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首页> 外文期刊>Journal of Physical Oceanography >Vortex Dipole Formation by Baroclinic Instability of Boundary Currents
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Vortex Dipole Formation by Baroclinic Instability of Boundary Currents

机译：边界流斜压不稳定性形成的涡旋偶极子

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摘要

In situ data of the Mediterranean Water undercurrents and eddies south of Portugal indicate that the undercurrents have a tubelike structure in potential vorticity and that dipole formation can occur when the lower undercurrent extends seaward below an offshore upper countercurrent. A two-layer quasigeostrophic model is used to determine the dynamical conditions under which dipole formation is possible. With piecewise-constant potential vorticity, the flow exhibits two linear modes of instability comparable to those found in the Phillips model with topography. Weakly nonlinear analysis and fully nonlinear simulations of the flow evolution agree on the regimes of either finite-amplitude perturbation saturation, corresponding to filamentation, or amplification, corresponding to vortex or dipole formation. This latter regime is more specifically studied: vortex dipole formation and ejection from the coast is obtained for long waves, with opposite-signed but similar amplitude layer potential vorticities. A simple point vortex model reproduces this phenomenon under the same conditions. It is then shown that dipole formation occurs for minimal wave dispersion, and hence for weak horizontal velocity shears. As observed at sea, dipoles are formed when the lower potential vorticity core extends seaward below a countercurrent.
机译：葡萄牙南部的地中海水暗流和涡流的原位数据表明，该暗流的潜在涡流具有管状结构，当下部暗流向海延伸至离岸上部逆流以下时，会发生偶极子形成。使用两层准地转模型来确定可能形成偶极子的动力学条件。具有分段恒定的势涡，该流表现出两种不稳定性的线性模式，可与具有地形的菲利普斯模型中发现的线性模式相比。流动演化的弱非线性分析和完全非线性模拟在有限振幅摄动饱和（对应于细丝化）或放大（对应于涡旋或偶极子形成）的状态上达成了一致。对后一种情况进行了更具体的研究：对于长波，具有相反符号但振幅层的电势相似，获得了涡旋偶极子的形成和从海岸的喷射。一个简单的点涡模型在相同条件下重现了这种现象。然后表明，偶极子的形成是为了使波的色散最小，从而使水平速度剪切变弱。如在海上观察到的那样，当较低电势涡旋磁芯向逆流下方向海延伸时，就会形成偶极子。

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