Study On The Circulation In Western North Pacific And The Water Exchange Between The Pacific And The South China Sea

The western North Pacific Ocean is of major climate importance, where activemass, heat, and salinity exchange between the subtropical and equatorial oceansoccurs. The South China Sea (SCS) is the largest marginal sea in the western PacificOcean. Effective water exchange between the SCS and the western North Pacific (NP)occurs through the Luzon Strait, which is important for both the circulation and heatbudget in the SCS and the circulation and biology in the western NP. Based on the in-situ intensive CTD and LADCP measurements during Oct.-Nov., 2005 and Aug.-Sep.,2008, this paper studies the circulation in the western NP and the water exchangesbetween the NP and the SCS, especially in the intermediate and deep layer whereprevious observations are limited. Some novel features are found.During the period of observations in 2005, the North Equatorial Current (NEC)splits at about 12°N near the sea surface, feeding the northward Kuroshio and thesouthward Mindanao Current (MC); this bifurcation shifts northward with depth,reaching about 20°N at 1000m, and then it remains nearly unchanged to as deep as2000 m; The Luzon Undercurrent (LUC), emerging below the Kuroshio from about21°N, intensifies southward, with its upper boundary surfacing around 12°N; Fromthere, part of the LUC separates from the coast, while the rest continues southward tojoin the MC; The MC extends to 2000m near the coast, and appears to be closelyrelated to a subsurface, cyclonic circulation; the Mindanao Undercurrent (MUC) shiftseastward upon approaching the Mindanao coast, and eventually becomes part of theeastward undercurrent between 10°and 12°N at 130°E; The Antarctic IntermediateWater (AAIW) originating in the South Pacific is observed off Mindanao, but itsnorthward intrusion is confined primarily to the south of 10°N.Through the analysis of the current structure and water mass distribution in the vicinity of the Luzon Strait in Oct. 2005, this study finds an anticyclonic eddy in theintermediate layer (26.8-27.3σθ, about 500-900 m) of the Luzon Strait. Due to theexistence of the eddy, part of the relatively high-salinity (34.427.64σθ) through the Bashi Channel is 1.14 Sv, increases to 1.44 Sv at thesouthern mouth of the Luzon Trough, and reacheas 2.3 Sv at the mouth to the SCS. Avolume flux of 0.1 Sv deep water flows southeastward from the SCS to the LuzonTrough at the gap along 121°E. Density decreases and stratification weakens from theNP to the Luzon Trough, thus ths pressure gradient drives the deep flow. Thetopography control is also important on the deep current structure. Hydraulic controloccurs when deep water flows over the sill in the Bashi Channel, and plays importantrole in the deep water exchange between the NP and the SCS.