Water Masses at the Surface of the Indian Ocean

Water masses in the surface layer of the ocean are harder to define in terms of hydrographic properties because they undergo large seasonal and interannual variations. Therefore, single TS-combinations cannot be used to define surface water masses. The solution to the definition of surface water masses is the temperature/salinity time diagram (TS-time diagram) and/or the temperature/chlorophyll time diagram (TC-time diagram). This work establishes a systematic definition of regional water masses in the surface layer of the tropical Indian Ocean based on TC-time and TS-time diagrams of mean and standard deviation for 5° squares. This classification is based on an analysis of the ratios between semi-annual and annual amplitudes of temperature and a multiple regression analysis between temperature, salinity, and chlorophyll, with Ekman pumping, components of pseudo-stress wind, wind magnitude and net-down-freshwater-flux. Four water masses are identified for the surface of the Indian Ocean: The Arabian Sea Surface Water (ASSW), the Bay of Bengal Surface Water (BBSW), the Equatorial Indian Surface Water (EISW) and the Southern Indian Surface Water (SISW). The geographical representation of the water masses agrees well with the theoretical frame of reference.