| The Beibu Gulf, located in the northwest of South China Sea, is a shallowsemi-closed gulf. Southern of this gulf faces the SCS, and the other three sides arecoastal land. The Beibu Gulf of Guangxi province is a major sea port of southwestChina, which has great economic and strategic significance. Ocean circulation plays animportant role in transfers of energy and mass, which can influence seawater quality,ocean self-regulation, and even national defense. The related study has been listed asfocus of ocean science. So far, limited to the patchier observation, understanding ofcirculation structure and its mechanisms in the Beibu Gulf remain uncertain. This studyis in effort to reproduce high-resolution seasonal variation of circulation structure inthe Beibu Gulf, considering key physical processes, and based on mechanism ofwave-tide-current.1. Based on the Princeton Ocean Model, a high resolution3D mode for tide bulgein the Beubu Gulf has been constructed, including the four main constituents K1、O1、M2、S2. The simulations show typical diurnal tides in most of this gulf. Amphidromicpoint of the K1partial tide appears near the coast of Hue, Vietnam, forming alevo-amphidromic tidal system. The amplitude is increasing from amphidromic pointto inner gulf, and up to90cm at the top of this gulf. Similar to K1, amphidromic pointof O1partial tide occurs near the coast of Hue, Vietnam. A retrogressive amphidromicpoint of M2partial tide is found at the north of Haiphong, Vietnam, with amplitude ofup to70cm. The amplitude of S2partial tide is below10cm, suggesting a very weaklevel. Most of the tide currents are typical reversing currents. Partial tides of K1and O1are faster than S2. The tidal residual currents generally show two anticlockwisecirculations in north and south respectively. Westward residual currents at theQiongzhou strait are the fastest, and the second is the northward currents along the West Hainan Island.2. A high resolution mode for ocean circulation in the Beibu Gulf has beenconstructed, based on the MASNUM (Laboratory of MArine Science and NUmericalModeling) developed by the First Institute of Cceanography, SOA. This mode isadopted to simulate seasonal variation of circulations in this Gulf and then to revealits dynamic mechanisms. The results show that the Beibu Gulf is controlled by anon-closed anticlockwise circulation and two small circulations inside during winter.The two small ones include a clockwise circulation in the eastern of BailongweiIsland and a anticlockwise circulation in the eastern of Vinh,Vietnam. But no oceancurrent form south mouth of the gulf is found in the north of18.5oN, which differfrom previous researches. Strong currents from the Qiongzhou Strait branch asouthward current along the Hainan Island. In spring, the circulations are generallyanticlockwise, with a few of small circulations inside, including anticlockwise ones inthe eastern of Bailongwei Island and in the south of the gulf. In summer, thecirculations mainly exhibit as two anticlockwise eddies. One eddy consists of asouthward current in the south of Red River, and a northward current in the trough ofthis gulf. Another eddy is around the cold water mass (19.5oN，108.2oE). An upwindsouthward current occur in the south of Red River, which result from baroclinic flowinduced by salinity front. In autumn, the pattern of circulation exhibit as a largenon-closed anticlockwise circulations and a Beibu Gulf loop. The westward currentsfrom Qiongzhou Strait enter the Beibu Gulf, running counterclockwise along the coastof Vietnam. The southward currents on the coast of Hainan Island enter this gulf fromits south mouth, running northward along the contour of sea ridge. After arriving onthe coast of Vietnam, they run anti-clockwise and leave the Gulf, thus forming a loopof the Beibu Gulf. Dynamic mechanisms of circulation in each season are different.Tidal residual current is the most important cause. The second forcing is densitycurrent in spring and summer, but shift to wind in autumn and winter, which hasminor influence in the rest seasons.3. Cold water mass in the north-central Beibu Gulf is reproduced to explore its seasonal variation and dynamic mechanism. The results show that the cold water massstarts to form in March, reaches the maximum strength during June and July, andfades away since October; the cold water mass is locally developed, which iscontrolled by sea surface heat flux, while terrain and tidal mixing also play importantroles.4. Seasonal variation of ocean currents of the Qiongzhou Strait and its influenceon structure of circulation in the Beibu Gulf are analyzed. The result shows awestward current through the Qiongzhou Strait for most of a year, which shiftsdirection when the south-west wind is strong in summer. This westward current isdriven by a westward tidal-induced residual current. Wind governs the current fluxand its direction. The Qiongzhou Strait can influence speed of current along the westcoast of Leizhou Peninsula, the north coast of Hainan Island, and the coast of Guangxi,especial in winter and summer, but has a minor influence in the south of19°N.5. This model is applied to simulate upwelling on the west side of Hainan Islandin summer and reveal its mechanisms. Tidal-mixing front is tested to be the maincause through sensitive experiment without considering wind and tide respectively.Using the numerical model, strength of upwelling and seasonal variation arequalitatively analyzed. In sum, the upwelling forms in spring, peaks at summer, weaksduring autumn, and disappears in winter.6. This model is applied to simulate the warm pool in winter near Yinggehai, andanalyze its evolution, incorporating satellite-derived SST data. The result shows thatthe warm pool forms in the October, becomes strongest in the November, declines inthe December, and disappears in the next January. A sensitivity experiment anddynamic analysis indicate that warmer water from South China Sea is the major causeof warm pool in the Beibu Gulf, and the clockwise eddy near Yinggehai can furtherpromote and maintain the warm pool. |
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