Breaking down the tropospheric circulation response by forcing

This study describes simulated changes in the general circulation during the twentieth and twenty -first centuries due to a number of individual direct radiat ive forcings and warming sea surface temperatures, by examining very long time-slice simulations created with an enhanced version of the Geophysical Fluid Dynamics Laboratories Atmospheric Model AM 2.1. We examine the effects of changing stratospheric ozone, greenhouse gas concentrations, and sea surface temperatures individually and in combination over both hemispheres . Data reveal robust poleward shifts in zonal mean circulation features in present -day simulations compared to a pre-industrial control, and in future simulations compared to present -day. We document the seasonality and significance of these shifts, and find that the combined response is well approximated by the sum of the individual responses. In contrast with other recent studies, we find that circulation shifts due to changing sea surface temperatures - not ozone depletion or recovery - dominate the combined southern hemisphere response during all seasons, and accordingly project that the southern hemisphere jet will continue to shift poleward during the twenty -first century.