Small-scale instabilities in the upper equatorial oceans

赤道海洋和大气通过马登朱利安振荡和厄尔尼诺-南方涛动（ENSO）等众所周知的现象，在年度到年际尺度上调节天气和气候方面发挥着至关重要的作用。赤道纬向流系统中的湍流混合是海洋-大气耦合系统的一个关键组成部分，但在厄尔尼诺/南方涛动的预测模型中，没有预测能力准确地表示它。间歇性的剪切不稳定性可以触发持续数小时的混合事件，并解释了表层和深海之间的大部分热量和其他流动性质的转移。由于海洋和大气混合的大多数过程都涉及类似的事件，因此更好地了解它们将具有重大价值。不稳定性之前的流动条件的线性稳定性分析（LSA）是对这种理解的必要的第一步。以前的LSA项目最多只覆盖了几周的海洋演变。在这里，将对赤道海洋中与气候有关的距离和时间尺度上的幕式切变不稳定性进行统计调查。由于剪切不稳定性在地球物理流动中普遍存在，LSA方法在各种各样的研究项目中具有价值。在该项目下，下一代LSA代码将被开发并提供给社区。该项目将增加不稳定性和湍流之间关系的知识，这在流体力学的所有领域都是至关重要的知识。自1980年代后期以来，热带大气海洋系泊阵列的观测记录提供了赤道东太平洋剪切不稳定性长期模式的诱人一瞥。这一记录包括1997 - 98年强大的厄尔尼诺现象和其他几次厄尔尼诺和拉尼娜事件。统计的理查森数（稳定浮力梯度的比例，不稳定的剪切）表明，潜在的不稳定的流动条件是常见的，在北方的夏季，秋季和冬季，但在北方的春季少得多。此外，这种情况在拉尼娜和ENSO中性状态下很常见，但在厄尔尼诺期间则要少得多。由于未知的原因，Richardson数没有区分夏季和冬季，也没有区分拉尼娜和ENSO中性条件。线性稳定性分析是一种预测小扰动增长的方法，从而描述混合事件的初始阶段。不稳定性的第一个衡量标准是它的指数增长率，它必须足够快，以超过任何混杂的外部影响;例如，夜间表面冷却引起的对流不稳定性只有到日出才能增长。LSA还预测了有限振幅结构的波长和取向。显式LSA是Richardson数分析的一种改进。它不仅可以解释这些区别，而且还可以告诉我们，在这些条件下，剪切不稳定性是否真的存在，而且还可以告诉我们参数化所产生的湍流所需的空间和时间尺度。迄今为止，大部分知识来自太平洋冷舌内的一个单一位置，西经140度。拟议的LSA将涵盖赤道太平洋以及大西洋和印度洋的系泊记录。

The equatorial ocean and atmosphere play a crucial role in modulating weather and climate on annual to inter-annual scale through well-known phenomena such as the Madden Julian Oscillation and the El Nino - Southern Oscillation (ENSO). Turbulent mixing in the equatorial zonal current system is a crucial component of that coupled ocean-atmosphere system, but no predictive capacity exists to represent it accurately in forecast models for ENSO. Episodic shear instabilities can trigger mixing events lasting up to a few hours and account for most of the transfer of heat and other flow properties between the surface and the deep ocean. Because similar events are involved in most of the processes that mix the oceans and the atmosphere, a better understanding of them will be of great value. Linear stability analysis (LSA) of the flow conditions that precede instability is a necessary first step toward such understanding. Previous LSA projects have covered at most a few weeks of ocean evolution. Here, a statistical survey of episodic shear instabilities over climatologically relevant distances and time scales in the equatorial oceans will be conducted. Because shear instability is ubiquitous in geophysical flows, LSA methods are of value in a wide variety of research projects. Under the project, the next-generation LSA codes will be developed and made available to the community. This project will add to the knowledge of the relationship between instability and turbulence, knowledge that is vital in all areas of fluid mechanics.Observational records from the Tropical Atmosphere Ocean mooring array since the late 1980s provide a tantalizing glimpse of the long-term patterns of shear instability in the eastern equatorial Pacific. This record includes the powerful El Niño of 1997-98 and several other El Niño and La Niña events. Statistics of the Richardson number (the ratio of stabilizing buoyancy gradient to destabilizing shear) indicate that potentially unstable flow conditions are common during boreal summer, fall and winter but much less so in boreal spring. Moreover, such conditions are common in the La Niña and ENSO-neutral states, but much less so during El Nino. For reasons unknown, the Richardson number does not distinguish between summer and winter, nor between La Niña and ENSO-neutral conditions. Linear stability analysis is a method for predicting the growth of small perturbations, and thereby describing the initial stage of a mixing event. The first measure of an instability is its exponential growth rate, which must be rapid enough to outpace any confounding external influences; for example, convective instability caused by nocturnal surface cooling has only until sunrise to grow. LSA also predicts the wavelength and orientation of the finite amplitude structures. Explicit LSA is a refinement of the Richardson number analysis. It promises not only to explain these distinctions, but also to tell us not only whether shear instability actually exists during these conditions, but also the space and time scales needed to parameterize the resulting turbulence. Most of the knowledge to date comes from a single location, 140W longitude, within the Pacific cold tongue. The proposed LSA will cover mooring records from across the equatorial Pacific and also from the Atlantic and Indian oceans.