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Yu, L. (16-Dec-16)

Studies of recent years have led to an increasing recognition that the evaporation-minus-precipitation (E-P) flux and SSS are not directly linked but through oceanic processes. Two fundamental characteristics define the complex relationship between SSS and E-P. First, E-P is a mass flux and does not stay locally. For instance, when rain adds to the mass of the water column, it causes a pressure perturbation and fast oceanic responses in terms of gravity waves and barotropic Rossby waves. Secondly, E-P does not have a feedback relationship with SSS. This is in stark contrast to the surface heat flux which serves as both forcing and damping mechanisms for SST. E-P forces SSS anomalies but does not dampen them. As a consequence, SSS anomalies tend to be carried out by oceanic processes and circulate around for a period of time. Here we present the analysis of the ocean dynamical control on E-P generated SSS anomalies in two contrasting regimes. One is the tropical salinity-minimum (Smin) zone where P dominates E. We found that the oceanic Ekman transport and convergence govern the maintenance and distribution of the rain-freshened surface waters. The other regime is the subtropical salinity-maximum (Smax) zone where E dominates P. We found that the vertical turbulent entrainment across the base of the mixed layer plays a leading role in Smax variability. By using a mixed-layer salinity model that is forced by satellite-derived surface wind and E-P forcing products, we diagnosed the balance between E-P and the oceanic processes and investigated the issues regarding the pattern of Smax changes, the role of E-P versus the subsurface processes, and the mechanism governing the seasonal and long-term Smax changes. In particular, our model experiments show that the oceanic processes could either enhance or erase the SSS anomalies generated by E-P.