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The stratospheric polar vortex (SPV) significantly influences current weather and climate patterns. However, its state in the geological past remains largely unexplored. This study investigates SPV variations in the past 250 million years, using a fully coupled Earth System Model. It is found that midlatitude paleogeography primarily drove substantial SPV variations in the deep time, while changes in CO2 concentrations and solar insolation play a minor role. Both the Arctic and Antarctic SPV were strengthened when the supercontinent Pangea broke up. The increased asymmetry of midlatitude land‐sea contrast tended to weaken the upward propagation of wavenumber‐1 planetary waves, thereby strengthening the SPV. This SPV strengthening correlated with the decelerated stratospheric Brewer‐Dobson circulation and uplift of the polar tropopause. Our results highlight the crucial role of paleogeography in regulating SPV variations and stratosphere–troposphere coupling in deep‐time climate. Plain Language Summary: At present, influences of the stratospheric polar vortex (SPV) on surface climate have attracted growing attention. In the geological past, however, how the SPV acted and how it impacted on the troposphere and surface climate remains unclear. This study explores SPV variations in both the Northern and Southern Hemispheres in the past 250 million years (Myr) using simulations of a fully coupled Earth System Model. Our results reveal that polar stratospheric temperatures fluctuated by up to 20 K in the geological past, which is much greater than the variations from 1850 to the present. Midlatitude paleogeography played a crucial role in SPV variations over tectonic timescales. The breakup and drift of the Pangea supercontinent altered the upward propagation of Rossby waves from the troposphere into the stratosphere. It influenced not only the SPV but also the strength of stratospheric Brewer‐Dobson circulation as well as the polar tropopause height. This work helps to understand the role of paleogeography in the stratosphere. Key Points: The stratospheric polar vortex (SPV) experienced dramatic variations in the past 250 million yearsMidlatitude paleogeography has a crucial impact on SPV variations by regulating upward propagation of planetary wavesThe SPV variation is closely associated with changes in stratospheric Brewer‐Dobson circulation over tectonic timescales [ABSTRACT FROM AUTHOR]

Climate models robustly project acceleration of the Brewer‐Dobson circulation (BDC) in response to climate change. However, the BDC trends simulated by comprehensive models are poorly constrained by observations, which cannot even determine the sign of potential trends. Additionally, the changing structure of the troposphere and stratosphere has received increasing attention in recent years. The extent to which vertical shifts of the circulation are driving the acceleration is under debate. In this study, we present a novel method that enables the attribution of advective BDC changes to structural changes of the circulation and of the stratosphere itself. Using this method allows studying the advective BDC trends in unprecedented detail and sheds new light into discrepancies between different data sets (reanalyses and models) at the tropopause and in the lower stratosphere. Our findings provide insights into the reliability of model projections of BDC changes and offer new possibilities for observational constraints. Plain Language Summary: The large‐scale interhemispheric meridional overturning circulation in the middle atmosphere determines the composition of this region, including the distribution of radiatively important trace gases. The long‐term change of this circulation is a subject of ongoing debate, and an area of disagreement between models and observations. In our study we present a method that provides an unprecedented insight into the change and disentangles the individual factors behind it. Hence, the method introduces new constraints on the circulation change and can aid the reconciliation between models and observations. Key Points: A method for attributing net upwelling trends to changes in circulation, air density and structure of the upwelling region is establishedModels agree largely on contributions from vertical shifts in pressure surfaces/tropopause and changes in vertical advectionFor reanalyses, on the other hand, the uncertainty is larger, not allowing direct constraints on model trends [ABSTRACT FROM AUTHOR]

• For the SH 2019 SSW, the wave driving observed was the highest recorded since 1979. • An enhanced upwelling led to cooling in the tropical stratosphere. • Tropical upper stratospheric water vapor increased while ozone decreased after SSW. The changes in the intensity of Brewer-Dobson circulation (BDC) in the record-breaking Southern Hemisphere sudden stratospheric warming (SH SSW) event in 2019 are investigated in the present study. Wave driving was observed to be at its highest in 2019 SH SSW, despite the event being classified as a minor warming event. The Eliassen-Palm flux divergence, indicative of wave driving, was observed to be increasing before the SSW, and subsiding afterwards. The stratospheric circulation followed the wave driving and intensified along the warming episode. The resulting stratospheric distribution from Aura Microwave Limb Sounder showed a negative ozone anomaly (∼20%) in the tropical lower stratosphere and a downward propagating positive anomaly in the Southern Hemisphere polar stratosphere around the central dates. The water vapor mixing ratio showed an increase of more than 25% in the polar lower stratosphere and 10 % in the tropical upper stratosphere. A cooling of about 5 K was observed in the tropical middle stratosphere. The results indicate that SH SSW can change the intensity of the stratospheric circulation which in turn affects the ozone and water vapor transport from the tropics to the pole. [ABSTRACT FROM AUTHOR]

Abstract Climate models robustly project acceleration of the Brewer‐Dobson circulation (BDC) in response to climate change. However, the BDC trends simulated by comprehensive models are poorly constrained by observations, which cannot even determine the sign of potential trends. Additionally, the changing structure of the troposphere and stratosphere has received increasing attention in recent years. The extent to which vertical shifts of the circulation are driving the acceleration is under debate. In this study, we present a novel method that enables the attribution of advective BDC changes to structural changes of the circulation and of the stratosphere itself. Using this method allows studying the advective BDC trends in unprecedented detail and sheds new light into discrepancies between different data sets (reanalyses and models) at the tropopause and in the lower stratosphere. Our findings provide insights into the reliability of model projections of BDC changes and offer new possibilities for observational constraints.

It is well established that the shallow branch of the Brewer‐Dobson circulation accelerates in a warming climate due to enhanced wave drag in the subtropical lower stratosphere. This has been linked to the strengthening of the upper flanks of the subtropical jets. However, the seasonality of the zonal wind trends, peaking in the winter hemisphere, is opposite to that of the Eliassen‐Palm flux convergence trends, peaking in summer. We investigate the seasonality in the wave drag trends and find a different behavior for each hemisphere. The Shepherd and McLandress (2011, https://doi.org/10.1175/2010jas3608.1) mechanism, involving transient wave dissipation at higher levels following the rise of the critical lines, is found to maximize in austral summer. On the other hand, in the Northern Hemisphere the wave drag increase peaks in summer primarily due to the changes in the stationary planetary waves (monsoonal circulations) associated with enhanced deep convection. Plain Language Summary: The Brewer‐Dobson circulation, responsible for mass, heat and constituents global transport in the stratosphere, is projected to accelerate in a warming climate. This circulation is driven by the momentum transferred by dissipating waves. We explore the seasonality of trends in wave dissipation in the subtropical lower stratosphere. First, we show that the largest changes in the wave dissipation take place in the summer hemisphere, opposite to the largest changes in the zonal wind, which is known to control wave dissipation conditions. We investigate this apparent contradiction and find that (a) the conditions are particularly favorable for the waves to be affected by the changing wind in summer, due to their spectral characteristics and the structure of the background zonal wind, in particular the proximity of the zero wind line; and (b) in the Northern Hemisphere the changes are primarily associated with stationary waves triggered by enhanced deep convection in a warmer climate. Key Points: Future subtropical trends in lower stratospheric wave drag are strongest in the summer hemisphere, whereas zonal wind trends peak in winterThe largest changes in transient wave drag due to critical line shift are found in the Southern Hemisphere summerThe Northern Hemisphere summer trends are mainly due to changes in stationary wave drag linked to stronger and higher deep convection [ABSTRACT FROM AUTHOR]

