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Sulfur‐based stratospheric aerosol intervention (SAI) can cool the climate, but also heats the tropical lower stratosphere if done with injections at low latitudes. We explore the role of this heating in the climate response to SAI, by using mechanistic experiments that remove the effects of longwave absorption of sulfate aerosols above the tropopause. If longwave absorption by stratospheric aerosols is disabled, the heating of the tropical tropopause and most of the related side effects are strongly alleviated and the cooling per Tg‐S injected is 40% bigger. Such side‐effects include the poleward expansion of eddy‐driven jets, acceleration of the stratospheric residual circulation, and delay of Antarctic ozone recovery. Our results add to other recent findings on SAI side effects and demonstrate that SAI scenarios with low‐latitude injections of absorptive materials may result in atmospheric effects and regional climate changes that are comparable to those produced by the CO2 warming signal. Plain Language Summary: We explore the effects of the injection of sulfur into the tropical stratosphere to lower global average temperatures from those projected under a high‐emission greenhouse gas scenario to the levels of a mid‐emission scenario. We found that some detrimental effects of this injection are of a similar magnitude to those from climate change itself in some regions. This includes a strong warming 15 km above the tropics, which alters large‐scale weather patterns in the atmosphere. In comparison to the mid‐emission scenario, we find enhanced surface warming in the polar regions and modification in regional precipitation patterns over land, therefore not completely alleviating the warming of the high‐emission scenario in high northern latitudes. We show that the tropical stratospheric heating is responsible for a large portion of these side effects on tropospheric climate. Key Points: Many side effects of sulfur‐based stratospheric aerosol intervention are caused by heating of the tropical lower stratosphereSome regional patterns of change tied to atmospheric circulation can be of the same magnitude as those that are CO2‐drivenThe absorptivity of the aerosol particles increases their lifetime but decreases their cooling efficiency per Tg‐S per year by 40% [ABSTRACT FROM AUTHOR]

The impacts of Stratospheric Aerosol Injection (SAI) on the atmosphere and surface climate depend on when and where the sulfate aerosol precursors are injected, as well as on how much surface cooling is to be achieved. We use a set of CESM2(WACCM6) SAI simulations achieving three different levels of global mean surface cooling and demonstrate that unlike some direct surface climate impacts driven by the reflection of solar radiation by sulfate aerosols, the SAI‐induced changes in the high latitude circulation and ozone are more complex and could be non‐linear. This manifests in our simulations by disproportionally larger Antarctic springtime ozone loss, significantly larger intra‐ensemble spread of the Arctic stratospheric jet and ozone responses, and non‐linear impacts on the extratropical modes of surface climate variability under the strongest‐cooling SAI scenario compared to the weakest one. These potential non‐linearities may add to uncertainties in projections of regional surface impacts under SAI. Plain Language Summary: The injection of reflective aerosols, or their precursors, into the lower stratosphere (Stratospheric Aerosol Injection, SAI) has been proposed as a temporary measure to offset some of the adverse impacts of climate change whilst atmospheric concentrations of greenhouses are being stabilized and, ultimately, reduced. The impacts of SAI on the atmosphere and surface climate would depend on when and where the sulfate aerosol precursors are injected, as well as on how much surface cooling is to be achieved. Here we analyze SAI impacts on stratospheric climate and ozone in a set of Earth system model simulations under varying magnitudes of the SAI‐induced global mean cooling. We demonstrate that unlike some of the direct surface climate impacts from the reflection of solar radiation by sulfate aerosols, the SAI‐induced changes in stratospheric circulation, chemistry and climate are more complex, with the model simulations pointing toward more non‐linear behavior of the high latitude circulation and ozone under higher SAI scenarios. These potential non‐linearities may add to uncertainties in projections of regional surface impacts under SAI. Key Points: Impacts of Stratospheric Aerosol Injection (SAI) depend on how much surface cooling is to be achievedHigh latitude circulation, ozone and modes of extratropical variability can vary non‐linearly with the SAI‐induced global surface coolingThese potential non‐linearities may add to uncertainties in projections of regional surface impacts under SAI [ABSTRACT FROM AUTHOR]

The dominant pattern of Northern Hemisphere extratropical climate variability is the Northern Annular Mode (NAM), which in wintertime represents the coupling of the stratospheric and tropospheric circulations. The internal variability associated with the NAM has been shown to give rise to substantial uncertainty in climate projections of regional surface air temperatures (SATs), but it's unclear how much of this variability arises from stratosphere‐troposphere coupling processes. Here, using three large‐ensemble coupled climate model simulations from 1850 to 2100 with varying fidelity of stratosphere‐troposphere coupling processes, we demonstrate that regional SAT variability across timescales is amplified when the tropospheric NAM is coupled to the stratospheric NAM. Moreover, models that lack adequate simulation of stratosphere‐troposphere coupling processes may not capture the magnitude of uncertainty in near‐term SAT projections associated with this coupling. Plain Language Summary: Large‐scale atmospheric circulation variability drives substantial variation in regional surface air temperatures (SATs), so that even on 30–40 year timescales local cooling trends are possible even as global‐mean temperatures increase. We provide evidence that when the stratospheric mean flow is coupled to the tropospheric mean flow, the decadal variability of SATs is amplified in some regions. Some models do not represent well the observed coupling of the stratosphere to the troposphere, and these models may underestimate decadal variability associated with this coupling. Key Points: The Northern Annular Mode (NAM) exhibits decadal variability that can accelerate or slow the rate of anthropogenic regional surface warmingAmplified decadal variability of extratropical surface temperatures occurs when the NAM is coupled from the stratosphere to the troposphereModels that fail to adequately capture vertical coupling may underestimate the magnitude of decadal variations in surface air temperatures [ABSTRACT FROM AUTHOR]

