Multidecadal to decadal variability in the equatorial Indian Ocean subsurface temperature and the forcing mechanisms

The Tropical Indian Ocean (TIO) is seen to exhibit robust warming after the 1950s. Most of the previous studies on the Indian Ocean (IO) surface and subsurface temperature focussed on long-term trends and interannual variability. The multidecadal to decadal variability in the Eastern Equatorial Indian Ocean (EEIO, 80 degrees E-110 degrees E, 8 degrees S-5 degrees N, 50-110 m) subsurface temperature is studied using reanalysis products and ocean general circulation model (OGCM) simulations. Prior to 1990s, the EEIO subsurface temperature is found to be dominated mostly by the multidecadal variability. However, since the mid-1990s, significant decadal variability is observed, something that has not been reported before. The mechanisms responsible for the observed decadal and multidecadal variability in the EEIO subsurface temperature are studied in detail. Wind forcing along the equator, and the associated ocean dynamics are found to be the major factors responsible for the EEIO subsurface temperature variability in the decadal and multidecadal time scales, emphasizing the importance of local atmospheric forcing on this variability. Alongshore winds along the Sumatra coast also contribute considerably to the decadal changes in the EEIO subsurface temperature. It is also found that the observed decadal and multidecadal changes are independent of the phase reversal of Interdecadal pacific oscillation (IPO). OGCM sensitivity experiments also suggest that the decadal variability of EEIO subsurface temperature is largely unaffected by the oceanic pathways such as Indonesian Through Flow (ITF). Hence, remote forcing through these pathways is insignificant in the TIO subsurface temperature north of 10 degrees S as compared to the TIO wind forcing. Consistent with EEIO subsurface temperature, equatorial IO (EIO) winds also exhibit decadal variability post mid-1990s. The major contributors for the decadal variability in EIO winds are found to be the decadal variability in the Mascarene high, the low level jet, and the large scale off-equatorial circulations.