Understanding the Timescales of the Tropospheric Circulation Response to Abrupt CO2 Forcing in the Southern Hemisphere

This study examines the timescales of the Southern Hemisphere (SH) tropospheric circulation response to increasing atmospheric CO₂ concentrations in Coupled Model Intercomparison Project Phase 5 (CMIP5) models. In response to an abrupt quadrupling of atmospheric CO₂, the mid-latitude jet stream and poleward edge of the Hadley circulation shift poleward on the timescale of the rising global-mean surface temperature during the summer and fall seasons, but on a much more rapid timescale during the winter and spring seasons. The seasonally varying timescales of the SH circulation response are closely tied to SH polar lower stratospheric temperatures. During summer and fall, SH polar lower stratospheric temperatures cool on the timescale of the global-mean surface temperature rise, as the lifting of the tropopause height with warming tropospheric temperatures produces cooling at lower stratospheric levels. However, during winter and spring, tropopause height rises are ineffective at cooling SH polar lower stratospheric temperatures, so the response of SH polar lower stratospheric temperatures (and hence the SH tropospheric circulation) is more closely tied to fast timescale radiative processes.

In contrast, the poleward edge of the SH subtropical dry zone shifts poleward on the timescale of the rising global-mean 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 eddy moisture 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.