress.

Because early winter SSWs occur primarily under easterly quasi-biennial oscillation (QBO) conditions and late winter SSWs occur most often under westerly QBO conditions, these results have implications for the origin of the observed modulation of the MJO by the stratospheric QBO.  Extratropical wave forcing events (including minor as well as major warmings) are typically stronger in early winter under easterly QBO conditions, as was originally reported by Holton and Tan (1980).  These events will reduce tropical lower stratospheric static stability primarily in boreal winter when the QBO-MJO connection is observed.  While most MJO convection extends only to lower altitudes in the troposphere, it is mainly the strongest MJO events that are modulated by the QBO.  These latter events may extend to higher altitudes and be more affected by stability conditions in the lowermost stratosphere.

How to cite: Hood, L. L., Trencham, N. E., Hoopes, C. A., and Galarneau, Jr., T. J.: Lagged Strengthening of the Tropical Madden-Julian Oscillation Following Early Winter Sudden Stratospheric Warmings:  Observational Analyses and CMIP6 Model Comparisons, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2935, https://doi.org/10.5194/egusphere-egu23-2935, 2023.

The Brewer-Dobson circulation is the wave driven meridional circulation of the stratosphere it plays an important role in determining the transport of trace gases and aerosols within the stratosphere, affecting the lifetimes of ozone depleting substances, and the global radiation budget. The overturning part of the circulation is usually split into a deep and a shallow branch. Here, we investigate the dynamical driving of these circulation branches, aiming for a dynamical separation of different circulation regimes.

For that purpose, we use data from fifth generation atmospheric reanalysis ERA5 by the European Centre for Medium-range Weather Forecasts (ECMWF) and apply the Transformed Eulerian Mean approach to estimate the overturning mass flux and the related wave forcing in the stratosphere. We find that reducing the horizontal resolution of the data from 0.3 to 1 degree does not affect results significantly. However, reducing temporal resolution from 1 to 6 hours has a significant effect on the structure of daily mean upwelling, but this effect is much less pronounced for monthly means. Eliassen-Palm flux divergence is used as a diagnostic to estimate the wave propagation in the stratosphere. Using Fourier transformation for spectral decomposition we estimate the contribution from different waves to the driving of the deep and shallow branches. In particular, it is found that the deep branch has a strong seasonality with maximum in winter, while the shallow branch is less affected by the change of seasons. On the one hand, we find a strong correlation between variability in residual circulation velocity along the deep branch and large scale waves with wavenumber 3 or smaller. On the other hand, variability of medium and small scale waves with wavenumber 4 or greater correlates strongly with variability in the shallow branch circulation velocities. The change between these two dynamical regimes happens at a level close to 37 hPa. These results were further tested by applying a downward control calculation, showing that indeed the large-scale, planetary waves (wavenumber less than 4) account for the largest part of deep branch variability, while smaller waves (wavenumber larger than 3) account for the largest part of shallow branch variability. Based on these results, we propose a physical definition of the different Brewer-Dobson circulation branches, with the deep branch defined as driven by planetary waves (wave numbers 1-3) and located above 35 hPa, whereas the shallow branch being located below that level and driven by smaller-scale waves (wave numbers 4 and greater).  

How to cite: Baikhadzhaev, R., Ploeger, F., Preusse, P., Ern, M., and Birner, T.: A dynamical separation of deep and shallow branches in the stratospheric circulation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4011, https://doi.org/10.5194/egusphere-egu23-4011, 2023.

Recent studies have suggested that extreme stratospheric wave activity is connected to surface temperature anomalies, with a dynamical mechanism distinct from the canonical downward influence of stratospheric polar vortex events. However, some key processes regarding the underlying dynamics and timescales are not well understood. In this study, we show in observations that the stratospheric events featured by weaker-than-normal wave activity are associated with increased cold extreme risks over North America before and near the event onset, accompanied by less frequent atmospheric river (AR) events on the west coast of the U.S. Strong stratospheric wave events, on the other hand, exhibit a tropospheric weather regime transition. North American warm anomalies and increased AR frequency over the west coast are observed before strong wave events, while an increased risk of cold extremes over North America and north-shifted ARs over the Atlantic occurs after the events. Historical simulations from CMIP6 can capture the extreme stratospheric wave events and their overall tropospheric fingerprints, with evident uncertainties across different models.