Tropospheric methane (CH4) shows large inter‐annual variability in its growth rates on top of decadal trends, which is usually interpreted as changes in sources or sinks. The contribution of the stratosphere‐troposphere exchange (STE) is often omitted in such analyses. Here, we quantify annual anomalies of stratosphere‐troposphere CH4 fluxes using varied model‐ or observation‐based methods. Globally, the inter‐annual variability (standard deviation) of this flux during 2000–2020 is 2.0 Tg a−1, producing a variability of ∼0.6 ppb a−1 at the surface, which is only ∼20% of observed surface CH4 growth rate variability. Compared to global averages, high latitudes experience a larger STE‐induced surface variability, which is 80% of the observed surface anomalies in the Antarctic and 44% in the Arctic. Our results suggest that although the STE process is a minor contributor globally, it has a considerable impact over polar regions on the inter‐annual variability of surface CH4 growth rates. Plain Language Summary: In addition to surface emissions and tropospheric removal, tropospheric CH4 concentrations are also affected by exchange with the stratosphere. However, the changes in the stratosphere‐troposphere flux are often unaccounted for in the analysis of surface CH4 observations. Here we evaluate this stratospheric influence by quantifying stratosphere‐troposphere CH4 fluxes. We find that the surface CH4 variability induced by the stratosphere‐troposphere exchange is relatively small globally but can be substantial in polar regions at anomalies of the stratospheric circulation. Key Points: The inter‐annual variability of the CH4 flux between the stratosphere and the troposphere is 2.0 Tg a−1Globally the stratosphere‐troposphere exchange is a minor contributor to observed inter‐annual variation of tropospheric CH4 growthThe surface impact can be substantial at high latitudes during the maximum or minimum of the Brewer‐Dobson Circulation [ABSTRACT FROM AUTHOR]

The 15 January 2022 eruption of the Hunga Tonga‐Hunga Ha'apai underwater volcano (20.5°S, 175.4°W) injected a record amount of water directly into the stratosphere. This study attempts to quantify this impact on the temperature, as well as the subsequent changes to the stratospheric circulation, during the months following the eruption based on reanalysis fields. The extreme nature of the temperature, wind, and circulation changes are tracked through comparisons of the first six months of 2022 with the previous 42 years. Examination of the data assimilation process shows that at 20 hPa the thermal observations are forcing significant cooling, compensating for the absence of the excess stratospheric moisture in the model used for the reanalysis. In response to this cooling the atmosphere adjusts by creating strong westerly winds above the temperature anomaly and large changes to the downward and poleward mean meridional circulation. Plain Language Summary: While the stratosphere (15–55 km) region of the atmosphere contains the ozone layer, it is typically very dry, especially when compared to the troposphere. However, the remarkable eruption of the Hunga Tonga‐Hunga Ha'apai underwater volcano on 15 January 2022 injected a record amount of water directly into the stratosphere. Winds in the stratospheric soon carried this excess water vapor around the globe to all longitudes and spread the water vapor in latitude as well. Since water vapor can cool to space more rapidly than the stratospheric air, enhanced cooling in the stratosphere is expected. This study quantifies this cooling, as well as the subsequent changes to the stratospheric circulation, during the months following the eruption. The extreme nature of the stratospheric temperature, wind, and circulation changes are tracked through comparisons of the first six months of 2022 with the previous 42 years. Details of the stratospheric perturbations in latitude and pressure are presented for June 2022, where anomalously low temperatures are found at near 20 km altitude from 60 to 30°S. In response to this cooling the atmosphere adjusts by creating strong westerly winds above the temperature anomaly and large changes to the downward and poleward mean meridional circulation. Key Points: Extreme perturbations in the stratospheric winds and temperatures are linked to a volcanic eruptionHunga Tonga‐Hunga Ha'apai eruption effects are unique in the stratospheric recordData assimilation can track temperature and wind perturbations, even when some physics is missing [ABSTRACT FROM AUTHOR]

Tropospheric trends in long‐lived source gases N2O and the chlorofluorocarbons cause trends in O3 through changes in their reactive product gases. Transport affects the product gases because it controls the distribution of the long‐lived source gases. We find that large changes in tropical upwelling 10–5 hPa since 2012 have strengthened the northern branch of the upper stratospheric (UpS) transport circulation, dramatically altering the abundances of N2O and its odd nitrogen product gases, NOx and HNO3. Increased upwelling is connected to stronger and more frequent Quasi‐Biennial Oscillation easterly winds at 10 hPa and above. We use simulations with and without time varying MERRA2 meteorology to quantify the impact of dynamical changes on O3 loss via the NOx and ClOx cycles. We find that dynamical impacts on these cycles explain the mid‐stratospheric tropical O3 increase and Arctic UpS O3 decrease since 2005. Plain Language Summary: Changes in the upper stratospheric circulation over the past decade have altered composition between 30 and 40 km altitude. In the tropics, faster ascent carried greater amounts of nitrous oxide (N2O), a gas produced at the surface, to the upper stratosphere. This increased the amount of the ozone‐destroying nitrogen radicals produced from N2O. Most of the extra nitrogen radicals traveled to the Arctic above ∼32 km, where they increased ozone destruction. Below the altitude where the radicals are produced (about 35 km), the faster tropical ascent carries reduced amounts of nitrogen radicals to the middle stratosphere (∼26 to 35 km), leading to an ozone increase. Observations by satellite instruments revealed changes in the composition. Model simulations showed that circulation change caused the composition changes that affected ozone. The circulation change was connected to changes in the tropical winds that circulate around the globe and change from easterly to westerly on a fairly regular basis. During the last 10 years, this long‐standing pattern changed. This could be variability or the start of a trend; it's too soon to tell. Either way, it complicates the identification and attribution of the ozone increases we expect due to international treaties that banned the use of manmade chlorocarbons. Key Points: Tropical upwelling 10–5 hPa increased up to 30% from 2005 to 2021, increasing upper stratospheric N2O and reactive nitrogen productionIncreased upwelling is connected to stronger and more frequent Quasi‐Biennial Oscillation easterlies at 10 hPa and aboveIncreased upwelling changed NOx‐driven O3 losses, increasing tropical O3 from 15 to 7 hPa while reducing Arctic O3 from 15 to 2 hPa [ABSTRACT FROM AUTHOR]

