Analysis on Characteristics and Formation Mechanism of the Winter Boreal Circulation in the Beibu Gulf

【Objective】In order to provide a scientific reference for the environment protection,it is significant to research on the characteristic and formation mechanism of the circulation in the Beibu Gulf.【Method】A three-dimensional hindcast model,which is forced by the daily averaged wind and heat flux,river discharges,as well as the tidal harmonic,is constructed via POM in this study,modeling the circulation in the Beibu Gulf during 2006~2007.Additionally,the sensitive experiment without the forcing of monsoon is utilized to further analyze the effect of the monsoon on the winter boreal circulation.On the other hand,one month observation data off the Yangpu port,from December 25,2005to January 25,2006,is used to examine the current features off the northwestern coast of Hainan Island.【Result】The model results are consistent well with the observations so that with the numerical model the circulation structure is revealed that the gulf-wide circulation is cyclonic nested a cyclonic gyre in the southern Gulf.In addition,there is seasonal oscillation between the autumn and winter circulation,that is,the southern gyre can intrude more northwards in autumn than that in winter,to 19°N,whereas the southward currents off the western Gulf coast and northwestern coast of Hainan in winter are stronger than that in autumn,and this seasonal oscillation exhibits from the surface to the bottom.Interestingly,the current off the northwestern coast of Hainan is found to be southwestward instead of northeastward.【Conclusion】It is showed that the northern gyre in the Beibu Gulf is driven by the local wind in winter boreal whereas the southern gyre is generated from the wind-induced South China Sea(SCS)shelf current.On the other hand,the seasonal oscillation between the autumn and winter circulation is caused by the high frequency component of the northeasterly monsoon,especially of the meridional wind,which indicates the temporal resolution of the wind field plays an important role on the circulation in the Gulf.Finally,it is detected that the southwestward current off the northwestern coast of Hainan is driven by the westward current in the Qiongzhou Strait whereas the northeasterly wind plays a supplement role.