Narrowing of the upwelling branch of the Brewer-Dobson circulation in the lower stratosphere

Strengthening of the Brewer-Dobson circulation has been simulated by nearly all middle atmosphere climate models as a robust stratospheric response to greenhouse gas (GHG) increase and stratospheric ozone change. This paper investigates another aspect of changes in the Brewer-Dobson circulation: the width of the tropical upwelling. Using three 100-year simulations of the 21st century under different scenarios of GHG increase and ozone depleting substance (ODS) from the Goddard Earth Observing System Coupled Chemistry-Climate Model (GEOSCCM), we investigate the shift of the turnaround latitudes and the underlying mechanism. Model results show a narrowing and strengthening of the tropical upwelling in the lower stratosphere during the course of the 21st century in all scenarios. The turnaround latitudes at 70 hPa in two ozone-recovery simulations (with the WMO ODS scenario Ab) shift equatorward by 4 and 6 degrees under the IPCC GHG scenarios A1B and A2, respectively. Another low-chlorine simulation with fixed 1960-level halogen amounts produces a narrowing of the tropical upwelling at 70 hPa by 4 degrees under the IPCC GHG scenario A1B. The impact of the narrowing of the tropical upwelling on the distributions of long-lived tracers will be presented.

It is found that the narrowing of the tropical upwelling is accompanied by an equatorward shift of the zero wind line in the lower stratosphere, which is due to a westerly acceleration of the zonal wind in the subtropical upper troposphere and lower stratosphere. Model results suggest that these wind changes result in an equatorward shift of the stratospheric wave driving, leading to a narrowing of the upwelling branch of the Brewer-Dobson circulation. Interestingly, the narrowing and strengthening of the tropical Brewer-Dobson circulation are opposite to the model projected widening of the Hadley circulation in the troposphere. Possible linkages between the responses of the Hadley circulation and the Brewer-Dobson circulation to climate change will be discussed.