Flushing the Lake Littoral Region: The Interaction of Differential
Cooling and Mild Winds
Abstract
The interaction of a uniform cooling rate at the lake surface with
sloping bathymetry efficiently drives cross-shore water exchanges
between the shallow littoral and deep interior regions. The faster
cooling rate of the shallows results in the formation of density-driven
currents, known as thermal siphons, that flow downslope until they
intrude horizontally at the base of the surface mixed layer. Existing
parameterizations of the resulting buoyancy-driven cross-shore transport
assume calm wind conditions which are, however, rarely observed in lakes
and thereby strongly restrict their applicability. Here we examine how
moderate winds (≲ 5 m s-1) affect this convective cross-shore transport.
We derive simple analytical solutions that we further test against
realistic three-dimensional numerical hydrodynamic simulations of an
enclosed stratified basin subject to uniform and steady surface cooling
rate and cross-shore winds. We show cross-shore winds modify the
convective circulation, stopping or even reversing it in the upwind
littoral region and enhancing the cross-shore exchange in the downwind
region. The magnitude of the simulated offshore unit-width discharges in
the upwind and downwind littoral regions was satisfactorily predicted by
the analytical parameterization. Our scaling expands the previous
formulation to a regime where both wind and buoyancy forces drive
cross-shore discharges of similar magnitude. This range is defined by
the non-dimensional Monin-Obukhov length scale, χMO: 0.1 ≲ χMO ≲0.5. The
information needed to evaluate the scaling formula can be readily
obtained from a traditional set of in-situ observations.