Salinity Variability and Its Role in the Barrier-Layer Formation during TOGA-COARE

Abstract

During the intensive observation period of TOGA-COARE between November 1992 and February 1993, two R/V Franklin cruises, FR09/92 and FR01/93, were carried out to study the response of oceanic surface layer temperature and salinity to atmospheric inputs such as wind, radiation, precipitation, etc. A total of seven Seasonar surveys (each has a square of about 50 km) were designed and performed to monitor the variations in oceanic heat and freshwater content. This paper focuses on the observed salinity variation in spatial and time scales and its association with the barrier layer production. During the first and second 1992 surveys, a salinity front was observed. The crossing-front salinity difference was at least 0.16 from the surface down to below 40 m, implying a freshwater influx of about 20 cm. MIT Doppler radar located on R/V Vickers showed very heavy and persistent storm rains covered a very large area of a few hundred square kilometers west of the surveys for two days before the surveys started, giving a possible source of that much freshwater intake by the ocean. The low salinity water pool formed by the storm rains then moved eastward under a relatively weak westerly wind and a wide eastward ocean current. The salinity front of the pool was observed during the first survey and again observed during the second survey, which was estimated to travel in the southeeast direction with a speed of 20 cm s?1. ADCP shear data show an eastward flow in the top 40 m of the front but a reverse flow in the bottom of the front, suggesting that the strong salinity stratification in the lower isothermal layer is most likely caused by the shears. It is this eastward-moving low salinity water in the upper level of the front over the high salinity water moving westward in the bottom of the front that is responsible for the production an observed barrier layer.at least 30 m thick, in which salinity changed 0.2?0.3 in the vertical, while vertical temperature changed only 0.1°?0.2°C for the same depth range. However, the situation was very different during one of the 1993 surveys, on which occasion the westerly wind was stronger with a speed of 15 knots on average and the rain-formed low salinity water pools penetrated deep1y. Between the bottom of the pools and of the isothermal layer there was a strong salinity stratified layer with a vertical salinity change of 0.1?0.15 and a vertical temperature change of 0.1°C. A barrier layer about 10 m thick was formed. This gives an example that direct rainfall under strong wind forcing can also generate a barrier layer, but one much thinner than the one produced by advection. The reason is probably that with an existing mixed layer, rain showers coming into the surface can easily reach the bottom of a mixed layer under turbulent mixing and entrainment, while the thickness of a barrier layer is proportional to the amount of rain input.