Effects of Ambient Turbulence on Interleaving at a Baroclinic Front
Effects of Ambient Turbulence on Interleaving at a Baroclinic Front
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DOI：
10.1175/2009jpo4297.1
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发表时间：
2010-04
影响因子：
3.5
通讯作者：
W. Smyth;B. Ruddick
中科院分区：
文献类型：
--
作者：
W. Smyth;B. Ruddick
本文采用理论和数值计算相结合的方法，结合梅迪沙龙和法罗锋的观测资料，研究了环境湍流对温盐交错的作用。通过允许动量、热量和盐的湍流扩散系数依赖于背景梯度并随着不稳定性的增长而发展，改进了以前使用的高度简化的环境湍流模型。以往的研究表明，环境湍流，在典型的海平面，可以淬灭斜压锋上的温盐交错不稳定。这些发现与斜压锋区尽管有环境湍流但交错是常见的观察相冲突。对现有理论的另一个挑战来自数值实验，实验表明剪切盐指的施密特数比以前假设的要小得多。在交织计算中使用修正值导致交织层比观察到的交织层更弱且更薄。本研究旨在解决这些悖论。作者表明，当湍流的普朗特数大于1时，湍流动量通量可以补偿减少的施密特数盐指。因此，环境湍流决定了交错的垂直尺度。在典型的海洋交错结构中，即使使用改进的湍流模型，观察到的属性梯度也不足以在可观察的水平上预测交错增长。不过，这个缺陷很小：梯度再尖锐几十个百分点就足以支持不稳定性。作者认为，这是由于交织在擦除这些属性梯度方面的效率。一类新的机制交错，湍流扩散系数的流动依赖性波动驱动，确定。其基本机制类似于众所周知的菲利普斯分层不稳定性;然而，由于科里奥利效应，它有一个明确的垂直尺度，也有一个倾斜角相反的指状驱动交错。
In this paper the authors investigate the action of ambient turbulence on thermohaline interleaving using both theory and numerical calculations in combination with observations from Meddy Sharon and the Faroe Front. The highly simplified models of ambient turbulence used previously are improved upon by allowing turbulent diffusivities of momentum, heat, and salt to depend on background gradients and to evolve as the instability grows. Previous studies have shown that ambient turbulence, at typical ocean levels, can quench the thermohaline interleaving instability on baroclinic fronts. These findings conflict with the observation that interleaving is common in baroclinic frontal zones despite ambient turbulence. Another challenge to the existing theory comes from numerical experiments showing that the Schmidt number for sheared salt fingers is much smaller than previously assumed. Use of the revised value in an interleaving calculation results in interleaving layers that are both weaker and thinner than those observed. This study aims to resolve those paradoxes. The authors show that, when turbulence has a Prandtl number greater than unity, turbulent momentum fluxes can compensate for the reduced Schmidt number of salt fingering. Thus, ambient turbulence determines the vertical scale of interleaving. In typical oceanic interleaving structures, the observed property gradients are insufficient to predict interleaving growth at an observable level, even when improved turbulence models are used. The deficiency is small, though: gradients sharper by a few tens of percent are sufficient to support instability. The authors suggest that this is due to the efficiency of interleaving in erasing those property gradients. A new class of mechanisms for interleaving, driven by flow-dependent fluctuations in turbulent diffusivities, is identified. The underlying mechanism is similar to the well-known Phillips layering instability; however, because of Coriolis effects, it has a well-defined vertical scale and also a tilt angle opposite to that of fingerdriven interleaving.