Sea-surface salinity fronts and associated salinity-minimum zones in the tropical ocean

Abstract

The Intertropical Convergence Zone (ITCZ) is a major source of the surface freshwater input to the tropical open ocean. Under the ITCZ, sea-surface salinity (SSS) fronts that extend zonally across the basins are observed by the Aquarius/SAC-D mission and Argo floats. This study examined the evolution and forcing mechanisms of the SSS fronts. It is found that, although the SSS fronts are sourced from the ITCZ-freshened surface waters, the formation, structure, and propagation of these fronts are governed by the trade wind driven Ekman processes. Three features characterize the governing role of Ekman forcing. First, the SSS fronts are associated with near-surface salinity-minimum zones (SMZs) of 50–80 m deep. The SMZs are formed during December–March when the near-equatorial Ekman convergence zone concurs with an equatorward displaced ITCZ. Second, after the formation, the SMZs are carried poleward away at a speed of ∼3.5 km d−1 by Ekman transport. The monotonic poleward propagation is a sharp contrast to the seasonal north/south oscillation of the ITCZ. Lastly, each SMZ lasts about 12–15 months until dissipated at latitudes beyond 10°N/S. The persistence of more than 1 calendar year allows two SMZs to coexist during the formation season (December–March), with the newly formed SMZ located near the equator while the SMZ that is formed in the previous year located near the latitudes of 10–15° poleward after 1 year's propagation. The contrast between the ITCZ and SMZ highlights the dominance of Ekman dynamics on the relationship between the SSS and the ocean water cycle.