Abstract:Based on the Hadley Center monthly sea surface temperature data,the NCEP/NCAR monthly sea level pressure data,sea surface wind data and the vertical velocity data at 500 hPa,the characteristics of Southern Indian Ocean Dipole(SIOD) variation and its relationship with ENSO are studied.The results showed that the SIOD is characterized by occurrence of a cold(warm) SST anomaly in the northeast Indian Ocean and a warm(cold) SST anomaly in the southwest,which are seasonal phase-locked.It develops from October to December,reaches a climax in the following January to March,and decays from April to June.The formation mechanism of SIOD is affected by the large-scale atmospheric circulation.The location and intensity variations of Mascarene high and Australian High(Low) contribute to the sea surface wind anomalies,which results in SST anomalies and then leads to the SIOD.There is a close relationship between the sea surface temperature in the austral subtropical Indian Ocean and that in the equatorial central eastern Pacific Ocean.The SIOD event is significantly negatively correlated with ENSO which occur in the previous winter.Most of the positive SIOD events occur after La Niña events,whereas most of the negative SIOD events occur after El Niño events.In some cases,SIOD appears after neither of the two events.The anomalous vertical motion and anomalous tropical zonal wind,which are induced by ENSO,can affect the intensity and location of sea surface pressure and sea surface wind fields in the austral subtropical Indian Ocean,and can further influence the evolution of the eastern and western poles of SIOD,respectively.Furthermore,SIOD can also occur in isolation,and the occurrence of the negative SIOD events is usually one month earlier than that of the positive SIOD events.The anomalous sea surface temperature cannot develop continuously without the effect of ENSO,which leads to a short life span of the SIOD.