Ageostrophic Secondary Circulation at a Submesoscale Front and the Formation of Gravity Currents
Ageostrophic Secondary Circulation at a Submesoscale Front and the Formation of Gravity Currents
复制标题
亚中尺度锋面的地转二次环流和重力流的形成
DOI：
10.1175/jpo-d-17-0271.1
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发表时间：
2018
影响因子：
3.5
通讯作者：
S. Sarkar
中科院分区：
文献类型：
--
作者：
H. Pham;S. Sarkar
利用大涡模拟方法研究了亚中尺度锋面非地转次级环流的发展及其输送。根据孟加拉湾北方和太平洋冷舌的观测结果，模式锋面有一个大的横向锋面密度差，这部分是由横向温度和盐度梯度补偿的。垂直分层是不同的情况下，以探讨其对ASC的影响。ASC的演变与分层不同。当锋面不成层时，由地转切变发展而来的切变不稳定性使锋面滑塌。来自光侧的冷水在表面的锋面上传播，而来自密集侧的温水在深度方向相反。在有分层的情况下，在表面上形成由横向前缘压力梯度驱动的剪切层，以启动ASC。剪切驱动的湍流与层中增强的剪切力相关联，导致前缘滑塌，并且ASC向前的发展与未分层的情况相似。不管这里模拟的强锋的初始分层，表层演变成重力流。ASC由表面重力流和逆流组成，两者之间被中间层隔开，中间层具有增强的分层和逆温。湍流耗散在重力流的鼻端和逆流前缘后面的剪切区得到增强。在没有分层的情况下，重力流的传播速度与穿过锋面的浮力差成比例。
Large-eddy simulations are performed to investigate the development of the ageostrophic secondary circulation (ASC) and associated transport in a submesoscale front. Based on the observations in the northern Bay of Bengal and in the Pacific cold tongue, the model front has a large cross-front density difference that is partially compensated with lateral temperature and salinity gradients. Vertical stratification is varied in different cases to explore its effect on the ASC. The evolution of the ASC differs with stratification. When the front is unstratified, shear instabilities, which develop from the geostrophic shear, cause the front to slump. Cold water from the light side propagates across the front on the surface, while warm water from the dense side spreads in the opposite direction at depth. In cases with stratifications, a shear layer driven by the cross-front pressure gradient forms at the surface to initiate the ASC. Shear-driven turbulence associated with the enhanced shear in the layer causes the front to slump, and the development of the ASC onward is similar to the unstratified case. Irrespective of the initial stratification of the strong fronts simulated here, the surface layer evolves into a gravity current. The ASC is composed of the surface gravity current and a countercurrent that are separated by a middle layer with enhanced stratification and a thermal inversion. Turbulent dissipation is enhanced at the nose of the gravity current and in a sheared region somewhat behind the leading edge of the countercurrent. The gravity current propagates at a speed proportional to the buoyancy difference across the front in the case with no stratification.