Study On The Fluxes In The Luzon Strait And Turbulent Mixing In The South China Sea

The South China Sea (SCS) is one of the broadest marginal seas adjacent to the Northwest Pacific, in which lots of oceanic dynamic processes occur. The Luzon Strait is the only deep channel that connects the SCS and the Pacific. The oceanic problems in the SCS and the Luzon Strait are the focuses, among others; the turbulent mixing in the SCS, the Luzon Strait Transport (LST) and the internal tide energy flux in the Luzon Strait are the most important. These topics are researched through the current velocity, hydrography and microstructure observations by LADCP, CTD and TurboMAP/VMP in fall 2005 and summer 2007.The results obtained in this paper suggest that the flow pattern in the upper and intermediate layers change largely from summer to fall, especially in the intermediate layer where the eastward flow existed in the southern part of the strait and the westward flow in the north in summer, while in fall, the westward flow occurred in the south and the eastward flow in the north. They are almost opposite to each other. Few changes occur in the deep layer for the two seasons. The results also show that the LST has a sandwiched vertical structure in fall, with water entering the SCS in the upper and deep layers and flowing back to the Pacific in the intermediate layer. In summer, water flows into the Pacific in the upper and intermediate layers, and comes into the SCS in deep layer, presenting a two-layer structure. The total net transport through the Luzon Strait is eastward in summer with a value of 5.4 Sv, while in fall, the total LST is westward with a value of 6 Sv. The similar results are also derived from geostrophic flow.The calculation of internal tide energy fluxes reveals that the Luzon Strait is one of the most important generation regions of internal tides, where much energy fluxes propagate into the SCS. In summer, the total internal tide energy flux generated in the Luzon Strait is estimated to be 6.0 GW, and the westward and eastward flux values are 5.6 and 0.4 GW, respectively. The diurnal tides and semidiurnal tides contribute equally to the energy flux propagating into the SCS. In fall, 7.5 GW energy flux is generated in the Luzon Strait, and the westward and eastward energy fluxes are 4.9 and 2.6 GW, respectively. The westward energy flux of diurnal tide changes largely in two seasons, which transforms from 0.3 GW in summer to 2.3 GW in fall. The mode decomposition result reveals that the energy mainly exist in the first several modes, while almost zero in the higher modes. The energy exists dramatically in the first mode, suggesting that the high vertical modes are dissipated close to the generation site, with the most energy being contained in the low modes.The turbulent mixing in the northern SCS, the Luzon Strait and the Pacific is examined and compared with different methods and microstructure observation. The results suggest that, the turbulent mixing below 1000 m depth in the SCS is enhanced greatly, which almost occupy 2000 m thickness, and the magnitude is about 10-3 m2 s-1 in average, 100 times larger than that of the ocean background mixing. Enhanced mixing in such a thick layer is first found. In the Pacific, the turbulent mixing decrease notably to the ocean background values. While in the transition region, between the SCS and the Pacific, the mixing values range that of the SCS and the Pacific. In addition, the mixing rate below 1500 m depth in the SCS basin is estimated through the energy balance and heat budget, and agrees well with the model results. In addition, the mode results show a consistency between the model results and the direct microstructure observation measured by TurboMAP/VMP at some stations. These comparisons reveal that the model results are credible.