Small-scale instabilities in the upper equatorial oceans

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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.