Observations of the Pacific North Equatorial Current Bifurcation at the Philippine Coast

Abstract

Hydrographic surveys were conducted off the Philippine coast in September 1987 and April 1988 as part of the United States/People's Republic of China cooperative research program. These cruises sampled the western Pacific Ocean where the North Equatorial Current (NEC) meets the western boundary and divides into the Kuroshio and Mindanao Currents. The requirement for mass conservation within a region enclosed by stations is utilized here to obtain absolute circulation fields for the two surveys. In both realizations, the surface flow of the NEC was observed to bifurcate near latitude 13°N; NEC flow poleward of this latitude turned north as the Kuroshio while flow to the south fed the Mindanao Current. Most striking was a twofold increase in the strength of the current system in spring 1988 as compared with fall 1987. We note that the observations in fall 1987 were obtained during the height of the 1986/87 El Niño, while those in spring 1988 were during a cold phase of the El Niño/Southern Oscillation. It is not clear how the observed current changes relate to the evolution of this event. The potential vorticity (Q) distributions of the surface waters were examined to explore the dynamics of the bifurcation. Within the NEC, Q was nearly constant (layer thickness change balanced meridional planetary vorticity variation). Within the Kuroshio and Mindanao currents, near constant Q (with magnitude comparable to that in the NEC) was also found with a balance between relative vorticity variation and layer depth change as would be expected for inertia] boundary currents.