A sea surface salinity dipole mode in the tropical Indian Ocean

Based on the ten-year sea surface salinity (SSS) data from Argo, we identified a salinity dipole mode in the tropical Indian Ocean, termed S-IOD: a pattern of interannual SSS variability with anomalously low-salinity in the central equatorial and high-salinity in the southeastern tropical Indian Ocean (IO). The S-IOD matures in November-December, lagging the Indian Ocean dipole (IOD) mode derived from sea surface temperature (SST) by two months. For the period of observation, the S-IOD persists longer than the IOD, untill the following September-October. Oscillations of the two S-IOD poles are governed by different processes. Ocean advection associated with the equatorial current variability dominates the SSS anomalies of the northern pole, while surface freshwater flux variability plays a key role in the SSS anomalies of the southern pole, where anomalous precipitation is sustained by preformed sea surface temperature anomalies. The S-IOD concurs with the IOD, reflecting an ocean-atmosphere coupling through the SST-precipitation-SSS feedback.