These links between the stratosphere and troposphere are attributed to the vertical structure of wave coupling. Weak wave events are accompanied by a wave structure tilting westwards with increasing altitude, while strong wave events show a shift from westward tilt to eastward tilt during the life cycle of events. This wave phase shift indicates vertical wave coupling and likely regional planetary wave reflection. Further examination shows that models with a degraded representation of stratospheric wave structure exhibit biases in the troposphere during strong wave events. More specifically, models with a stratospheric ridge weaker than the reanalysis exhibit a weaker tropospheric signal. Our findings suggest that the vertical coupling of extreme stratospheric wave activity should be evaluated in the model representation of stratosphere-troposphere coupling. 

How to cite: Ding, X. and Chen, G.: Assessing Stratosphere-troposphere Coupling of Extreme Stratospheric Wave Activity in CMIP6 Models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4526, https://doi.org/10.5194/egusphere-egu23-4526, 2023.

Sudden stratospheric warming (SSW) events can form a window of forecast opportunity for polar vortex predictions on subseasonal-to-seasonal time scales. Analysing numerical ensemble simulations, we show that negative wind anomalies in the polar stratosphere following SSWs lead to a reduction in upward planetary wave propagation and hence a reduction in the dynamical variability of the polar vortex. Ensembles that predict an SSW show reduced ensemble spread in terms of polar vortex strength for several weeks to follow, as well as a corresponding reduction in forecast errors. The associated increase in predictability is particularly pronounced for strong SSWs and even occurs if not all ensemble members predict a major SSW. The decrease in upward wave fluxes and polar vortex variability following the event then manifests in a delay of the final warming during years that experience an SSW and potentially has further implications for the tropospheric or mesospheric circulation.

How to cite: Rupp, P., Spaeth, J., Garny, H., and Birner, T.: Reduced stratospheric variability following sudden stratospheric warming events, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9027, https://doi.org/10.5194/egusphere-egu23-9027, 2023.

Atmospheric climate change is characterized by global warming above the Earth’s surface associated with the increase of the greenhouse gas population since the start of the industrial era. In the upper atmosphere where both neutral and plasma gases are subject to substantial variability due to space and terrestrial weather, including low atmospheric forcing. Direct and long-term ionospheric observations with the incoherent scatter radar (ISR) technique provide an efficient way to quantify and understand the variation trend of the thermal status in the upper atmosphere. Since 2008, the ISRs have been providing some of the key evidence for the cooling in the ionosphere and thermosphere, particularly, its altitude dependence above 100 km. This cooling was increasing as a function of height and was initially interpreted as a greenhouse gas effect in the upper atmosphere which was also observed in satellite drag observations, however, almost all the ISR results suggested that the cooling appeared substantially large and therefore additional cooling processes beyond the greenhouse gases as a thermospheric cooling agent are needed. These include potential wave activity changes arising from climate change as well as secular changes in Earth’s main magnetic field. In this presentation, we provide an updated analysis of ISR-measured trends and discuss some progresses in understanding these results.

How to cite: Zhang, S.-R., Wang, W., Aa, E., Erickson, P., Qian, L., and Goncharenko, L.: Monitoring and understanding upper atmospheric long-term cooling, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10155, https://doi.org/10.5194/egusphere-egu23-10155, 2023.

Sudden stratospheric warming (SSW) events, followed by a characteristic circulation regime in the lower troposphere, are crucial for the subseasonal weather prediction. What remains controversial is whether or not increasing the height of top layer in a climate model and the vertical resolution improve the representation of SSW events and the subsequent influences on the near-surface climate. In this study, we examine the SSW events simulated in a high-top climate model (the Whole Atmosphere Community Climate Model version6, WACCM6) and those simulated in a low-top model (Community Atmosphere Model version 6, CAM6) with 30 ensemble members. The two sets of experiments are forced by identical observational sea-surface temperature and sea-ice concentration, as well as the radiative forcings, during the 1979-2013 period. We find that WACCM6 produces about two times more SSWs than CAM6 (i.e., 759 v.s. 357 events), and, in terms of occurring frequency, SSWs in WACCM6 happen about 7 times per decade, closer to the SSW frequency in reanalysis datasets. Analyses on the thermodynamical and dynamical components of SSWs, including the sea-level pressure precursor, the preceding Eliassen-Palm fluxes into the stratosphere, the stratospheric temperature increases, and the downward propagation features, reveal that WACCM6 in general gives weaker signals than CAM6. This is likely attributed to the weaker mean stratospheric circulation in WACCM6. We also find that the WACCM6-CAM6 differences can be amplified during the years of El Niño and La Niña events. Finally, we perform the vortex moments diagnostics to gain further insights into the vortex structure and separate the splitting and displacement SSWs. The diagnostics shows that CAM6 generates more symmetric polar vortices than WACCM6, while the vortices from CAM6 deviate less often from the North Pole. However, the ratio of displacement and splitting SSWs in both model is about 70%, larger than about 60% in reanalysis datasets. Our study suggests that the high-top configuration leads to better performance of stratospheric circulation variability.

