AN OGCM STUDY FOR THE TOGA DECADE - PART II - BARRIER-LAYER FORMATIONAND VARIABILITY

A set of OGCM experiments is used to investigate the processes respons ible for barrier layer (BL) formation in the Pacific Ocean. As in exis ting datasets, BL appears in the present experiments both in the weste rn Pacific (WP) and under the intertropical convergence zone (ITCZ). I n the WP, the BL displays a strong interannual variability linked to E NSO variability, in qualitative agreement with the observations of And o and McPhaden. In both the equatorial and 3 degrees-8 degrees S bands , a subduction process is responsible for BL formation. In the equator ial region, it results from a strong downwelling near the salinity fro nt created by convergence between central Pacific salty water and WP f reshwater In the southern region, the subduction of the South Equatori al Current salty water involves mainly mixed layer thinning due to the freshening of the surface layer by rain and equatorial divergence of water from the eastward fresh equatorial jets. The formation of BL und er the ITCZ is found to be mostly related to local precipitation. The impact of the BL presence is then investigated. The BL interannual var iability modifies the surface layer heat budget by switching on and of f the entrainment cooling. The haline stratification traps most of the wind stress in the surface layer of the fresh and warm pool and induc es strong eastward currents in response to westerly wind bursts (WWBs) . The overall effect of salinity stratification is to retain heat and momentum in the upper layer of the WP by restraining the exchanges wit h the cooler waters from below and from the central Pacific. The combi ned effect of zonal advection and mixing after a WWB results in an eas tward shift of the thick BL regions along the equator. These propertie s of the BL structure might favor the growth of unstable air-sea inter actions in the central Pacific after a WWB.