南海内潮对台风下海洋响应过程的调制机理研究

Less knowledge on the oceanic response and feedback to the typhoon severely restricted the forecast level of typhoons. The South China Sea (SCS), not only suffers the most frequently typhoons in the world, but also contains actively multi-scale motions, especially for the strong internal tides. Recent research reported that the internal tides could obviously modulate the oceanic response and feedback to a typhoon: through the nonlinear coupling between internal tides and typhoon-generated near-inertial inertial wave, a secondary wave was generated and hence complicate the dynamical response; due to the nonlinear wave-wave interaction, near-inertial energy was transformed to high mode internal wave energy, enhanced the local turbulent mixing, modulated the upper ocean temperature evolving under typhoon forcing and potentially affect the oceanic feedback to a typhoon. Therefore, investigating the upper ocean response to typhoons with strong background internal tides present, could not only contribute to the understanding of physics of typhoon-ocean interaction in the SCS, but also be important to improve the typhoon's forecast level. Based on the abundant ocean current and temperature profiles under typhoons from continuously 3-year mooring arrays, associated with the ROMS model, the present project investigate the processes and mechanism of the nonlinear wave-wave interaction between typhoon-generated near-inertial wave, internal tides and the correspondingly secondary wave. Through this project, we also intend to quantify the modulation of internal tides on the upper ocean temperature evolving under typhoons from observation and sensitive model experiments (with or without background internal tides present), and finally discuss the potential influence of internal tides on the oceanic feedback to typhoons.

海洋对台风的响应与反馈过程的认知水平薄弱是制约台风预报的关键因子。南海是全球台风发生频率最高的海区之一，同时蕴含着活跃的内潮等多尺度动力过程。最新研究表明，背景内潮对海洋的响应与反馈过程具有重要的调制作用：内潮与台风激发近惯性内波发生非线性相互作用，产生了次级波动，调制了海洋对台风的响应过程；驱使了内波能量向高模态转化，加强了海洋混合过程，调制了海洋上层温度结构的演变过程，潜在影响了海洋对台风的反馈过程。因此，开展在南海强内潮背景场下海洋对台风的响应过程研究，对增进南海台风-海洋相互作用认识具有重大的科学意义，对提高我国台风预报水平和防灾减灾具有重要的应用价值。本课题以台风潜标阵列观测资料分析为出发点，结合数值模拟，探究台风激发近惯性内波与内潮非线性相互作用及次级波动的生消过程，阐明内波非线性相互作用及能量传递机制，探明内潮对台风过境期间海洋上层温度结构演变的调制作用与机理。

近几十年来，台风强度预报水平进展缓慢，主要限制因子之一是海洋对台风的响应与反馈过程的认知水平薄弱。南海是全球台风发生频率最高的海区之一，同时蕴含着活跃的内潮等多尺度动力过程，开展在南海强内潮背景场下海洋对台风的响应过程研究，对增进南海台风-海洋相互作用认识具有重大的科学意义。.本项目基于多源现场观测资料，并结合海洋数值模式实验，开展强内潮背景场下台风期间上层海洋的动力学和热力学响应研究，主要取得如下进展：（1）观测发现与开阔大洋相比，南海台风过境期间上层海洋流场更为复杂，由近惯性内波、全日和半日内潮共同主导，并存在着活跃的次级波动；（2）阐明内潮通过与近惯性内波发生非线性相互作用，能量向高频谐波次级波动转移，非线性相互作用主要由垂直非线性项主导；（3）强背景内潮与次级波动可增强台风期间跨等密面混合，增大南海台风致海表面降温；同等台风条件下，南海台风引起的海表面降温全球最强；（4）台风路径左侧中尺度冷涡可以造成局地混合层变浅，使得海表面降温出现异常“左偏”现象；（5）与台风前相比，台风过后上层海洋混合率大大增强，最大增幅可达10倍以上。.本课题研究成果阐明了在强内潮背景场条件下，南海台风-海洋相互作用的独特特征与机理，对进一步研究内潮对上层海洋负反馈作用起到了指引性作用，对提高我国南海台风预报水平和防灾减灾具有重要的潜在应用价值。