What drives the decadal variability in sea surface salinity and stratification over the tropical Indian Ocean?

The decadal variability in the surface salinity and stratification over the tropical Indian Ocean (TIO) is investigated for the period 1979–2017 using analysis and reanalysis products and ocean model simulations. The present study reveals that upper ocean (from surface to 50 m depth) stratification exhibits decadal and multidecadal variability in the eastern equatorial Indian Ocean (EEIO), southwestern tropical Indian Ocean (SWTIO), and head Bay of Bengal. The forced ocean model successfully simulated the decadal variability in stratification and is consistent with analysis and reanalysis products though with some underestimation of amplitude. The study further shows that upper ocean salinity has a stronger influence on the EEIO stratification variability whereas stratification over SWTIO is largely affected by the upper ocean temperature through Rossby wave propagation. The present study also highlights the growing role of equatorial currents in modulating the stratification over EEIO in the recent decades. The anomalous equatorial currents amplify the stratification variability along EEIO especially in the recent decades. Apart from the atmospheric and surface oceanic impact, subsurface ocean dynamics do impact stratification over EEIO and SWTIO regions by modulating subsurface temperature and salinity. Model sensitivity experiments reveal that decadal variability in both stratification and surface salinity over the head Bay of Bengal is modulated by river runoff. On the other hand, TIO stratification is largely unaffected by the Pacific Ocean pathways except for the southeastern Indian Ocean. Moreover, the climate modes, such as the decadal Indian Ocean Dipole and Interdecadal Pacific Oscillation, modulate stratification over the TIO through modulation of precipitation and evaporation.