Evidence for water exchange between the South China Sea and the Pacific Ocean through the Luzon Strait

An analysis of historical oxygen data provides evidence on the water exchange between theSouth China Sea (SCS) and the Pacific Ocean (PO). In the vicinity of the Luzon Strait (LS) , the dissolved oxygen concentration of sea water is found to be lower on the Pacific side than on the SCS side at depths between 700 and 1500 m (intermediate layer) , while the situation is reversed above 700 m (upper layer) and below 1 500 m (deep layer). The evidence suggests that water exits the SCS in the intermediate layer but enters it from the Pacific in both the upper and the deep layers, supporting the earlier speculation that the Luzon Strait transport has a sandwiched structure in the vertical. Within the SCS basin, the oxygen distribution indicates widespread vertical movement, including the upwelling in the intermediate layer and the downwelling in the deep layer.

A numerical study of the South China Sea deep circulation and its relation to the Luzon Strait transport

A fine-resolution MOM code is used to study the South China Sea basin-scale circulationand its relation to the mass transport through the Luzon Strait. The model domain includes the South China Sea, part of the East China Sea, and part of the Philippine Sea so that the currents in the vicinity of the Luzon Strait are free to evolve. In addition, all channels between the South China Sea and the Indonesian seas are closed so that the focus is on the Luzon Strait transport. The model is driven by specified Philippine Sea currents and by surface heat and salt flux conditions. For simplicity, no wind-stress is applied at the surface.The simulated Luzon Strait transport and the South China Sea circulation feature a sandwich vertical structure from the surface to the bottom. The Philippine Sea water is simulated to enter the South China Sea at the surface and in the deep ocean and is carried to the southern basin by western boundary currents. At the intermediate depth, the net Luzon Strait transport is out of the South China Sea and is fed by a western boundary current flowing to the north at the base of the thermocline. Corresponding to the western boundary currents, the basin circulation of the South China Sea is cyclonic gyres at the surface and in the abyss but an anti-cyclonic gyre at the intermediate depth. The vorticity balance of the gyre circulation is between the vortex stretching and the meridional change of the planetary vorticity. Based on these facts, it is hypothesized that the Luzon Strait transports are determined by the diapycnal mixing inside the entire South China Sea. The South China Sea plays the role of a 'mixing mill' that mixes the surface and deep waters to return them to the Luzon Strait at the intermediate depth. The gyre structures are consistent with the Stommel and Arons theory (1960), which suggests that the mixing-induced circulation inside the South China Sea should be cyclonic gyres at the surface and at the bottom but an anti-cyclonic gyre at the intermediate depth. The simulated gyre circulation at the intermediate depth has been confirmed by the dynamic height calculation based on the Levitus hydrography data. The sandwich transports in the Luzon Strait are consistent with recent hydrographical observations.Model results suggest that the Kuroshio tends to form a loop current in the northeastern South China Sea. The simulated Kuroshio Loop Current is generated by the pressure head at the Pacific side of the Luzon Strait and is enhanced by the β-plane effects. The β - plane appears to be of paramount importance to the South China Sea circulation and to the Luzon Strait transports. Without the β-plane, theLuzon Strait transports would be greatly reduced and the South China Sea circulation would be complete-ly different.

Water exchange and circulation structure near the Luzon Strait in early summer

Using hydrographic data covering large areas of ocean for the period from June 21 to July 5 in 2009, we studied the circulation structure in the Luzon Strait area, examined the routes of water exchange between the South China Sea （SCS） and the Philippine Sea, and estimated the volume transport through Luzon Strait. We found that the Kuroshio axis follows a e-shaped path slightly east of 121°E in the upper layer. With an increase in depth, the Kuroshio axis became gradually farther from the island of Luzon. To study the water exchange between the Philippine Sea and the SCS, identification of inflows and outflows is necessary. We first identified which flows contributed to the water exchange through Luzon Strait, which differs from the approach taken in previous studies. We determined that the obvious water exchange is in the section of 121°E. The westward inflow from the Philippine Sea into the SCS is 6.39 Sv in volume, and mainly in the 100-500 m layer at 19.5°-20°N （accounting for 4.40 Sv）, while the outflow from the SCS into the Philippine Sea is concentrated in the upper 100 m at 19°-20°N and upper 400 m at 21°-21.5°N, and below 240 m at 19°-19.5°N, accounting for 1.07, 3.02 and 3.43 Sv in volume transport, respectively.

