Degradation of an internal wave beam by parametric subharmonic instability in an upper ocean pycnocline
Degradation of an internal wave beam by parametric subharmonic instability in an upper ocean pycnocline
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DOI：
10.1002/jgrc.20321
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发表时间：
2013-09
影响因子：
--
通讯作者：
B. Gayen;S. Sarkar
中科院分区：
文献类型：
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作者：
B. Gayen;S. Sarkar
[1] 通过数值模拟来研究半日内波 (IW) 光束与上层海洋密斜层的不均匀分层的相互作用。在相互作用的初始阶段，高次谐波在 IW 光束在焦散处反射后产生，并被捕获在密斜层中。在密度斜层厚度足够大（大约大于光束主波长的两倍）的情况下，入射光束在稍后通过密度斜层折射期间会经历参数次谐波不稳定性（PSI）。因此，存在显着的能量转移到次谐波运动，并以 2/3 day−1 的速率呈指数增长。通过波频率和波数空间的诊断，证明这种不稳定性是共振三元组相互作用。较小的垂直尺度起源于 PSI，导致波陡化，并产生对流翻转。这项工作确定了一种潜在机制，该机制会导致 IW 光束在上层海洋传播过程中退化。
[1] Numerical simulations are performed to investigate the interaction of a semidiurnal internal wave (IW) beam with the nonuniform stratification of an upper ocean pycnocline. During the initial stage of the interaction, higher harmonics originate after reflection of the IW beam at the caustic and are trapped in the pycnocline. In cases where the pycnocline thickness is sufficiently large (approximately larger than twice the dominant wavelength in the beam), the incoming beam undergoes a parametric subharmonic instability (PSI) during refraction through the pycnocline at later time. Consequently, there is significant energy transfer to subharmonic motions that exhibit exponential growth with a rate of 2/3 day−1. The instability is demonstrated to be a resonant triad interaction by diagnostics in both wave frequency and wave number spaces. Smaller vertical scales originate during the PSI, lead to wave steepening, and produce convective overturns. This work identifies a potential mechanism that drives IW beam degradation during its propagation through the upper ocean.