Understanding the timescales of the tropospheric circulation response to abrupt CO2 forcing in the Southern Hemisphere: Seasonality and the role of the stratosphere

This study examines the time scales of the Southern Hemisphere (SH) tropospheric circulation response to increasing atmospheric CO2 concentrations in models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). In response to an abrupt quadrupling of atmospheric CO2, the midlatitude jet stream and poleward edge of the Hadley circulation shift poleward on the time scale of the rising global-mean surface temperature during the summer and fall seasons but on a much more rapid time scale during the winter and spring seasons. The seasonally varying time scales of the SH circulation response are closely tied to the meridional temperature gradient in the upper troposphere–lower stratosphere and, in particular, to temperatures in the SH polar lower stratosphere. During summer and fall, SH polar lower-stratospheric temperatures cool on the time scale of warming global surface temperatures, as the lifting of the tropopause height with tropospheric warming is associated with cooling at lower-stratospheric levels. However, during winter and spring, SH polar lower-stratospheric temperatures cool primarily from fast time-scale radiative processes, contributing to the faster time-scale circulation response during these seasons. The poleward edge of the SH subtropical dry zone shifts poleward on the time scale of the rising globalmean surface temperature during all seasons in response to an abrupt quadrupling of atmospheric CO2. The dry zone edge initially follows the poleward shift in the Hadley cell edge but is then augmented by the action of eddymoisture fluxes in a warming climate. Consequently, with increasing atmospheric CO2 concentrations, key features of the tropospheric circulation response could emerge sooner than features more closely tied to rising global temperatures.