基于实测资料的南海北部台风“海鸥”致近惯性振荡研究

A study of near-inertial oscillations in the northern South China Sea based on in-situ observations during the passage of Typhoon Kalmaegi

文章利用2014年9月南海东北部附近海域的A、B两个相距约20km的潜标数据,分析了台风"海鸥"在南海东北部激发的近惯性振荡的垂向分布特征。结果表明,台风过境在两站点激发了强烈的近惯性振荡,其能量由海表向海洋内部传播;近惯性能量在表层随着深度增加而增大,最大值出现在次表层,此后随着深度增加迅速衰减。但是两个观测站点的近惯性振荡垂向分布特征有较大的差别:A站点的近惯性振荡在不同深度上存在三个不同的垂向相速度,而B站点的垂向相速度没有变化;两站点的最大近惯性动能的大小及其所在深度不同,B站点比A站点的最大近惯性动能大15%左右。台风过后两个站点均出现由近惯性波f和半日内潮波D_2非线性相互作用产生的次级波动fD_2,但其强度存在差异。台风后A站点fD_2在不同深度上有不同的变化,f和D_2相互作用较弱;而B站点的fD_2在整个近惯性振荡影响深度内都是增强,其fD_2和f频带的动能和流速在时空分布以及变化趋势上有较好的相关性。两站点的近惯性振荡垂向相速度以及次级波动fD_2的不同可能是水体层结的差异和涡旋的影响所导致的。

In this study,the vertical characteristics of near-inertial oscillations(NIOs)induced by Typhoon Kalmaegi are analyzed by using data from two moorings about20km apart in the northeastern South China Sea in Sept.2014.The results show that the energy of typhoon-induced NIOs,which propagates downward into the ocean interior,increases with depth in the upper layer and becomes maximum in the mid layer before dissipating with depth.However,we find that there is a large difference in vertical characteristics of typhoon-induced NIOs between the two moorings.The NIOs at mooring A had three different vertical phase velocities at different depths while the NIOs at mooring B showed invariable phase velocity.The value and depth of the maximum near-inertial kinetic energy(NIKE)are also different at the two moorings.The maximum NIKE was15%greater at mooring B than at mooring A.After the passage of Kalmaegi,fD2via nonlinear wave interaction between f and D2occurred at the two moorings,except that the intensities of fD2at the two moorings were different.At mooring A,fD2changed with depth and its interaction between f and D2was weak.The energy of fD2at mooring B enhanced at the full depth,and the kinetic energy and velocities of fD2and f had a good correlation for their time-space distributions and variations.The difference of vertical phase velocity of NIOs and fD2at the two moorings may be caused by the effects of eddy and different stratifications.