Evidence for the origin of the subsurface salinity maximum in the subtropical North Atlantic

Abstract

The subtropical North Atlantic exhibits the saltiest surface waters of the open ocean, the sea surface salinity maximum (sss-max). Eventually that water is exported towards the Equator as a subsurface salinity maximum (s-max), forming the lower limb of the shallow overturning circulation (SOC). Climatologically the sss-max in the winter, coinciding with the deepest mixed layer, is too salty and cold compared to the s-max. Towed CTD measurements during the field program SPURS (March/April 2013) within the center of the sss-max reveal several fresh and warm anomalies, which deviate strongly from the climatological conditions in the sss-max. Source regions for the anomalies are found in the south and west, collocated to areas of elevated Eddy Kinetic Energy and strong lateral gradients in salinity. Regional model output produces an abundance of fresh features similar in space and time scales. It is hypothesized that a surface layer rich in meso-scale activity, in an otherwise very quiescent mean flow is predominantly responsible for setting mixed layer characteristics in the sss-max, which translate to s-max characteristics, hence influence the variability of the SOC