Interactions basses fréquences océan-atmosphère dans l’océan Austral

Patterns of interannual variability of the ocean- atmosphere coupled system in the Southern Hemisphere ex- tratropics are studied with a simple dynamical model, in or- der to determine the basic physical processes of interaction independently of tropical forcing. The model used is an at- mospheric quasi-geostrophic model coupled to a "slab" oceanic mixed layer, which includes mean geostrophic advection by the Antarctic Circumpolar Current (ACC). The ocean-atmosphere coupling occurs through surface heat fluxes and Ekman cur- rent heat advection. In a fully coupled simulation, the atmos- pheric part of the model, which includes high frequency tran- sient eddies at midlatitudes, exhibits a strong Southern Annu- lar Mode (SAM) as the first mode of variability at interannual time-scales. The SAM-related wind anomalies induce Ekman currents in the mixed layer which produce sea surface tempe- rature anomalies. These are then advected along by the ACC. A forced mechanism where the ocean role is reduced to advect the SST appears sufficient to reproduce the main features of the variability. Nevertheless, a positive feedback of the ocean was also found. It operates through anomalous Ekman currents heat advection and contributes to the maintenance of the SST anomaly. The stationary atmospheric response to an idealised SST ano- maly (SSTa) is next studied in the same simple model of the Southern Hemisphere. Sensitivity of the stationary atmosphe- ric response to the SSTa location is determined. Differentiating the barotropic from the baroclinic responses, it was found that for extratropical SSTa, the baroclinic atmospheric response is independent of the SSTa longitude whereas the barotropic res- ponse can take two different patterns. The SSTa induces a warm air anomaly through the thermal wind balance, which gives a baroclinic response that creates a trough 45 degrees eastward. This response is simply due to the advection of the SSTa induced anomalous vortex stretching by quasi-stationary westerlies. Baroclinic transients eddies were found to have a dissipative role. The barotropic response consists of midlatitudes ridge and South Pole trough for SSTa localised from the western Atlan- tic to the Indian center ocean; and of South Pole ridge for SSTa localised from the Australo-Antarctic basin to the cen- ter of the Pacific ocean.The barotropic responses have a similar eddy component. The difference in the response pattern is se- lected by the zonally symmetric component, which is similar to opposite phases of the Southern Annular Mode (SAM). These SAM-like responses are driven by the anomalous advection of relative vorticity which in turn depends on the position of the SSTa relatively to the geopotential quasi-stationary wave mean field of the model.