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Developing a framework for a field campaign in the cold tongue: Analysis of Pacific Upwelling and Mixing Physics from models and observations

The intense oceanic heat uptake and air-sea coupling of the eastern equatorial Pacific cold tongue are enabled and maintained by efficient, turbulence-driven communication between the surface and the shallow thermocline. The cold tongue is dynamically special: the shoaling thermocline and resulting strong shear between the westward surface current and the shallow eastward equatorial undercurrent (EUC) result in flow that is persistently susceptible to shear instability below the mixed layer (marginally unstable flow). Shear turbulence is triggered every night, transporting heat from the ocean surface to the upper flanks of the EUC. This shear turbulence exposes the upper thermocline as a sink of heat (and a source of carbon). The former cools SST resulting in a large flux of heat from the atmosphere into the ocean.
Our previous work with high resolution models demonstrates that the long-term mean ocean mixing and strong water mass transformation is concentrated in a narrow eastern equatorial zone between approximately 2 S and 2 N above the EUC, where upwelling brings the thermocline close to the surface. On shorter time scales, intense mixing exists outside this narrow band: flow in the cold cusps of simulated Tropical Instability Waves is marginally unstable and mixing can extend as far north as 5 N. These results demand deeper investigation of the near-equatorial marginally unstable flow, its spatial extent, temporal variability, and connection to the background near-equatorial circulation.
In this response to the competition CVP: Observation and Modeling Studies in Support of Tropical Pacific Process Studies, Pre-Field-II: 2943817 we propose research to advance our understanding of upwelling and mixing physics in the cold tongue. Our motivating questions are: What is the influence of variations in near equatorial mixing on the geography of air-sea fluxes and large scale features such as the tropical cells? Which macroscale phenomena modulate near-equatorial mixing and water mass transformation? What sampling strategies will capture the essential dynamics of these interactions in the context of both the PUMP field campaign and long-term TPOS backbone array?
To address these questions we will: Elucidate and diagnose oceanic processes, and scales of those processes, that control mixing and upwelling in the eastern tropical Pacific thermocline; Characterize the spatial and temporal variability of shear, stratification, and mixing high-resolution models, large eddy simulations, and observations; Perform Observing System Simulation Experiments to test the proposed frameworks; Construct testable hypotheses and associated sampling strategies for a potential PUMP field experiment to constrain equatorial upwelling and mixing that will ultimately provide guidance for improvement of model parameterizations; Develop indirect and bulk indicators of diabatic mixing processes that can be inferred from routinely observed quantities.
The outcome of this effort will further our scientific understanding of vertical exchange in the cold tongue and facilitate the interpretation of observations from a future PUMP field campaign and optimally utilize the backbone array. Ultimately, observations of these processes will serve to evaluate, constrain and guide the development of ocean models.

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