Boundary layer-mediated vorticity generation in currents over sloping bathymetry
Boundary layer-mediated vorticity generation in currents over sloping bathymetry
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DOI：
10.1175/jpo-d-20-0253.1
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发表时间：
2021-03
影响因子：
3.5
通讯作者：
Arjun Jagannathan;K. Srinivasan;J. McWilliams;M. Molemaker;A. Stewart
中科院分区：
文献类型：
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作者：
Arjun Jagannathan;K. Srinivasan;J. McWilliams;M. Molemaker;A. Stewart
海流-地形相互作用在海洋中产生了跨越各种时空尺度的涡旋。最新的数值模拟结果表明，海岸和水下地形是亚中尺度相干涡（SCV）的重要生成场所，其特征是水平尺度为（0.1 - 10）km。使用理想化的，亚中尺度和BBL解决模拟，并采用一个集成的涡度平衡制定，我们精确地量化的底部边界层（BBL）的作用，在涡度生成过程。特别是，我们表明，地形斜坡上的涡量产生主要是由于在底部的应力的垂直发散所施加的扭矩。我们称之为底部应力分散扭矩（BSDT）。BSDT是一个基本上非保守的扭矩，它在综合涡度收支中作为源项出现，并与更熟悉的底部应力旋度（BSC）相区别。它是密切相关的底部压力扭矩（BPT）通过在底部的水平动量平衡，事实上是一个很好的解决方案，解决方案中的BPT的主要组成部分。这表明BPT解释为粘性涡量生成分量（BSDT）和无粘性“流动转向”分量的总和。没有底阻力的伴随模拟表明，尽管涡量的产生仍然可以通过涡拉伸和倾斜的无粘机制发生，但尾涡往往具有较弱的环流，能量大大降低，空间尺度较小。
Current-topography interactions in the ocean give rise to eddies spanning a wide range of spatial and temporal scales. Latest modeling efforts indicate that coastal and underwater topography are important generation sites for submesoscale coherent vortices (SCVs), characterized by horizontal scales of (0.1 – 10) km. Using idealized, submesoscale and BBL-resolving simulations and adopting an integrated vorticity balance formulation, we quantify precisely the role of bottom boundary layers (BBLs) in the vorticity generation process. In particular, we show that vorticity generation on topographic slopes is attributable primarily to the torque exerted by the vertical divergence of stress at the bottom. We refer to this as the Bottom Stress Divergence Torque (BSDT). BSDT is a fundamentally nonconservative torque that appears as a source term in the integrated vorticity budget and is to be distinguished from the more familiar Bottom Stress Curl (BSC). It is closely connected to the bottom pressure torque (BPT) via the horizontal momentum balance at the bottom and is in fact shown to be the dominant component of BPT in solutions with a well-resolved BBL. This suggests an interpretation of BPT as the sum of a viscous, vorticity generating component (BSDT) and an inviscid, ‘flow-turning ’ component. Companion simulations without bottom drag illustrate that although vorticity generation can still occur through the inviscid mechanisms of vortex stretching and tilting, the wake eddies tend to have weaker circulation, be substantially less energetic, and have smaller spatial scales.