Recorded presentation10.1
Eddy-driven buoyancy gradients on eastern boundaries
Paola Cessi, SIO/Univ. Of California, La Jolla, CA; and C. L. Wolfe
It is demonstrated that eddy fluxes of buoyancy at the eastern and
western boundaries maintain alongshore buoyancy gradients along the
coast. Eddy-fluxes arise near the eastern and western boundaries
because on both coasts buoyancy gradients normal to the boundary are
strong. The eddy fluxes are accompanied by mean vertical flows that
take place in narrow boundary layers next to the coast where the
geostrophic constraint is broken. These ageostrophic cells have a
velocity component normal to the coast that balances the geostrophic
mean velocity. It is shown that the dynamics in these thin
ageostrophic boundary layers can be replaced by {\sl effective
boundary conditions} for the interior flow, relating the eddy flux of
buoyancy at the seaward edge of the boundary layers to the buoyancy
gradient along the coast. These effective boundary conditions are
applied to a model of the thermocline linearized around a mean
stratification and a state of rest. The linear model parametrizes the
eddy fluxes of buoyancy as isopycnal diffusion. The linear model
produces horizontal gradients of buoyancy along the eastern coast on a
vertical scale that depends on both the vertical diffusivity and the
eddy diffusivity. The buoyancy field of the linear model agrees very
well with the mean state of an eddy-resolving computation. Because the
East-West difference in buoyancy is related to the zonally integrated
meridional velocity, the linear model successfully predicts the
meridional overturning circulation.
Session 10, Vortex Dynamics II
Thursday, 11 June 2009, 10:20 AM-12:20 PM, Pinnacle BC
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