Stratosphere‐Troposphere exchange (STE) of air mass and ozone in ERA5 and Modern Era Retrospective analysis for Research and Application, version 2 (MERRA2) reanalyses from 1980 to 2022 are investigated on their seasonal cycle, annual‐mean climatology, and monthly anomalies smoothed using a 1‐year Lanczos low‐pass filter. We employ a lowermost stratosphere mass budget approach with dynamic isentropic surfaces fitted to tropical tropopause as the upper boundary of lowermost stratosphere. The annual‐mean ozone STEs over the NH extratropics, SH extratropics, tropics, extratropics, and globe in ERA5 are −342, −239, 201, −581, and −380 Tg year−1, respectively, versus −305, −224, 168, −529, −361 Tg year−1 from MERRA2. The annual‐mean global ozone STE difference between ERA5 and MERRA2 is dominated by the diabatic heating difference, partly compensated by the ozone concentration difference. There are about 40% (−40%) differences between ERA5 and MERRA2 in global ozone STEs in boreal summer (autumn), mainly due to the difference in seasonal breathing of the lowermost stratosphere ozone mass between reanalyses. The correlation coefficient between ERA5 and MERRA2 global ozone mass STE monthly anomalies is 0.57 and thus ERA5 and MERRA2 can only explain each other's variance by 33%. Multiple linear regression analysis shows that El Niño–Southern Oscillation, quasi‐biennial oscillation, and Brewer‐Dobson circulation explain the variance in the ERA5 (MERRA2) global ozone STE monthly anomalies by 17.3 (5.0), 5.4 (7.2), and 1.0 (3.1)%, respectively. The volcanic aerosol impacts on ozone STEs from ERA5 and MERRA2 have opposite signs and thus are inconclusive. Cautions are therefore needed when using ERA5 and MERRA2 to investigate the STE seasonal cycle and interannual variability. Plain Language Summary: Stratosphere‐troposphere exchange (STE) of ozone can impact surface ozone concentration and air quality. We investigate air mass and ozone STEs using two state‐of‐art and widely used reanalyses, that is, ERA5 and Modern Era Retrospective analysis for Research and Application, version 2 (MERRA2) from 1980 to 2022, on annual‐mean climatology, seasonal cycle, and monthly anomalies. The magnitudes of annual‐mean global ozone STEs in ERA5 and MERRA2 are 380 and 361 Tg year−1, respectively. The relative differences between ERA5 and MERRA2 in global ozone STEs can be up to 40% (−40%) in boreal summer (autumn). In addition, we find the ERA5 and MERRA2 STE monthly anomalies are quite different: the correlation coefficient between ERA5 and MERRA2 global ozone STE monthly anomalies is only 0.57. We further investigate the impacts of climate variabilities and perturbations, including El Niño–Southern Oscillation, quasi‐biennial oscillation, Brewer‐Dobson circulation, solar cycle, and volcanic aerosols on the air mass and ozone STE interannual variabilities. We find that the Brewer‐Dobson circulation can only explain very small variances in ERA5 and MERRA2 air mass and ozone STE monthly anomalies. We show that 67%–77% of the global ozone STE interannual variances from ERA5 and MERRA2 cannot be explained by the climate variabilities and perturbations considered. Key Points: Annual‐mean global ozone stratosphere‐troposphere exchanges (STEs) from ERA5 and Modern Era Retrospective analysis for Research and Application, version 2 (MERRA2) are 380 and 361 Tg year−1, with difference mainly due to diabatic heating40% (−40%) difference between ERA5 and MERRA2 in global ozone STEs in boreal summer (fall) is mainly due to lowermost stratosphere ozone mass change rate differenceERA5 and MERRA2 can only explain each other's variance in global ozone STE monthly anomalies by 33%, and the variance explained by Brewer‐Dobson circulation is <5% [ABSTRACT FROM AUTHOR]

Abstract It is well established that the shallow branch of the Brewer‐Dobson circulation accelerates in a warming climate due to enhanced wave drag in the subtropical lower stratosphere. This has been linked to the strengthening of the upper flanks of the subtropical jets. However, the seasonality of the zonal wind trends, peaking in the winter hemisphere, is opposite to that of the Eliassen‐Palm flux convergence trends, peaking in summer. We investigate the seasonality in the wave drag trends and find a different behavior for each hemisphere. The Shepherd and McLandress (2011, https://doi.org/10.1175/2010jas3608.1) mechanism, involving transient wave dissipation at higher levels following the rise of the critical lines, is found to maximize in austral summer. On the other hand, in the Northern Hemisphere the wave drag increase peaks in summer primarily due to the changes in the stationary planetary waves (monsoonal circulations) associated with enhanced deep convection.

The Pinatubo eruption in 1991 injected 10–20 Tg SO2 into the stratosphere, which formed sulfate aerosols through oxidation. Our modeling results show that volcanic heating significantly perturbs the heterogeneous and homogeneous chemistry including NOx and HOx catalytic cycles in the tropical stratosphere. The simulated tropical chemical ozone tendency is positive at 20 mb while negative at 10 mb in the tropics. The simulated ozone chemical tendency is of the same magnitude as the dynamical ozone tendency caused by the accelerated tropical upwelling, but with the opposite sign. Our study finds that the tropical ozone chemical tendency due to homogeneous chemistry becomes more important than heterogeneous chemistry 3 months after eruption. Sensitivity simulations further suggest that the tropical ozone tendency through heterogeneous chemistry is saturated when the injected amount exceeds 2 Tg. Plain Language Summary: In 1991, a large volcanic eruption injected 10–20 Tg SO2 into the stratosphere and perturbed the stratospheric chemistry and dynamics. In this study, we use a climate model to quantify the chemical and dynamical influence of the volcano on tropical stratospheric ozone. Model suggests that the ozone chemical tendency is positive around 28 km while negative around 35 km. Our study also suggests that both heterogeneous and homogeneous chemical reactions contribute to the ozone anomalies. With sensitivity studies, we show that the tropical ozone changes due to heterogeneous chemistry is saturated if the injected amount exceeds 2 Tg. Key Points: The simulated ozone tendency due to chemistry is of the same order of magnitude but of the opposite sign than that due to dynamicsThe chemistry‐driven change in the tropical ozone tendency is dominated by the gas‐phase rather than heterogeneous chemistryThe ozone tendency change due to heterogeneous chemistry is saturated when the injected SO2 amount exceeds 2 Tg [ABSTRACT FROM AUTHOR]

In this study, we have investigated the oxygen isotope compositions (δ17O and δ18O) of modern rain and ice cores using published isotopic data. We find that, contrary to existing interpretations, precipitation δ17O is influenced by two factors: mass-dependent fractionation (MDF), which occurs during ocean evaporation, and mass-independent fractionation (MIF), which happens in the stratosphere. The MDF contribution remains constant and can be understood by studying tropical rain, as the overall movement of mass in the tropics is upward toward the stratosphere. On the other hand, the MIF effect comes from the mixing of stratospheric air in the troposphere, which is a result of the Brewer–Dobson circulation. This MIF effect on precipitation 17O increases from the tropics toward the poles and is observed consistently in modern precipitation and ice cores. The relative δ17O and δ18O composition, denoted as ∆'17O, in modern precipitation can be calibrated with surface air temperature, creating a new and independent tool for estimating past temperatures. We used this calibration along with the ∆'17O of Antarctic and Greenland ice cores, and our reconstructed past temperatures are in excellent agreement with those derived from borehole thermometry or gas phase analysis of air trapped in the ice. The ∆'17O method overcomes the problems associated with using δ18O alone for paleothermometry. Our findings align with climate models that suggest a weakening of the Brewer–Dobson circulation during the Last Glacial Maximum. Furthermore, our approach could be used to monitor future changes in stratosphere–troposphere mass exchange in response to a warming climate caused by increasing greenhouse gases. [ABSTRACT FROM AUTHOR]

Abstract The Pinatubo eruption in 1991 injected 10–20 Tg SO2 into the stratosphere, which formed sulfate aerosols through oxidation. Our modeling results show that volcanic heating significantly perturbs the heterogeneous and homogeneous chemistry including NOx and HOx catalytic cycles in the tropical stratosphere. The simulated tropical chemical ozone tendency is positive at 20 mb while negative at 10 mb in the tropics. The simulated ozone chemical tendency is of the same magnitude as the dynamical ozone tendency caused by the accelerated tropical upwelling, but with the opposite sign. Our study finds that the tropical ozone chemical tendency due to homogeneous chemistry becomes more important than heterogeneous chemistry 3 months after eruption. Sensitivity simulations further suggest that the tropical ozone tendency through heterogeneous chemistry is saturated when the injected amount exceeds 2 Tg.

