A North-South Contrast of Subsurface Salinity Anomalies in the Northwestern Pacific From 2002 to 2013

Abstract

This paper finds a north‐south contrast of subsurface salinity trend during 2002–2013 in the northwestern Pacific. Both Argo float data and long‐term repeat hydrographic measurements along the 137°E section show that salinity anomalies along the isopycnals of 24.5–25.4 kg/m3 exhibit a pronounced decreasing trend north of 15°N and an increasing trend south of 15°N. We perform a quantitative analysis based on satellite‐derived data and a qualitative analysis that used a lower‐order isopyncal salinity model that represents key balance terms (i.e., evaporation E, precipitation P, and wind forcing advection). Both of the analyses consistently show that the subsurface salinity anomalies in the north and south of 15°N are induced by different physical processes. Fresher surface waters in the northwestern subtropical outcrop region due to an excess freshwater supply (E − P < 0) contribute to the freshening of subsurface waters north of 15°N. In contrast, saltier surface waters in the northeastern subtropical subduction region induced by a deficit of freshwater (E − P > 0) and anomalous ocean circulation associated with the recent accelerated trade winds of the tropical Pacific cause the saltiness of subsurface waters south of 15°N. The results imply that the salinity north‐south contrast may play an important role in changing ocean thermocline structure and upper ocean stratification in the northwestern Pacific.