Presented at the 2013 SMOS-Aquarius Science WorkshopThe Indian Ocean Dipole (IOD) is the dominant mode of interannual climate variability in the equatorial Indian Ocean. It consists of a basin-scale modification of the upper ocean thermal structure, associated with drastic changes in the rainfall patterns. During a positive IOD year, a pool of warm water forms in the western half of the basin in fall, and triggers convective rainfall there. Conversely, during a negative IOD year, warm waters build up at the eastern edge of the basin off Indonesia, along with the convective rains. This basin-scale coupled oscillation is irregular in time, with typical occurrence periods of a few years.In fall 2010, a year after the launch of SMOS, a negative IOD event struck the Indian Ocean. A year later, in fall 2011, it was replaced by a positive IOD event. Our study investigates the signature of this oscillation in sea surface salinity (SSS), as revealed by SMOS level-3 products. Depending on the version of level-3 processing, the SSS difference between these contrasting years in the Indian Ocean may be the largest interannual signal captured by SMOS since its launch, consistently with available in situ observations. We then investigate the possible mechanisms responsible for the salinity anomalies, based on a suite of in situ and spaceborne observational datasets. Rainfall appears as the main driver of salinity anomalies. This opens up bright prospects for a routine spaceborne monitoring of SSS and related climate variability in the tropical oceans.