Sudden stratospheric warmings (SSWs) can have a strong impact on the troposphere. Their fingerprint is often associated with the negative phase of the Northern Annular Mode (NAM) and the North Atlantic Oscillation (NAO), and extreme weather with high societal impact. However, the mechanisms behind this downward impact are not well understood. We investigate this surface impact through its associated predictability limits, by studying the 2018 SSW event. We search for predictability barriers that occurred after the onset of the SSW and before its surface impact. It is found that dynamical tropospheric events consisting of two cyclogenesis events were the main reasons for these predictability barriers in the prediction of negative NAM/NAO anomalies reaching the surface. This work corroborates that individual synoptic events might constitute predictability barriers during the downward impact of SSW events, and thereby sheds light on stratosphere‐troposphere coupling. Plain Language Summary: Sudden stratospheric warmings (SSWs) can have a strong impact on near‐surface weather. They have the potential to alter the atmospheric circulation and associated extreme events, thus affecting society. However, the way the stratosphere couples with surface weather is not fully understood. In this work, we investigate this relationship by analyzing the predictability of the surface impact of a stratospheric warming event that occurred in 2018, and which left widespread societal impacts over Europe due to a severe cold spell and extraordinary rainy and windy conditions. We find that the impact of this SSW event on the surface was regulated by two low‐pressure systems that developed over the North Atlantic. Even though the SSW event was well developed, alternative behavior of these systems could have led to drastically different weather conditions over Europe. Thus, this study highlights the role of the tropospheric circulation in the effectiveness of the stratospheric impact on surface weather. Key Points: The surface impact of the sudden stratospheric warming in 2018 was associated with a Greenland blocking in the troposphereMedium‐range uncertainties in the prediction of the Greenland blocking were linked to two cyclogenesis events over the North AtlanticIndividual synoptic events constitute "predictability barriers" for subsequent forecast lead times after sudden stratospheric warming events [ABSTRACT FROM AUTHOR]

The teleconnection between the Quasi‐Biennial Oscillation (QBO) and the Arctic polar vortex is investigated using Coupled Model Intercomparison Project 6 (CMIP6) models. Output from 14 CMIP6 models is compared with reanalysis, three experiments with prescribed QBOs, one of which has no free polar stratospheric variability, and transient experiments in which a QBO is prescribed in runs previously devoid of a QBO. Each CMIP6 model underestimates the Holton‐Tan effect (HTE), the weakening of the polar vortex expected with QBO easterlies in the tropical lower stratosphere. To establish why, potential vorticity maps are used to investigate longitudinal variations in the teleconnection. Prescribing easterly QBO in the transient experiments promotes more high‐latitude planetary wave breaking by influencing the mid‐latitude stratospheric circulation, particularly over Asia. CMIP6 models that better simulate this response over Asia better simulate the HTE. These models also have stronger 10 hPa QBO westerlies. Plain Language Summary: In the tropics at altitudes between two and three times as high as commercial airplanes fly (10–30 kilometers), successive sets of easterly and westerly winds propagate downward approximately every 28 months. This pattern of winds is called the Quasi‐Biennial Oscillation (QBO). Despite being in the middle to lower tropical stratosphere, the QBO affects the global circulation in ways that ultimately influence regional weather. One example of this occurs during winter, when the QBO changes the strength of the westerly flow that forms in the Arctic polar stratosphere in the absence of solar radiation, the polar vortex. Although scientists have known about this phenomenon for 40+ years, this long‐distance relationship is complicated. Models of the coupled land, ocean, atmosphere system have steadily improved at representing the QBO. These models also represent the QBO's relationship with the polar vortex, but each model does it differently. The difference between models that represent the QBO‐polar vortex relationship well and models that do not, teaches us more about the relationship. We learn here that the QBO communicates with the polar stratosphere partly by changing the atmospheric circulation 30 kilometers above Asia. This phenomenon is best represented by models that have stronger QBOs. Key Points: Coupled Model Intercomparison Project 6 models underestimate the weakening of the polar vortex expected with Quasi‐Biennial Oscillation (QBO) easterlies in the tropical lower stratosphereThe mid‐latitude stratospheric response to the QBO over Asia influences how the QBO affects the strength of the polar vortex in the modelsModels simulating more realistic extratropical responses to the QBO have stronger QBO westerlies in the tropical middle stratosphere [ABSTRACT FROM AUTHOR]

In the extratropical atmosphere, Rossby waves (RWs) and internal gravity waves (GWs) propagating from the troposphere mediate a coupling with the middle atmosphere by influencing the dynamics therein. In state‐of‐the‐art chemistry‐climate models (CCMs), RW effects are well resolved while the majority of GW effects have to be parameterized. Here, we analyze orographic GW (OGW) interaction with resolved dynamics in a comprehensive CCM on daily time scales. For this, we apply a recently developed method of strong OGW drag event composites for three pronounced northern hemisphere OGW hotspots. We show that strong OGW drag events are associated with anomalous resolved wave propagation in the stratosphere. Causal links are inferred from previously published work and are supported by the anomalies in zonal circulation and wave activity tendencies. The nature of these anomalies varies depending on the hotspot region, which underlines the parameterized OGW‐resolved dynamics interaction being a two‐way process. Plain Language Summary: The majority of atmospheric waves are generated near the surface and propagate subsequently upward in the atmosphere. This includes Rossby waves that are resolved in climate models and small‐scale gravity waves (GWs) that commonly have to be parameterized. In the middle atmosphere, the waves eventually break, thereby dissipating their momentum and energy, which influences atmospheric dynamics. The interaction of GWs with the large‐scale circulation is to date poorly understood. In this study, we associate regionally enhanced GW drag with anomalies in resolved wave propagation in the stratosphere in a comprehensive chemistry‐climate model on the time scale of days. For this, we identify strong orographic GW (OGW) events for the three most pronounced northern hemisphere OGW hotspots and construct composites of anomalies of selected variables with respect to the climatological mean. We find that the response of the resolved wavefield strongly depends on the hotspot region, leading to diverse consequences on the large‐scale stratospheric circulation. Strong OGW events in the hotspots are in turn determined by the resolved fields from the surface to the level of OGW dissipation, which highlights the two‐way coupling between OGWs and resolved flow. Key Points: Intermittent parameterized orographic gravity wave drag affects stratospheric large‐scale dynamics in a chemistry‐climate modelDependence of the resolved dynamical response on the hotspot in question underlines the two‐way nature of the interactionUnsteady nature of the dynamical response sheds new light on how parameterized orographic gravity waves affect stratospheric transport [ABSTRACT FROM AUTHOR]

