Baroclinic instability and submesoscale eddy formation in weakly stratified oceans under cooling
Baroclinic instability and submesoscale eddy formation in weakly stratified oceans under cooling
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冷却下弱分层海洋的斜压不稳定性和亚中尺度涡流形成
DOI：
10.1029/2010jc006125
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发表时间：
2010
影响因子：
--
通讯作者：
K
中科院分区：
文献类型：
--
作者：
Akitomo;K
利用一个三维非静力模式对冷却条件下弱层结海洋中的斜压不稳定和次中尺度涡旋的形成进行了数值试验。两种类型的斜压不稳定性可以存在于两层海洋中，其中对流形成的深层混合层覆盖在弱层化的下层上。一种是在混合层中快速发展的短波长模式（浅模式），另一种是占据整个海洋深度的长波长模式（深模式）。当背景流较深时，浅模态首先发展，但几天后深模态取代它。相反，只有浅模式被激发的混合层覆盖的强分层层。线性稳定性分析很好地解释了这些结果。尽管主动冷却，随后形成的涡流抑制混合层和修改水的密度在整个深度。当背景流较浅时，斜压不稳定性仅限于混合层，并产生一个由表面增强的气旋性涡旋和中层增强的反气旋性涡旋组成的偶极子，这两个涡都是亚中尺度的（约10 km）。由对流形成但弱分层的水组成的反气旋涡流穿过锋面，进入对流无法到达的深层。以这种方式，密度改变延伸到混合层下方。地面冷却和斜压性通过激活次中尺度涡旋的形成增强了混合层的抑制作用和深度的密度修正。这些结果可以解释在冷却季节观测到的深层混合层的抑制现象和最近在极地海洋探测到的次中尺度相干涡旋的起源。
Numerical experiments with a three‐dimensional nonhydrostatic model have been performed to investigate baroclinic instability and submesoscale eddy formation in weakly stratified oceans under cooling. Two types of baroclinic instability can exist in a two‐layered ocean where the convectively formed deep mixed layer overlies the weakly stratified lower layer. One rapidly develops in the mixed layer with short wavelengths (shallow mode), and the other occupies the whole ocean depth with long wavelengths (deep mode). When a background flow is deep, the shallow mode develops first, but the deep mode replaces it in several days. In contrast, only the shallow mode is excited in the mixed layer overlying the strongly stratified layer. The linear stability analysis explains these results well. Despite active cooling, subsequently formed eddies restratify the mixed layer and modify water density over the whole depth. When a background flow is shallow, baroclinic instability is confined to the mixed layer and produces a dipole of surface‐intensified cyclonic eddy and middepth‐intensified anticyclonic eddy, both of which are of submesoscale (∼10 km). The anticyclonic eddy consisting of convectively formed but weakly stratified water moves across the front to ventilate the deep layer, where convection does not reach locally. In this way, density modification extends below the mixed layer. Surface cooling as well as baroclinicity enhances restratification of the mixed layer and density modification at depths by activating submesoscale eddy formation. The results can explain the restratification of the deep mixed layer observed during a cooling season and the origins of submesoscale coherent vortices recently detected in polar oceans.