Atmosphere Ocean Science Colloquium

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Beyond Horizontal Convection: Laboratory and Numerical Experiments

Speaker: Stefan Llewellyn-Smith, UCSD

Location: Warren Weaver Hall 1302

Date: Wednesday, April 8, 2015, 3:30 p.m.

Synopsis:

Horizontal convection (HC) is the generic designation for the circulation resulting from differential heating along one horizontal boundary of a fluid. We discuss recent work aimed at moving from the overly simplified problem of HC toward the richer dynamical setting of the ocean. We present laboratory experiments in which buoyancy forcing and mechanical forcing are both imposed on the same horizontal surface of a rectangular tank. If the stress is strong enough to overturn surrounding stratified waters, we observe the development of a region of localized mixing.In all experiments that involve a region of enhanced mixing, there exists a boundary layer flow along the wall where the plume descends, which funnels dense fluid to the base of the tank. In order for the general circulation to remain approximately steady, instability mechanisms leading to "loss of balance" (LOB) have been hypothesized to take place so that the eddy kinetic energy (EKE) may be transferred to small scales where it can be dissipated. We examine the kinetic energy pathways in direct numerical simulations The flow is allowed to reach a statistical steady state at which point it exhibits both a forward and an inverse energy cascade. We show that EKE is dissipated preferentially at small scales near the surface via frontal instabilities associated with LOB and a forward energy cascade rather than by bottom drag after an inverse energy cascade These results suggest that LOB caused by frontal instabilities near the ocean surface could provide an efficient mechanism, independent of boundary effects, by which EKE is dissipated.