Effects of Velocity Shear in Advective Mixed-Layer Models

Abstract

Abstract A general multidimensional model of the upper mixed layer of oceans and lakes is presented. The density profile is approximated as uniform over the depth of the layer. Such an assumption is not made for the distribution of the horizontal velocity component, as there is no observational evidence for it. In fact, many observations indicate that important velocity shears exist in regions where advection can be expected to play an important role in the dynamics of the mixed layer (such as lakes, near upwelling and frontal areas, and in equatorial oceanic regions). It is shown that vertical shear in the horizontal velocity field combined with a horizontal density gradient can result in the production or absorption of mechanical energy. Production results if the velocity shear tends to destabilize the density profile, consumption if the tendency is stabilizing so that work against gravity must be done to mix the stabler density profile to uniformity. In the mechanical energy equation, this effect of th...