AN OGCM STUDY FOR THE TOGA DECADE - PART I - ROLE OF SALINITY IN THE PHYSICS OF THE WESTERN PACIFIC FRESH POOL

A set of numerical simulations of the tropical Pacific Ocean during th e 1985-94 decade is used to investigate the effects of haline stratifi cation on the low-frequency equilibrium of the Coupled Ocean-Atmospher e Response Experiment region. The simulated sea surface salinity struc ture is found to be quite sensitive to the freshwater forcing and to t he 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 stratif ication is the result of a balance between precipitations and entrainm ent 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 str ong fresh equatorial jets. The model is able to produce a barrier laye r between 5 degrees N and 10 degrees S in the western Pacific and unde r the intertropical convergence zone (as in the Ando and McPhaden comp osites), but also around 10 degrees S, 120 degrees 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 spat ial structure is governed by the large-scale circulation. The heat bud get of the upper-ocean mixed layer is analyzed in these barrier-layer regions. The Lukas and Lindstrom hypothesis that the surface heat flux should be near zero in these regions in order to maintain the weak te mperature 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 l ayer and allowing barrier layer maintenance in the presence of a posit ive net heat flux. Conversely, the development of the barrier layer is associated with a dramatic decrease of the entrainment cooling, or ev en entrainment heating, especially near the equator. On the whole, the barrier layer seems to insulate the SST from the effects of atmospher ic forcing.