The influence of meteorological variation on the upwelling system off eastern Hainan during summer 2007–2008 |
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Authors: | Jian Su Jun Wang Thomas Pohlmann Dongfeng Xu |
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Institution: | (1) Institute of Oceanography, Centre for Marine and Climate Research, University of Hamburg, Bundesstr. 53, 20146 Hamburg, Germany;(2) Institute for Coastal Research, GKSS Research Center, Max-Planck-Str. 1, 21502 Geesthacht, Germany;(3) State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, State Oceanic Administration, Hangzhou, 310012, China |
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Abstract: | The influence of meteorological variation, i.e., typhoon and precipitation events, on the coastal upwelling off the eastern
Hainan Island was studied based on observations taken during two upwelling seasons. The observations were made in August 2007
and July 2008, respectively. We found that, in principle, similar structure of sea surface temperature and bottom temperature
prevailed in both observational periods, providing evidence that upwelling events occur frequently during the summer monsoon
along the eastern Hainan shelf. Based on a simple momentum balance theory, we studied the balances between momentum fluxes,
wind stress, and bottom stress. The results showed that the Burger number is S ≈ 1, indicating that the cross-shelf momentum flux divergence was balanced by the wind stress and the onshore return flow
occurred in the interior of the water column. Hence, a conceptual model of the upwelling structure was built for further understanding
of upwelling events. In addition, it was also observed that variations in the strength of upwelling are controlled by storm
events, i.e., strong northerly winds change the structure of the thermocline on the shelf significantly. The strong mixing
caused by wind reduces the strength of the thermocline, in particular in coastal seas. Based on our conceptual model, a frontal
zone between mixed coastal water and offshore water develops which destabilizing the water column and hence decreases the
upwelling strength. Freshwaters from the two main rivers in the Wenchang Bay are confined to the coastal area less than 20–30
m deep, as confirmed by our water mass analysis. Freshwater discharge stabilized the water column, inhibiting the upwelling
as shown by the potential energy calculation. Consequently, estuarine water only inhibits the upwelling in the near coastal
area. Therefore, it can be concluded that estuarine water does not have a significant impact on upwelling strength on the
shelf. |
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