Further studies of plankton ecosystems in the eastern Indian Ocean. II. Seasonal variations in water mass distribution (upper 150 m) along 110°E. (August 1962-August 1963)

From temperature-salinity relations, with approximate corrections for localized heat exchange, and from evaporation-precipitation records, in an upper mixed layer of 50 m (tropical) and of 75 m (subtropical region), the variations throughout the year of the principal surface and subsurface water masses along 110°E. have been examined. In winter-spring the west-flowing South Equatorial Current and the east-drifting subtropical waters maintain a tropical convergence around 20°S. Doming of deeper tropical waters creates a nutrient-enriched near-surface zone to the north of this South Equatorial Current. In summer-autumn the east-flowing Monsoon Current creates a more sluggish uplift of deeper waters around 20°. These deeper waters, however, contain little of the deeper nutrient-rich tropical waters. Around 30-35°S. nutrient-richer deeper water is carried into near-surface layers by doming within cyclonic eddies or by horizontal entrainment from the south. A number of other surface water bodies of tropical origin occur irregularly throughout the year at the northern end of the 110°E. section.