Surface mixed layer deepening through wind shear alignment in a seasonally stratified shallow sea
Surface mixed layer deepening through wind shear alignment in a seasonally stratified shallow sea
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DOI：
10.1002/2015jc011382
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发表时间：
2016-08
影响因子：
--
通讯作者：
B. Lincoln;T. Rippeth;J. Simpson
中科院分区：
文献类型：
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作者：
B. Lincoln;T. Rippeth;J. Simpson
惯性振荡是海洋表面普遍存在的特征。在这里，我们结合联合收割机新的观测与数值模式，探讨惯性振荡的作用，在驱动加深的表面混合层在季节性分层的海。在西爱尔兰海的一个系泊处对温度和海流结构的观测显示，强海流（>0.3 m s−1）持续数天，导致温跃层上的切变增强。虽然强海流的发生与大风期一致，但切变的变化与风应力没有直接关系。切变在亚惯性时间尺度上变化，切变最大值的形成持续几个小时，发生在当地惯性周期14.85小时。这些切变最大值与表面流的方向一致，与风向的右侧成大约90°的角度。在多风期间的水柱结构的观察揭示了加深的表面混合层在一系列的步骤，这与一段时间的增强剪切。在切变梯度增强期间，理查森数估计表明，在地面混合层底部的Ri−1 ≥ 4，这意味着切变不稳定性导致的加深。一维垂直交换模式成功地再现了剪切峰的大小和相位以及台阶状加深。因此，观测和模型结果确定风切变对齐的作用，作为一个关键的夹带机制驱动表面混合层加深在一个浅，季节性分层的海。
Inertial oscillations are a ubiquitous feature of the surface ocean. Here we combine new observations with a numerical model to investigate the role of inertial oscillations in driving deepening of the surface mixed layer in a seasonally stratified sea. Observations of temperature and current structure, from a mooring in the Western Irish Sea, reveal episodes of strong currents (>0.3 m s−1) lasting several days, resulting in enhanced shear across the thermocline. While the episodes of strong currents are coincident with windy periods, the variance in the shear is not directly related to the wind stress. The shear varies on a subinertial time scale with the formation of shear maxima lasting several hours occurring at the local inertial period of 14.85 h. These shear maxima coincide with the orientation of the surface current being at an angle of approximately 90° to the right of the wind direction. Observations of the water column structure during windy periods reveal deepening of the surface mixed layer in a series of steps which coincide with a period of enhanced shear. During the periods of enhanced shear gradient, Richardson number estimates indicate Ri−1 ≥ 4 at the base of the surface mixed layer, implying the deepening as a result of shear instability. A one-dimensional vertical exchange model successfully reproduces the magnitude and phase of the shear spikes as well as the step-like deepening. The observations and model results therefore identify the role of wind shear alignment as a key entrainment mechanism driving surface mixed layer deepening in a shallow, seasonally stratified sea.