南海冬季风潮背景下热带气旋诱导的近惯性振荡:Mirinae(0921)个例分析

Near-inertial oscillations induced by tropical cyclones under the background of South China Sea winter monsoon surge: A case study of Mirinae(0921)

南海是热带气旋多发的海域,热带气旋过境在海洋上层激发的近惯性振荡近年来备受关注。文章利用一个位于南海西北部的锚定声学多普勒流速剖面仪(ADCP),检测到了热带气旋Mirinae由东向西横穿南海时在锚定点产生的显著的近惯性振荡。与以往研究热带气旋激发显著近惯性振荡的文献相较,Mirinae强度较低,移动速度较快,距离观测点较远。对ADCP所测流速进行旋转小波分析后的结果显示,处于这种条件下的Mirinae也在观测点激发了显著的近惯性振荡。研究发现,Mirinae自东向西横穿南海时,正处于南海冬季风盛行的时期,Mirinae北部旋臂与该盛行的东北季风方向一致,二者相融,东北季风强度增大,方向发生变化,间接使得主要在其控制下的观测点上方的风场在大小和方向上产生了突变,而且风速矢量随着时间也呈顺时针偏转,从而在锚定点产生显著的近惯性振荡。平板模式的结果支持这一观点。

A number of tropical cyclones are active in the South China Sea each year. Near-inertial oscillations (NIOs) induced by the passing of tropical cyclones in the upper ocean became a hot research topic recently. Based on an acoustic Doppler current profiler (ADCP) moored in the northwestern South China Sea, we observed prominent NIOs during tropical cyclone Mirinae crossing the South China Sea. Comparing with the tropical cyclones that induced significant NIOs in previous studies, Mirinae was weak, moving quickly and far from the mooring site. After rotary wavelet analysis on currents observed by the moored ADCP, we found that significant NIOs were also triggered by Mirinae. How did Mirinae trigger these significant NIOs? We found that winter monsoon prevailed in the South China Sea during Mirinae. The northerly wind of Mirinae had the same direction as the winter monsoon. So, the arrival of Mirinae enhanced the magnitude of winter monsoon and changed its direction, which indirectly changed the magnitude and direction of local winds above the mooring site mainly controlled by winter monsoon and also made the local winds turn clockwise. Prominent NIOs at the mooring site were thus induced by the changing of local winds. These results were verified by the results of a damped-slab model.