Coherent evidence from Aquarius and Argo for the existence of a shallow low-salinity convergence zone beneath the Pacific ITCZ

Abstract

Aquarius observations feature a prominent zonal sea-surface salinity (SSS) front that extends across the tropical Pacific between 2–10°N. By linking to Argo subsurface salinity observations and satellite-derived surface forcing datasets, the study discovered that the SSS front is not a stand-alone feature; it is in fact the surface manifestation of a low-salinity convergence zone (LSCZ) located within 100 m of the upper ocean. The near-surface salinity budget analysis suggested that, although the LSCZ is sourced from the rainfall in the Inter-tropical convergence zone (ITCZ), its generation and maintenance are governed by the wind-driven Ekman dynamics, not the surface evaporation-minus-precipitation flux. Three distinct features highlight the relationship between the oceanic LSCZ and the atmospheric ITCZ. First, the seasonal movement of the LSCZ is characterized by a monotonic northward displacement starting from the near-equatorial latitudes in boreal spring, unlike the ITCZ that is known for its seasonal north-south displacement. Second, the lowest SSS waters in the LSCZ are locked to the northern edge of the Ekman salt convergence throughout the year, but have no fixed relationship with the ITCZ rain band. Collocation between the LSCZ and ITCZ occurs only during August-October, the time that the ITCZ rain band coincides with the Ekman convergence zone. Lastly, the SSS front couples with the Ekman convergence zone but not the ITCZ. The evidence reinforces the findings of the study that the Ekman processes are the leading mechanism of the oceanic LSCZ and the SSS front is the surface manifestation of the LSCZ.