Relationships between the Brewer-Dobson circulation and the southern annular mode during austral summer in coupled chemistry-climate model simulations

Abstract

The Brewer-Dobson circulation (BDC) is the mean meridional mass circulation in the stratosphere and the southern annular mode (SAM) is the prime variability pattern of the Southern Hemisphere extratropical troposphere. Motivated by previous studies showing that both the strengths of the BDC and the SAM have the largest trends in the austral summer in the recent past, this paper investigates the relationships between the BDC and the SAM using coupled chemistry-climate model simulations. The model results show that the strengthening of the BDC in the Southern Hemisphere during November–February (NDJF) is strongly projected onto the high index of the SAM. The trends in the BDC and the SAM are driven by Antarctic ozone depletion, which increases stratosphere-troposphere interactions through a delayed Antarctic vortex breakup. The prolonged persistence of stratospheric westerlies enhances upward propagation of tropospheric wave activity into the stratosphere and strengthens the BDC. The wave flux and westerly anomalies in the stratosphere in turn drive a SAM trend toward its high index. Model results also show that the BDC-SAM relationship is robust on the interannual time scale.