Dynamics of turbulent western boundary currents at low latitude in a shallow water model
C. Q. C. Akuetevi1,2 and A. Wirth11Univ. Grenoble Alpes, CNRS, LEGI UMR5519, Grenoble, France 2Univ. Grenoble Alpes, CNRS, LGGE UMR5183, Grenoble, France
Received: 19 Dec 2013 – Accepted for review: 11 Feb 2014 – Discussion started: 05 Mar 2014
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 1000 m2 s−1 and above, the boundary layer dynamics compares well to the 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 (violent detachments of the viscous sub-layer) 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.
Akuetevi, C. Q. C. and Wirth, A.: Dynamics of turbulent western boundary currents at low latitude in a shallow water model, Ocean Sci. Discuss., 11, 753-788, doi:10.5194/osd-11-753-2014, 2014.