Interannual subsurface variability in the tropical Indian Ocean with a special emphasis on the Indian Ocean Dipole

Interannual variability in the subsurface tropical Indian Ocean (TIO) is studied using three independent data sets: satellite derived sea-level data, an ocean general circulation model simulation, and in situ upper-ocean temperature data. It is found that significant interannual variability in the TIO is confined to the north of 15°S. Unlike the Pacific Ocean, the dominant modes of interannual variability in the Indian Ocean do not show co-variability between the surface and the subsurface. In contrast to the sea-surface temperature variability dominated by the El Niٌo and Southern Oscillation, subsurface variability is governed by the Indian Ocean Dipole in the TIO. The dominant mode of the interannual variability in the subsurface TIO is characterized by a dipole. Its evolution is controlled by equatorial ocean dynamics forced by zonal winds in the equatorial region. The subsurface dipole provides the delayed time required to reverse the phase of the surface dipole in the following year. The second dominant mode of interannual variability shows the interesting quasi-biennial tendency. It is found that the turnabout of the phase of the subsurface dipole leads to the quasi-biennial behavior of the TIO. Analysis of in situ subsurface temperature data substantiates this finding.