北部湾冬半年环流特征及驱动机制分析

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

【目的】为了给北部湾的环境保护提供科学依据，研究北部湾冬半年的环流变化特征及其驱动机制。【方法】通过POM模型构建三维斜压后报模型，模拟200642007年南海西北部的环流，模拟时考虑了日平均风场、日平均热通量、河流径流和潮汐等强迫场；通过无季风驱动的敏感性实验分析东北季风对北部湾冬半年流场的作用；通过洋浦港外海在2005年12月25日到2006年1月25日的一个月海流观测数据来分析冬季海南岛西北岸的海流特征。【结果】模拟结果与观测数据吻合较好，北部湾冬半年环流呈气旋式，嵌套着南部的气旋式环流，而且秋季与冬季环流存在季节性振荡特征：在秋季，南部的气旋式环流可向北侵入至19°N，比冬季深入得多；在冬季，湾西岸和海南岛西北岸的南向流比秋季要强；这种季节性变化特征可从表层直达深层。另外，海南岛西北岸流在冬季为西南向而非东北向。【结论】冬半年北部湾的北部环流为局地风驱动，南部环流是由东北季风驱动的南海陆架流侵入北部湾而形成的；这种季节性涛动受东北季风的高频振荡分量驱动，尤其风场的南北向分量贡献较大，也说明风场的时间分辨率对北部湾环流影响较大；海南岛西北岸的西南向流受琼州海峡的西向流驱动，而东北季风只起辅助作用。

[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, 2005 to 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 struc ture 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 the southern gyre in autumn than the southward and winter circulation, that is, can intrude more northwards that in winter, to 19°N,whereas 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.