In the extratropical atmosphere, Rossby waves (RWs) and internal gravity waves (GWs) propagating from the troposphere mediate a coupling with the middle atmosphere by influencing the dynamics therein. In state‐of‐the‐art chemistry‐climate models (CCMs), RW effects are well resolved while the majority of GW effects have to be parameterized. Here, we analyze orographic GW (OGW) interaction with resolved dynamics in a comprehensive CCM on daily time scales. For this, we apply a recently developed method of strong OGW drag event composites for three pronounced northern hemisphere OGW hotspots. We show that strong OGW drag events are associated with anomalous resolved wave propagation in the stratosphere. Causal links are inferred from previously published work and are supported by the anomalies in zonal circulation and wave activity tendencies. The nature of these anomalies varies depending on the hotspot region, which underlines the parameterized OGW‐resolved dynamics interaction being a two‐way process. Plain Language Summary: The majority of atmospheric waves are generated near the surface and propagate subsequently upward in the atmosphere. This includes Rossby waves that are resolved in climate models and small‐scale gravity waves (GWs) that commonly have to be parameterized. In the middle atmosphere, the waves eventually break, thereby dissipating their momentum and energy, which influences atmospheric dynamics. The interaction of GWs with the large‐scale circulation is to date poorly understood. In this study, we associate regionally enhanced GW drag with anomalies in resolved wave propagation in the stratosphere in a comprehensive chemistry‐climate model on the time scale of days. For this, we identify strong orographic GW (OGW) events for the three most pronounced northern hemisphere OGW hotspots and construct composites of anomalies of selected variables with respect to the climatological mean. We find that the response of the resolved wavefield strongly depends on the hotspot region, leading to diverse consequences on the large‐scale stratospheric circulation. Strong OGW events in the hotspots are in turn determined by the resolved fields from the surface to the level of OGW dissipation, which highlights the two‐way coupling between OGWs and resolved flow. Key Points: Intermittent parameterized orographic gravity wave drag affects stratospheric large‐scale dynamics in a chemistry‐climate modelDependence of the resolved dynamical response on the hotspot in question underlines the two‐way nature of the interactionUnsteady nature of the dynamical response sheds new light on how parameterized orographic gravity waves affect stratospheric transport [ABSTRACT FROM AUTHOR]

Observational and modeling evidence suggest a recent acceleration of the stratospheric Brewer-Dobson circulation (BDC), driven by climate change and stratospheric ozone depletion. However, slowly varying natural variability can compromise our ability to detect such forced changes over the relatively short observational record. Using observations and chemistry-climate model simulations, we demonstrate a link between multi-decadal variability in the strength of the BDC and the Interdecadal Pacific Oscillation (IPO), with knock-on impacts for composition in the stratosphere. After accounting for the IPO-like variability in the BDC, the modeled trend is approximately 7%-10% dec-1 over 1979-2010. Furthermore, we find that sea surface temperatures explain up to 50% of the simulated decadal variability in tropical mid-stratospheric ozone. Our findings demonstrate strong links between low-frequency variability in the oceans, troposphere and stratosphere, as well as their potential importance in detecting structural changes in the BDC and future ozone recovery. Plain Language Summary Natural variability is a key element of the climate system and can mask human-induced changes. Here, we are interested in the naturally varying strength of the stratospheric global circulation and how this impacts the composition of the stratosphere. Using observations and model simulations, we show that slowly changing (multi-decadal) natural variability in the Pacific Ocean is reflected in the stratospheric circulation. This link helps us to better understand structural changes in the stratospheric circulation arising due to human interferences. In turn, slow transport variability reconciles recent low levels of ozone in the middle tropical stratosphere, which otherwise are in disagreement with the expected ozone recovery. These results have implications for both reconciling theory and observed changes in the stratospheric circulation, as well as better understanding the rate of stratospheric ozone recovery. [ABSTRACT FROM AUTHOR]

Planetary waves (PWs) and gravity waves (GWs) are the key drivers of middle atmospheric circulation. Insufficient observations and inaccurate model representation of GWs limit our understanding of their stratospheric contributions, especially during the Antarctic polar vortex breakdown. This study employs the strength of the high‐resolution ERA5 reanalysis in resolving a broad spectrum of GWs in southern midlatitudes and its ability to estimate their forcing during the breakdown period. Most of the resolved southern hemisphere GWs deposit momentum around 60°S over the Southern Ocean. Further, a zonal momentum budget analysis during the breakdown period reveals that the resolved GW forcing in ERA5 provides as much as one‐fourth of the necessary wind deceleration at 60°S, 10 hPa. The parameterized GW drag, mostly from non‐orographic sources, provides more than half of the wind deceleration. Both findings highlight the key role of GWs in the vortex breakdown and discuss possibilities for further stratospheric GW analysis. Plain Language Summary: Strong flow over mountains during winters and instabilities in the southern hemisphere troposphere can excite gravity waves that propagate from near‐surface all the way to atmospheric heights of 50–80 km. At these heights, they dissipate momentum and decelerate the strong eastward winds. Knowing the structure and extent of the forcing by these waves can help to better understand their role in driving the stratospheric and mesospheric circulation. However, the net forcing by these waves is not accurately known on account of limited observations and because global climate models cannot sufficiently resolve them. This study illustrates that the high‐resolution ERA5 reanalysis, which forms a natural bridge between observations and free running climate models, can be used to estimate the mean forcing due to such gravity waves and can help assess their role in springtime deceleration of polar vortex. Such an analysis was not possible using previous reanalysis datasets due to low resolution. The findings show that, indeed, gravity wave forcing can provide a large fraction of the deceleration needed to slow down the strong westerly winds in late winters. Key Points: ERA5 partially resolves gravity waves allowing evaluation of their contribution to the springtime polar vortex breakdownClimatology of the gravity wave forcing highlights relevance of gravity wave refraction and oblique propagation and suggests deceleration of up to −8 m/s/day around 60°SDeceleration of the vortex due to gravity wave dissipation can be as high as the total necessary deceleration for the vortex breakdown [ABSTRACT FROM AUTHOR]

