南海西边界急流影响下的近惯性振荡特征分析

Characteristics of Near-inertial Oscillation Influenced by Western Boundary Current of South China Sea

在利用南海西沙海域多年声学多普勒流速剖面仪(ADCP)观测流速资料分析近惯性内波运动的生成、传播等演变特征的过程中,发现冬季和夏季的近惯性振荡频率存在整体性的偏移效应。ADCP所处海域位于南海西边界急流区,冬季和夏季的背景环流分别为东北向和西南向,背景环流的方向性差异可能影响近惯性振荡频率的偏移效应。针对背景环流的不同方向,筛选出其对应的近惯性振荡信号,进行合成分析,探讨南海西边界急流如何影响近惯性振荡特征。从背景涡度的角度对这一影响机制做出了解释:夏季时,东北方向背景环流的水平剪切使得局地产生负的背景涡度,近惯性振荡频率往低频方向偏移("红移"现象);冬季时,西南向背景环流的水平剪切导致局地产生正的背景涡度,近惯性振荡频率往高频方向偏移("蓝移"现象)。

Based on observed velocity data of ADCP for near-inertial wave evolution characteristics of genera- tion and propagation, the difference of shifting effect was found in winter and summer. The background circulation in winter and summer is northeast and southwest, respectively. The diversity of the background flow directions may take an important part in shifting effect of the near-inertial wave motion. According to the different directions of the background circulation, the signals of near-inertial oscillation were screened out and analyzed by synthesis for dis- cussing how the directions of background circulation affected the characteristics of near-inertial oscillation. The mechanism of this effect was explained from the perspective of background vorticity ： As the direction of background circulation is the northeast, the background vorticity becomes negative and the effective frequency decreases, the near-inertial oscillation frequency shifts to low frequency band （red shift） ; contrarily, the background vorticity be- comes positive and the effective frequency increases with the southwest background circulation, the near-inertial os- cillation frequency shifts to high frequency band （blue shift）.