On the Impact of Salinity Barrier Layer on the Pacific Ocean Mean State and ENSO

Observational studies of the western Pacific Ocean have suggested since the mid-1980s that the barrier layer resulting from the salinity stratification within the mixed layer could influence significantly the ocean-atmosphere interactions. Numerical experiments based on a CGCM are designed and analyzed in such a goal. The formation of the barrier layer is primarily identified as resulting from a tilting/shearing mechanism in which horizontal and vertical gradients of salinity, as well as the dynamical response of the ocean to westerly winds, are tightly coupled. When the contribution of salinity to the computation of the horizontal pressure gradient force in the ocean model is cancelled within the equatorial warm pool, both the mean climatology and the low frequency variability are affected as the result of a complete annihilation of the barrier layer. The decreased sensitivity of the coupling between the SST, winds and atmospheric deep convection is likely due to the deepening of the ocean mixed layer that cools the SST and weakens the amplitude of its variability. These local changes within the western Pacific warm pool also induce a basin scale response that weakens the amplitude of ENSO variability. These results suggest that the formation of the barrier layer is a key element of the whole Pacific ocean-atmosphere coupled system.