Circulation Response to Climate Forcing: Uniform SST Warming

7 The mechanisms of the atmospheric response to climate forcing are analyzed using an ex8 ample of uniform SST warming in an idealized aqua-planet model. A 200-member ensemble 9 of experiments is conducted with an instantaneous uniform SST warming. The zonal mean 10 circulation changes display a rapid poleward shift in the midlatitude eddy-driven westerlies 11 and the edge of the Hadley cell circulation, and a slow equatorward contraction of the circu12 lation in the deep tropics. The shift of the poleward edge of the Hadley cell is predominantly 13 controlled by the eddy momentum flux. It also shifts the eddy-driven westerlies against the 14 surface friction, at a rate much faster than the expectation from the natural variability of the 15 eddy-driven jet (i.e., the e-folding time scale of the annular mode), with much less feedback 16 between the eddies and zonal flow. 17 The transient eddy-zonal flow interactions are delineated using a newly developed finite18 amplitude wave activity diagnostics of Nakamura. Applying it to the transient ensemble 19 response to uniform SST warming reveals that the eddy-driven westerlies are shifted poleward 20 by permitting more upward wave propagation in the middle and upper troposphere rather 21 than reducing the lower tropospheric baroclinicity. The increased upward wave propagation 22 is attributed to a reduction in eddy dissipation of wave activity as a result of a weaker 23 meridional PV gradient. The reduction allows more waves to propagate away from the 24 latitudes of baroclinic generation, which, in turn, leads to more poleward momentum flux 25 and a poleward shift of eddy-driven winds and Hadley cell edge. 26