Comparisons between isothermal depth to the top of the thermocline, and the mixed layer depth based on a ot criterion were undertaken for the tropical world oceans. In three equatorial regions, a shallower mixed layer than isothermal layer occurs, implying the presence of a strong halocline above the thermocline. This distance separating the top of the thermocline and the bottom of the mixed layer is referred to as the "barrier layer", in relation to its impediment to vertical heat flux out of the base of the mixed layer. Different mechanisms are responsible for maintaining the barrier layer in each of the three regions. In the western equatorial Pacific Ocean a salinity budget confirmed that heavy local precipitation most likely results in the isothermal but salt-stratified layer. In the northwest equatorial Atlantic, it is hypothesized that high salinity waters are subducted at the subtropics during winter and advected westward as a salinity maximum in the upper layers of the tropics, resulting in the barrier layer. In the eastern equatorial Indian Ocean, monsoonal related rainfall and river runoff contribute significantly to the freshwater flux, producing salt stratification in the surface. These results suggest the need to include the effects of salinity stratification when determining mixed layer depth.

A method of water mass analysis is developed which optimizes the use of all information contained in a hydrographic data set by solving an overdetermined linear set of mixing equations. It requires representation of water masses by a finite number of water types and observations from at least as many oceanographic parameters as water types. The procedure to establish the water types, find the correct weights for the parameters, and solve the mixing equations through minimization of the residuals is described in detail. The method is applied to a data set from the eastern Indian Ocean, where Indian Central Water and Australasian Mediterranean Water meet to produce a strong front in the thermocline. It is shown that although most of the water from the Indonesian archipelago is carried westward north of 11°S, contributions from the Australasian Mediterranean Water are found as far south as 25°S, particularly along the Australian shelf as a result of advection with the Leeuwin Current.

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