How to cite: Liang, Y.-C., Wang, Y., Kwon, Y.-O., Frankignoul, C., Polvani, L., and Suo, L.: Exploring the Sudden Stratospheric Warming Events in High-top and Low-top Climate Model Large Ensembles, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10986, https://doi.org/10.5194/egusphere-egu23-10986, 2023.

Previous studies have documented the impact of the Arctic sea ice loss and associated warming on the midlatitude weather and climate, especially the influence of sea ice retreat over the Barents-Kara Sea on the North Atlantic and Europe regions. However, less attention has been given to other geographical locations over the Arctic, and to the linear additivity of the circulation response to regional Arctic sea ice loss and temperature anomalies. Using a simplified dry dynamical core model, we demonstrate that responses to regional Arctic temperature anomalies over the Barents-Kara Sea, Baffin Bay-Davis Strait-Labrador Sea, and East Siberia-Chukchi Sea, separately, cause similar equatorward shift of the tropospheric jet, but different stratospheric polar vortex responses. Furthermore, responses to regional Arctic temperature anomalies are not linearly additive, and the residual resembles a positive Northern Annular Mode-like structure. Additional targeted experiments highlight the stratospheric influence in the non-additivity of the midlatitude tropospheric response.

How to cite: De, B., Wu, Y., Polvani, L., and Elsaesser, G.: Non-Additivity of the Midlatitude Circulation Response to Regional Arctic Temperature Anomalies: The Role of the Stratosphere, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13741, https://doi.org/10.5194/egusphere-egu23-13741, 2023.

Recent work using models from the Chemistry-Climate Model Initiative shows that the stratopause is descending and that the stratosphere has contracted substantially over the last decades, being the increasing concentrations of greenhouse gases (GHGs) the main driver. This stratospheric contraction is not a mere response to stratospheric cooling, as changes in both tropopause and stratopause pressure contribute significantly to it.

Here we present the evolution over the last decades of the stratopause and stratospheric thickness in the main reanalyses datasets: MERRA-2, JRA-55, and ERA5.1. We compare them to model results with WACCM-X and satellite observations (GOMOS, MLS and SABER). For our computations, we consider the discontinuity problems in temperature at stratopause heights suitable to affect its structure and behaviour due to the assimilation of new observational data as they become available.

Our results show that there is high variability in the percentages of stratopauses and their height, a significant dependence on the latitude and dataset for the stratopause height and limitations of JRA-55 to represent elevated stratopause cases (probably linked to sudden stratospheric warnings) over the polar regions because of its low-top compared to other reanalyses.

How to cite: de la Torre, L., Añel, J. A., Kuchař, A., and Sacha, P.: Representation, evolution and validation of the stratopause and stratospheric contraction in reanalyses data, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9666, https://doi.org/10.5194/egusphere-egu23-9666, 2023.

The Constellation of High-performance Exosphere Science Satellites (CHESS) mission is a dedicated CubeSat mission designed to analyse the upper atmosphere of Earth in situ. The status of this dynamic region is driven by external and internal forces, causing variations in temperature, the chemical composition, total number density, and their altitude profiles. Although such measurements are key for our understanding of the origin and evolution of the habitable atmosphere of Earth, the community lacks updated, local, detailed measurements. Recent technological advancements in instrumentation enable sensitive in situ measurements of the exosphere–thermosphere–ionosphere region with small satellites. On one hand, the miniaturisation of a high-performance mass spectrometer enables measurements of the chemical composition and density. On the other hand, a new generation of dual-frequency Global Navigation Satellite System (GNSS) receivers enable precise orbit determination. From the knowledge of the evolution of the orbits, the atmospheric drag and hence, the density can be derived complementary. Additionally, the GNSS receivers provide the dispersive line-of-sight total electron content from the linear combination of dual-frequency carrier phase measurements that can be converted into total column density between the spacecraft in low Earth orbit and the satellites of the GNSS in higher orbits. Thanks to the elliptic orbit of the spacecraft, the altitude profiles of these number density measurements of species can be converted into exospheric temperatures. Measuring with a constellation will allow for overcoming the space-time degeneracy to analyse the drivers and mechanisms including basic physics in detail. The CHESS mission is scheduled for launch in early 2026. Once available, the collected data will also be compared with atmospheric escape data on, for example, Venus and Mars to provide insights into the different evolution of these rocky, initially comparable planets.

