On the Role of Extratropical Air-Sea Interaction in the Persistence of the Southern Annular Mode

The Southern Annular Mode (SAM), representing the meridional shift of the eddy-driven jet, is the dominant low-frequency variability in the Southern Hemisphere (SH) extratropics and plays a primary role for variability of weather and climate of the SH. The persistence of the SAM is thought to be maintained by the positive feedback between the zonal mean zonal wind and the eddy momentum forcing. Besides the pure internal eddy-zonal flow interactions in the atmosphere, the extratropical air-sea interaction is also suggested play a role in the SAM variability. However, it remains elusive on how the SAM-induced SST anomalies feed back to the atmosphere.

Using the daily atmosphere and ocean reanalysis data, this study shows that the SAM-induced meridional dipolar sea surface temperature (SST) anomalies, through surface heat fluxes, can maintain persistent lower tropospheric temperature anomalies, which further results in anomalous eddy momentum forcing enhancing the persistence of the SAM. With the Finite Amplitude Wave Activity diagnosis, we illustrate that response of the eddy momentum forcing to SST anomalies can be attributed to changes in both baroclinic processes as baroclinic eddy generation and barotropic processes as wave breaking thus resultant diffusive eddy mixing, with the former confined at high latitudes and the latter strongest at midlatitudes. Spectral analysis further suggests that the above air-sea interactions are important for bimonthly and longer time scale SAM variations. The dipolar SST pattern may be an indicator for predicting subseasonal and interseasonal variabilities of the SAM.