Quantitative relationships of near-surface salinity variations and their causes in an area of the western tropical Atlantic

Abstract

The vertical salinity distribution in the upper strata of the ocean area near Fernando de Noronha is characterized by a nearly homohaline surface, layer of about 60 m depth and a salinity maximum at about 75 m depth followed by a strong decrease of salinity with depth. The annual march of salinity in the upper mixed layer reveals a maximum between November, and December and a minimum in May. Although the evaporation minus precipitation difference in this area has a annual range of 13.2 cm/month, the annual range of salinity remains relatively small, that is 0.33‰. A quantitative analysis of mean monthly salinity variations suggests that horizontal advection in the upper mixed layer and vertical mixing at the lower boundary of this layer are the most important “forcing functions” for the observed annual march, of salinity along with evaporation and precipitation. In this area evaporation exceeds precipitation by as much as 42 cm/year, which leads to an increase of salinity in the upper layer during the year. Moreover, this increase is augmented by an additional salt gain as a result of vertical mixing at the lower boundary of the top layer. In order to maintain a constant mean annual salinity in this region, other factors must be involved that balance this salt gain. The significant role of horizontal advection of water with lower salinity in the annual balance is demonstrated, and the combined effects of the three forcing functions yield a fair explanation, of the observed mean salinity variation during the year.