Initiation of diffusive layering by time-dependent shear
Initiation of diffusive layering by time-dependent shear
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DOI：
10.1017/jfm.2018.790
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发表时间：
2018-11
影响因子：
3.7
通讯作者：
Justin M. Brown;T. Radko
中科院分区：
文献类型：
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作者：
Justin M. Brown;T. Radko
北极盐跃层对于双扩散和动态不稳定性的发展总体上是稳定的，这是中纬度海洋小规模混合的两个主要来源。尽管如此，观测结果显示北冰洋存在丰富的双扩散阶梯，这表明存在一些不稳定过程，促进从平滑梯度到层状分层的过渡。最近的研究表明，如果存在弱静态剪切，即使流动是动态和扩散稳定的，在这种情况下也会产生不稳定性。然而，对于扩散情况，与内部重力波的存在相关的振荡剪切的影响尚未得到解决。通过扩散对流的二维模拟，我们研究了外部强迫振荡剪切的幅度和频率对温盐剪切不稳定性的影响。随机剪切模拟（其特征是从惯性到浮力的连续频率谱）表明，由于不稳定模式（接近浮力频率的振荡）的存在，温盐分层确实发生。这些模拟表明，这些层在温度和盐度剖面中显示为明确定义的步骤。因此，温盐剪切不稳定性是北极阶梯形成的一种合理机制，值得未来进行大量研究。
The Arctic halocline is generally stable to the development of double-diffusive and dynamic instabilities – the two major sources of small-scale mixing in the mid-latitude oceans. Despite this, observations show the abundance of double-diffusive staircases in the Arctic Ocean, which suggests the presence of some destabilizing process facilitating the transition from smooth-gradient to layered stratification. Recent studies have shown that an instability can develop in such circumstances if weak static shear is present even when the flow is dynamically and diffusively stable. However, the impact of oscillating shear, associated with the presence of internal gravity waves, has not yet been addressed for the diffusive case. Through two-dimensional simulations of diffusive convection, we have investigated the impact of the magnitude and frequency of externally forced oscillatory shear on the thermohaline-shear instability. Simulations with stochastic shear – characterized by a continuous spectrum of frequencies from inertial to buoyancy – indicate that thermohaline layering does occur due to the presence of destabilizing modes (oscillations of near the buoyancy frequency). These simulations show that such layers appear as well-defined steps in the temperature and salinity profiles. Thus, the thermohaline-shear instability is a plausible mechanism for staircase formation in the Arctic and merits substantial future study.