Typhoon Megi passed between two subsurface moorings in the northern South China Sea in October 2010 and the upper ocean thermal and dynamical response with strong internal tides present was examined in detail. The entire observed water column (60–360 m) was cooled due to strong Ekman-pumped upwelling (up to 50 m in the thermocline) by Megi, with maximum cooling of 4.2°C occurring in thermocline. A relatively weak (maximum amplitude of 0.4 m s−1) and quickly damped (e-folding time scale of 2 inertial periods) near-inertial oscillation (NIO) was observed in the mixed layer. Power spectrum and wavelet analyses both indicated an energy peak appearing at exactly the sum frequency fD1 (with maximum amplitude up to 0.2 m s−1) of NIO (f) and diurnal tide (D1), indicating enhanced nonlinear wave-wave interaction between f and D1 during and after typhoon. Numerical experiments suggested that energy transfer from NIO to fD1 via nonlinear interaction between f and D1 may have limited the growth and accelerated the damping of mixed layer NIO generated by Megi. The occurrence of fD1 had a high correlation with NIO; the vertical nonlinear momentum term, associated with the vertical shear of NIO and vertical velocity of D1 or vertical shear of D1 and vertical velocity of NIO, was more than 10 times larger than the horizontal terms and was responsible for forcing fD1. After Megi, surface-layer diurnal energy was enhanced by up to 100%, attributed to the combined effect of the increased surface-layer stratification and additional Megi-forced diurnal current.
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机译:台风“梅吉”于2010年10月在南海北部的两个水下系泊之间通过,并详细研究了存在强内潮的上层海洋的热力和动力响应。梅吉(Megi)强力进行埃克曼泵送上升流(在热跃层中长达50 m),从而冷却了整个观察到的水柱(60-360 m),在热跃层中出现了最大4.2°C的冷却。在混合层中观察到相对较弱(最大振幅为0.4 m s-1)和快速衰减(2个惯性周期的电子折叠时间标度)的近惯性振动(NIO)。功率谱和小波分析都表明能量峰值恰好出现在NIO(f)和日潮(D1)的总和频率fD1(最大振幅高达0.2 ms-1)上,表明f和台风期间及之后的D1。数值实验表明,通过f和D1之间的非线性相互作用,从NIO到fD1的能量转移可能限制了Megi生成的混合层NIO的生长并加速了阻尼。 fD1的发生与NIO高度相关;垂直非线性动量项与NIO的垂直剪切力和D1的垂直速度或D1的垂直剪切力和NIO的垂直速度有关,比水平项大10倍以上,并且是强迫fD1的原因。 Megi之后,由于增加的表层分层和额外的Megi强迫的昼夜电流共同作用,表层的昼夜能量增加了高达100%。
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