Seasonal variations of sea-surface salinity and temperature in the tropical Indian Ocean

Measurements of sea-surface temperature and sea-surface salinity obtained from ships-of-opportunity are used to map the fields in the tropical Indian Ocean. The seasonal variation is described in detail along the six shipping tracks that have the best data coverage: Gulf of Aden to La Reunion island, Persian Gulf to Cape Town, Gulf of Aden to east Africa, Gulf of Aden to Indonesia, Sri Lanka to Torres Strait through Malacca Strait, and along the west coast of Australia. Seasonal variation with large amplitude is found in an extensive area in the western Indian Ocean. In the eastern Indian Ocean, seasonal variation is small, except where it is linked to local features such as coastal upwelling, local wind or rainfall-runoff. Water masses, defined from the surface T− S diagram, are related to winds and currents or are formed locally. The movement of these water masses is linked to currents driven by the monsoon circulation.