Epochal variability in the Equatorial Indian Ocean subsurface temperature and the forcing mechanisms.

Tropical Indian Ocean (TIO) exhibits a robust warming since the 1950s. Most of the previous studies were focused on the long term trends and interannual variability of Indian Ocean (IO) surface and subsurface temperature. But the decadal, multidecadal and epochal changes in TIO subsurface temperature especially north of 10^oS and the underlying mechanisms are yet to be explored in detail. The long term trend and decadal variability in TIO temperature plays a major role on regional as well as global climate variability.The reanalysis products and ocean general circulation model (OGCM) simulations show a significant decadal to multi decadal subsurface temperature variability in the Eastern Equatorial Indian Ocean (EEIO, 80°E-110°E, 8°S-5°N, 50-110m). Prior to the 1990s, EEIO subsurface temperature was highly dominated by the multidecadal variability, whereas the decadal variability is found to be dominating after the mid 1990s. It is also observed that the decadal variability in EEIO is superimposed by significant epochal variability with epoch-1 (1958-1980) experiencing strong subsurface cooling and a significant warming in epoch-2 (1981-2014). The wind forcing along the equator (EIO wind) and the associated ocean dynamics are found to be the major factors responsible for the EEIO subsurface temperature epochal and decadal variability. Alongshore winds and coastal welling along the Sumatra coast also contribute considerably to the decadal changes in the EEIO subsurface temperature. It is further evident from the OGCM sensitivity experiments that EEIO subsurface variability is unaffected by the oceanic pathways such as Indonesian Through Flow (ITF) and such remote forcing is insignificant in the TIO subsurface temperature north of 10^oS. Consistently with subsurface EEIO temperature, EIO winds undergo a multidecadal to decadal shift in the recent decades. Our further analysis suggests that the EIO winds are largely influenced by the Equatorial Pacific winds prior to the 1990s whereas posterior to the mid 1990s the relationship between equatorial winds over the two basins has been weakened. The above study clearly indicates that the recent decade changes in TIO temperature and winds are largely influenced by the local forcing rather than the remote forcing.