Based on the statistical analyses using ERA data sets, this study reveals a significant linkage between the tropospheric Western Hemisphere (WH) circulation pattern and the stratospheric EOF1 mode (North America [NA] pattern) of the wintertime daily height field at 50 hPa, which exhibits the displacement of the polar vortex to NA. The results of composite analysis indicate that the WH pattern leads the NA pattern at about 5 days. This time‐lagged linkage between the two patterns is further confirmed by the results of time‐lagged singular value decomposition (SVD) analyses. Through the linear interference with climatological waves, the WH pattern contributes to the enhanced (suppressed) wave‐2 (wave‐1) activities propagating into the stratosphere. The characteristics of planetary waves, with opposite sign of anomalous wave‐1 and wave‐2 activities, lead to the formation of the stratospheric NA pattern. We suggest the need for caution in interpreting the stratospheric influences on cold air outbreaks over NA. Key Points: There is an obvious time‐lagged linkage between the WH circulation pattern and the EOF1 mode of NH 50‐hPa daily height anomaliesThe WH pattern can enhance (suppress) the planetary wave‐2 (wave‐1) fluxes into the stratosphere through the linear interferenceThe anomalous wave‐1 and wave‐2 fluxes into the stratosphere with opposite sign are both important for the polar vortex morphology [ABSTRACT FROM AUTHOR]

Total columns of the trace gases nitric acid (HNO3) and hydrogen chloride (HCl) are sensitive to variations in the lower stratospheric age of air, a quantity that describes transport time scales in the stratosphere. Analyses of HNO3 and HCl columns from the Network for the Detection of Atmospheric Composition Change panning 77°S to 79°N have detected changes in the extratropical stratospheric transport circulation from 1994 to 2018. The HNO3 and HCl analyses combined with the age of air from a simulation using the MERRA2 reanalysis show that the Southern Hemisphere lower stratosphere has become 1 month/decade younger relative to the Northern Hemisphere, largely driven by the Southern Hemisphere transport circulation. The analyses reveal multiyear anomalies with a 5‐ to 7‐year period driven by interactions between the circulation and the quasi‐biennial oscillation in tropical winds. This hitherto unrecognized variability is large relative to hemispheric transport trends and may bias ozone trend regressions. Plain Language Summary: Our analyses of the 25‐year Network for the Detection of Atmospheric Composition Change column HNO3 and HCl data records from nine stations provide observational evidence that air in the Southern Hemisphere lower stratosphere has been getting younger relative to the Northern Hemisphere at a rate of 1 month/decade since 1994. This stands in contrast to several model studies that predict that Antarctic ozone hole recovery in this century will increase the Southern Hemisphere age of air relative to the Northern Hemisphere. The analyses also reveal extratropical variability with a 5‐ to 7‐year period driven by interactions between the circulation and tropical winds. This previously unrecognized, low‐frequency variability is much larger than hemispheric transport trends and is likely to cause bias in trends calculated using data records shorter than about two decades. Understanding and quantifying changes in the transport circulation matters to our ability to model how our protective O3 layer will evolve in the future. Key Points: Trace gas data show southern stratospheric air is getting younger relative to the Northern Hemisphere, in contrast to model predictionsBoth extratropical hemispheres have dynamically driven 5‐ to 7‐year periodicity with large amplitude relative to the age trendMultidecadal data sets are required to quantify transport trends that are not confounded by large extratropical variability [ABSTRACT FROM AUTHOR]

By injecting SO2 into the stratosphere at four latitudes (30°, 15°N/S), it might be possible not only to reduce global mean surface temperature but also to minimize changes in the equator‐to‐pole and inter‐hemispheric gradients of temperature, further reducing some of the impacts arising from climate change relative to equatorial injection. This can happen only if the aerosols are transported to higher latitudes by the stratospheric circulation, ensuring that a greater part of the solar radiation is reflected back to space at higher latitudes, compensating for the reduced sunlight. However, the stratospheric heating produced by these aerosols modifies the circulation and strengthens the stratospheric polar vortex which acts as a barrier to the transport of air toward the poles. We show how the heating results in a feedback where increasing injection rates lead to stronger high‐latitudinal transport barriers. This implies a potential limitation in the high‐latitude aerosol burden and subsequent cooling. Plain Language Summary: If we were to inject aerosols at high altitudes in order to reflect some incoming solar radiation and cool the planet, it would result in a localized warming at those altitudes. This would affect the circulation of air masses, and we show here that it would reduce the intensity of the transport of air from the mid‐latitudes to the poles. If this transport is reduced, less aerosol can reach the high latitudes, making it harder to achieve a distribution of aerosols that would offset global warming evenly. Key Points: The stratospheric injection of SO2 would result in localized stratospheric warmingThis warming would modify the thermal wind balance, strengthening the stratospheric polar vortexesThis results in less poleward transport of the aerosols, requiring more SO2 to achieve the desired cooling [ABSTRACT FROM AUTHOR]

