Energy Exchanges between Density Fronts and Near-Inertial Waves Reflecting off the Ocean Surface
Energy Exchanges between Density Fronts and Near-Inertial Waves Reflecting off the Ocean Surface
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DOI：
10.1175/jpo-d-15-0072.1
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发表时间：
2016-02-01
影响因子：
3.5
通讯作者：
Thomas, Leif N.
中科院分区：
文献类型：
--
作者：
Grisouard, Nicolas;Thomas, Leif N.
在地转平衡锋中向上传播的惯性波在海面上经历临界反射。这种反射自然会产生小垂直尺度的振荡，粘性摩擦成为主导过程。在这里，摩擦改变了内波的极化关系，并允许来自平衡锋的能量与非地转运动交换并最终消散。此外，在众所周知的无粘性情况下，内波只以两个特征传播，而本研究使用解析模型证明，强粘性效应引入了额外的振荡模式，可以与锋面交换能量。此外，在线性的近临界反射中，这些振荡的叠加引起了与锋面更强的能量交换。当基于锋面热风切变的Richardson数为0(1)时，能量交换率在接近惯性的波频率处达到峰值，其量级与入射向上传播波的能量通量相当。二维线性数值实验证实了这一发现。分析模型还表明，该过程对实际粘度值或表面边界条件的形式在定性上不敏感。在完全非线性的实验中，作者恢复了这些定性结论。然而，非线性波-波相互作用，特别是湍流，强烈地改变了与锋面交换的能量。实际上，只有当入射波的频率高于科里奥利频率时，这种非线性效应才会起作用，因为这些结构有利于入射波和反射波之间的近共振三分量相互作用。
Inertial waves propagating upward in a geostrophically balanced front experience critical reflections against the ocean surface. Such reflections naturally create oscillations with small vertical scales, and viscous friction becomes a dominant process. Here, friction modifies the polarization relations of internal waves and allows energy from the balanced front to be exchanged with the ageostrophic motions and eventually dissipated. In addition, while in the well-known inviscid case internal waves propagate on only two characteristics, this study demonstrates using an analytical model that strong viscous effects introduce additional oscillatory modes that can exchange energy with the front. Moreover, during a linear, near-critical reflection, the superposition of several of these oscillations induces an even stronger energy exchange with the front. When the Richardson number based on the frontal thermal wind shear is O(1), the rate of energy exchange peaks at wave frequencies that are near inertial and is comparable in magnitude to the energy flux of the incident, upward-propagating waves. Two-dimensional, linear numerical experiments confirm this finding. The analytical model also demonstrates that this process is qualitatively insensitive to the actual value of the viscosity or the form of the boundary condition at the surface. In fully nonlinear experiments, the authors recover these qualitative conclusions. However, nonlinear wave-wave interactions and turbulence in particular, strongly modify the amount of energy that is exchanged with the front. In practice, such nonlinear effects are only active when the incident waves have frequencies higher than the Coriolis frequency, since these configurations are conducive to near-resonant triad interactions between incident and reflected waves.