Tropical Oceanic Intraseasonal Variabilities associated with Central Indian Ocean Mode

The oceanic intraseasonal variabilities (ISVs) are pronounced over the tropical Indian Ocean. Recently, the Central Indian Ocean (CIO) mode has been proposed as an intrinsic ocean-atmosphere coupled mode at intraseasonal timescales. It has a close relation with northward-propagating ISVs and intraseasonal precipitation during the Indian summer monsoon. In this study, the dynamics of tropical oceanic ISVs associated with the CIO mode are analyzed using reanalysis products and observations. A complete heat budget analysis shows that intraseasonal SST anomalies which propagate westward from eastern to central Indian Ocean in the tropics during the CIO mode are mainly attributable to zonal temperature advection. The component due to surface heat flux is the second largest contributor. This is distinct from the traditional tropical oceanic ISVs as a response to the Madden-Julian Oscillation (MJO), in which surface heat flux is usually the dominant component. In addition, the mixed layer depth (MLD) over central Indian Ocean is influenced by upwelling and downwelling Rossby waves and the perturbations in MLD propagate eastward from the central to the eastern Indian Ocean as equatorial Kelvin waves. Overall, a three-dimensional circulation system in the tropics is identified. Current results along with previously reported atmospheric dynamics during the CIO mode depict a framework for the ocean-atmosphere coupled mode over the tropical Indian Ocean which is expected to advance a more comprehensive understanding of tropical ISVs and will ultimately contribute to the improvement in process studies, simulations, and forecasts of the Indian summer monsoon.