| Among the most societally important trends in observations and models are poleward shifts in global circulation features. In order to investigate the magnitude and mechanisms of such circulation shifts, we explore the general circulation response to imposed forcings, using the Geophysical Fluid Dynamics Laboratories Atmospheric Model version 2.1.;Present-day simulations exhibit robust poleward shifts in zonal mean circulation features, compared to a preindustrial control, as do future simulations compared to present-day. These responses vary by season, and the response to combined forcings is well-approximated by the sum of the individual responses. Results suggest that warming sea surface temperatures are the main driver of circulation change over both hemispheres. This work also projects that the southern hemisphere jet will continue to shift poleward, albeit more slowly during the summer due to expected ozone recovery in the stratosphere.;The relationship between shifts in the position of the eddy-driven jet and of the Hadley cell edge are examined. From year to year, the eddy-driven jet shifts more than the Hadley cell edge, with a ratio of approximately 1.5:1 between the two depending on season, hemisphere, and simulation. Furthermore, the mean position of the Hadley cell edge explains a substantial portion of the variability of this ratio. The author attributes this to the varying susceptibility of the Hadley cell to the influence of midlatitude eddies.;Finally, the transient response to suddenly imposed forcings is examined. For both direct radiative forcings and sea surface warming, broad tropospheric and stratospheric temperature changes can be seen almost immediately. Once zonal winds near the tropopause accelerate a few days later, zonal winds and eddies respond throughout the rest of the troposphere, shifting the surface circulation. The transient changes in the Ferrel cell center and the Hadley cell edge appear simultaneous, illustrating the need for consideration of both tropical and extratropical processes in the theory for the latitude of either the Hadley cell edge or the eddy-driven jet. |
