Small-scale instabilities in the upper equatorial oceans

赤道海洋和大气通过马登-朱利安涛动和厄尔尼诺-南方涛动(ENSO)等众所周知的现象，在年际尺度上对天气和气候起着至关重要的调节作用。赤道纬向流系统中的湍流混合是海-气耦合系统的重要组成部分，但在ENSO预报模式中没有准确描述它的预报能力。阶段性的切变不稳定性可以触发持续数小时的混合事件，并解释了表面和深海之间热量和其他流动特性的大部分转移。由于大多数混合海洋和大气的过程中都涉及到类似的事件，因此更好地了解它们将是非常有价值的。对失稳之前的流动条件进行线性稳定性分析(LSA)是通向这种理解的必要的第一步。之前的LSA项目最多只涵盖了几周的海洋演化。在这里，将对赤道海洋气候相关距离和时间尺度上的幕式切变不稳定进行统计调查。由于剪切不稳定性在地球物理流动中普遍存在，因此LSA方法在各种研究项目中都有价值。根据该项目，将开发下一代LSA代码并向社区提供。这个项目将增加关于不稳定和湍流之间关系的知识，这一知识在流体力学的所有领域都是至关重要的。自1980年代末以来，热带大气海洋系泊阵列的观测记录提供了赤道东太平洋切变不稳定的长期模式的诱人一瞥。这一记录包括1997-98年的强大厄尔尼诺现象，以及其他几次厄尔尼诺和拉尼娜事件。对Richardson数(稳定浮力梯度与不稳定切变的比率)的统计表明，在北方夏季、秋季和冬季，潜在的不稳定流动状况很常见，但在北方春季，潜在不稳定流动状况要少得多。此外，这种情况在拉尼娜和ENSO中性状态中很常见，但在厄尔尼诺期间要少得多。由于未知的原因，理查森指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。不稳定的第一个衡量标准是它的指数增长率，它必须足够快，足以超过任何令人困惑的外部影响；例如，由夜间表面冷却引起的对流不稳定只有在日出之前才会增长。LSA还可以预测有限振幅结构的波长和方向。显式LSA是Richardson数分析的改进。它不仅承诺解释这些区别，而且不仅告诉我们在这些条件下是否确实存在剪切不稳定，而且还告诉我们将所产生的湍流参数化所需的空间和时间尺度。到目前为止，大多数知识都来自太平洋冷舌内的一个位置，即西经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.