A linkage between subtropical SST dipole event in southern Indian Ocean and ENSO developing

Sea surface temperature (SST) anomalies in the Subtropical Southern Indian Ocean show interannual dipole events which are quite different from that of the near-equatorial Indian Ocean dipole events. Note that cold (warm) SST anomalies are found to the east of Australia, and warm (cold) SST anomalies are near the southern part of Madagascar based on the second mode of Empirical Orthogonal Function (EOF). A subtropical dipole index (SDI) is then proposed by the SST anomaly difference between the western (40-75E, 45-30S) and eastern (75-95E, 25-10S) subtropical Indian Ocean.

It is found that, in years associated with a La Nina (El Nino) event, the subtropical Southern Indian Ocean tends to have a positive (negative) dipole with warm (cold) SST anomalies in the western part and cold (warm) SST anomalies in the eastern part. A possible mechanism responsible for such cycle linkage between subtropical dipole and ENSO events might be related to both atmospheric processes (such as shifts in the walker circulation) and oceanic processes (such as transportation changes in the Indonesian throughflow).