Unexpectedly strong sub-seasonal SST variability over the tropical Indian Ocean has been observed by the TRMM microwave imager, which provides unprecedented tropical SST information in the presence of clouds. In January--February 1999 SST averaged over an area 20¿ longitude by 10¿ latitude cooled by ~1.5 ¿C over 10 days and warmed by ~1.25 ¿C in the following week. Local changes were up to ¿3 ¿C. There were strong variations in atmospheric convection and surface wind stress associated with the SST variability. The evolution of the convection suggests a feedback from the SST changes into atmospheric convection. Net air-sea heat-flux cannot account for the observed cooling; oceanic processes are fundamental to this event. The roles of such sub-seasonal ocean-atmosphere interactions in the evolution of the Madden-Julian Oscillation, the seasonal and the interannual variability of Indian Ocean SST deserve further investigation. ¿ 2001 American Geophysical Union