Chemistry–Climate Model Simulations of Twenty-First Century Stratospheric Climate and Circulation Changes

Abstract

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.

Earth System Chemistry integrated Modelling (ESCiMo) with the Modular Earth Submodel System (MESSy) version 2.51

“…All simulations show an increase in F trop with rising atmospheric mixing ratios of GHGs, which is well known from earlier CCM simulations (e.g. Butchart et al, 2010;Oberlän-der et al, 2013). In the lower and middle stratosphere, the model simulations are grouped with respect to their underlying SSTs, whereas the vertical resolution has a minor influence on the performance of the RC.…”

Effects of meridional sea surface temperature changes on stratospheric temperature and circulation

“…Deckert and Dameris (2008) suggested that warmer tropical SSTs will strengthen the lower-stratospheric BDC in the tropics in the summer hemisphere via enhancing deep-convective generation of upward propagating quasi-stationary eddies. Some other studies have shown that SST changes can modulate BDC by changing propagation properties of synoptic and planetary-scale waves both in the troposphere and stratosphere (e.g., Butchart et al, 2006Butchart et al, , 2010Shepherd and McLandress, 2011).…”

Sensitivity of 21st century stratospheric ozone to greenhouse gas scenarios

“…This cooling slows gas-phase ozone loss cycles [Haigh and Pyle, 1982], thereby increasing ozone concentrations. Other studies have highlighted increased tropical vertical velocities (upwelling) under elevated GHG concentrations leading to reduced transport time scales and a decrease in mean age of air [Butchart and Scaife, 2001;Butchart et al, 2010]. Increases in nitrous oxide (N 2 O) and methane (CH 4 ) could also impact ozone by accelerating catalytic ozone destruction cycles [e.g., Ravishankara et al, 2009], but changes in abundances of reactive nitrogen (NO y ) and hydrogen (HO x ) play only a minor role in long-term ozone changes under the A1B GHG scenario [Oman et al, 2010].…”