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.
Contributions of ENSO and East Indian Ocean SSTA to the Interannual Variability of Northwest Pacific Tropical Cyclone Frequency*