Dynamics of turbulent western boundary currents at low latitude in a shallow water model

Abstract

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.

Interactions between the Somali Current eddies during the summer monsoon: insights from a numerical study

“…Although the basic formation mechanism and physical characteristics of these lowlatitude eddies are similar, their interactions with the general circulation and regional topography differ substantially. Indeed the GW is unique as it is first generated by remote Rossby waves (see Akuetevi, 2014) and later amplified by the monsoon winds via an intensification and a retroflection of the Somali Current. Somewhat differently, the SG is the result of pure retroflection of the East African Coastal Current, several retroflection events of that type eventually occurring during the monsoon cycle.…”

“…• model simulations can be found in Akuetevi (2014). The model surface circulation during the summer monsoon is compared with an observational reference in Fig.…”

“…At the scale of the whole Indian Ocean, the year-round large circulation patterns and the planetary wave dynamics (not shown, Akuetevi, 2014) simulated by the 1/4 • and 1/12…”