Seasonal variations in the Indian Ocean along 110°E. I. Hydrological structure of the upper 500 m

Tropical and subtropical water masses at surface and subsurface depths were separated by their salinity, temperature, oxygen, and nutrient characteristics. The annual mean depths and latitudinal extent of these water masses were determined. Annual changes in the upper 50 m were generally so small relative to those found in other oceans that advection and mixing must have been less important in their genesis than local climatic changes. There was a barely significant seasonal rhythm in surface phosphate and nitrate, with peak occurrences of each some 6 months apart. At each latitude the permanent thermal discontinuity centred around a particular isotherm varied little in intensity during the year, but rose and fell in accordance with surface currents. The thermocline south of c. 18�S. varied little in depth but greatly in intensity during the summer. The depth of the mixed layer was much less in summer and at all times shallower in the tropics. The depth of this layer was governed more by the accumulation of surface waters by zonal currents and eddies, than by wind stress or convective overturn. Therefore there was little difference from south to north, or month to month, in average nutrient values of this mixed column. The movement of the various surface waters, deduced from salinity and temperature changes during the year, usually agrees with geostrophic currents across 110�E, and ships' observations of surface currents in the south-east Indian Ocean.