Generation of Bulk Shear Spikes in Shallow Stratified Tidal Seas
Generation of Bulk Shear Spikes in Shallow Stratified Tidal Seas
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DOI：
10.1175/2008jpo4074.1
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发表时间：
2009-04-01
影响因子：
3.5
通讯作者：
Rippeth, Tom P.
中科院分区：
文献类型：
--
作者：
Burchard, Hans;Rippeth, Tom P.
近年来对温跃层剪切和层结的精细观测表明，温跃层具有边缘稳定性，这表明增强的剪切事件可能导致剪切不稳定和底辟混合。季节性分层的陆架海的上下边界层之间的整体剪切显示出显着的变化，潮汐，惯性和天气的时间尺度，尚未得到解释。本文介绍了当水柱具有明显的两层结构时，北海北方季节性分层的观测结果。基于ADCP测量的整体剪切估计显示了在当地惯性周期顺时针方向旋转的整体剪切矢量，具有大约每天两次的整体剪切峰值，并且发生在持续数天的突发中。为了解释这一观察，一个简单的两层模型的基础上层平均的一维动量方程的开发，迫使在表面的风应力和阻尼（潮汐为主）海床摩擦。这两个层然后通过界面应力项连接。该模型再现的意见，表明批量剪切尖峰的风应力，潮床应力和（顺时针旋转）批量剪切矢量的对齐的结果。速度微观结构的测量，然后用于确认增强的混合水平在一段时间的批量剪切峰值。数值研究表明，穗产生机制的敏感性，当地的潮汐条件和定相和风事件的持续时间。
Recent finescale observations of shear and stratification in temperate shelf sea thermoclines show that they are of marginal stability, suggesting that episodes of enhanced shear could potentially lead to shear instability and diapcynal mixing. The bulk shear between the upper and lower boundary layers in seasonally stratified shelf seas shows remarkable variability on tidal, inertial, and synoptic time scales that has yet to be explained. In this paper observations from the seasonally stratified northern North Sea are presented for a time when the water column has a distinct two-layer structure. Bulk shear estimates, based on ADCP measurements, show a bulk shear vector that rotates in a clockwise direction at the local inertial period, with episodes of bulk shear spikes that have an approximately twice daily period, and occur in bursts that last for several days. To explain this observation, a simple two-layer model based on layer averaging of the one-dimensional momentum equation is developed, forced at the surface by wind stress and damped by (tidally dominated) sea bed friction. The two layers are then linked through an interfacial stress term. The model reproduces the observations, showing that the bulk shear spikes are a result of the alignment of the wind stress, tidal bed stress, and (clockwise rotating) bulk shear vectors. Velocity microstructure measurements are then used to confirm enhanced levels of mixing during a period of bulk shear spikes. A numerical study demonstrates the sensitivity of the spike generation mechanism to the local tidal conditions and the phasing and duration of wind events.