Optimal estimation of absolute geostrophic velocity field in vicinity of the Luzon Strait using variational data assimilation technique

A P - vector method is optimized using the variational data assimilation technique(VDAT). The absolute geostrophic velocity fields in the vicinity of the Luzon Strait (LS) are calculated, the spatial structures and seasonal variations of the absolute geostrophic velocity field are investigated. Our results show that the Kuroshio enters the South China Sea (SCS) in the south and middle of the Luzon Strait and flows out in the north, so the Kuroshio makes a slight clockwise curve in the Luzon Strait, and the curve is strong in winter and weak in summer. During the winter, a westward current appears in the surface, and locates at the west of the Luzon Strait. It is the north part of a cyclonic gyre which exits in the northeast of the SCS; an anti-cyclonic gyre occurs on the intermediate level, and it exits in the northeast of the SCS, and an eastward current exits in the southeast of the anti-cyclonic gyre.

Currents in the Luzon Strait during the spring of 2002: observation and computation by modified inverse model

On the basis of the current measurements at 200,500 and 800 m from moored current meters with the time series data from March 17 to April 15 at the mooring station (20°49′57″N, 120°48′ 12″E) and the hydrographic data obtained in the Luzon Strait during the spring of 2002 cruise, the circulation in the investigated area is computed by using the modified inverse method. The major observed results are as follows: (1) the average velocity and the flow direction in the observing days are (47.4 cm/s, 346°) at the 200 m level. The average velocity in the observing days is (20.3 cm/s, 350? at the 500 m level. These mean that the Kuroshio intrudes into the South Chin Sea to flow northwestward through the Luzon Strait at 200 and 500 m levels. (2) The average velocity in the observing days is (1.2 cm/s, 35°) at the 800 m level, i. e., its direction is northeastward. This means that the flow condition at the 800 m level very differs from mat at the 200 and 500 m levels. (3) There is the high density and cold water (HDCW) in the middle of western part of in the investigated region, and its center is located near the hydrological station 3 at Section A. (4) There is the lower density and warm water (LDWW) in the southeastern part of investigated region. (5) The currents in April 2002 are stronger than those in March 2002.The major computed results are as follows: (1) The northwestward and southeastward VTs through Section B are 32.48×106 m3/s (inclusive of VT of anticyclonic eddy) and 3.34×106m3/s, respectively. The net northwestward VT through Section B in the investigated area is about 29.14×106 m3/s. (2) The eastern and western VTs through Section A are about 16.71×106 and 8.57×106 m3/s, respectively. Thus, the net eastward VT through Section A is about 8.14×106 m3/s. (3) The net northward VT through Section M is about 24.68×106 m3/s. (4) After about 24.68×106 m3/s flows through Section M, most of it, about 16.54×106 m3/s, flows northward through the eastern part of Section C and then flows northward into the region east Taiwan Island. The other part of it, about 8.14×106 m3/s, branches out from the main Kuroshio and then flows meanderingly through the western part of Section C. Thus, the Kuroshio has the two cores of current at Section C. (5) The direction of the computed current near the mooring station M agrees with the direction of the current measurements at 200 and 500 m from moored current meters, i.e., their directions both are northwestward. (6) About 3.34×106 m3/s of the South Chin Sea water probably flows slowly from the northwest to the southeast in the layer below 550 m at the western part of Section B.