We quantify the changes in the intensity of Brewer‐Dobson Circulation (BDC) during sudden stratospheric warming (SSW) and its impact on the tropical stratospheric thermal structure and ozone distribution by composite analysis using observations and a chemical‐transport model. An increase in the planetary wave activity and enhancement in BDC intensity before the central date of SSW is noticed. A positive ozone anomaly is observed in the tropical upper stratosphere. The tropical lower stratosphere shows a cooling (1–2 K) and negative ozone anomaly (∼0.1 ppmv) after ∼10 days from the central date. The polar stratosphere experiences a positive ozone anomaly, whereas the upper stratosphere shows ozone depletion due to the downwelling of NOx‐rich mesospheric air. The cold‐point tropopause temperature shows a cooling of ∼0.5 K for major warming which in turn dries the lower stratosphere. Plain Language Summary: The Brewer‐Dobson circulation transports tropical air toward the polar stratosphere. During sudden stratospheric warming, the associated wave activity and changing zonal wind direction in the stratosphere alter the intensity of the circulation. The strength of the circulation increases before the warming, resulting in enhanced upwelling over the tropics. The upwelling leads to cooling across the stratosphere and decreased ozone concentrations in the lower stratosphere over the tropics. The lower temperatures over the upper stratosphere reduce the ozone depletion rate. Over the polar upper stratosphere, ozone depletion is observed due to the downwelling of NOx‐rich air and an increase in the lower stratosphere due to the transport of ozone‐rich air from higher levels. Key Points: Change in the intensity of Brewer‐Dobson Circulation (BDC) due to sudden stratospheric warmingOzone transport due to change in the intensity of BDC using observations and modelingCooling across the tropical stratosphere and decreased ozone concentrations due to upwelling [ABSTRACT FROM AUTHOR]

This study investigates changes in stratosphere‐troposphere exchange (STE) of air masses and ozone concentrations from 1960 to 2099 using multiple model simulations from Chemistry Climate Model Initiative (CCMI) under climate change scenario RCP6.0. We employ a lowermost stratosphere mass budget approach with dynamic isentropic surfaces fitted to the tropical tropopause as the upper boundary of lowermost stratosphere. The multi‐model mean (MMM) trends of air mass STEs are all small over all regions, which are within 0.3 (0.1) % decade−1 for 1960–2000 (2000–2099). The MMM trends of ozone STE for 1960–2000 are 0.3%, −2.7%, 3.4%, −0.9%, and −2.7% decade−1 over the Northern hemisphere (NH) extratropics, Southern hemisphere (SH) extratropics, tropics, extratropics, and globe, respectively. The corresponding ozone STE trends for 2000–2099 are 3.0%, 4.3%, 0.8%, 3.5%, and 4.7% decade−1. Changes in ozone STEs are dominated by ozone concentration changes, driven by climate‐induced changes and ozone‐depleting substance (ODS) changes. For 1960–2000, small changes in ozone STEs in the NH extratropics are due to a cancellation between effects of climate‐induced changes and ODS increases, while the ODS effect dominates in the SH extratropics, leading to a large ozone STE magnitude decrease. Increased ozone transport from tropical troposphere to stratosphere for 1960–2000 is due to increased tropospheric ozone. A decreased global ozone STE magnitude for 1960–2000 was largely caused by ODS‐induced ozone loss that is partly compensated by climate‐induced ozone changes. For 2000–2099, about two‐thirds of global ozone STE magnitude increases are caused by ozone increases in the extratropical lower stratosphere due to climate‐induced changes. The remaining one‐third is caused by ozone recovery due to the phaseout of ODS. Plain Language Summary: Stratosphere‐troposphere exchange (STE) of ozone is an important source of tropospheric ozone and can impact the tropospheric air quality. This study investigates changes in STE of ozone concentrations for 1960–2099 using multiple climate model simulations from the Chemistry Climate Model Initiative under the RCP6.0 scenario. It is shown that the trends in global ozone STEs are −2.7% decade−1 for 1960–2000, and 4.7% decade−1 for 2000–2099 in. In other words, global ozone STEs are decreased by 11% from 1960 to 2000, but increased by 47% from 2000 to 2099. We separate the contributions from climate change and ozone‐depleting substance (ODS) effects to the ozone STE changes. A decreased global ozone STE magnitude for 1960–2000 was caused by ODS‐induced ozone loss that is partly compensated by climate‐induced ozone changes. For 2000–2099, about two‐thirds of the global ozone STE magnitude increases are caused by ozone increases in the extratropical lower stratosphere due to climate changes. The remaining one‐third is caused by ozone recovery due to the phaseout of ODS. Key Points: Global ozone stratosphere‐troposphere exchange (STE) trends from Chemistry Climate Model Initiative models are −2.7% decade−1 for 1960–2000, and 4.7% decade−1 for 2000–2099 in RCP6.0 scenarioFor 1960–2000, a decreased global ozone STE magnitude was caused by ozone‐depleting substance (ODS)‐induced ozone loss, partly compensated by climate‐induced changesFor 2000–2099, climate‐induced changes and phaseout of ODS contribute about two‐thirds and one‐third of the global ozone STE magnitude increase, respectively [ABSTRACT FROM AUTHOR]

The Hunga Tonga‐Hunga Ha'apai (HTHH) eruption on 15 January 2022 was one of the most explosive volcanic events of the 21st century so far. According to satellite‐based measurements, 0.4 Tg of sulfur dioxide (SO2) was injected into the stratosphere during the eruption. By using observations and model simulations, here we investigate changes in the chemical compositions of the stratosphere 1 year after the HTHH eruption and examine the key physical and chemical processes that influence the ozone (O3) concentrations. Injected SO2 was oxidized into sulfate during the first 2 months, and transported from the tropics to the Antarctic by the Brewer‐Dobson circulation within 1 year. In mid‐to‐low latitudes, enhanced sulfate aerosol increased O3 concentrations in the middle stratosphere but declined in the lower stratosphere. In addition to the chemical processes, sulfate aerosols also reduced polar low‐stratospheric O3 concentrations through enhanced Antarctic upwelling anomalies. Plain Language Summary: The Hunga Tonga‐Hunga Ha'apai (HTHH) eruption on 15 January 2022 was one of the most explosive volcanic eruptions of the 21st century and has attracted global attention. Volcanic ash and gases entering the atmosphere could affect weather and climate processes. Recent studies have largely explored the effects on global warming of HTHH eruption, and have founded that its climate impact is not very strong. However, its impacts on ozone (O3) remains unclear. We used observations and models to analyze how HTHH eruption could influence O3. It confirms that stratospheric O3 can be affected when volcano‐induced aerosols are transported. We suggest that physical and chemical processes combine together to influence stratospheric O3 after HTHH eruption. Moreover, the effect on O3 of HTHH eruption is probably one of the reasons for the recent discovery of a larger O3 hole in Antarctica. Key Points: Volcano‐induced stratospheric sulfate aerosols are transported toward the South Pole and downwards by the Brewer‐Dobson circulationCatalytic nitrogen oxide ozone loss cycles and sulfate aerosols' radiative effects cause extra‐polar stratospheric ozone anomaliesVolcanic aerosol‐induced heterogeneous chemistry and enhanced upward transport causes polar stratospheric ozone anomalies [ABSTRACT FROM AUTHOR]

Abstract We quantify the changes in the intensity of Brewer‐Dobson Circulation (BDC) during sudden stratospheric warming (SSW) and its impact on the tropical stratospheric thermal structure and ozone distribution by composite analysis using observations and a chemical‐transport model. An increase in the planetary wave activity and enhancement in BDC intensity before the central date of SSW is noticed. A positive ozone anomaly is observed in the tropical upper stratosphere. The tropical lower stratosphere shows a cooling (1–2 K) and negative ozone anomaly (∼0.1 ppmv) after ∼10 days from the central date. The polar stratosphere experiences a positive ozone anomaly, whereas the upper stratosphere shows ozone depletion due to the downwelling of NOx‐rich mesospheric air. The cold‐point tropopause temperature shows a cooling of ∼0.5 K for major warming which in turn dries the lower stratosphere.

Abstract The Hunga Tonga‐Hunga Ha'apai (HTHH) eruption on 15 January 2022 was one of the most explosive volcanic events of the 21st century so far. According to satellite‐based measurements, 0.4 Tg of sulfur dioxide (SO2) was injected into the stratosphere during the eruption. By using observations and model simulations, here we investigate changes in the chemical compositions of the stratosphere 1 year after the HTHH eruption and examine the key physical and chemical processes that influence the ozone (O3) concentrations. Injected SO2 was oxidized into sulfate during the first 2 months, and transported from the tropics to the Antarctic by the Brewer‐Dobson circulation within 1 year. In mid‐to‐low latitudes, enhanced sulfate aerosol increased O3 concentrations in the middle stratosphere but declined in the lower stratosphere. In addition to the chemical processes, sulfate aerosols also reduced polar low‐stratospheric O3 concentrations through enhanced Antarctic upwelling anomalies.

