G. Maze, F. D'Andrea, A. Colin de Verdiere and P. Klein
Journal of Climate
The stationary atmospheric response to an idealized sea surface temperature anomaly (SSTa) is studied with a quasi-geostrophic atmospheric model of the Southern Hemisphere. Sensitivity of the stationary response to mid-latitude SSTa location is determined and responses decomposed on vertical modes.
The SSTa almost directly forces baroclinic responses, inducing warm air anomalies 40 to 50 degrees downstream, eastward, to the SSTa. These baroclinic responses arise from an equilibrium between the SSTa-induced anomalous vortex stretching and (i) advection by the quasi-stationary flow and (ii) dissipation by high frequency eddies.
The barotropic response consists of a midlatitude ridge (trough) and a South Pole trough (ridge) for SSTa localized from the Drake Passage to the Western Indian ocean (from South of Australia to the center of the Pacific ocean). This response can be further decomposed into (i) a zonally asymmetric component – a quasi-stationary wave train forced by a barotropic ridge downstream of the SSTa and (ii) a zonal mean component similar to a meridional shift of westerlies and hence a Southern Annular Mode (SAM) like pattern. The former component is phase locked with the SSTa position while the latter has a phase which depends on the relative SSTa position with regard to the background quasi-stationary wave pattern. We show that the barotropic downstream ridge response is responsible for modifying the low frequency eddy-mean flow interactions through relative vorticity fluxes and inducing the bipolar projection of the zonal mean response.
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