The tropical Indian Ocean has been warming at an alarming rate in the recent decade with the southern tropical Indian Ocean (STIO; 15oS-35oS) being the hotspot storing more than 40% of the heat absorbed by the Indian Ocean since the 1990s. This also coincides with the “global warming hiatus” marked by an increase of Pacific warm water transport into the Indian Ocean through the Indonesian throughflow (ITF). However, the cause for the basin-wide spread of this warm water has remained unknown. Using observations and reanalysis, we identify the positive phase of the Southern Annular mode and intensification of extratropical westerlies caused by the Southern Ocean warming as the major drivers. We show that westerlies over the Southern Ocean create a strong positive curl and thereby generate westward propagating Rossby waves. Consequently, STIO exhibits a rapid increase in the sea level (4 mm/yr) and a deepening of the thermocline. This leads to an anomalous east-west tilt in the thermocline which drives the spread of warm water into the interior of the basin. Such Southern Ocean forcing of the tropical Indian Ocean assumes a new significance since the Indian Ocean warming is now known to influence the Pacific Ocean response to global warming and accelerate the Atlantic Meridional Overturning Circulation.