The Brewer‐Dobson Circulation (BDC) determines the distribution of long‐lived tracers in the stratosphere; therefore, their changes can be used to diagnose changes in the BDC. We evaluate decadal (2005–2018) trends of nitrous oxide (N2O) in two versions of the Whole Atmosphere Chemistry‐Climate Model (WACCM) by comparing them with measurements from four Fourier transform infrared (FTIR) ground‐based instruments, the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE‐FTS), and with a chemistry‐transport model (CTM) driven by four different reanalyses. The limited sensitivity of the FTIR instruments can hide negative N2O trends in the mid‐stratosphere because of the large increase in the lowermost stratosphere. When applying ACE‐FTS measurement sampling on model datasets, the reanalyses from the European Center for Medium Range Weather Forecast (ECMWF) compare best with ACE‐FTS, but the N2O trends are consistently exaggerated. The N2O trends obtained with WACCM disagree with those obtained from ACE‐FTS, but the new WACCM version performs better than the previous above the Southern Hemisphere in the stratosphere. Model sensitivity tests show that the decadal N2O trends reflect changes in the stratospheric transport. We further investigate the N2O Transformed Eulerian Mean (TEM) budget in WACCM and in the CTM simulation driven by the latest ECMWF reanalysis. The TEM analysis shows that enhanced advection affects the stratospheric N2O trends in the Tropics. While no ideal observational dataset currently exists, this model study of N2O trends still provides new insights about the BDC and its changes because of the contribution from relevant sensitivity tests and the TEM analysis. Plain Language Summary: The circulation in the stratosphere is characterized by upward motion above the Tropics, followed by poleward and downward motions above the high latitudes. Changes in the pattern of this stratospheric circulation are currently a challenging topic of research. We investigate the decennial changes of this stratospheric circulation using observations and numerical simulations of the long‐lived tracer nitrous oxide. Observations are obtained from ground‐based and satellite instruments. Numerical simulations include complex atmospheric models that reproduce the chemistry and dynamics of the stratosphere. Both observations and models show differences between the hemispheres in the nitrous oxide decennial changes. Unfortunately, the current observations of nitrous oxide are not perfect. The ground‐based instruments cannot correctly measure the changes of nitrous oxide in the northern hemisphere. The satellite does not measure at all times, and it spatially covers more the high latitudes, which negatively affects the measurements of nitrous oxide. On the other hand, model simulations can provide valuable insights into the changes in the stratospheric circulation. They show that changes in the stratospheric circulation cause the differences between hemispheres in the nitrous oxide trends and show that the circulation changes can be associated with different physical processes. Key Points: Sparse sampling of Atmospheric Chemistry Experiment Fourier Transform Spectrometer exaggerates the stratospheric nitrous oxide trendsTransformed Eulerian Mean analysis shows that the residual mean advection contributes to the positive nitrous oxide trend in the TropicsThe Whole Atmosphere Community‐Climate Model simulates weaker hemispheric asymmetries of the nitrous oxide trends compared to reanalyses [ABSTRACT FROM AUTHOR]

The study presents the results of analysis of spatiotemporal distribution of updated total ozone climate normals for the period of 1991–2020. It analyzes the changes since the last total ozone climate normals estimate conducted for the period of 1981–2010. The daily data retrieved using TOMS and OMI satellite instruments over the territory of Ukraine allowed calculation of multiyear average values, climate normals for each day of the year, amplitudes, phases and determination coefficients for total ozone seasonal variations. Use of the NCEP/NCAR reanalysis data allowed establishment of the relation between total ozone and meridional wind changes in the lower stratosphere. The research shows the existence of the regions with higher/lower total ozone content that are not in line with the characteristics of latitudinal distribution. They are formed due to more frequent recurrence of air advection with ozone content that is not typical for the Ukrainian territory, mostly from January to March. The study describes a typical temporal distribution of the total ozone climate normals varying from 285 to 375 Dobson units (D.u.) and analyzes the statistic patterns of deviations distribution and recurrence of extremely high/low total ozone content. The paper emphasizes a small variation of total ozone over the territory of Ukraine. The authors also researched the features of spatial distribution of seasonal variation amplitudes varying within the range of 42–46 D.u. and the specifics of maximum values periods observed from 19 to 30 of March, depending on a region. It was established that, since the previous update of the climate normals, the total ozone decreased for all months with its maximum decrease of about 8 D.u. in winter. However, no increase of recurrence of ultraviolet radiation high levels was observed. Total ozone changes are mainly associated with shifts of meridional wind values to negative ones. This process indicates the increase of recurrence of air advection from the north. The paper also emphasizes the complexness of total ozone changes during the period of research and the lack of consistency of such changes with the circulation factor of summer months.

In response to global warming, ozone is predicted to increase aloft due to stratospheric cooling but decrease in the tropical lower stratosphere. The ozone reductions have been primarily attributed to a strengthening Brewer‐Dobson circulation, which upwells ozone‐poor air. Yet, this paper finds that strengthening upwelling only explains part of the reduction. The reduction is also driven by tropospheric expansion under global warming, which erodes the ozone layer from below, the low ozone anomalies from which are advected upwards. Strengthening upwelling and tropospheric expansion are correlated under global warming, making it challenging to disentangle their relative contributions. Therefore, chemistry‐climate model output is used to validate an idealized model of ozone photochemistry and transport with a tropopause lower boundary condition. In our idealized decomposition, strengthening upwelling and tropospheric expansion both contribute at leading order to reducing tropical ozone. Tropospheric expansion drives bottom‐heavy reductions in ozone, which decay in magnitude into the mid‐stratosphere. Plain Language Summary: The ozone layer absorbs ultraviolet light otherwise harmful to life. Due to compliance with the Montreal Protocol, the ozone layer is generally recovering from depletion. But, at the same time, global warming is predicted to impact ozone, increasing ozone in the upper stratosphere and decreasing ozone in the tropical lower stratosphere. These decreases are typically argued to result from a strengthening of tropical stratospheric upwelling under global warming. Yet, this paper shows that in addition to contributions from strengthening upwelling, much of the ozone loss arises from a deepening of the troposphere under global warming. The deepening of the troposphere erodes the ozone layer from below, with the low ozone anomalies in the eroded region subsequently transported upwards by the background upwelling. Deepening of the troposphere therefore helps to explain the predicted ozone reductions throughout the tropical lower stratosphere. Key Points: Global warming reduces ozone in the tropical lower stratosphere, an effect typically attributed to strengthening stratospheric upwellingYet, global warming also deepens the troposphere, which erodes the ozone layer and reduces transport of ozone into the lower stratosphereAlong with strengthening upwelling, tropospheric expansion contributes at leading order to reductions in tropical lower stratospheric ozone [ABSTRACT FROM AUTHOR]

A westerly phase of the quasi‐biennial oscillation (QBO) was disrupted in 2015/16. Previous studies have primarily focused on the cause of the localized negative momentum forcing, initiating the QBO disruption. However, the upward displacement of the westerly QBO followed by the negative momentum forcing, clearly observed in 2015/16, has received less attention. This study shows that the distinct upward propagation of the westerly winds during the 2015/16 QBO disruption was contributed by the strong Brewer‐Dobson circulation (BDC). Strong Rossby waves with wavenumber 1 propagating from the troposphere mainly drove the strong BDC. Potential contributions of El Niño and Barents–Kara sea ice reduction to Rossby waves are also discussed. Plain Language Summary: The quasi‐biennial oscillation (QBO), describing alternate easterly and westerly winds in the tropical stratosphere, generally shows downward phase propagation with time. However, downward‐propagating westerly winds were split into two in February 2016, with one propagating upward and the other propagating downward. It is found that the upward propagation of the westerly winds was contributed from the stronger upwelling in the tropical stratosphere, mainly attributed to the strong planetary‐wave activity in the midlatitude troposphere. Key Points: Strong Brewer‐Dobson circulation contributed to the distinct upward propagation of the westerly QBO during the 2015/16 QBO disruptionMidlatitude Rossby waves with wavenumber 1 mainly induced the strong Brewer‐Dobson circulation in 2015/16 [ABSTRACT FROM AUTHOR]

Recent studies have suggested that water isotopologues in snow pits from remote East Antarctica can be influenced by the input of stratospheric water, which has anomalously high 17O‐excess values. However, it remains unclear whether the 17O‐excess records preserved in snow and ice from this region can be used to reconstruct stratosphere‐troposphere exchange (STE). In this study, we present high‐resolution 17O‐excess records from two snow pits at Dome A, the highest point of the Antarctic ice sheet. The 17O‐excess records show a significant positive correlation with the strength of the Brewer‐Dobson circulation (BDC), the hemispheric‐scale troposphere‐stratosphere overturn circulation. Stronger BDC leads to more stratospheric water input over Antarctica and higher 17O‐excess, and vice versa. In addition, the 17O‐excess records also have a significant positive correlation with the Southern Annular Mode (SAM) index, because SAM modulates Antarctic precipitation, which has a dilution effect on the stratospheric water input. The 17O‐excess records do not show significant correlations with local temperature and relative humidity in the moisture source region. These results suggest the dominant effect of BDC on 17O‐excess and indicate the potential for using 17O‐excess records in ice cores from remote sites in East Antarctica for reconstructing long‐term variations of STE, and understanding their mechanisms and climate effects. Plain Language Summary: Previous studies suggest the influence of stratospheric water on 17O‐excess variations in snow pits from the inland Ppateau of East Antarctica, and the potential for using ice‐core 17O‐excess to trace historical stratosphere‐troposphere exchange. However, it remains unclear whether ice‐core 17O‐excess records from this remote part of East Antarctica can be used as a proxy for stratosphere‐troposphere exchange. In this study, we presented high‐resolution 17O‐excess records from two snow pits at Dome A, the highest point of the Antarctic ice sheet. We found that the 17O‐excess variations over the past several decades were mainly controlled by the strength of the Brewer‐Dobson circulation. This discovery could open up an opportunity for new methods to reconstruct long‐term variations of the Brewer‐Dobson circulation. Key Points: The 17O‐excess records in snow pits at Dome A show a significant correlation with the strength of the Brewer‐Dobson circulation (BDC)Stronger BDC leads to more stratospheric water input over Antarctica and higher 17O‐excess, and vice versaThe 17O‐excess records are also significantly correlated with the Southern Annular Mode, which dilutes the stratospheric water input [ABSTRACT FROM AUTHOR]

