Mixed-Layer Salinity Budget in the SPURS Region on Seasonal to Interannual Time Scales and Their link to the Lager Scale Dynamics

Previous studies suggested that regions with high and low salinity are better indicator for global water cycle changes. Understanding the physical processes governing the salinity changes in those regions is particularly important. Surface salinity variations and processes affecting surface salinity in the SPURS region, i.e. the high salinity region of the subtropical North Atlantic, are investigated combining data from in situ observations and satellite remote sensing measurements. On time-average, the freshwater flux term (evaporation minus precipitation) in the SPURS region is positive, i.e., increase mixed-layer salinity. The oceanic advection plays the largest role in compensating the salinity increase from the freshwater flux. On seasonal time scale, the mixed-layer salinity increases from April to August and decreases from September to March. This seasonal evolution of the mixed-layer salinity is largely controlled by the air-sea freshwater flux term, with vertical entrainment plays a secondary role. Whereas, the domain-averaged oceanic advection and diffusion terms do not show significant seasonal cycle. The domain-averaged salinity variations on interannual time scale are largely captured by the sum of all terms. Different from the seasonal cycle, the interannual variations in the freshwater flux, oceanic advection, and vertical entrainment all contribute to the interannual variations in the surface salinity. The oceanic advection plays a larger role in the salinity changes during 2008-2012, whereas the surface freshwater flux term dominates the surface salinity evolution during 2004-2007 and in 2013. Separating the advection term into the geostrophic and Ekman components indicates that the Ekman component dominates the total advection term. Although the mean freshwater flux is dominated by the evaporation in the SPURS region, the interannual variations in the freshwater flux term mainly come from changes in the precipitation. The possible link of the salinity in the SPURS with the large-scale circulation is investigated.