Laboratory experiments are used to investigate the processes governing steady convectively
driven circulation in a basin that communicates with a large external reservoir
over a shallow sill. The motion is maintained by a steady loss of buoyancy distributed
over the surface of the basin. Turbulent convection associated with the forcing produces
a horizontal buoyancy gradient across the sill and the resulting mean flow
consists of a layer directed into the basin near the surface with a dense counter flow
below.
To first order, the magnitude of the exchange flow over the sill is determined by
the horizontal momentum balance within the basin. Measurements of the mean and
turbulent flow fields are used to show that inertia, buoyancy and friction may each
contribute significantly to the balance. The interior flow produces a horizontal pressure
gradient near the surface which must also contribute to the momentum balance. The
density of the lower layer at the sill reflects the cumulative effect of interior processes,
such as mixing, and these in turn influence the hydraulically controlled exchange
flow over the sill. The basin dynamics are therefore coupled in a nonlinear fashion
with the submaximal sill exchange. This coupling is investigated first by showing how
interior processes are affected by changes in the magnitude of the forcing, and then
by observing the associated variation of the flow state at the sill. The flow state is
defined in terms of its relative proximity to the theoretical maximal exchange limit.
Results show that the exchange flows are submaximal with flow rate approximately
85% of the maximal limit. This state appears to change very little in response to
increasing forcing.
For a stratified basin, which exhibits a deep stagnant layer under the convectively
driven near-surface exchange flow, the possibility of basin ventilation or erosion
of deep fluid exists in the long term. This process and its dependence on external
parameters is also explored.