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Submesoscale instabilities at ocean fronts

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Climate prediction is on a lot of our minds. Businessmen want to plan for the future, reinsurance companies hope to capitalise on extreme weather events and scientists seek to motivate their often abstract research ideas :) The challenge with climate prediction, however, is that it requires sufficient understanding of a vast array of small- and large-scale processes in order to have good predictive skill. A good example in the atmosphere is knowledge of cloud-physics. Here, we add to this vast array by describing a physical process that occurs at small scales in the ocean and not yet represented in climate-scale ocean models: winds blowing down ocean fronts. Using (1) a simple model of wind-driven buoyancy flux, (2) European Centre for Medium-range Weather Forecast (ECMWF) model reanalysis and (3) an unprecedented suite of upper ocean observations from the open-ocean North Atlantic (this rather “mundane” location offers us the ability to study the physics in a regime not dominated by strong mean flows.) including potential vorticity and turbulent dissipation rate, we demonstrate that winds blowing down ocean fronts catalyse convective instabilities that then generate turbulence, with implications for mixing and associated changes in upper ocean stratification. Importantly, this mechanism is found to dominate over surface buoyancy fluxes in terms of turbulence production, turning our understanding of ocean-atmosphere buoyancy flux on its head. Taken together with the upper ocean’s role in imparting deep ocean properties to the atmosphere (and vice-versa), these results point to the importance of parameterising submesoscale instabilities at ocean fronts in climate models.

This talk is part of the British Antarctic Survey - Polar Oceans seminar series series.

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