The transient circulation response to radiative forcings and sea surface 1 warming

5 Tropospheric circulation shifts have strong potential to impact surface climate. But the magnitude 6 of these shifts in a changing climate, and the attending regional hydrological changes, are difficult 7 to project. Part of this difficulty arises from our lack of understanding of the physical mechanisms 8 behind the circulation shifts themselves. In order to better delineate circulation shifts and their 9 respective causes, we decompose the circulation response into (1) the “direct” response to radiative 10 forcings themselves, and (2) the “indirect” response to changing sea surface temperatures. Using 11 ensembles of 90-day climate model simulations with immediate switch-on forcings, including 12 perturbed greenhouse gas concentrations, stratospheric ozone concentrations, and sea surface 13 temperatures, we document the direct and indirect transient responses of the zonal mean general 14 circulation, and investigate the roles of previously proposed mechanisms in shifting the midlatitude 15 jet. 16 17 We find that both the direct and indirect wind responses often begin in the lower stratosphere. 18 Changes in midlatitude eddies are ubiquitous and synchronous with the midlatitude zonal wind 19 response. Shifts in the critical latitude of wave absorption on either flank of the jet are not indicted 20 as primary factors for the poleward shifting jet, although we see some evidence for increasing 21 equatorward wave reflection over the southern hemisphere in response to sea surface warming. 22 Mechanisms for the northern hemisphere jet shift are less clear. 23