An OGCM Study for the TOGA Decade. Part II: Barrier-Layer Formation and Variability

Abstract

A set of OGCM experiments is used to investigate the processes responsible for barrier layer (BL) formation in the Pacific Ocean. As in existing datasets, BL appears in the present experiments both in the western Pacific (WP) and under the intertropical convergence zone (ITCZ). In the WP, the BL displays a strong interannual variability linked to ENSO variability, in qualitative agreement with the observations of Ando and McPhaden. In both the equatorial and 3°?8°S bands, a subduction process is responsible for BL formation. In the equatorial region, it results from a strong downwelling near the salinity front created by convergence between central Pacific salty water and WP freshwater. In the southern region, the subduction of the South Equatorial 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 under the ITCZ is found to be mostly related to local precipitation. The impact of the BL presence is then investigated. The BL interannual variability modifies the surface layer heat budget by switching on and off the entrainment cooling. The haline stratification traps most of the wind stress in the surface layer of the fresh and warm pool and induces 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 with the cooler waters from below and from the central Pacific. The combined effect of zonal advection and mixing after a WWB results in an eastward shift of the thick BL regions along the equator. These properties of the BL structure might favor the growth of unstable air?sea interactions in the central Pacific after a WWB.