A mechanism for the effect of tropospheric jet structure on the annular mode-like response to stratospheric forcing

For many climate forcings the dominant response of the extratropical circulation is a latitudinal shift of the troposphericmidlatitude jets. Themagnitude of this response appears to depend on climatological jet latitude in general circulation models (GCMs): lower-latitude jets exhibit a larger shift. The reason for this latitude dependence is investigated for a particular forcing, heating of the equatorial stratosphere, which shifts the jet poleward. Spinup ensembles with a simpliedGCMare used to examine the evolution of the response for ve different jet structures. These differ in the latitude of the eddy-driven jet but have similar subtropical zonal winds. It is found that lower-latitude jets exhibit a larger response due to stronger tropospheric eddy-mean ow feedbacks. A dominant feedback responsible for enhancing the poleward shift is an enhanced equatorward refraction of the eddies, resulting in an increased momentum ux, poleward of the low-latitude critical line. The sen-sitivity of feedback strength to jet structure is associated with differences in the coherence of this behavior across the spectrum of eddy phase speeds. In the congurations used, the higher-latitude jets have a wider range of critical latitude locations. This reduces the coherence of the momentum ux anomalies associated with different phase speeds, with low phase speeds opposing the effect of high phase speeds. This suggests that, for a given subtropical zonal wind strength, the latitude of the eddy-driven jet affects the feedback through its inuence on the width of the region of westerly winds and the range of critical latitudes on the equatorward ank of the jet.