Tropical tropospheric responses to a sudden stratospheric warming (SSW) event that occurred over Antarctica in September 2019 have been investigated by conducting a series of ensemble forecast experiments. Comparative examinations between the normal forecast and the partially nudged forecast, whose stratospheric circulation is constrained to a reanalysis, reveal a significant enhancement/suppression in the convective activity (upwelling/downwelling) over the northern/southern part of the tropics. Such an acceleration of the Hadley circulation is set up by thermal structural changes in the upper troposphere and lower stratosphere. The tropical cooling caused by the enhanced Brewer‐Dobson circulation destabilizes the environment there and stimulates deep cumulus convections. In this case, the southern area of the Asian monsoon region is particularly sensitive. Although details of the triggered response depend on the adopted cumulus parameterization scheme, a boosting tendency of the convective upwelling via the SSW is robust. Plain Language Summary: A rare Antarctic sudden stratospheric warming (SSW) event that occurred in September 2019 provides an opportunity to investigate its impact on convective activity in the tropics. By comparing the normal forecast and a "perfect stratosphere" forecast, this study confirmed the robust influence of the SSW on the cumulus convection over the Northern Hemisphere tropics. In other words, the Antarctic SSW could enhance the probability of extreme weather in boreal summer to early autumn, including the genesis and development of tropical cyclones. Therefore, this process of the teleconnection between the Antarctic stratosphere and the tropical troposphere has the potential to be a valuable source of extended‐range predictability in practical seasonal forecasts. Key Points: Enhancement in the tropical convective activity during the 2019 Antarctic sudden warming is confirmed by ensemble nudging integrationsEnhanced Brewer‐Dobson circulation is linked to Hadley cells via thermal structure changes in the upper troposphere and lower stratosphereFor certain parameterizations, the large stratospheric effect is observed in cumulus convections at the south of the Asian monsoon zone [ABSTRACT FROM AUTHOR]

The observational impacts of satellite data assimilation on extended‐range forecasts of sudden stratospheric warmings (SSWs) are investigated by conducting ensemble forecast experiments. We use two Japanese novel reanalysis products: the Japanese 55‐year reanalysis (JRA‐55) and its subset that assimilates conventional observations only (JRA‐55C). A comparative examination on the reproducibility for SSWs between the two ensemble forecasts reveals that the impact of satellite observations is significant for forecasts starting 5 days before the SSW onset, with 20% less accuracy in the JRA‐55C forecasts. Moreover, some of forecasts of vortex‐splitting SSWs show a sudden appearance of deep difference, which lasts over a few months in the lower stratosphere and significantly affects the surface climate. These results highlight an important role of mesospheric and upper stratospheric circulations on the onset and development of SSWs. Plain Language Summary: Satellite observations are valuable for producing initial conditions for numerical weather prediction (NWP) systems, especially over the upper stratosphere that typical upper‐air observations cannot cover. However, many NWP models suffer from biases associated with unresolved processes. This study explores how the NWP system benefited from satellite data in forecasting the breakdown events of stratospheric polar vortexes/sudden stratospheric warmings (SSWs) by making many forecasts from typical initial conditions, both with and without satellite data. Due to unresolved bias over the upper stratosphere, some forecasts from no‐satellite initial conditions miss the onset of SSWs and subsequent anomalous tropospheric conditions. Thus, the deteriorated grasp of the upper atmosphere in the absence of satellite observations degrades the deterministic predictability of extreme stratospheric events and following downward‐propagating signals. Key Points: Mesospheric and upper stratospheric initial conditions play an important role in forecasting the onset and development of sudden warmingsThe 5‐day lead capture rate of the onset of major sudden stratospheric warmings degrades about 20% if satellite data are not assimilatedThe absence of satellite observations could also affect the extended‐range forecast skill related to downward‐propagating signals [ABSTRACT FROM AUTHOR]

The Brewer‐Dobson circulation during the Last Glacial Maximum (LGM) is investigated in simulations using the Whole Atmosphere Community Climate Model version 6. We examine vertical mass fluxes, age of stratospheric air, and the transformed Eulerian mean stream function and find that the modeled annual‐mean Brewer‐Dobson circulation during the LGM is almost everywhere slower than that in the modern climate (with or without anthropogenic ozone depleting substances). Compared to the modern climate, the annual‐mean tropical upwelling in the LGM is 11.3–16.9%, 11.2–15.8%, and 4.4–10.2% weaker, respectively, at 100, 70, and 30 hPa. Simulated decreases in annual‐mean mass fluxes at 70 and 100 hPa are caused by a weaker parameterized orographic gravity wave drag and resolved wave drag, respectively. Plain Language Summary: The Brewer‐Dobson Circulation (BDC) is the large‐scale stratospheric circulation that transports stratospheric ozone from the tropics to poles and transports ozone from the stratosphere to troposphere in middle‐ and high‐latitudes. During the Last Glacial Maximum (LGM), the BDC could have been very different from the modern climate, due to different radiative constituent concentrations, the presence of large ice sheets in the Northern Hemisphere, and lower sea surface temperatures but an increased latitudinal sea surface temperature gradients. Here we investigate the BDC during the LGM using the Whole Atmosphere Community Climate Model version 6 with fully interactive chemistry for the first time. We find that the annual‐mean BDC during the LGM is everywhere weaker than that in the modern climate. A decreasing BDC in the LGM will have implications for the spatial distribution of ozone in the stratosphere, as well as stratosphere‐troposphere exchange and surface ultraviolet radiation. The latter two changes can be expected to affect tropospheric oxidant abundances, with potential implications for the lifetime of trace gases such as methane. Because of impacts on the climate sensitivity and methane, dynamically consistent O3 fields in the LGM from the present study provide an improved framework for an accurate simulation of the LGM climate. Key Points: A state‐of‐the‐art atmosphere model shows a slower Brewer‐Dobson circulation during the Last Glacial Maximum than the modern climateCompared to modern climate, the annual‐mean tropical upwelling in the Last Glacial Maximum is 14%, 14%, and 7% weaker at 100, 70, and 30 hPa, respectivelyDecrease in mass fluxes at 70 and 100 hPa is caused by weaker parameterized orographic gravity wave and resolved wave drags, respectively [ABSTRACT FROM AUTHOR]

