Observational Study Of Deep Circulation In The Northeastern South China Sea

The Luzon Strait (LS) is the only deep connection between the Pacific and South China Sea (SCS). The deep water in the Pacific penetrates into the SCS through the LS, upwells in the interior SCS, and flows out through the intermediate layer of the LS and the other shallow straits around the SCS. This is called the SCS throughflow, which influences the Indonesia throughflow substantially and further plays a significant role in modulating the global climate. And the deep circulation in the northeastern SCS, including the LS, is the key part of it. Besides, it is also the key factor modulating the balance of heat and buoyancy, regional sediment transport, and abyssal carbon cycle of the SCS. Considering the significant role the deep circulation in the northeastern SCS plays, a series of studies are conducted here, to investigate its spatial structure, temporal variability, as well as the dynamic mechanism.Based on the observations at the Bashi Channel (BC), Luzon Trough (LT), and the Heng-Chun Ridge (HCR), the pathway, volume transport, spatial structure and temporal variability of the deep circulation in the LS is revealed, as well as its driving mechanism. The consistent results of observation and simulation indicate that deep water in the Northern Pacific penetrates into the Luzon Strait mainly through the BC, involves some deep water from the Taltung Canyon, flows southward along the LT, and spills over into the deep SCS mainly through the sills at the middle and south of the HCR. The maximum velocity of deep circulation in the LS is found near to the bottom (about 120 m above the bottom). The mean maximum velocity is 19.9±6.5 cm/s at the BC and 23.0±11.8 cm/s at the LT, corresponding to deep water volume transport of 0.83±0.46 Sv and 0.88±0.77 Sv at BC and LT, respectively. Significant seasonal variability of the deep circulation in the LS is revealed, which is strong in autumn/winter and relatively weak in spring/summer. The amplitude of seasonal variability is 1.6 cm/s at the BC and 3.3 cm/s at the LT. Besides, energetic intraseasonal variations with periods of around 30 days and 100 days is also indicated by the velocity time series of deep circulation in the LS. The 30-day oscillation get enhanced at every spring, during which deep water in the SCS could go back all the way through the deep LS. Further analysis implies that the intraseasonal variations could be related to the deep eddies in the LS. Based on simulation of HYCOM, as well as observed temperature and salinity profiles, it is indicated that the enhanced mixing in the deep LS and SCS is the primary mechanism of driving the deep circulation in the LS. The enhanced mixing weakens the stratification of the deep SCS, results into the pressure gradient between the deep Pacific and SCS, and further drives the penetration of north Pacific deep water into the deep SCS. And the seasonal variability of mixing in the deep SCS could be the reason modulating the seasonal variability of the deep circulation in the LS.Based on the mooring observations and temperature and salinity climatology datasets, the deep current at the western boundary of northern Philippine Basin is also investigated, including its spatial structure, temporal variability as well as its contribution to the sediment provenances of the northeastern SCS. Although no typical strong deep western boundary current is observed, the deep current exhibits significant seasonal variability. In winter/spring, the deep boundary current flows northward, with mean velocity of 1.7 cm/s, while in summer/autumn, the deep boundary current flows southward, with mean velocity of -2.4 cm/s below about 1800 m. The hydrographic characteristic also varies in seasonal period, with warmer and saltier water corresponding to the northward flow and relatively colder and fresher water along with the southward flow. This results from the temperature and salinity field in the deep northern Philippine Basin, both with negative gradient polarward. The annual reversal and energetic intraseasonal variability could play a significant role in the sediment provenance in the northeastern SCS. In winter/spring, the northward deep current can carry the suspended sediment with high concentration of illite to the mouth of the BC, while in summer/autumn, the southward deep current can bring the suspended sediment with high concentration of smectite to the mouth of the BC. With the transport of the deep circulation in the LS, these two kinds of deep water with different constitute of suspended sediment can go through the LS and contribute to the sediment provenance in the northeastern SCS.Based on observations of a series of mooring arrays, structure and variability of the northeastern deep SCS is also investigated. After flowing mainly through the two sills at the HCR, deep water in the LS turn immediately to the north along the slope. Constraint by the topography, it turns left at the northern end of the Manila Trench and further goes southwestward along the topography. The deep current is accompanied with relatively cold and salty deep water. Significant near inertial oscillation and intraseasonal variability with periods between 30 and 50 days is revealed. Besides, the current direction between the cores of the northward and southward deep current oscillates between northward and southward, which could result from the deep mesoscale processes.