Water vapor in the middle atmosphere plays an essential role in global warming, ozone depletion, and the formation of polar stratospheric and mesospheric clouds. We show that tropical middle atmospheric water vapor simulated with the specified‐dynamics version of the Whole Atmosphere Community Climate Model (SD‐WACCM) is consistent with changes observed in a merged satellite data set, which encompasses the period 1993–2020. Consistent with previous work, we find no significant trend in the stratosphere in either the observations or the simulation; in the mesosphere, we find a long‐term trend of 0.1 ppmv per decade, but only in the observations. We also analyze an SD‐WACCM simulation for the longer period 1980–2019 to quantify the contribution of various factors to the decadal variation of middle atmospheric water vapor. Over 1980–1995, the simulated water vapor in the upper stratosphere and mesosphere, averaged zonally and over ±30° latitude, increases by 0.30 ppmv per decade due to increasing methane emissions. After 1995, a significant abrupt decrease of water vapor of 0.37 ppmv per decade and then a gradual increase of 0.33 ppmv per decade result from changes in stratospheric cold point temperature. The cold‐point temperature is strongly influenced by the strength of the Brewer‐Dobson circulation. The acceleration of the Brewer‐Dobson circulation before about 2003 leads to a cooler tropical tropopause and a decrease of water vapor, and the deceleration thereafter leads to corresponding warming of the tropopause and an increase in water vapor. Plain Language Summary: Water vapor in the middle atmosphere is important to global warming and ozone depletion. We analyze both satellite data and climate model output to understand its variation in the past four decades. We conclude that there is a slight increasing trend in observed mesospheric water vapor, but no significant trend in stratospheric water vapor. Model simulation results indicate that methane oxidation explains most of the increase of water vapor in the upper stratosphere and mesosphere over 1980–1995. Changes in the meridional circulation of the middle atmosphere lead to changes in the tropical tropopause temperature, which is the main factor that influences middle atmospheric water vapor over 1995–2020. Key Points: Our paper focuses on the long‐term trend and decadal variation of water vapor in the tropical middle atmosphereMethane oxidation explains most of the water vapor increases in the upper stratosphere and mesosphere over 1980–1995Changes in residual circulation lead to changes in the tropical tropopause temperature, and middle atmospheric water vapor over 1995–2020 [ABSTRACT FROM AUTHOR]

Water stable isotopes are crucial for paleoclimate reconstruction and water cycle tracing in Antarctica. Accurate measurement of atmospheric water vapor isotopic composition of hydrogen and oxygen is required urgently for understanding the processes controlling the atmosphere–snow interaction and associated isotope fractionation. This study presents in situ real-time measurements of water vapor isotopes along the transect from Zhongshan Station to Dome Argus (hereafter Dome A) in East Antarctica for the first time. The results reveal that the surface vapor stable isotopes of δ18O and δ D showed a gradual decreasing trend in the interior plateau region with the distance away from the coast, with significant δ18O-temperature correlation gradient of 1.61‰°/C and δ18O-altitude gradient of –2.13‰/100 m. Meanwhile, d-excess gradually arises with elevation rise. Moreover, the spatial variation of vapor isotopic composition displays three different characters implying different atmosphere circulation backgrounds controlling the inland water cycle; it can be divided as the coastal steep area below 2,000 m, a vast inland area with an elevation varied between 2,000 and 3,000 m, and high central plateau. Thirdly, observed high inland Antarctica water vapor d-excess quantitatively confirms stratosphere air intrusion and vapor derived from low latitudes by Brewer–Dobson circulation. Finally, the diurnal cycle signals of interior area water vapor isotopes δ18O, δ D, and air temperature highlighted the substantial domination of the supersaturation sublimation/condensation effect in inland, and this suggests that fractionation occurs during sublimation and vapor–snow exchanges should no longer be considered insignificant for the isotopic composition of near-surface snow in Antarctica.

Using multidecadal time series of ground‐based and satellite Fourier transform infrared measurements of inorganic fluorine (i.e., total fluorine resident in stratospheric fluorine reservoirs), we investigate stratospheric circulation changes over the past 20 years. The representation of these changes in five modern reanalyses is further analyzed through chemical‐transport model (CTM) simulations. From the observations but also from all reanalyses, we show that the inorganic fluorine is accumulating less rapidly in the Southern Hemisphere than in the Northern Hemisphere during the 21st century. Comparisons with a study evaluating the age‐of‐air of these reanalyses using the same CTM allow us to link this hemispheric asymmetry to changes in the Brewer‐Dobson circulation (BDC), with the age‐of‐air of the Southern Hemisphere getting younger relative to that of the Northern Hemisphere. Large differences in simulated total columns and absolute trend values are, nevertheless, depicted between our simulations driven by the five reanalyses. Superimposed on this multidecadal change, we, furthermore, confirm a 5–7‐year variability of the BDC that was first described in a recent study analyzing long‐term time series of hydrogen chloride (HCl) and nitric acid (HNO3). It is important to stress that our results, based on observations and meteorological reanalyses, are in contrast with the projections of chemistry‐climate models in response to the coupled increase of greenhouse gases and decrease of ozone‐depleting substances, calling for further investigations and the continuation of long‐term observations. Plain Language Summary: The overturning circulation of the stratosphere is projected to change in response to increases in greenhouse gases and decreases in ozone‐depleting substances, with the Southern Hemisphere branch expected to get weaker relative to the Northern Hemisphere. Here, we use 30‐year time series of observations and simulations of a long‐lived tracer to investigate stratospheric circulation changes. The observations analyzed are from ground‐based and satellite instruments. We compare them with simulations that use a chemical‐transport model driven by winds from meteorological reanalyses of 1990–2019. A reanalysis assimilates meteorological measurements into a forecast model to simulate the best possible representation of the atmospheric state. All five simulations, which use different reanalyses, agree with the observational analysis showing that the analyzed tracer is accumulating less rapidly in the Southern Hemisphere than in the Northern Hemisphere through 2019. This hemispheric asymmetry is attributed to changes in the stratospheric transport circulation, with the Southern Hemisphere branch getting stronger relative to the Northern Hemisphere. Our results support the conclusions of a recent observational study but do not support circulation changes projected by chemistry‐climate models. This study highlights the crucial importance of long‐term observation time series. Key Points: Fluorine in the stratosphere is accumulating less rapidly in the Southern Hemisphere than in the Northern HemisphereThe observed fluorine trend asymmetry, attributed to transport circulation changes, opposes trends predicted by chemistry‐climate modelsObservations and five modern reanalyses are qualitatively in agreement with these changes, calling for maintaining long‐term observations [ABSTRACT FROM AUTHOR]

Stratosphere-troposphere exchange (STE) of ozone (O3) is key in the budget of tropospheric O3, in turn affecting climate forcing and global air quality. We compare three commonly used diagnostics meant to quantify cross-tropopause O3 fluxes with a Chemistry-Transport Model driven by two distinct European Centre forecast fields. Our reference case calculates accurate, geographically resolved net transport across an isosurface in artificial tracer e90 representing the tropopause. Hemispheric fluxes derived from the ozone mass budget of the lowermost stratosphere yield similar results. Use of the Brewer-Dobson residual vertical velocity as a scaled proxy for ozone flux, however, fails to capture the interannual variability. Thus, the common notion that the strength of stratospheric overturning circulation is a good measure for global STE does not apply to O3. Climatic variability in the modeled O3 flux needs to be diagnosed directly rather than indirectly through the overturning circulation. Key Points O3 residual flux by BDC does not sync with cross-tropopause O3 fluxesMore accurate diagnostics of ozone STE are needed in climate-chemistry modelsTC and LS methods are suitable for diagnosing STE of ozone

Global electric circuits could be the key link between space weather and lower atmosphere climate. It has been suggested that the ultrafine erosol layer in the middle to upper stratosphere could greatly contribute to local column resistance and return current density. In previous work by Tinsley, Zhou, and Plemmons (Atmos. Res., 2006, 79 (3–4), 266–295), the artificial ultrafine layer was addressed and caused a significant symmetric effect on column resistance at high latitudes. In this work, we use an updated erosol coupled chemistry-climate model to establish a new global electric circuit model. The results show that the ultrafine aerosol layer exits the middle stratosphere, but due to the Brewer-Dobson circulation, there are significant seasonal variations in the ion loss due to variations in the ultrafine aerosol layer. In the winter hemisphere in the high latitude region, the column resistance will consequently be higher than that in the summer hemisphere. With an ultrafine aerosol layer in the decreasing phase of solar activity, the column resistance would be more sensitive to fluctuations in the low-energy electron precipitation (LEE) and middle-energy electron precipitation (MEE) particle fluxes.

