南海北部台风和中尺度暖涡对近惯性振荡的影响

Effects of typhoon and the mesoscale warm eddy on the near-inertial oscillations in the northern of the South China Sea

基于2014年8-9月南海北部东沙群岛附近海域两个临近站位(站位A,20.736°N,117.745°E,水深1 249 m;站位B,20.835°N,117.56°E,水深848 m)的潜标数据,研究了台风过境所激发的近惯性振荡的特征,分析了中尺度暖涡对近惯性频率的调制及其对近惯性动能分布和传播的影响。站位A(B)142(175) m以浅,近惯性频率由0.710 1(0.713 3)周/d红移至0.659 2周/d,频率减小了7.2%(7.6%),观测结果与两个站位所处的背景涡度相吻合。中尺度暖涡改变了水体层结状态,两个站位的近惯性动能在不同层结中被改变了0.5~3倍。水体层结对能量的折射作用使得站位B的近惯性动能在深度158~223 m之间衰减较少,而站位A的近惯性动能则随着深度的增加快速减小。站位A和站位B近惯性内波的垂向群速度分别约为15.2 m/d和14.1 m/d。如果忽略近惯性动能的水平辐散,近惯性内波的垂向传播分别造成了两个站位垂向上约47%和38%的近惯性动能衰减。

Two moorings(Station A, 20.736°N, 117.745°E, 1 249 m;Station B, 20.835°N, 117.56°E, 848 m) were located near the Dongsha Islands in the northern of the South China Sea from August to September 2014. Based on the measured data, we have studied the characteristics of near-inertial oscillations(NIOs) induced by typhoons, and analyzed the modulation of mesoscale warm eddy on the near-inertial frequency and its influence on the distribution and propagation of near-inertial kinetic energy(NIKE).The frequency of the NIOs at Station A(B) is 0.659 2 circle per day< at depths shallower than 142(175) m with a redshift of 7.2%(7.6%) from the local initial frequency of 0.710 1(0.713 3) circle per day, which is consistent with their background vorticities. The mesoscale warm eddy changes the stratification, which causes the NIKE of the two stations modified by 0.5-3 times in different stratifications. The refraction effect of stratification on energy makes the NIKE relatively concentrate with small decay between 158 and 223 m at Station B but rapidly decrease at Station A. The vertical group velocities of near-inertial internal waves at Station A and Station B are approximately 15.2 m/d and 14.1 m/d. Ignore the loss of NIKE in horizontal direction, their vertical propagation causes approximately 47% and 38% of NIKE decay in vertical direction,respectively.spectively.