The response of surface buoyancy flux-driven convection to localized mechanical forcing

We present laboratory experiments in which both a buoyancy and mechanical forcing are imposed on the surface of a rectangular tank filled with freshwater. The buoyancy forcing is generated by a saltwater source at the surface that drives a sinking half-line plume along one endwall, and the mechanical forcing is generated by a continuous flow of freshwater across the surface of the tank. A steady-state circulation is achieved when the advection of salt by the plume is matched by the diffusion of salt through the upper boundary. The surface stress drives flow in the same direction as the plume, resulting in a convective cell whose depth is determined by the interplay of the two forcings. When the surface stress is relatively weak, the steady-state flow is described by a high-Rayleigh number ‘recycling box’ model for horizontal convection (Hughes et al. in J Fluid Mech 581:251–276, 2007). Once the stress is strong enough to overturn the stratified waters, a region of localized mixing develops. The immediate consequence of this regional turbulence is a net input of stabilizing buoyancy in the form of fresher water into the plume, which renders it too weak to penetrate to the bottom boundary. In general, the plume is unable to recover a full-depth circulation within the experiment time frame. The resulting flow can be described by the recycling box model with a spatially varying turbulent diffusivity parameterized by the characteristics of the turbulent eddy that develops in the mixing region. This work applies experimental techniques to show that, with adequate mechanical forcing, a buoyancy-driven circulation will develop localized mixing that significantly alters the overall structure and density distribution of the circulation for relatively long timescales. The experimental results corroborate the recycling box model as a valid descriptor of the flow structure in such systems.