Ocean–atmosphere coupled processes in the tropical Indian Ocean region prior to Indian summer monsoon onset over Kerala

Abstract

The present study examines atmosphere–ocean interaction before MOK using various observational data sets during the last 35 years (1982–2016). The analyses suggest a new mechanism for the evolution of MOK involving the coupling of radiation, sea surface temperature (SST), wind, evaporation, SST gradient, wind stress, total precipitable water (TPW), and convection processes. During the pre-monsoon period, Arabian Sea (ArS) starts warming and reaches a maximum just four pentads before MOK. Hence, a meridional SST gradient develops that pulls air from the colder southern Indian Ocean, which leads to enhanced wind speed and thus wind stress towards the warmer region. This is followed by the cooling of SST over southwestern ArS due to the wind-induced evaporation and upwelling along Somali coast in response to the enhanced wind stress curl. The low-level anticyclonic circulation over north ArS also aids this Somali coast upwelling, which builds up a zonal SST gradient between East and West ArS. In response, surface wind speed enhances further. The north-eastward directed gradient drives the low-level wind north-eastward towards the eastern ArS. The increase in evaporation enhances moisture content of the air which is carried by low-level wind and accumulates in a column of high TPW near the Kerala coast and adjoining south-east ArS. That moist air column fosters rainfall and thus monsoon onset over Kerala. In short, ocean warming by short-wave radiation during pre-monsoon sets the background, then the ocean surface provides the necessary energy to the atmosphere in the form of latent heat flux in response to wind driven by the developing SST gradient. Significant inter-annual correlations are found between wind stress, TPW and precipitation over the North Indian Ocean during this period, supporting the interpretation detailed above.