Salinity anomaly as a brake on the positive Indian Ocean Dipole-Experimental assessments using a regional ocean model-

The Indian Ocean Dipole (IOD), which is a dominant climate mode in the tropical Indian Ocean, is accompanied by significant variation of the upper ocean salinity. By changing the density of seawater, these salinity anomalies alter pressure field and stratification, and thereby have potentials to affect the evolution of SST anomalies associated with the IOD. Though several previous studies have investigated the roles played by salinity in the IOD, no quantitative assessments have been conducted so far. To this end, a series of sensitivity experiments using a regional ocean model is conducted with a novel experimental strategy.

Results from these sensitivity experiments show that interannual salinity anomalies associated with the positive IOD tend to suppress the cooling in the eastern equatorial Indian Ocean by about 1.0°C. Through an on-line heat budget analysis of each experiment, it is shown that weakening of the upwelling in the eastern Indian Ocean, which originates from the upward shift of the Equatorial Undercurrent, is the primary cause of the suppressed cooling there. These changes are induced by enhanced stratification in the eastern equatorial Indian Ocean associated with negative (positive) salinity anomalies near the surface (pycnocline). Momentum budget analysis and additional sensitivity experiments using a linear continuously stratified ocean model further corroborate the above arguments. Our results suggest that salinity anomalies play an active role in the evolution of the IOD and act to damp its further amplification.