Atmosphere Ocean Science Colloquium

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On the interaction between submesoscales and turbulence: from theory to implementation in global climate models

Speaker: Abigail Bodner, Brown University

Location: Warren Weaver Hall 1302

Date: Wednesday, October 30, 2019, 3:30 p.m.

Synopsis:

The ocean mixed layer plays a key role in transferring momentum and tracers such as heat and carbon, from the atmosphere to the ocean interior, and the effectiveness of such depends largely on the depth of the mixed layer itself. Variations in the mixed layer depth can be attributed to surface forcing, as well as dynamical processes such as turbulent mixing, submesoscale frontal instabilities and mixed layer eddies. Current submesoscale parameterizations, which help set mixed layer depth in global climate models, depend on a simplistic scaling of frontal width. The presence of turbulence and instabilities are likely responsible for keeping fronts at the scale observed, yet a complete understanding of how and why this happens has been a long standing problem.

Building toward a more complete understanding of the processes that set this scale, a theoretical approach of perturbation analysis is used to include the effects of parameterized turbulence as a first order correction to classic frontogenesis (frontal sharpening) theory. A modified solution is obtained by using potential vorticity (PV) and surface conditions, which exhibit the complex nonlinear behavior of frontal dynamics. It is found that vertical processes merely delay frontal sharpening, whereas horizontal processes may completely oppose frontogenesis. In a more realistic environment, using a suite of Large Eddy Simulations spanning the submesoscale and into the boundary layer turbulence scale, a variety of processes--winds, waves, convection, and mixed layer instabilities—are found to compete with frontogenesis. Furthermore, a surprising result emerges, revealing the limitations of PV below the submesoscale.