An OGCM Study for the TOGA Decade. Part I: Role of Salinity in the Physics of the Western Pacific Fresh Pool

Abstract

A set of numerical simulations of the tropical Pacific Ocean during the 1985?94 decade is used to investigate the effects of haline stratification on the low-frequency equilibrium of the Coupled Ocean?Atmosphere Response Experiment region. The simulated sea surface salinity structure is found to be quite sensitive to the freshwater forcing and to the other fluxes. Despite this sensitivity, several robust features are found in the model. Sensitivity experiments illustrate the important role of the haline stratification in the western Pacific. This stratification is the result of a balance between precipitations and entrainment of subsurface saltier water. It inhibits the downward penetration of turbulent kinetic energy. This results notably in a trapping of the westerly wind burst momentum in the surface layer, giving rise to strong fresh equatorial jets. The model is able to produce a barrier layer between 5°N and 10°S in the western Pacific and under the intertropical convergence zone (as in the Ando and McPhaden composites), but also around 10°S, 120°W, where there are no data to validate its presence. The barrier layer thickness in these regions is found to be sensitive to local water forcing, and its spatial structure is governed by the large-scale circulation. The heat budget of the upper-ocean mixed layer is analyzed in these barrier-layer regions. The Lukas and Lindström hypothesis that the surface heat flux should be near zero in these regions in order to maintain the weak temperature gradient between the mixed layer and the barrier layer does not seem necessary. A significant part of the solar heat flux is lost beneath the thin mixed layer, attenuating the heating of the surface layer and allowing barrier layer maintenance in the presence of a positive net heat flux. Conversely, the development of the barrier layer is associated with a dramatic decrease of the entrainment cooling, or even entrainment heating, especially near the equator. On the whole, the barrier layer seems to insulate the SST from the effects of atmospheric forcing.