The response of stratospheric climate and circulation to increasing amounts of greenhouse gases (GHGs) and ozone recovery in the twenty-first century is analyzed in simulations of 11 chemistry–climate models using near-identical forcings and experimental setup. In addition to an overall global cooling of the stratosphere in the simulations (0.59 ± 0.07 K decade−1 at 10 hPa), ozone recovery causes a warming of the Southern Hemisphere polar lower stratosphere in summer with enhanced cooling above. The rate of warming correlates with the rate of ozone recovery projected by the models and, on average, changes from 0.8 to 0.48 K decade−1 at 100 hPa as the rate of recovery declines from the first to the second half of the century. In the winter northern polar lower stratosphere the increased radiative cooling from the growing abundance of GHGs is, in most models, balanced by adiabatic warming from stronger polar downwelling. In the Antarctic lower stratosphere the models simulate an increase in low temperature extremes required for polar stratospheric cloud (PSC) formation, but the positive trend is decreasing over the twenty-first century in all models. In the Arctic, none of the models simulates a statistically significant increase in Arctic PSCs throughout the twenty-first century. The subtropical jets accelerate in response to climate change and the ozone recovery produces a westward acceleration of the lower-stratospheric wind over the Antarctic during summer, though this response is sensitive to the rate of recovery projected by the models. There is a strengthening of the Brewer–Dobson circulation throughout the depth of the stratosphere, which reduces the mean age of air nearly everywhere at a rate of about 0.05 yr decade−1 in those models with this diagnostic. On average, the annual mean tropical upwelling in the lower stratosphere (∼70 hPa) increases by almost 2% decade−1, with 59% of this trend forced by the parameterized orographic gravity wave drag in the models. This is a consequence of the eastward acceleration of the subtropical jets, which increases the upward flux of (parameterized) momentum reaching the lower stratosphere in these latitudes.

The strength of the Brewer–Dobson circulation (BDC) in a changing climate is studied using multidecadal simulations covering the 1960–2100 period with a coupled chemistry–climate model, to examine the seasonality of the change of the BDC. The model simulates an intensification of the BDC in both the past (1960–2004) and future (2005–2100) climate, but the seasonal cycle is different. In the past climate simulation, nearly half of the tropical upward mass flux increase occurs in December–February, whereas in the future climate simulation the enhancement of the BDC is uniformly distributed in each of the four seasons. A downward control analysis implies that this different seasonality is caused mainly by the behavior of the Southern Hemisphere planetary wave forcing, which exhibits a very different long-term trend during solstice seasons in the past and future. The Southern Hemisphere summer planetary wave activity is investigated in detail, and its evolution is found to be closely related to ozone depletion and recovery. In the model results for the past, about 60% of the lower-stratospheric mass flux increase is caused by ozone depletion, but because of model ozone trend biases, the atmospheric effect was likely smaller than this. The remaining fraction of the mass flux increase is attributed primarily to greenhouse gas increase. The downward control analysis also reveals that orographic gravity waves contribute significantly to the increase of downward mass flux in the Northern Hemisphere winter lower stratosphere.

[1] The strength of the Brewer-Dobson circulation is computed for multi-decadal simulations of a coupled chemistry-climate model covering the period 1960 to 2100. The circulation strength, as computed from the tropical mass upwelling, generally increases throughout the simulations. The model also includes an age of air tracer which generally decreases during the simulations.

[1] The stratospheric age spectrum is the probability distribution function of the transit times since a stratospheric air parcel had last contact with a tropospheric boundary region. Previous age spectrum studies have focused on its annual mean properties. Knowledge of the age spectrum's seasonal variability is very limited. In this study, we investigate the seasonal variations of the stratospheric age spectra using the pulse tracer method in the Goddard Earth Observing System Chemistry Climate Model (GEOSCCM). The relationships between the age spectrum and the boundary impulse response (BIR) are reviewed, and a simplified method to reconstruct seasonally varying age spectra is introduced. The age spectra in GEOSCCM have strong seasonal cycles, especially in the lowermost and lower stratosphere and in the subtropical overworld. These changes reflect the seasonal evolution of the Brewer-Dobson circulation, isentropic mixing, and transport barriers. We also investigate the seasonal and interannual variations of the BIRs. Our results clearly show that computing an ensemble of seasonally dependent BIRs is necessary in order to capture the seasonal and annual mean properties of the stratospheric age spectrum.