Trichlorofluoromethane (CFC‐11, CFCl3) is a major anthropogenic ozone‐depleting substance and greenhouse gas, and its production and consumption are controlled under the Montreal Protocol. However, recent studies show that CFC‐11 emissions increased during 2014–2017 relative to 2008–2012. In this study, we use a chemistry‐climate model to investigate the stratospheric impacts of potential CFC‐11 emissions continuing into the future. As a sensitivity test, we use a high CFC‐11 scenario in which the inferred 2013–2016 average emissions of 72.5 Gg/yr is sustained to year 2100. This increases equivalent effective stratospheric chlorine by 15% in 2100, relative to the WMO (2018) baseline scenario in which future emissions decay with a bank release rate of 6.4%/year. Consistent with recent studies, the resulting ozone response has a linear dependence on the accumulated CFC‐11 emissions, yielding global and Antarctic spring total ozone sensitivity per 1,000 Gg of −0.37 and −3.9 DU, respectively, averaged over 2017–2100. The deepened ozone hole reduces UV heating, causing a colder Antarctic lower stratosphere in spring/early summer. Through thermal wind balance, this accelerates the circumpolar jet which in turn alters planetary and gravity wave propagation through the Southern Hemisphere stratosphere, and modifies the Brewer‐Dobson circulation. Age of air in the high scenario is slightly younger than the baseline in the lower stratosphere globally during 2090–2099, with a maximum change of −0.1 years. Coupled atmosphere‐ocean model simulations show that the resulting greenhouse gas impact of CFC‐11 is small and not statistically significant throughout the troposphere and stratosphere. Key Points: Continuing trichlorofluoromethane (CFC‐11) emissions will cause additional future stratospheric ozone depletionLargest statistically significant ozone depletion occurs in the global and Antarctic spring total column, and global upper stratosphereAdditional ozone depletion due to continuing CFC‐11 emissions will cause small changes in stratospheric temperature and circulation [ABSTRACT FROM AUTHOR]

Previous studies paid more attention on the winter‐mean relationship between the surface Arctic Oscillation (AO) and stratospheric ozone over the Arctic. However, few studies focused on the subseasonal relationship between the stratospheric ozone and surface AO in boreal winter. We investigated the subseasonal relationship between the AO and stratospheric ozone over the Arctic in each boreal winter month during 1980–2017 that is independent on the El Niño–Southern Oscillation and Quasi‐Biennial Oscillation signals using Modern‐Era Retrospective Analysis for Research and Applications version 2 reanalysis data sets. Results showed that the positive AO phases correspond to two negative anomalous ozone centers located in the middle stratosphere (∼30 hPa) and upper troposphere–lower stratosphere over the Arctic in each month. There is an in‐phase relationship between the AO and the Arctic ozone at 70–100 hPa in December, which is opposite to the out‐of‐phase relationship between these two metrics in mid‐to‐late winter. In December, the subtropical jet in the Pacific under the positive AO phase shifts poleward with strengthened planetary wavenumber‐2 waves in the lower stratosphere. The strengthened wavenumber‐2 wave flux contributes to the positive ozone anomalies at 70–100 hPa via modifying the asymmetric component of polar vortex. However, in January and February, the subtropical jet weakens during the positive AO years, along with less planetary wavenumber‐1 waves propagating into the Arctic lower stratosphere. This weakened wavenumber‐1 wave tends to result in the negative ozone anomalies via weakening the downwelling branch of Brewer–Dobson circulation and strengthening the stratospheric Arctic vortex. Key Points: The relationship between the Arctic ozone in the lower stratosphere and AO in December is in‐phase, opposite to that in mid‐to‐late winterThere are positive ozone anomalies in December during the positive AO years, which are related to the asymmetric part of polar vortexIn mid‐to‐late winter, the linkage of the ozone with the AO mainly depends on Brewer–Dobson circulation and intensity of polar vortex [ABSTRACT FROM AUTHOR]

The Earth's three poles, the North Pole, South Pole, and Third Pole (i.e., the Tibetan Plateau and its surroundings), hold the largest amount of fresh water on Earth as glaciers, sea ice, and snow. They are sensitive to climate change. However, the linkages between climate variations of the three poles, particularly between the South Pole and Third Pole, remain largely unknown. The temperatures at 200 hPa over the three poles are the highest in the summer and are less affected by surface conditions, which could reflect large-scale dynamic linkages. Temperatures at 200 hPa peak the three poles during their respective hemispheric summer and exhibit in-phase variations on interdecadal timescales (10-100 years). The 200 hPa temperatures over the North Pole and South Pole were significantly correlated with the Brewer-Dobson circulation (BDC), which transports stratospheric ozone poleward, heating the air at 200 hPa. Tropopause warming over the Third Pole was found to enhance the poleward BDC, particularly to the South Pole, linking the Third Pole's climate to the other two poles. Additionally, the Interdecadal Pacific Oscillation (IPO) also exhibits links with the 200 hPa temperatures of the three poles. [ABSTRACT FROM AUTHOR]

The transport of trace gases by the atmospheric circulation plays an important role in the climate system and its response to external forcing. Transport presents a challenge for Atmospheric General Circulation Models (AGCMs), as errors in both the resolved circulation and the numerical representation of transport processes can bias their abundance. In this study, two tests are proposed to assess transport by the dynamical core of an AGCM. To separate transport from chemistry, the tests focus on the age‐of‐air, an estimate of the mean transport time by the circulation. The tests assess the coupled stratosphere–troposphere system, focusing on transport by the overturning circulation and isentropic mixing in the stratosphere, or Brewer–Dobson Circulation, where transport time‐scales on the order of months to years provide a challenging test of model numerics. Four dynamical cores employing different numerical schemes (finite‐volume, pseudo‐spectral, and spectral‐element) and discretizations (cubed sphere versus latitude–longitude) are compared across a range of resolutions. The subtle momentum balance of the tropical stratosphere is sensitive to model numerics, and the first intercomparison reveals stark differences in tropical stratospheric winds, particularly at high vertical resolution: some cores develop westerly jets and others easterly jets. This leads to substantial spread in transport, biasing the age‐of‐air by up to 25% relative to its climatological mean, making it difficult to assess the impact of the numerical representation of transport processes. This uncertainty is removed by constraining the tropical winds in the second intercomparison test, in a manner akin to specifying the Quasi‐Biennial Oscillation in an AGCM. The dynamical cores exhibit qualitative agreement on the structure of atmospheric transport in the second test, with evidence of convergence as the horizontal and vertical resolution is increased in a given model. Significant quantitative differences remain, however, particularly between models employing spectral versus finite‐volume numerics, even in state‐of‐the‐art cores. [ABSTRACT FROM AUTHOR]

Tropospheric warming and stratospheric cooling influence the vertical structure of the atmosphere. Numerous studies have analysed the thermal expansion of the troposphere, however, stratospheric cooling reverses the sign of this shift in the middle stratosphere, causing a downward shift in the upper stratosphere and mesosphere. This is a robust feature in transient climate model simulations, but its impact is commonly unappreciated. Here, we quantify the trend difference of the residual mean vertical velocity (w‾∗), a proxy for diagnosing the advective Brewer–Dobson circulation (BDC) strength, which arises from implicit neglect of the shrinking distance between stratospheric pressure levels in the CCMI‐1 (Chemistry‐Climate Model Initiative part 1) data request. There, a log‐pressure formula with constant scale height is recommended to compute w‾∗. However, stratospheric cooling in transient climate simulations causes a reduction of the geometrical distance between pressure levels and thereby also the scale height significantly decreases over time. Using the general scale height definition for the transformation, the w‾∗ trends are therefore smaller. In both cases, the units are m·s−1, but in the latter case it is the constant measure of length geopotential metres and not log‐pressure metres. We quantify that, due to the temperature dependence of log‐pressure metres, past studies that based w‾∗ trend analyses on log‐pressure w‾∗ overestimated the advective BDC acceleration by ∼ 20%. This result is consistent among the CCMI‐1 projections over the 1960–2100 period. We highlight that other diagnostics can also be affected by the neglect of the declining stratospheric pressure level distance. A detailed description of the diagnostics is necessary for consistent assessments of trends. Data processing tools should generally not include the constant scale height assumption if the data are used for trend analyses. [ABSTRACT FROM AUTHOR]

