Strengthening Kuroshio observed at its origin during November 2010 to October 2012

Abstract

Direct measurements of Kuroshio at its origin (18N, east of the Luzon Island) are conducted from November 2010 to October 2012. It is found that the depth-averaged Kuroshio between 200 and 700 m has increased over 15 cm s-1 during the 2 year observational period and it is accompanied by the pronounced southward shift of the North Equatorial Current (NEC) bifurcation. Further analysis indicates that the Kuroshio's strengthening is confined to the upstream segment east of the Luzon Island while the Kuroshio decreased as it passed the Luzon Strait due to a dipole-like sea surface height (SSH) trend between 15N and 23N. It is demonstrated that the 2 year strengthening of the Kuroshio, as well as the dipole-like SSH trend can be adequately reproduced by a 1.5 layer nonlinear reduced gravity model, suggesting an important role of upper ocean response to low-frequency wind forcing in the western Pacific. Salinity at 500 m depth is also found to increase during the concurrent 2 years. This subthermocline salinity increase is a combined outcome of vertical (basin-scale isopycnal surface movement) and horizontal advections (i.e., strengthened Kuroshio) due to changes in the large-scale wind-driven ocean circulation.