Mardi 24 Septembre - Dynamics of turbulent western boundary currents at low latitude in a shallow water model
Sep 24, 2013
from 03:00 PM to 04:00 PM
Cyrille Akuetevi, LEGI, Grenoble
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Dynamics of turbulent western boundary currents at low latitude in a shallow water model
Abstract: The dynamics of low latitude turbulent western boundary currents, subject to two different types of idealized wind forcing, Monsoon Wind and Trade Wind, is considered using numerical results from integrations of a reduced gravity shallow-water model. For viscosity values of 1000m2s-1 and above, the boundary layer dynamics compares well tohe analytical solutions of the Munk-layer and the inertial-layer, derived from quasigeostrophic theory. Modifications due to variations in the layer thickness (vortex stretching) are only important close to the boundary. When the viscosity is reduced the boundary layer becomes turbulent and coherent structures in form of anticyclonic eddies, bursts and dipoles appear. Three distinct boundary layers emerge, the viscous sub-layer, the advective boundary layer and the extended boundary layer. The first is characterized by a dominant vorticity balance between the viscous transport and the advective transport of vorticity. The second by a balance between the advection of planetary vorticity and the advective transport of relative vorticity. The extended boundary layer is the area to which turbulent motion from the boundary extends.The scaling of the three boundary layer thicknesses with viscosity is evaluated.A pragmatic approach to determine the eddy viscosity diagnostically for coarse resolution numerical models is proposed.