Salinity variability in the surface layer of the tropical western Pacific Ocean

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Abstract
Observations of surface layer salinity and temperature, rainfall, wind and upper ocean currents from two surveys during the Tropical Ocean Global Atmosphere‐Coupled Ocean‐Atmosphere Response Experiment are presented. The surveys covered a region of 25×25 n. mi (46.3×46.3 km) with hydrographic sections of 5 n. mi (9.3 km) separation, using a Seasoar towed conductivity‐temperature‐depth profiler. One survey was dominated by the presence of several rain‐induced patches of low‐salinity water with slightly reduced temperature. These patches extended to the bottom of the mixed layer, which at the time, was 20–30 m deep, and appeared to be the result of convection driven by their reduced temperature. Below the mixed layer was a 15 to 20 m thick barrier layer. By distributing the freshwater over the mixed layer almost instantly, the convecting patches appeared to contribute to the maintenance of the barrier layer. This survey is typical for conditions found during most of the observation period. The second survey was dominated by a salinity front and advection of low‐salinity water with the large‐scale westerly wind that prevailed at the time. A single event of very heavy rain produced a shallow lens of freshened water; but the reduction in salinity was so strong that the associated cooling was insufficient to initiate convection. No barrier layer was seen ahead of the front but the observations indicate the development of a barrier layer after the passage of the front. The work demonstrates the difficulties in establishing a freshwater budget for the tropical ocean. Ship borne rain gauge measurements, even though becoming more accurate, seem not to be representative for an area much beyond a few kilometers or less. Sometimes the surface salinity distribution shows better visual correlation with the wind gust pattern than with the rainfall distribution, indicating that the wind gusts associated with atmospheric convection cells are less intermittent and more coherent in space than the rain.

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