A laboratory model of benthic fronts

Abstract

Experiments are described which model some features of anticyclonic eddies in the benthic boundary layer. Eddies were created in a two-layer stratified rotating fluid, by “squashing” a column of water in the centre of a tank on a rotating table. Ekman suction into the bottom boundary layer caused thinning of the lower layer in the core of the eddy. If the eddy was sufficiently strong, a “bare-spot” (i.e. a region over which the lower layer was no longer present) bounded by a front where the interface intersected the lower boundary was formed. These are similar to the “bare-spots” described by Linden and Van Heijst (1984, Journal of Fluid Mechanics, 143, 69–94). Formation of a bare-spot reduced the time required for the spin-up of the upper layer. When F = V/(g′H 2) 1 2 ≳ 1 (V = azimuthal velocity, g′ = reduced gravity of the interface and H 2 = the initial depth of the lower layer) , upper-layer fluid in the bottom Ekman layer was observed to intrude under the denser lower layer. Instabilities, initially in the form of two-dimensional rolls, soon developed on the intruding layer. This resulted in mixing between the initial upper and lower layers. Baroclinic instabilities of the front were observed and it is shown that Ekman dissipation increases the wavelength of the fastest growing disturbances. The possibility of the formation of fronts in the benthic boundary layer is discussed.