A Climatology of the Circulation and Water Mass Distribution near the Philippine Coast

Abstract

This study provides a climatology of the circulation and water mass distribution by using historical data combined with observations from dozens of recent cruises near the Philippine coast. The most striking results are related to the poleward contraction of the subtropical gyre on denser surfaces, with the bifurcation of the North Equatorial Current moving from about 15°N in the upper thermocline to about 20°N at intermediate depths. Though time variability and the possible errors in the data are rather large, the Halmahera eddy (HE) is clearly seen in the climatic mean fields, lying at about 3°N, 130°E near the surface and reaching the Mindanao coast on density surfaces around 27.2σ?. It seems that the previously observed Mindanao Undercurrent is merely a component of the recirculation associated with the HE. North Pacific Tropical Water (NPTW) and Intermediate Water (NPIW) enter the western ocean with their extreme properties centered at 15° and 20°N, respectively, and continue southward as far as the southern tip of Mindanao along the western boundary. The influence of South Pacific sources becomes increasingly important with depth. Antarctic Intermediate Water (AAIW) is traced to about 12°N off Mindanao; but, there is little indication of a northward flow of AAIW farther north. Salinity extremes are also used as an indicator of NPTW and NPIW, and the primary result is that mixing of potential temperature and salinity are not jointly compensated, thus leading to an increase of density in NPTW and a decrease of density in NPIW in the flowpath from the North Pacific subtropical gyre to the Tropics along the Philippine coast.