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Abstract Water vapor and cirrus clouds in the tropical tropopause layer (TTL) are important for the climate and are largely controlled by temperature in the TTL. On interannual timescales, both stratospheric and tropospheric modes of the large‐scale variability could affect temperatures in the TTL. Here multiple linear regression (MLR) is used to investigate explained variance in the cold point tropopause temperature (CPT), cold point tropopause height (CPZ), 83 hPa water vapor (WV83), 83 hPa ozone (O₃83), and total cirrus cloud fraction with cloud base (TTLCCF) and top (ALLCF) above 14.5 km, all averaged over 15°S‐15°N. Predictors of the MLR are a set of stratospheric and tropospheric large‐scale modes of variability. The MLR explains significant variance in CPT (76%), CPZ (78%), WV83 (65%), O₃83 (62%), TTLCCF (52%), and ALLCF (36%). The interannual variability of CPT and WV83 is dominated by stratospheric processes associated with the Quasi‐Biennial Oscillation (QBO) and Brewer‐Dobson Circulation (BDC), whereas the variability of CPZ, O₃83, TTLCCF and ALLCF is also controlled by 500 hPa temperature (T500). Residual variability in CPT and CPZ not captured by the MLR are further significantly correlated to stratospheric temperature. It is shown that the portion of the BDC's shallow branch missed by the eddy heat flux based BDC index contributes significant amounts of the explained variances. 

Abstract The Quasi‐Biennial Oscillation (QBO) dominates the interannual variability in the tropical lower stratosphere and is characterized by the descent of alternating easterly and westerly zonal winds. The QBO impact on tropical clouds and convection has received great attention in recent years due to its implications for weather and climate. In this study, a 15‐year record of high vertical resolution cloud observations from CALIPSO and a 50 hPa zonal wind QBO index from ERA5 are used to document the QBO impact on equatorial (10°S–10°N) clouds. Observations from radio occultations, the CERES instrument, and the ERA5 reanalysis are also used to document the QBO impact on temperature, cloud radiative effect (CRE), and zonal wind, respectively. It is shown that the QBO impact on zonal mean equatorial cloud fraction has a strong seasonality. The strongest cloud fraction response to the QBO occurs in boreal spring and early summer, which extends from above the mean tropopause to ∼12.5 km and results in a significant longwave CRE anomaly of 1 W/m². The seasonality of the QBO impact on cloud fraction is synchronized with the QBO impacts on temperature and zonal wind in the tropical upper troposphere. 

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 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.