SST variability in the eastern intertropical Indian Ocean: On the search for trigger mechanisms of IOD events

This study focusses on the variability of sea surface temperature in the eastern intertropical Indian Ocean off the coasts of Sumatra and Java - a region that is central to the Indian Ocean Dipole (IOD). Using multiplatform satellite and ARGO data, here we show that the appearance of vast areas of anomalously cold surface water off the southwest Sumatran coast during positive IOD (pIOD) events is preconditioned by upwelling favourable winds along Sumatra's central west coast (1–3°S). A simple theory of coastal upwelling along coasts of finite horizontal extent, developed in this work, can explain this SST response. While this theory is consistent with the pIOD events of the years 1994, 1997 and 2006, it uncovers the existence of another strong upwelling event in the year 2011 that is missed when calculating average SST anomalies in the wider region (i.e. the Dipole Mode Index). In addition, we also derived the surface circulation establishing in the study region during pIOD events. This circulation pattern indicates that equatorward transport of colder water by a classical upwelling jet (that we call the South West Sumatra Current) reinforces negative (cold) SST anomalies off southwest Sumatra.