Low‐frequency variability in the Southern Ocean region in a simplified coupled model

International audiencePatterns of interannual variability of the ocean‐atmosphere coupled system in the Southern Hemisphere extratropics are studied with a simple dynamical model in order to determine the basic physical processes of interaction independently of tropical forcing. The model used is an atmospheric 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 current heat advection. In a fully coupled simulation, the atmospheric part of the model, which includes high‐frequency transient eddies at midlatitudes, exhibits a strong Southern Annular Mode (SAM) as the first mode of variability at interannual timescales. The SAM‐related wind anomalies induce Ekman currents in the mixed layer which produce sea surface temperature anomalies. These are then advected along by the ACC. A forced mechanism where the ocean role is reduced to advect the sea surface temperature (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