Observational and model studies of the circulation in the Gulf of Tonkin, South China Sea

Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 118 (2013): 6495–6510, doi:10.1002/2013JC009455.Moored current measurements were made at one mooring site in the northern Gulf of Tonkin for about 1 year during 1988–1989. Analyses were performed to examine characteristics and variability of tidal and subtidal flows. Rotary spectra showed two peaks at diurnal and semidiurnal periods, with higher diurnal energy. Complex demodulations of diurnal and semidiurnal tidal currents indicated that the tidal current magnitudes varied significantly with seasons: more energetic in the stratified summer than in the vertically well-mixed winter. The observed subtidal currents were highly correlated with the surface wind in winter but not in summer; challenging the conceptual summertime anticyclonic circulation pattern derived using wind-driven homogenous circulation theory. The computed currents from a global ocean model were in good agreement with the observed currents. Similar to the current observations, the model-computed flow patterns were consistent with the conceptual wind-driven circulation pattern in winter but opposite in summer. Process-oriented experiments suggest that the summertime cyclonic circulation in the northern Gulf of Tonkin forms as a result of the combination of stratified wind-driven circulation and tidal-rectified inflow from Qiongzhou Strait. The interaction between the southwest monsoon and buoyancy-driven flow from Hong River can significantly intensify the cyclonic circulation near the surface, but its contribution to the vertically averaged flow of the cyclonic circulation is limited.Y. Ding has been supported by the State Scholarship Fund from the China Scholarship Council. C. Chen serves as chief scientist for the International Center for Marine Studies, Shanghai Ocean University, and his contribution has been supported by the Program of Science and Technology Commission of Shanghai Municipality (09320503700). C. Chen serves as the Zi Jiang Scholar at the State Key Laboratory for Estuarine and Coastal Research (SKLEC) of East China Normal University (ECNU) and Visiting Professor at School of Marine Sciences, Sun Yat-Sen University. C. Chen would like to credit this research to these two universities. Z. Lai’s contribution is supported by NSFC project 41206005 and Sun Yat-Sen University 985 grant 42000–3281301. The development of Global-FVCOM was funded by the US National Science Foundation Office of Polar Programs through grants ARC0712903, ARC0732084, ARC0804029, and ARC1203393.2014-06-0

Observed wintertime tidal and subtidal currents over the continental shelf in the northern South China Sea

Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 119 (2014): 5289–5310, doi:10.1002/2014JC009931.Synthesis analyses were performed to examine characteristics of tidal and subtidal currents at eight mooring sites deployed over the northern South China Sea (NSCS) continental shelf in the 2006–2007 and 2009–2010 winters. Rotary spectra and harmonic analysis results showed that observed tidal currents in the NSCS were dominated by baroclinic diurnal tides with phases varying both vertically and horizontally. This feature was supported by the CC-FVCOM results, which demonstrated that the diurnal tidal flow over this shelf was characterized by baroclinic Kelvin waves with vertical phase differences varying in different flow zones. The northeasterly wind-induced southwestward flow prevailed over the NSCS shelf during winter, with episodic appearances of mesoscale eddies and a bottom-intensified buoyancy-driven slope water intrusion. The moored current records captured a warm-core anticyclonic eddy, which originated from the southwestern coast of Taiwan and propagated southwestward along the slope consistent with a combination of β-plane and topographic Rossby waves. The eddy was surface-intensified with a swirl speed of >50 cm/s and a vertical scale of ∼400 m. In absence of eddies and onshore deep slope water intrusion, the observed southwestward flow was highly coherent with the northeasterly wind stress. Observations did not support the existence of the permanent wintertime South China Sea Warm Current (SCSWC). The definition of SCSWC, which was based mainly on thermal wind calculations with assumed level of no motion at the bottom, needs to be interpreted with caution since the observed circulation over the NSCS shelf in winter included both barotropic and baroclinic components.R. Li was supported by the SOA 908 Special Project Foundation of China (908-01-ST07 and 908-01-BC10), the National High Tech Project Foundation (863) of China (2008AA09A401), the Administrator Foundation of South Branch, SOA (0683). The development of FVCOM was funded by the US NSF Office of Polar Programs through grants ARC0712903, ARC0732084, ARC0804029, and ARC1203393.2015-02-1

Physical mechanisms for the offshore detachment of the Changjiang Diluted Water in the East China Sea

Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 113 (2008): C02002, doi:10.1029/2006JC003994.Physical mechanisms for the summertime offshore detachment of the Changjiang Diluted Water (CDW) into the East China Sea are examined using the high-resolution, unstructured-grid, Finite-Volume Coastal Ocean Model (FVCOM). The model results suggest that isolated low salinity water lens detected west of Cheju Island can be formed by (1) a large-scale adjustment of the flow field to the Changjiang discharge and (2) the detachment of anticyclonic eddies as a result of baroclinic instability of the CDW front. Adding the Changjiang discharge intensifies the clockwise vorticity of the subsurface current (originating from the Taiwan Warm Current) flowing along the 50-m isobath and thus drives the low-salinity water in the northern coastal area of the Changjiang mouth offshore over a submerged plateau that extends toward Cheju Island. Given a model horizontal resolution of less than 1.0 km, the CDW front becomes baroclinically unstable and forms a chain of anticyclonic and cyclonic eddies. The offshore detachment of anticyclonic eddies can carry the CDW offshore. This process is enhanced under northward winds as a result of the spatially nonuniform interaction of wind-induced Ekman flow and eddy-generated frontal density currents. Characteristics of the model-predicted eddy field are consistent with previous theoretical studies of baroclinic instability of buoyancy-driven coastal density currents and existing satellite imagery. The plume stability is controlled by the horizontal Ekman number. In the Changjiang, this number is much smaller than the criterion suggested by a theoretical analysis.The development of FVCOM is supported by the Massachusetts Fisheries Institute through NOAA grants DOC/ NOAA/NA04NMF4720332 and DOC/NOAA/NA05NMF4721131 and also the U.S. GLOBEC Northwest Atlantic/Georges Bank program through NSF grants OCE-0234545 and OCE-0227679, NOAA grant NA160P2323 and ONR subcontract grant from Woods Hole Oceanographic Institution. P. Ding is supported by the Chinese National Key Basic Research Project grant 2002CB412403. X. Mao is supported by the National Natural Science Foundation of China (NSFC) grant 40576079

Probability Researches of Design Basis Flood Level in Shidao Bay

Design Basis Flood Level(DBFL) is one of the important parameters in nuclear power plant siting in coastal area. This paper proposes a new model of Instantaneous Sea Surface Height(ISSH) to calculate DBFL. The model is validated and compared with bivariate extreme distribution model and independent variables model, using observed series from Shidao bay in Shandong province. Results show that the model is more suitable for representing the DBFL. The joint probability model gets greater return period and smaller return level than ISSH model, because it adopts an association parameter derived from the linear correlation coefficient. Independent variables model is the most reliable in safety but wasteful in economy

The Role of Qiongzhou Strait in the Seasonal Variation of the South China Sea Circulation

An analysis of the water level and current data taken in Qiongzhou Strait in the South China Sea (SCS) over the last 37 years (1963 to 1999) was made to examine the characteristics of tidal waves and residual flow through the strait and their roles in the seasonal variation of the SCS circulation. The observations reveal that Qiongzhou Strait is an area where opposing tidal waves interact and a source of water transport to the Gulf of Beibu (Gulf of Tonkin), SCS. A year-round westward mean flow with a maximum speed of 10-40 cm s(-1) is found in Qiongzhou Strait. This accounts for water transport of 0.2-0.4 Sv and 0.1-0.2 Sv into the Gulf of Beibu in winter-spring and summer-autumn, respectively. The outflow from Qiongzhou Strait may cause up to 44% of the gulf water to be refreshed each season, suggesting that it has a significant impact on the seasonal circulation in the Gulf of Beibu. This finding is in contrast to our current understanding that the seasonal circulation patterns in the South China Sea are primarily driven by seasonal winds. Several numerical experiments were conducted to examine the physical mechanisms responsible for the formation of the westward mean flow in Qiongzhou Strait. The model provides a reasonable simulation of semidiurnal and diurnal tidal waves in the strait and the predicted residual flow generally agrees with the observed mean flow. An analysis of the momentum equations indicates that the strong westward flow is driven mainly by tidal rectification over variable bottom topography. Both observations and modeling suggest that the coastal physical processes associated with tidal rectification and buoyancy input must be taken into account when the mass balance of the SCS circulation is investigated, especially for the regional circulation in the Gulf of Beibu