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Abstract 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⁻¹for 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⁻¹over 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. 

Abstract 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⁻¹from 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. 

Abstract This study estimates the stratosphere–troposphere exchange (STE) of air masses and ozone concentrations averaged over 2007 to 2010 using the Modern Era Retrospective‐Analyses for Research and Applications 2 (MERRA2) and ERA5 reanalyses, and observations. The latter includes Microwave Limb Sounder (MLS) for ozone, MLS and Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) for temperatures, and A‐Train measurements for diabatic heating. The extratropical downward ozone fluxes are 538 Tg year⁻¹from the ERA5 reanalysis, 543 Tg year⁻¹from the MERRA2 reanalysis, and 528–539 Tg year⁻¹from the observations, consistent with previous studies. Previous studies, however, did not consider tropical upward ozone flux. Here we show that the tropical upward ozone flux is 183–193 Tg year⁻¹, which compensates about 35% of the extratropical downward ozone fluxes and should not be neglected. After considering the tropical upward ozone flux, the global ozone STE is 346 Tg year⁻¹from the ERA5 reanalysis, 360 Tg year⁻¹from the MERRA2 reanalysis, and 336–346 Tg year⁻¹from the observations. Those estimates (347 ± 12 Tg year⁻¹) can be used as the contribution of ozone STE to the tropospheric ozone budget. We also investigate cloud radiative effects on the STE of air mass and ozone. At 380 K, cloud radiative effects enhance downward fluxes in the extratropics from both reanalyses and observation, but reduce and enhance upward fluxes in the tropics from reanalyses and observation, respectively. The discrepancy in the tropics is related to the tropical tropopause layer thin cirrus that is missing in the reanalyses. We find that cloud radiative effects enhance the global ozone STE by about 21%–29%. 

The quasi-biennial oscillation (QBO) and sudden stratospheric warmings (SSWs) during the Last Glacial Maximum (LGM) are investigated in simulations using the Whole Atmosphere Community Climate Model version 6 (WACCM6). We find that the period of QBO, which is 27 months in the preindustrial and modern climate simulations, was 33 months in the LGM simulation using the proxy sea surface temperatures (SSTs) and 41 months using the model-based LGM SSTs. We show that the longer QBO period in the LGM is due to weaker wave forcing. The WACCM6 simulations of the LGM, preindustrial, and modern climates do not support previous modeling work that suggests that the QBO amplitude is smaller (larger) in a warmer (colder) climate. We find that SSWs in the LGM occurred later in the year, as compared to the preindustrial and modern climate, but that time of the final warming was similar. The difference in SSW frequency is inconclusive. 

Abstract Using the Whole Atmosphere Community Climate Model version 6, stratospheric ozone in the Last Glacial Maximum (LGM) is investigated. It is shown that, compared with preindustrial (PI) times, LGM modeled stratospheric temperatures are increased by up to 8 K, leading to faster ozone destruction rates for gas phase reactions, especially via the Chapman mechanism. On the other hand, stratospheric hydroxyl radical (OH) and nitrogen oxides (NO_x) concentrations are decreased by 10–20%, which decreases catalytic ozone destruction, thereby decreasing ozone loss rates. The net effect of these two compensating mechanisms in the upper stratosphere (above 15 hPa) is a vertically integrated 1–3 Dobson unit (DU) decrease during the LGM. In the lower stratosphere (tropopause to 15 hPa), changes in the stratospheric overturning circulation and resulting transport dominate changes in ozone. Consistent with a weakening of the residual circulation in the LGM, lower stratospheric ozone is increased by 2–5 DU in the tropics and decreased by 5–10 DU in the extratropics, but the latter is partly compensated by ozone increases due to a lower tropopause. It is found that tropospheric ozone is decreased by about 5 DU in the LGM versus PI. Combined changes in stratospheric and tropospheric ozone lead to a decrease in total ozone column everywhere except over the northeast North America, equatorial Indian and West Pacific Oceans. Surface ultraviolet radiation in the LGM versus PI is increased over the Northern Hemisphere middle and high latitudes, especially over the ice caps, and over the Southern Hemisphere near 60°S. 

Abstract 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.