Numerical Simulation Of Seasonally Circulation And Water Exchange In The Beibu Gulf

Beibu Gulf is a natural semi-enclosed shallow bay located in the northwest of theSouth China Sea. It faces the Leizhou Peninsula and Hainan Island on the east,Vietnam on the west, Guangxi Province on the north and links to the South China Seato the south. It process12.8×104km2water area, and has an average water depth of38m, while the maximum water depth is106m. Because of numerous coastal rivers,unique geographical location, Beibu Gulf turns to be a special environment consistingof both the estuarine ecosystem and the gulf ecosystem. Theoretically, it helps betterunderstanding the dynamic mechanism and physical self-purification capacity of theBeibu Gulf by studying circulation and water exchange in this area. It also providesscientific background and technical support for environment protection, and assistsrelevant department better making use of natural environment, ecologicalenvironment in the Beibu Gulf. A three-dimensional hydrodynamic model FVCOMwas built to numerically simulate seasonal circulation in the Beibu Gulf. Waterexchange was also studied base on Tracer-Tracking module. The main work andcorrespondent results are as follows:FVCOM is used to build a three-dimension hydrodynamic model, and its resultis verified by a consecutive nine-day tidal current and elevation observed data. Theresults are turned to be correct. Results are as follows:(1) Tidal wave propagates into the gulf from the mouth of the gulf, meanwhile,the amplitude reaches its maximum on the north of the Beibu Gulf. Diurnal tide K1，O1play a major role in the Gulf. The maximum amplitude of K1is above90cm,while O1is beyond100cm. The tide line rotates anti-clockwise around theamphidromic points of K1, O1, which are formed near Shunhua, Vietnam. Amplitudesof M2, S2are smaller than those of diurnal tides. Phases of M2, S2increasinglypropagate to the top of the Beibu Gulf. (2) The main tide form of the Beibu Gulf is reciprocating, whose direction isconsistent with the shoreline. Waters near Leizhou Peninsula move along with thecostal line. In the southern20°north latitude, from north to south in the Gulf, thedirection of tide current changes from northeast-southwest to northwest-southeast.The max velocity happens in the west exit of Qiongzhou Strait, than is westernHainan Island.(3) Tide-induced current from enclosed anti-clockwise circulation in northwestand Yinggezui of Hainan Island respectively. The current moves from QiongzhouStrait to the Beibu Gulf, running along the coast of Guangxi Province, then turnssouthward. One part of the current meet a northward current forming ananti-clockwise circulation, the other part turn southward after moving westward,producing a small anti-clockwise circulation near Yinggezui.Based on monthly average wind data from the National Centers forEnvironmental Prediction (NCEP), wind-tide coupled circulation is simulated usingFVCOM hydrodynamic mode. Results are as follows:(1) A long anti-clockwise circulation exists along Vietnam. Surface circulationruns westward into the Beibu Gulf, then moves along Guangxi Province, Vietnam,meeting the northwestward circulation from southern Hainan Island, then generate along anti-clockwise circulation. Middle layer circulation has a great difference withsurface circulation, while the same as the bottom layer circulation.(2) In summer, circulation still runs into the Gulf through Qiongzhou Strait, andforms two anti-clockwise circulations. Circulation near western Hainan Island movesnorthward, forming an anti-clockwise circulation in19°north latitude，108°eastlongitude. An eastward flow near19°north latitude moves out of the Beibu Gulfalong the south of Hainan Island, then forms a clockwise circulation near the southexit of the Beibu Gulf. The location of circulation in the middle layer changessomewhat. The circulation pattern in the bottom layer is the same as that in the middlelayer.In this paper, the line from Xindi village in Leizhou Peninsula to Macungang village in Chengmai County of Hainan Province and the other one from Sanya toDanang City of Vietnam are considered as water exchange boundaries. With themethod of tracer, the water exchange of the Beibu Gulf is simulated numerically.The half-time of exchange and residual time of Beibu Gulf are calculated. Here arethe results:In winter, the residual time of Beibu Gulf is430days, while this numberbecomes588days in summer. The similarity between Beibu Gulf’s residual time insummer and in winter is that the capacity of the water exchange weaken from theboundaries to the inner side, which means that the nearer the water masses get to theexchange boundaries, the stronger water exchange capacity they will be. In otherwords, waters get close to the boundaries exchange faster than the inner ones. Thedifference of water exchange between summer and winter is that the circulationpattern has significant difference seasonally. The different capacity of water exchangeresults from the different circulation pattern. The tracers transfer much slower in thesea area with cyclonic circulation which blocks the exchange of the sea water.