How to cite: Fausch, R., Moeller, G., Belkhiria, A., Kneib, J.-P., and Wurz, P.: In situ chemical composition and density measurements of Earth’s thermosphere, exosphere, and ionosphere with CHESS, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12717, https://doi.org/10.5194/egusphere-egu23-12717, 2023.

The Montreal Protocol and its amendments (MPA) have been a major success in preventing serious health damage from the destruction of the stratospheric ozone layer by chlorofluorocarbons (CFCs). Additionally, unabated CFC emissions would have significantly accelerated global warming and affected large-scale tropospheric circulation. CFC-induced ozone depletion would have contributed to global climate change through reduced absorption of UV radiation, resulting in weaker stratospheric circulation, affecting the dynamical coupling to the troposphere. With the Earth System Model SOCOLv4, we study an extreme condition where the MPA is absent to disentangle the radiative and chemical (i.e., ozone-mediated) effects of CFCs and their impacts on stratosphere-troposphere coupling. Our results show that at the end of the 21st century, unabated CFC emissions would have largely destroyed the global ozone layer, which would have strongly affected the large-scale stratospheric and tropospheric circulation. In the stratosphere, contrary to historical ozone destruction, the polar vortices severely weaken due to low-latitude ozone depletion. In the Northern Hemisphere (NH), the weakening of the vortex leads to a pronounced negative phase of the North Atlantic Oscillation (NAO) in boreal winter and spring due to the chemical CFC effect. Similarly, the stratosphere also affects the Southern Annular Mode (SAM) to be in a more negative phase in austral winter and spring. However, tropospheric warming from CFCs largely dominates the overall SAM response to be in a more positive phase, whereas in the NH it compensates for the NAO negative phase. Additionally to the circulation changes, uncontrolled CFC emissions would have led to around 2.5 K additional global surface warming, being partially compensated by a cooling of around 0.6 K due to ozone depletion, leading to an overall warming of around 1.9 K. Our study strongly emphasizes the importance of the MPA for our climate and its mitigation of stratospheric circulation changes and their effects on tropospheric variability.

How to cite: Sukhodolov, T., Zilker, F., Chiodo, G., Egorova, T., Friedel, M., Rozanov, E., Sedlacek, J., Seeber, S., and Peter, T.: Stratosphere-troposphere coupling under the extreme conditions of the No-Montreal-Protocol scenario, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8908, https://doi.org/10.5194/egusphere-egu23-8908, 2023.

Given the couplings between the circulation of the stratosphere and its composition, tracking the evolution of both is crucial. At present however, much remains to be learned about long term trends in the composition of the stratosphere, and there is still little to no agreement between the modeled trends in the Brewer Dobson Circulation and their observational counterparts; while models indicate that the BDC is accelerating at a pace of 2-3 %/decade, observational estimates suggest that the BDC is slowing down. These shortcomings are attributable in part to the relatively short length of the historical record and in part to difficulty characterizing the BDC using observations.

To alleviate these shortcomings, we propose to re-visit historical and projected BDC trends using the metric time of emergence (ToE), defined as the length of record needed to separate long-term trends from internal variability with a chosen degree of statistical confidence. We use ToE as it enables the evaluation of current observational capabilities for the detection and validation of BDC trends predicted by models. ToE also provides tangible motivation for the continued monitoring of the composition of the stratosphere by space borne platforms, a topic recently brought to light by the planned decommissioning of the Aura satellite in the absence of a follow-up flight mission.

