The Whole Atmosphere Community Climate Model version 6 (WACCM6) is a major update of the whole atmosphere modeling capability in the Community Earth System Model (CESM), featuring enhanced physical, chemical and aerosol parameterizations. This work describes WACCM6 and some of the important features of the model. WACCM6 can reproduce many modes of variability and trends in the middle atmosphere, including the quasi‐biennial oscillation, stratospheric sudden warmings, and the evolution of Southern Hemisphere springtime ozone depletion over the twentieth century. WACCM6 can also reproduce the climate and temperature trends of the 20th century throughout the atmospheric column. The representation of the climate has improved in WACCM6, relative to WACCM4. In addition, there are improvements in high‐latitude climate variability at the surface and sea ice extent in WACCM6 over the lower top version of the model (CAM6) that comes from the extended vertical domain and expanded aerosol chemistry in WACCM6, highlighting the importance of the stratosphere and tropospheric chemistry for high‐latitude climate variability.

Riding the Monsoon: Most air transport from the troposphere to the stratosphere occurs in the tropics, but additional transport may occur in areas of strong upward convection. Randel et al. (p. 611 , published online 25 March) report satellite measurements of atmospheric hydrogen cyanide over the region where the Asian summer monsoon occurs, which indicate that air is transported from the surface to deep within the stratosphere. This mechanism represents a pathway for pollutants to enter the global stratosphere, where they might affect ozone chemistry, aerosol characteristics, and radiative properties.

Time series of stratospheric water vapor measurements by satellites and balloons show persistent low values beginning in 2001. Temperature observations show that the tropical tropopause has been anomalously cold during this period, and the observed water vapor changes (approximately −0.4 ppmv) are consistent with the temperature decreases (approximately −1 K). The cold anomalies occur in the tropics over a narrow vertical layer near 15–20 km. There have been corresponding changes in the tropical ozone profile over the same period, with ∼10% reductions over a similar narrow layer near the tropopause. The variations in temperature and ozone appear coupled, and the spatial patterns of the changes since 2001 are consistent with an increase in the mean tropical upwelling (Brewer‐Dobson) circulation. Estimates of tropical upwelling derived from eddy statistics (“downward control”) show coherence with interannual temperature changes, including a consistent increase after 2001. Part of the temperature changes may also be explained as a radiative response to the observed ozone decreases. The results paint a consistent picture of enhanced tropical upwelling after 2001, resulting in colder temperatures, lower water vapor and lower ozone near the tropical tropopause. The recent low values significantly influence estimates of decadal‐scale trends in temperature and ozone near the tropical tropopause.

An updated analysis of observed stratospheric temperature variability and trends is presented on the basis of satellite, radiosonde, and lidar observations. Satellite data include measurements from the series of NOAA operational instruments, including the Microwave Sounding Unit covering 1979–2007 and the Stratospheric Sounding Unit (SSU) covering 1979–2005. Radiosonde results are compared for six different data sets, incorporating a variety of homogeneity adjustments to account for changes in instrumentation and observational practices. Temperature changes in the lower stratosphere show cooling of ∼0.5 K/decade over much of the globe for 1979–2007, with some differences in detail among the different radiosonde and satellite data sets. Substantially larger cooling trends are observed in the Antarctic lower stratosphere during spring and summer, in association with development of the Antarctic ozone hole. Trends in the lower stratosphere derived from radiosonde data are also analyzed for a longer record (back to 1958); trends for the presatellite era (1958–1978) have a large range among the different homogenized data sets, implying large trend uncertainties. Trends in the middle and upper stratosphere have been derived from updated SSU data, taking into account changes in the SSU weighting functions due to observed atmospheric CO₂ increases. The results show mean cooling of 0.5–1.5 K/decade during 1979–2005, with the greatest cooling in the upper stratosphere near 40–50 km. Temperature anomalies throughout the stratosphere were relatively constant during the decade 1995–2005. Long records of lidar temperature measurements at a few locations show reasonable agreement with SSU trends, although sampling uncertainties are large in the localized lidar measurements. Updated estimates of the solar cycle influence on stratospheric temperatures show a statistically significant signal in the tropics (∼30°N–S), with an amplitude (solar maximum minus solar minimum) of ∼0.5 K (lower stratosphere) to ∼1.0 K (upper stratosphere).