Study On The Deep Western Boundary Current Of The South China Sea

The South China Sea (SCS), the largest marginal sea in the western tropic Pacific Ocean, is one of the deep-water marginal seas. The deep-water feature makes it possible that the dynamic processes occurred in the open ocean is likely to be found in the SCS. According to the former studies,0.8～2.5 Sv pacific deep water will penetrate into SCS through Luzon Strait and induce a cyclonic circulation in the deep layer afterward. Due to the intense mixing in the SCS, deep water will be carried back to the middle layer and flow out from SCS through Luzon Strait. The path of the water frames the pattern of the South China Sea Throughflow(SCSTF), which is playing a significant part in keeping heat flux and water flux balance of the SCS. However, the absence of the in-situ observation frustrated people attempting to make clear of the deep circulation in the SCS. Circulation in the deep ocean is directly related to deepwater formation. The mean flow in the deep ocean interior is very slow, with the horizontal velocity on the order of mm/s. However, the deep western boundary current associated with the deepwater sources are the most significant features in the world oceans. Therefore, three mooring sections are deployed during the period from September 2012 to December 2013 to research the deep boundary current in the SCS. Meanwhile, numerical experiments are also to configured to study the temporal variability and driving mechanism of deep boundary current in the SCS.The result of the in-situ observation shows the existence of the deep boundary current in the SCS with an annual average flux of 0.94 Sv. The width of the boundary is about 40 km, extending from 2000 m to the bottom vertically. Compared with the mean flow, the variability is dominated, which is the result of the Rossby waves propagating westward. The Rossby wave is at a period of 100 days, and the amplitude is about 50 m.General ocean model HYCOM is also applied to simulate the circulation in the SCS, and notable intraseasonal oscillation at a period of 90 days is also observed within the velocity field. CEOF analysis is also used to characterize the oscillation more accurately. The oscillation originates along the east boundary of SCS and propagates westward. The oscillation originates along the east boundary of SCS and propagates westward. The wavelength is 269 km, about one fourth of the basin width. Amplitude of the oscillation reaches its maximum at upper layer and decreases with increasing depth. The wave is first baroclinic Rossby waves by the analysis of WOA climatological dataset and theories of Rossby waves. The shape of basin restricts the application of long-wave approximation and results in a smaller phase speed in SCS compared with its counterparts in the open ocean. There is a discrepancy between interior region and boundary region in phase speed of the Rossby waves. The phase speed in the boundary region is relatively smaller, which is the result of the reflection of Rossby waves near the western boundary.80-120 day band-pass filter is applied to the SLA time series to extract the baroclinic Rossby waves. The short Rossby waves we observed in the deep western boundary current may be the reflection of surface Rossby waves on the boundary.