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Abstract An observationally based global climatology of the temperature diurnal cycle in the lower stratosphere is derived from 11 different satellites with global positioning system–radio occultation (GPS-RO) measurements from 2006 to 2020. Methods used in our analysis allow for accurate characterization of global stratospheric temperature diurnal cycles, even in the high latitudes where the diurnal signal is small but longer time-scale variability is large. A climatology of the synthetic Microwave Sounding Unit (MSU) and Advanced MSU (AMSU) Temperature in the Lower Stratosphere (TLS) is presented to assess the accuracy of diurnal cycle climatologies for the MSU and AMSU TLS observations, which have traditionally been generated by model data. The TLS diurnal ranges are typically less than 0.4 K in all latitude bands and seasons investigated. It is shown that the diurnal range (maximum minus minimum temperature) of TLS is largest over Southern Hemisphere tropical land in the boreal winter season, indicating the important role of deep convection. The range, phase, and seasonality of the TLS diurnal cycle are generally well captured by the WACCM6 simulation and ERA5 dataset. We also present an observationally based diurnal cycle climatology of temperature profiles from 300 to 10 hPa for various latitude bands and seasons and compare the ERA5 data with the observations. 

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%. 

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. 

Abstract The degree of Hadley cell expansion under global warming will have a substantial impact on changing rainfall patterns. Most previous studies have quantified changes in total tropical width, focused on the Southern Hemisphere Hadley cell or considered each hemisphere's response to a multitude of anthropogenic forcings. It is shown here that under exclusive CO₂forcing, climate models predict twice as much Hadley cell expansion in the Southern Hemisphere relative to the Northern Hemisphere. This asymmetry is present in the annual mean expansion and all seasons except boreal autumn. It is robust across models and Hadley cell edge definitions. It is surprising since asymmetries in simulated Hadley cell expansion are typically attributed to stratospheric ozone depletion or aerosol emission. Its primary cause is smaller sensitivity of the Northern Hemisphere Hadley cell to static stability changes. The pattern of sea surface warming and the CO₂direct radiative effect also contribute to the asymmetry. 

Abstract This paper describes stratospheric waves in ERA5 and evaluates the contributions of different types of waves to the driving of the quasi-biennial oscillation (QBO). Because of its higher spatial resolution compared to its predecessors, ERA5 is capable of resolving a broader spectrum of waves. It is shown that the resolved waves contribute to both eastward and westward accelerations near the equator, mainly by the way of the vertical flux of zonal momentum. The eastward accelerations by the resolved waves are mainly due to Kelvin waves and small-scale gravity (SSG) waves with zonal wavelengths smaller than 2000 km, whereas the westward accelerations are forced mainly by SSG waves, with smaller contributions from inertio-gravity and mixed Rossby–gravity waves. Extratropical Rossby waves disperse upward and equatorward into the tropical region and impart a westward acceleration to the zonal flow. They appear to be responsible for at least some of the irregularities in the QBO cycle. 

Abstract The dynamics and momentum budget of the quasi-biennial oscillation (QBO) are examined in ERA5. Because of ERA5’s higher spatial resolution compared to its predecessors, it is capable of resolving a broader spectrum of atmospheric waves and allows for a better representation of the wave–mean flow interactions, both of which are of crucial importance for QBO studies. It is shown that the QBO-induced mean meridional circulation, which is mainly confined to the winter hemisphere, is strong enough to interrupt the tropical upwelling during the descent of the westerly shear zones. Since the momentum advection tends to damp the QBO, the wave forcing is responsible for both the downward propagation and for the maintenance of the QBO. It is shown that half the required wave forcing is provided by resolved waves during the descent of both westerly and easterly regimes. Planetary-scale waves account for most of the resolved wave forcing of the descent of westerly shear zones and small-scale gravity (SSG) waves with wavelengths shorter than 2000 km account for the remainder. SSG waves account for most of the resolved forcing of the descent of the easterly shear zones. The representation of the mean fields in the QBO is very similar in ERA5 and ERA-Interim but the resolved wave forcing is substantially stronger in ERA5. The contributions of the various equatorially trapped wave modes to the QBO forcing are documented in Part II. 

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.