Atmosphere Ocean Science Colloquium

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Ageostrophic dynamics in the ocean interior

Speaker: Lia Siegelman, Scripps

Location: Warren Weaver Hall 1302

Date: Thursday, May 14, 2020, 11:15 a.m.

Synopsis:

Submesoscale ocean processes, characterized by order one Rossby and Richardson numbers, are currently thought to be mainly confined to the ocean surface mixed layer, while the ocean interior is commonly assumed to be in quasi-geostrophic equilibrium. Here, a realistic numerical simulation in the Antarctic Circumpolar Current is used to demonstrate that the ocean interior departs from the quasi-geostrophic regime down to depths of 900 m, i.e., well below the mixed-layer. Results highlight that, contrary to the classical paradigm, the ocean interior is strongly ageostrophic, with a pronounced cyclone-anticyclone asymmetry and a dominance of frontogenesis over frontolysis. Numerous vortices and filaments, from the surface down to 900 m, are characterized by large Rossby and low Richardson numbers, strong lateral gradients of buoyancy and vigorous ageostrophic frontogenesis. These deep submesoscales fronts are only weakly affected by internal gravity waves and drive intense upward vertical heat fluxes, consistent with recent observations in the  Antarctic Circumpolar Current and the Gulf Stream. As such, deep submesoscale fronts are an efficient pathway for the transport of heat from the ocean interior to the surface, suggesting the presence of an intensified oceanic restratification at depth.