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General Session 1: Physical oceanic processes: Dynamics and physical-biological-biogeochemical interactions |
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Process and dynamic linkage of the circulation along the northern South China Sea shelf
GS1-46-S
Chenmin Yu* , Hong Kong University of Science and Technology
Jianping Gan, Hong Kong University of Science and Technology
Presenter Email: cyuai@ust.hk
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| Over the continental shelf in the northern South China Sea (NSCS), the shelf circulation is controlled by buoyancy flux from Pearl River, monsoonal wind forcing, bottom topography and external forcing. Based on the dynamic characteristics in response to local/remote forcing, circulation over the NSCS can be uniquely divided into 4 zones: east of Hainan Island (EHI) with steep shelf (Zone 1), quasi-two-dimensional flow regime between the Pearl River Estuary (PRE) and EHI (Zone 2), freshwater-influenced shelf off the PRE (Zone 3) and a narrowed-widened shelf in the eastern NSCS (Zone 4). These 4 zones are linked by the shelf circulation, and they interact each other to jointly shape the spatiotemporal variations of the circulations over the entire NSCS. Based on a three-dimensional model, we investigate the unique circulation in each of these zones as well as their dynamic linkage as a result of time-dependent, three-dimensional along- and cross-shelf circulation over the shelf. Seasonally, the dense water confines in more inshore region in Zone 1 and 4 as compared with other two zones in summer, and the water becomes vertically uniform over the shelf except in Zone 1 where downslope water transport occurs in winter. We found that the alongshore transport connects the adjacent zones, which is commensurate with the local cross-isobath transport, characterizes each zone. It is shown that the upwelling structures in Zones 2 and 3 during summer are largely modulated by upstream transport because of the relatively low upwelling intensity and the strong alongshore transports. In contrast, strong upwellings in Zones 1 and 4 are mainly locally forced by the respective steep and widened shelf. During winter, the conditions are depth-independent and horizontally-uniform, because the strong along-shelf transport exceeds the local cross-shelf transport, and the stronger mixing is strong. The exception occurs in Zone 1 where the downstream alongshore transport is blocked by Hainan Island and a downwelling cross-shelf feature is prominent. There is a weaker spatial variation of shelf circulation in winter than in summer when the spatially distinct upwelling structure appears along the shelf. The synoptic atmospheric variations have great impacts on the modulation of shelf circulations, which onset and offset upwelling/downwelling favorable winds and cause the discontinuous in both along- and cross-shelf circulation and thus in the linkage of the zones. During the discontinuity incidence, however, the freshwater generally transport farther downstream into Zone 2 due mainly to the excitement of the coastal trapped wave. |
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