Characteristics of atmospheric forcing and SST cooling events in the Gulf of Mannar during winter monsoon

Abstract

This work addresses the analysis of winter monsoon forcing and sea surface temperature (SST) cooling events in the Gulf of Mannar, which is situated between the southeast of the Indian tip and northwest of Sri Lanka, using a 7-year data set derived from satellite sensors. The surface forcing consists of wind stress and turbulent heat flux, which were estimated through the TOGA/COARE algorithm using Special Sensor Microwave Imager (SSM/I) wind, Advanced Very High Resolution Radiometer (AVHRR) SST, and surface atmospheric conditions derived from National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalyses data. Net heat flux was derived by combining the turbulent heat flux with net short- and long-wave radiation from the NCEP/NCAR reanalysis data set. SST cooling was monitored by using 9-km spatial resolution pathfinder SST that was derived from the AVHRR. The 7-year weekly mean maps of wind stress, net heat flux, and SST indicate that SST cooling is locally influenced by surface forcing in- and offshore of the Gulf of Mannar. A time series of these parameters near the Indian tip reveal that the strong wind stress and high surface heat loss, which are punctuated in time varying from 15 days to more than a month, occur every winter and lower SST by ∼1.5°C. The occurrence of such a phenomenon is referred to as an event in this work. The bell-shaped events, which have a periodicity of 15 days with a maximum wind stress around Day 8, occur every winter and are characterized by the SST cooling of less than a degree. The features of these events are studied by using the statistical correlation and composite technique. It is inferred that the SST cooling is strongly correlated with the surface forcing.