This study investigates the intermodel spread of the Northern Hemisphere winter stratospheric polar vortex change to anthropogenic greenhouse gas increase in Coupled Model Intercomparison Project Phase 5 (CMIP5) models. Previous proposed mechanisms for the polar vortex response to climate change, based on analysis of atmosphere‐only models, are found inadequate to explain the intermodel spread in the coupled models in CMIP5. It is further found that resolved stationary wave driving in the polar vortex region accounts for less than 30% of the intermodel spread, and intermodel differences in both the vertical and meridional wave propagation contribute to differences in the wave driving. The results call for a detailed budget analysis of the stratospheric circulation response by including both the resolved and parameterized processes through the Dynamics and Variability Model Intercomparison Project. The results also highlight a need for an improved theoretical understanding of future projected polar vortex change and intermodel spread. Plain Language Summary: It has been increasingly recognized that the stratospheric circulation can significantly affect the troposphere and the surface through a downward influence, and thus, it is important to predict and understand how the stratospheric circulation will respond to anthropogenic greenhouse gas increase. However, there is a large disagreement in the stratospheric circulation response to climate change among the current generation of climate models. This study determines that previously proposed mechanisms to explain the intermodel spread are inadequate and, therefore, motivates an improved understanding of the stratospheric circulation response and model‐to‐model differences. This is a goal that should be achieved through forthcoming model intercomparison efforts. Key Points: Resolved stationary wave driving accounts for less than 30% of the intermodel spread in the stratospheric polar vortex change in CMIP5Both altered vertical wave propagation into the stratosphere and altered meridional propagation within the stratosphere contributeProposed theories for the stratospheric polar vortex response to climate change appear inadequate to explain the intermodel spread in CMIP5 [ABSTRACT FROM AUTHOR]

We show that a weakened Beaufort High in late winter is becoming more common since the late 1980s, and this weakening is related to decadal changes in Northern Hemisphere ozone depletion. During this time, the stratospheric polar vortex has experienced a decadal northeastward displacement. Such displacement is associated with the variability of the East Asia‐eastern North America dipole. A positive Asia‐eastern North America dipole leads to zonal asymmetry in the ozone depletion with maximum loss over eastern Eurasia. Positive feedback with the zonally asymmetric ozone loss contributes to increased occurrence of the northeastward displacement of the stratospheric polar vortex. The northeastward displaced polar vortex enhances baroclinicity over the Eurasian Arctic and causes an increase in the frequency and intensity of cyclones and a decrease in anticyclones over the Eurasian Arctic. Concomitantly, the Beaufort High tends to have weakened. Key Points: A weakened winter Beaufort High has become more common since the late 1980s, which is related to decadal change of ozone depletionThe northeastward displaced stratospheric polar vortex results from the interactions of the polar vortex with the ozone depletionNortheastward displaced polar vortex causes an increase (decrease) in cyclones (anticyclones) over the Eurasian Arctic [ABSTRACT FROM AUTHOR]

Previous studies have reported that the tropospheric climate shifted around the 2000s, but the recent trend in the stratospheric wave intensity (SWI) is still unclear. The results in this study show that the SWI in December during 2001–2015 weakens, which is opposite to that during 1979–2000, implying a shift around the 2000s. The weakened SWI is dominated by its wave number‐1 component, which is related to the wave propagation impeded by the decreased refractive index at high latitudes and the weakened wave activity in the troposphere. Changes in refractive index are mainly contributed by changes in the meridional potential vorticity gradient via the barotropic term. This shift in the wave number‐1 waves leads to a shift in the stratospheric Arctic temperature, that is, warming (cooling) during 1979–2000 (2001–2015), implying that similar shifting phenomena may appear regardless of the continued greenhouse gas emissions. Plain Language Summary: Recent studies have reported that some tropospheric climate (e.g., global mean surface temperature, the Pacific trade wind, and the Walker circulation) and stratospheric circulations (e.g., Arctic vortex and Brewer‐Dobson circulation) have shifted around the 2000s. The planetary waves, which play a key role in the stratosphere‐troposphere dynamical coupling, can not only impact the stratospheric temperature and circulation via wave‐mean interactions but also modify the tropospheric weather and climate via downward wave coupling. Therefore, it is important to examine whether the planetary waves have shifted around the 2000s. In this study, we investigated the trends in stratospheric wave intensity in December during 1979–2000 and 2001–2015 and found that the stratospheric wave intensity during 2001–2015 weakens, which is opposite to that during 1979–2000, implying a shift around the 2000s. This shift in planetary waves suggests that temperature, circulation, and tracers, associated with planetary waves may have also shifted around the 2000s, even though global warming continues. Key Points: The stratospheric wave intensity after the 2000s has a weakening trend, different from the strengthening trend during 1979–2000The weakened stratospheric wave intensity is dominated by its wave number‐1 component that is related to the impeded wave propagationThis shift in the wave number‐1 may lead to a shift in the stratospheric Arctic temperature [ABSTRACT FROM AUTHOR]

Abstract: The impact of gravity wave drag on the Antarctic sudden stratospheric warming (SSW) in 2002 is examined through a mechanistic middle atmosphere model combined with a variational data assimilation system. Significant differences in the SSW representation are found between a model integration that uses reference gravity wave parameters and one that uses parameters estimated using data assimilation. Upon identical wave forcings at 100 hPa, the vortex breakdown may arise as either a vortex splitting event or a displacement vortex event depending on gravity wave parameters. A local enhancement of Rossby waves is found in the integration with estimated parameters, leading to a split SSW. The changes in the vortex breakdown are associated with changes in the vortex geometry caused entirely by modifying the gravity wave parameters. Gravity wave drag proved to play an instrumental role in preconditioning the stratosphere near a resonant excitation point that triggers the split SSW. [ABSTRACT FROM AUTHOR]

Abstract: The impact of mountains on stratospheric circulation is explored using the Whole Atmosphere Community Climate Model. The “Mongolian mountains” decrease the boreal winter stratospheric jet strength by ∼1/3 and increase the frequency of major sudden stratospheric warmings from 0.08 year−1 to the observed 0.60 year−1. These changes are twice the magnitude of the impacts of the Tibetan plateau and Himalayas. Consistent with the decrease in the zonal jet, there is enhanced Eliassen‐Palm flux convergence; this is predominantly from changes in wave propagation pathways through changes to the upper troposphere circulation, not from an increased amplitude of planetary waves reaching the stratosphere. The Mongolian mountains have the greater impact on upper tropospheric circulation due to their meridional location. The Rocky Mountains have no significant impact on the stratospheric jet. Changes in wave propagation in response to the Mongolian mountains are similar to those associated with major sudden stratospheric warming events in observations. [ABSTRACT FROM AUTHOR]