ToE is calculated using two methods, for which results are compared: a) classic bootstrapping based on a reference CMIP6 run (a pre-industrial run, or a run with fixed contemporary greenhouse gas concentrations), and b) an analytical method published by Li et al. (2017) that does not require a very long reference run. We focus the analysis on a comparison of ToE for trends in a) the diabatic circulation, taken as reference for the “true” BDC, and b) the BDC metric based on age of air developed by Linz et al. (2016), used as a proxy for observational trend estimates. The results shed light on how internal variability shapes our understanding of long term trends, and provide minimum requirements for the robust detection of trends in the BDC using observations. 

Li, J., Thompson, D.W., Barnes, E.A. and Solomon, S., 2017. Quantifying the lead time required for a linear trend to emerge from natural climate variability. J. of Climate.

Linz, M., Plumb, R.A., Gerber, E.P. and Sheshadri, A., 2016. The relationship between age of air and the diabatic circulation of the stratosphere. JAS.

How to cite: Rivoire, L., Linz, M., Li, J., and Abalos, M.: Brewer Dobson Circulation trends from Age of Air in models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17451, https://doi.org/10.5194/egusphere-egu23-17451, 2023.

Arctic ozone is subject to large interannual variability, and severe ozone minima can occur through chemical ozone depletion and dynamical variability. Such Arctic ozone minima have been shown to bear a great societal relevance due to their impacts on health and climate. Following the success of the Montreal Protocol, ozone depleting substances (ODSs) in the stratosphere are declining, implying an expected weakening of chemical ozone destruction. However, continuing greenhouse gas (GHG) emissions cool the stratosphere, which might lead to an enhanced formation of polar stratospheric clouds (PSCs) and thus more efficient chemical depletion of ozone. Due to these opposing processes, there is currently no consensus on the fate of Arctic ozone minima in future climate.

Here, we investigate the future evolution of Arctic ozone minima over the 21^st century under different emission pathways in simulations conducted for the Chemistry-Climate Model Initiative (CCMI), CCMI-1 and CCMI-2022, and constrain these projections based on the models’ skill in reproducing present-day Arctic ozone variability. We find a large model discrepancy in the magnitude of ozone minima under present-day climate, caused by biases in the underlying model climatology. Models that simulate large ozone minima in present-day climate generally have a cold bias, and consequently large concentrations of active chlorine species (ClOx); these are the models that project the largest decline in the magnitude of ozone minima in the future. Conversely, models simulating weak Arctic ozone minima under present-day conditions generally have a warm bias and small ClOx concentrations; these are models with the smallest sensitivity of ozone minima to changes in ODS and GHG emissions. Consequently, inter-model spread in the magnitude of springtime Arctic ozone minima is projected to decline in the future. Through comparison with reanalysis, we identify the most realistic models. These models show a weakening in stratospheric ozone minima of about 1 DU/decade and with little sensitivity to the GHG emission scenario, which is deemed as the most likely projection. Taken together, these results indicate that Arctic ozone minima will likely become weaker in the future, largely due to the decline in ODS abundances, while stratospheric cooling due to GHGs is expected to play a secondary role.

How to cite: Friedel, M., Chiodo, G., and Peter, T.: Weakening of springtime Arctic ozone depletion with climate change, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13525, https://doi.org/10.5194/egusphere-egu23-13525, 2023.

Following the success of the Montreal Protocol, stratospheric ozone is projected to recover over the coming decades as halogenated ozone depleting substances decline. However, future projections of stratospheric ozone recovery are also dependent on assumptions made about the emissions of other gases such as CO2, CH4, and N2O. As a result, the pathway of ozone recovery is sensitive to the choice of future emissions scenario. Here we explore ozone recovery under different Shared Socioeconomic Pathways (SSPs) in 6 CMIP6 models that include interactive chemistry schemes: CESM2-WACCM, CNRM-ESM2-1, GFDL-ESM4, GISS-E2-1-G, MRI-ESM2-0, UKESM1-O-LL. We explore the impact of different SSP scenarios on projections of ozone recovery, the date at which total column ozone returns to historic values, and the healing of the ozone hole. Additionally, we compare global mean return dates with regional return dates and explore the different processes affecting the timing of ozone recovery in these different regions.

How to cite: Keeble, J., Hassler, B., and Schlund, M.: Sensitivity of ozone return dates to shared socioeconomic pathway in CMIP6 models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14758, https://doi.org/10.5194/egusphere-egu23-14758, 2023.