On the role of air-sea interactions in the persistence of the Southern Annular Mode

Abstract:

The Southern Annular Mode (SAM) is the dominant mode of Southern Hemisphere extratropical variability on intraseasonal time scales. The persistence of the SAM is suggested due to the positive eddy-zonal flow feedbacks. In this study, the influence of extratropical air-sea interactions in the eddy-mean flow interactions in the SAM is investigated by using the ERA-40 reanalysis data(1982-2001) ,GODAS ocean data and the OISST-V2 SST data. Our lagged regression analysis show that the poleward shift of the barotropic eddy driven jet induces a robust dipolar SST anomaly pattern with cold anomalies at high latitude around 60S and warm anomalies at midlatitude near 40S .This is mainly attributed to the anomalous Ekman heat transport and surface turbulent heat flux induced by the anomalous zonal wind in the SAM. Further analysis shows that this SST anomaly may have a positive feedback to the persistence of the SAM by maintaining the lower-tropospheric temperature anomalies which further results in anomalous eddy momentum forcing. Furthermore, our analysis suggests two possible mechanisms through which the lower-level temperature anomalies can affect the eddy momentum forcing. With a hybrid Eulerian-Lagrangian Finite Amplitude Wave Activity diagnostic method, it is found that the SAM induced SST anomaly can influence both the upper-layer eddy propagation and the low-troposphere eddy generation. Both of these two processes can lead to anomalous eddy momentum flux and eventually extend the persistence of the SAM.