Basinwide connections of upper-ocean temperature variability in the equatorial indian ocean

Abstract

In this article, the interannual variability of upper-ocean temperature in the equatorial Indian Ocean (IO) and its basinwide connections are investigated using 58-yr (1958-2015) comprehensive monthly mean ocean reanalysis data. Three leadingmodes of an empirical orthogonal function (EOF) analysis dominate the variability of upper-ocean temperature in the equatorial IO over a wide range of time scales. A coherent interannual band within the first two EOF modes identifies an oscillation between the zonally tilting thermocline across the equatorial IOin its peak phases and basinwide displacement of the equatorial thermocline in its transitional phases. Consistent with the recharge oscillation paradigm, this oscillation is inherent in the equatorial IO with a quasi-periodicity around 15 months, in which the wind-induced off-equatorial Rossby waves near 58-108S provide the phase-transition mechanism. This intrinsic IO oscillation provides the biennial component in the observed IOD variations. The third leadingmode shows a nonlinear long-term trend of the upper-ocean temperature, including the nearsurface warming along the equatorial IndianOcean, accompanied by cooling trend in the lower thermocline originating farther south. Such vertical contrary trends may lead to an enhanced stratification in the equatorial IO.