Seasonal Variability of the Large-Scale Currents near the Coast of the Philippines

Abstract

The mean seasonal cycle of the western boundary currents in the tropical North Pacific Ocean is studied diagnostically by combining atmospheric climatologies with drifter, satellite altimetry, and hydrographic data in the framework of a simplified numerical model incorporating geostrophy, hydrostatics, continuity, and tracer conservation. The approach enables the authors to diagnose the absolute 3D velocity field and to assess the seasonal cycle of sea surface height (SSH)/total transports. Errors are estimated by considering multiple datasets and averaging over the results of the corresponding diagnostic computations. Analysis shows that bifurcation of the North Equatorial Current (NEC) occurs at 14.3° ± 0.7°N near the Philippine coast. Meridional migration of the NEC bifurcation latitude is accompanied by quantitative changes in the partitioning of the NEC transport between the Kuroshio and Mindanao Current. In February?July, when the NEC transport is 58 ± 3 Sv (Sv ≡ 106 m3 s?1), the Kuroshio transport is 12%?15% higher than the Mindanao Current (MC) transport. In the second half of the annual cycle the situation is reversed: in October the NEC transport drops to 51 ± 2 Sv with the MC transport exceeding the Kuroshio transport by 25%. The net westward transport through the Luzon Strait is characterized by a minimum of 1.2 ± 1.1 Sv in July?September and a maximum of 4.8 ± 0.8 Sv in January? February. A statistically significant correlation is established between the monthly SSH/streamfunction anomalies north of 10°N and the Ekman pumping rate associated with the northeast monsoon developing in the region in October?December. The result provides an indication of the fact that local monsoon is likely to be an important mechanism governing seasonal variation of the NEC partitioning and water mass distribution between the tropical and subtropical North Pacific.