南海西北部台风激发近惯性波频移研究

Study on frequency shift of typhoon-excited near-inertial waves in northwestern South China Sea

近惯性波是海洋对台风响应过程中最为重要的能量传递方式之一,其频率随深度的分布会影响近惯性波能量向海洋内部传播的速度。基于台风期间南海西北部的潜标观测资料,对涡度效应和背景流引起的多普勒效应进行分析,发现背景流引起的多普勒效应是近惯性波频率“蓝移”的主要因素,近惯性波频率大于当地惯性频率,并且频率随着深度增加而增加。深度在200 m以浅,背景流多普勒效应为负;深度在200 m左右,多普勒效应趋于0;深度在230~400 m,多普勒效应为正。在230~400 m深度,背景流强度达到最大,其方向与近惯性波传播方向接近,因此由其引起的多普勒正频移也最大,观测结果显示近惯性波频率偏移也最大。该研究对加深海洋对台风响应过程的认识,特别是近惯性波在背景流结构复杂海域(如西边界流区域)的传播具有重要作用。

Near-inertial waves(NIWs)play an important role in the response of ocean to typhoon.Their frequency varies with the depth and is the main factor in determining the propagation rate of near-inertial energy to the ocean interior.Based on the observation data from mooring,the factors affecting the blue-shift frequency of NIWs excited by typhoon were investigated in northwestern South China Sea.By analyzing the vorticity effect and Doppler effect caused by background currents,this study suggests that the Doppler effect of background currents was the main factor in the blue-shift frequency of NIWs.As depth increased,inertial wave frequencies increased.Quantitative calculations further demonstrated that within the upper 200 meters,the Doppler effect of the background currents was negative,approaching zero in depth around 200 meters.However,in the depth range of 230 to 400 meters,the Doppler effect became positive.This depth range exhibited the maximum strength of the background currents,with their direction aligned with the propagation direction of inertial waves.Consequently,the positive Doppler shift induced by the background currents was most pronounced.The results of this study are important for improving the understanding of the ocean response to typhoons,especially the propagation of near-inertial waves in areas with complex background current structure(e.g.,the western boundary current region).