Sudden stratospheric warmings (SSWs) are significant source of enhanced subseasonal predictability, but whether this source is untapped in operational models remains an open question. Here we report on the prediction of the SSW on 12 February 2018, its dynamical precursors, and surface climate impacts by an ensemble of dynamical forecast models. The ensemble forecast from 1 February predicted 3 times increased odds of an SSW compared to climatology, although the lead time for SSW prediction varied among individual models. Errors in the forecast location of a Ural high and underestimated magnitude of upward wave activity flux reduced SSW forecast skill. Although the SSW's downward influence was not well forecasted, the observed northern Eurasia cold anomaly following SSW was predicted, albeit with a weaker magnitude, due to persistent tropospheric anomalies. The ensemble forecast from 8 February predicted the SSW, its subsequent downward influence, and a long‐lasting cold anomaly at the surface. Plain Language Summary: Predicting climate anomalies more than 2 weeks ahead remains a challenging task. However, there are periods when climate is more predictable. In particular, it is known that in the wintertime Northern Hemisphere midlatitudes, climate may be more predictable during several weeks following sudden stratospheric warmings (SSWs), when a cold stratospheric polar vortex suddenly breaks and the stratosphere warms. In this article we evaluate quality of monthly forecasts by several up‐to‐date forecast systems during a sudden stratospheric warming observed in February 2018 focusing both on predictability of the stratospheric anomalies and on predictability of their surface impacts. We find that models predicted enhanced probability of sudden stratospheric warming at least 11 days before the observed event. Four days before the event it was forecast by all models with a very high certainty. We further show that successful prediction of the SSW required correct forecasting of anticyclonic anomaly over Ural and associated upward propagation of planetary‐scale atmospheric waves. The observed cold anomaly across Eurasia in February 2018 was predicted more than 2 weeks ahead although at this lead time the predictability was more likely associated with persistence of tropospheric anomalies in the forecast, with smaller downward influence of the stratospheric anomalies. Key Points: Subseasonal multimodel ensemble predicted increased odds of a large sudden stratospheric warming in February 2018 at a lead time of 11 daysExtended cold anomaly over Eurasia in February 2018 was predicted more than 2 weeks ahead, but its intensity varied among individual modelsAccurate forecasting of a Ural high and associated upward wave activity flux are essential for improved prediction of stratospheric warming [ABSTRACT FROM AUTHOR]

During late February and early March 2018, an unusual polynya was observed off the north coast of Greenland. This period was also notable for the occurrence of a sudden stratospheric warming. Here we use satellite and in situ data, a reanalysis and an ice‐ocean model to document the evolution of the polynya and its synoptic forcing. We show that its magnitude was unprecedented and that it was associated with the transient response to the sudden stratospheric warming leading to anomalous warm southerly flow in north Greenland. Indeed, regional wind speeds and temperatures were the highest during February going back to the 1960s. There is evidence that the thinning sea ice has increased its wind‐driven mobility. However, we show that the polynya would have developed under thicker ice conditions representative of the late 1970s and that even with the predicted trend toward thinner sea ice, it will only open during enhanced southerly flow. Plain Language Summary: In late February 2018, satellite imagery revealed the presence of a large polynya (a region of reduced sea ice cover within the pack), in the Wandel Sea off the north coast of Greenland. Since this region is not known for the development of polynyas, this discovery generated interest among Arctic observers and in the science community, raising questions about the nature and cause of this unusual event. In this paper, we show that its opening coincided with a period of sustained and unusually warm winds from the south, with above‐freezing temperatures and wind speeds in excess of 25 m/s reported at local weather stations. February 2018 was also notable for a sudden stratospheric warming event, in which an abrupt warming of the atmosphere between 10‐ and 50‐km altitudes occurred in conjunction with a reversal of the stratospheric winds. We show that this event was responsible for the polynya. We also use a computer model to confirm the dominant role of the winds in creating the polynya. Finally, we show that even with future thinning of sea ice due to climate change, extreme winds will remain necessary to create a polynya in this region over the next few decades. Key Points: During late February and early March 2018 a polynya opened off of north Greenland in a region not previously known for polynya formationThis event occurred after a sudden stratospheric warming that we show was responsible for the conditions that resulted in its developmentWe show that the high winds during the event played a dominant role in its formation and sea ice thickness played a less important role [ABSTRACT FROM AUTHOR]

Teleconnections between lower and higher latitude regions are widely known in both the Northern and Southern Hemispheres. To broaden our view of these teleconnections, we searched a reanalysis data set for evidence of a teleconnection between the Arctic Oscillation (AO) and the Antarctic Oscillation (AAO), two widely separated circumpolar phenomena. Statistical analysis of the Japanese 55‐year reanalysis data set showed significant in‐phase synchronization between the AO and AAO, particularly in October and February, with a vertical structure extending from the troposphere to the stratosphere. This vertical structure may suggest a stratospheric control, and we did not find a significant signature indicating a tropical ocean control. We also observed decadal‐scale modulation of the synchronicity. Observational evidence implies that the stratospheric meridional circulation may be responsible for AO‐AAO synchronization. Plain Language Summary: The Arctic Oscillation (AO) and the Antarctic Oscillation (AAO) are dominant atmospheric variability patterns in the Northern and Southern Hemispheres, respectively. Each is a pressure seesaw between the pole and the midlatitudes that remotely affects weather, climate, and environment around the world. We showed interhemispheric in‐phase synchronization between the AO and AAO in October and February, and we also found decadal‐scale variation of the synchronicity. Because the vertical structure of the AO‐AAO synchronization extends from the troposphere to the stratosphere, stratospheric variations may be responsible for the synchronization. This finding of AO‐AAO synchronization points the way to a better understanding of past, present, and future pole‐to‐pole climatic relationships and improvements in long‐term weather forecasts. Key Points: In‐phase synchronicity between the Arctic and Antarctic Oscillations was found in February and OctoberTropical ocean variations such as El Niño‐Southern Oscillation seem to be of little or no importance to this synchronicityThe stratospheric meridional circulation may be a key factor leading to synchronization [ABSTRACT FROM AUTHOR]