Understanding the Direct and Indirect Circulation Response to Radiative Forcings

A confounding factor in understanding the mechanisms behind tropospheric circulation change is that the atmosphere responds not only to radiative forcings but also to the resulting sea surface temperature (SST) changes. The additional effect of SST changes may be termed the indirect response. Previous research has documented mainly the net atmospheric response to anthropogenic forcings. However, the direct and indirect circulation responses may be associated with different mechanisms, and understanding these mechanisms individually may be crucial to understanding circulation change as a whole. To this end, we perform extended time-slice simulations using an uncoupled stratosphere-resolving model and a combination of different prescribed forcings. Our approach allows us to examine the individual and combined effects of ozone depletion and recovery, past and projected greenhouse gas forcings, and the SST response as derived from a coupled companion experiment. We analyze the direct atmospheric response to the various forcings, as well as the indirect atmospheric response using SST changes. We also examine the mechanisms behind both the direct and the indirect responses, evaluating in particular the mechanisms proposed in previous studies.