Propagation of the subsurface freshening water and its major source in the northwestern P acific

Abstract Previous studies of the exchanges of the subtropical‐tropical salinity in the North Pacific are still not conclusive. Some studies suggest that surface salinity anomalies formed in the northeastern subtropics can subduct and penetrate to the subsurface of the northwestern Pacific, whereas others demonstrate that subsurface salinity anomalies in the northwestern Pacific cannot be traced back to those of the eastern subtropics. This study examined the variability of subsurface salinity in the northwestern Pacific, with focus on its propagation characteristic and forcing mechanism in its outcrop region. It is found that salinity anomalies on 24.5–25.4 kg m−3 isopycnals exhibited a pronounced freshening trend in the northwestern Pacific during 2003–2012. This freshening can be traced back to the surface salinity anomalies in the outcrop zone of the northwestern subtropical gyre, rather than that of northeastern subtropics. The representative transit time from its outcrop region to the eastern Luzon Strait is about 1–3 years. The surface salinity anomalies in the outcrop zone are mainly controlled by air‐sea freshwater flux change, although the contribution of horizontal advection is also important, in particular for year‐to‐year variation. Compared to the air‐sea freshwater flux and horizontal advection, the contribution of mixed‐layer depth to the surface salinity anomalies is smaller and can be neglected in the outcrop zone of the northwestern Pacific.