The stratospheric water vapor feedbacks on climate for abrupt CO2 quadrupling are investigated with the coupled atmosphere–ocean Goddard Earth Observing System Chemistry-Climate model. A feedback suppression method is used to quantify the stratospheric water vapor climate feedback parameter and the impacts of stratospheric water vapor increases on temperature and circulation. It is found that increases in stratospheric water vapor change the model's net climate feedback parameter by 0.11 W m−2 K−1, contributing to 0.5 K, or 10%, of the global-mean surface warming under abrupt CO2 quadrupling. Stratospheric water vapor increases lead to significant impacts on stratospheric temperature and dynamics. The increases induce stratospheric dynamical changes that strongly modify stratospheric cooling patterns. About 30% of the acceleration of the stratospheric Brewer-Dobson circulation under 4 × CO2 is attributed to the stratospheric water vapor increases. In the troposphere, the stratospheric water vapor feedback plays a role in Arctic amplification and is responsible for 14% of the Arctic warming. It also affects tropospheric circulation, causing greater poleward shift of the northern hemisphere tropospheric midlatitude jet. [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 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]

Energy and momentum deposition from planetary-scale Rossby waves as well as from small-scale gravity waves (GWs) largely control stratospheric dynamics. Interactions between these different wave types, however, complicate the quantification of their individual contribution to the overall dynamical state of the middle atmosphere. In state-of-the-art general circulation models (GCMs), the majority of the GW spectrum cannot be resolved and therefore has to be parameterised. This is commonly implemented in two discrete schemes, one for GWs that originate from flow over orographic obstacles and one for all other kinds of GWs (non-orographic GWs). In this study, we attempt to gain a deeper understanding of the interactions of resolved with parameterised wave driving and of their influence on the stratospheric zonal winds and on the Brewer–Dobson circulation (BDC). For this, we set up a GCM time slice experiment with two sensitivity simulations: one without orographic GWs and one without non-orographic GWs. Our findings include an acceleration of the polar vortices, which has historically been one of the main reasons for including explicit GW parameterisations in GCMs. Further, we find inter-hemispheric differences in BDC changes when omitting GWs that can be explained by wave compensation and amplification effects. These are partly evoked through local changes in the refractive properties of the atmosphere caused by the omitted GW drag and a thereby increased planetary wave propagation. However, non-local effects on the flow can act to suppress vertical wave fluxes into the stratosphere for a very strong polar vortex. Moreover, we study mean age of stratospheric air to investigate the impact of missing GWs on tracer transport. On the basis of this analysis, we suggest that the larger ratio of planetary waves to GWs leads to enhanced horizontal mixing, which can have a large impact on stratospheric tracer distributions. [ABSTRACT FROM AUTHOR]

Previous studies showed significant stratospheric warming at the Southern-Hemisphere (SH) high latitudes in September and October over 1979–2006. The warming trend center was located over the Southern Ocean poleward of the Western Pacific in September, with a maximum trend of about 2.8 K/decade. The warming trends in October showed a dipole pattern, with the warming center over the Ross and Amundsen Sea, and the maximum warming trend is about 2.6 K/decade. In the present study, we revisit the problem of the SH stratospheric warming in the recent decade. It is found that the SH high-latitude stratosphere continued warming in September and October over 2007–2017, but with very different spatial patterns. Multiple linear regression demonstrates that ozone increases play an important role in the SH high-latitude stratospheric warming in September and November, while the changes in the Brewer-Dobson circulation contributes little to the warming. This is different from the situation over 1979–2006 when the SH high-latitude stratospheric warming was mainly caused by the strengthening of the Brewer-Dobson circulation and the eastward shift of the warming center. Simulations forced with observed ozone changes over 2007–2017 shows warming trends, suggesting that the observed warming trends over 2007–2017 are at least partly due to ozone recovery. The warming trends due to ozone recovery have important implications for stratospheric, tropospheric and surface climates on SH. [ABSTRACT FROM AUTHOR]

Based on ERA-Interim and JRA-55 daily reanalysis datasets, connections among the variations in the Brewer–Dobson circulation (BDC) intensity, stratospheric air mass, surface pressure, and the tropospheric meridional circulation during the period from 1979 to 2015 are analyzed. The results show that the variations in the surface pressure, particularly at middle and high latitudes, have a close correlation with the variations in the stratospheric BDC intensity. When the upwelling at 450 K intensifies, the surface pressure increases in the high latitudes (poleward of 60°) and decreases in the adjacent mid-latitudes in both hemispheres. And these correlations are most significant during boreal winter in the Northern Hemisphere and during austral autumn in the Southern Hemisphere. It is found that the high latitude surface pressure changes follow the anomalous BDC by about 30 days. The increase in polar surface pressure associated with the stronger BDC is due to an increase in stratospheric air mass, while the decrease in mid-latitude surface pressure is related to a decrease in tropospheric air mass. These air mass changes are caused by the anomalous meridional transport of air mass by the residual mean circulation in the stratosphere and troposphere. In addition, we found a significant connection between the intensity of the BDC and the Ferrel cell, i.e., when the BDC strengthens (weakens), there is a weakened (strengthened) Ferrel cell with its position shifting equatorward (poleward). [ABSTRACT FROM AUTHOR]

Surface ozone, which is a major air pollutant, has gained widespread public attention due to its significant harm to human health and ecosystems. Here we find that long-term trends of surface ozone are largely influenced by stratosphere-troposphere exchange (STE) using the historical simulations in the CMIP6 multi-model ensemble. It is found that STE contributes up to 39.5% of climatological surface ozone in the extratropics of the winter hemisphere. In the Southern Hemisphere (SH), prominent stratospheric ozone depletion tends to reduce tropospheric and surface ozone due to STE over 1960–1997, which has been reversed during 1997–2014. In the Northern Hemisphere (NH), STE enhances surface ozone during both ozone depletion and recovery periods. About 29.5% of the positive trends of surface ozone over 1960–2014 can be attributed to STE processes in boreal winter in the NH extratropics. The enhancement of STE is related to the changes in atmospheric circulation associated with global warming. Our results suggest that STE will further worsen surface ozone pollution in the future associated with the stratospheric ozone recovery and global warming. • Stratosphere-troposphere exchange (STE) of ozone is an important source of surface ozone, especially during winter. • The changes in STE tend to decrease surface ozone during 1960–1997 and increase surface ozone during 1997–2014 in the SH. • In the NH, STE contributes to positive trends in surface ozone over both 1960–1997 and 1997–2014. [ABSTRACT FROM AUTHOR]

We investigate the impact of tropical tropopause warming on the stratospheric water vapor using the Specified-Dynamics version of the NCAR Whole Atmosphere Community Climate Model. We find that the tropical tropopause warming results in a strengthening of the Brewer–Dobson circulation (BDC). The strengthening of BDC induced by a narrow warming of tropical tropopause within 12° latitude, which is much stronger in boreal winter than that in boreal summer, propagates more dry air from the tropical tropopause into the stratosphere and thus causes a reduction of water vapor in the middle stratosphere. On the contrary, the seasonal difference of the BDC strengthening is weaker in the experiment with a broader tropical tropopause warming within 25° latitude. The drying effect of the BDC is counteracted by the moistening effects of the tropical tropopause warming and methane oxidation. This leads to the moistening in both the lower and upper stratosphere. The results suggest the control of the stratospheric humidity by the tropical tropopause temperature could be significantly offset by the associated BDC changes. [ABSTRACT FROM AUTHOR]

Substantial increases in the atmospheric concentration of well‐mixed greenhouse gases (notably CO2), such as those projected to occur by the end of the 21st century under large radiative forcing scenarios, have long been known to cause an acceleration of the Brewer‐Dobson circulation (BDC) in climate models. More recently, however, several single‐model studies have proposed that ozone‐depleting substances might also be important drivers of BDC trends. As these studies were conducted with different forcings over different periods, it is difficult to combine them to obtain a robust quantitative picture of the relative importance of ozone‐depleting substances as drivers of BDC trends. To this end, we here analyze—over identical past and future periods—the output from 20 similarly forced models, gathered from two recent chemistry‐climate modeling intercomparison projects. Our multimodel analysis reveals that ozone‐depleting substances are responsible for more than half of the modeled BDC trends in the two decades 1980–2000. We also find that, as a consequence of the Montreal Protocol, decreasing concentrations of ozone‐depleting substances in coming decades will strongly decelerate the BDC until the year 2080, reducing the age‐of‐air trends by more than half, and will thus substantially mitigate the impact of increasing CO2. As ozone‐depleting substances impact BDC trends, primarily, via the depletion/recovery of stratospheric ozone over the South Pole, they impart seasonal and hemispheric asymmetries to the trends which may offer opportunities for detection in coming decades. Key Points: Impacts of ozone‐depleting substances (ODS) on Brewer‐Dobson circulation trends are analyzed in 20 chemistry‐climate modelsFor the period 1980–2000 ODS have contributed more than half (roughly 60%) of the stratospheric age‐of‐air trendsFor the period 2000–2080 models show that decreasing ODS levels will substantially decelerate the BDC [ABSTRACT FROM AUTHOR]

Abstract We consider physical mechanisms responsible for forming plain-layered jet streams in the winter stratosphere. Unlike the conventional notion about the balance between the energy of the solar UV radiation energy absorbed by the stratospheric ozone within the Hartley band and the energy of loss due to infrared emission from СО 2 , О 3 , and Н 2 О molecules, such a balance is shown not to persist. It is shown that the bias of these energies observed in satellite experiments can be explained by dynamic mechanism increasing the air gravity potential in the tropical stratosphere and forming equator/winter pole baroclinic instability, which generates the jet stream. Jet streams transport energy and pulse from equatorial to polar region and facilitate the descending part of the Brewer-Dobson global circulation. Potential energy release, when the stratospheric jet stream lowers, is ∼1018 W/day; the air mass transported by the jet stream to the winter tropopause region is estimated as being ∼1014 kg/day. Based on the ECMWF ERA-Interim reanalysis data, we analyzed the temporal characteristics of the stratospheric air motion from the region of gravity potential abundance generation in the summer tropical stratosphere to the polar winter tropopause altitudes, where the stratospheric air ends its motion, thus participating in cyclogenesis. Duration of the descending part of the Brewer-Dobson circulation in the winter stratosphere/troposphere averages 50–70 days. Highlights • Energy balance of the middle atmosphere considered within jet stream concept. • ECMWF ERA-Interim reanalysis data on stratosphere/mesosphere wind are used. • Stratosphere jet stream parameters are investigated. • Structure and drivers of stratosphere circulation are considered. [ABSTRACT FROM AUTHOR]

It has recently been recognized that, in addition to greenhouse gases, anthropogenic emissions of ozone‐depleting substances (ODS) can induce long‐term trends in the Brewer‐Dobson circulation (BDC). Several studies have shown that a substantial fraction of the residual circulation acceleration over the last decades of the twentieth century can be attributed to increasing ODS. Here the mechanisms of this influence are examined, comparing model runs to reanalysis data and evaluating separately the residual circulation and mixing contributions to the mean age of air trends. The effects of ozone depletion in the Antarctic lower stratosphere are found to dominate the ODS impact on the BDC, while the direct radiative impact of these substances is negligible over the period of study. We find qualitative agreement in austral summer BDC trends between model and reanalysis data and show that ODS are the main driver of both residual circulation and isentropic mixing trends over the last decades of the twentieth century. Moreover, aging by isentropic mixing is shown to play a key role on ODS‐driven age of air trends. Plain Language Summary: Concentrations of human‐emitted ozone‐depleting substances in the stratosphere have increased substantially during the last decades of the twentieth century, which are found in the Southern Hemisphere. These substances have caused stratospheric ozone depletion, which not only has important direct impacts on human health but also leads to important changes in the atmospheric circulation and climate. Here the impact of these substances on the stratospheric mean transport circulation is examined using chemistry‐climate model simulations and reanalysis data. In addition to destroying ozone, these substances act as greenhouse gases. In this paper, we find that the radiative forcing impacts of human‐emitted ozone‐depleting substances on the stratospheric circulation over the last decades of the twentieth century are negligible. In contrast, the Antarctic ozone hole has contributed substantially to the acceleration of stratospheric tracer transport not only in the polar region but globally. The impacts, largest in the austral summer stratosphere, are examined in detail separating the overturning circulation and irreversible mixing. It is shown that trends in the model compare qualitatively well with estimates from two different reanalyses. This paper contributes to advance our understanding of the impact of human emissions on the stratospheric circulation and to untangle the forcings of recent trends. Key Points: Increasing ODS have played a key role in observed BDC trends over the last decades of the twentieth centuryOzone depletion dominates the ODS impact on BDC trends, while the radiative effect of these substances is negligibleIncluding changes in mixing in addition to the residual circulation is crucial to interpreting past mean age trends [ABSTRACT FROM AUTHOR]

We compare the relationship between the Arctic Oscillation (AO) and ozone concentration in the lower stratosphere over the Arctic during 1980–1994 (P1) and 2007–2019 (P2) in January and February using reanalysis datasets. The out-of-phase relationship between the AO and ozone in the lower stratosphere is significant in January during P1 and February during P2, but it is insignificant in January during P2 and February during P1. The variable links between the AO and ozone in the lower stratosphere over the Arctic in January and February are not caused by changes in the spatial pattern of AO but are related to the anomalies in the planetary wave propagation between the troposphere and stratosphere. The upward propagation of the planetary wave in the stratosphere related to the positive phase of AO significantly weakens in January during P1 and in February during P2, which may be related to negative buoyancy frequency anomalies over the Arctic. When the AO is in the positive phase, the anomalies of planetary wave further contribute to the negative ozone anomalies via weakening the Brewer–Dobson circulation and decreasing the temperature in the lower stratosphere over the Arctic in January during P1 and in February during P2.

The positive trend of tropospheric column ozone (TCO) in China has been confirmed by numerous observations, particularly in the rapidly developing city clusters such as Beijing-Tianjin-Hebei (BTH), the Yangtze River Delta (YRD) and the Pearl River Delta (PRD). An unignorable but poorly known function may be played by the Brewer-Dobson circulation (BDC) and the Hadley circulation (HC), both of which are in their strengthening phase. This study discusses their connection to TCO in these three city clusters based on a novel and powerful causal analysis method Convergent Cross Mapping (CCM). It revealed that both BDC and HC exerted substantial causal effects on the variations of the TCO in BTH, the YRD, and the PRD. The upwelling moving northward in the summer substantially lowered TCO in the PRD, contributing to the difference in the TCO annual cycle characteristics between northern and southern China. Besides, CCM distinguished that hemispheric HC impacts TCO greatly more than total HC, and the most essential branches of BDC were stratospheric or shallow branches. The BDC and HC indices determined by CCM were used for building a multiple linear regression model, which assessed their importance for long-term changes in regional TCO. The intensity of BDC is the most important influencing factor in each region, particularly in the YRD. BTH is mainly affected by the intensity of the two meridian circulations. The influence of the widths of meridional circulation upwellings on TCO in the PRD cannot be ignored. Furthermore, TCO displayed significant positive trends during 2005–2020, with rates of 2.3 ± 0.4, 2.5 ± 0.4, and 2.7 ± 0.5 DU/decade in BTH, the YRD, and the PRD, respectively. • BDC and HC contribute to the different annual cycle patterns of TCO in northern and southern China. • Hemispheric HC impacts TCO greatly more than total HC. • The upwelling moving northward in the summer substantially lowered TCO in the PRD • TCO showed positive trends of around 2.5 DU/decade in China from 2005 to 2020 [ABSTRACT FROM AUTHOR]

The Antarctic stratospheric final warming (SFW) is usually simulated with a substantial delay in climate models, and the corresponding temperatures in austral spring are lower than observations, implying insufficient stratospheric wave drag. To investigate the role of orographic gravity wave drag (GWD) in modeling the Antarctic SFW, in this study the orographic GWD parameterization scheme is modified in the middle-atmosphere version of the Beijing Climate Center Atmospheric General Circulation Model. A pair of simulations are conducted to compare two orographic GWD schemes in simulating the breakdown of the stratospheric polar vortex over Antarctica. The control simulation with the default orographic GWD scheme exhibits delayed vortex breakdown and the cold-pole bias seen in most climate models. In the simulation with modified orographic GWD scheme, the simulated vortex breaks down earlier by 8 days, and the associated cold-pole bias is reduced by more than 2 K. The modified scheme provides stronger orographic GWD in the lower stratosphere, which drives an accelerated polar downwelling branch of the Brewer–Dobson circulation and, in turn, produces adiabatic warming. Our study suggests that modifying orographic GWD parameterizations in climate models would be a valid way of improving the SFW simulation over Antarctica.