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 rarely observed in lakes and thereby 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 analytical parameterization satisfactorily predicted the magnitude of the simulated offshore unit‐width discharges in the upwind and downwind littoral regions. 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.

Lake surface cooling drives littoral-pelagic exchange of dissolved gases

“…S2B) and convection dominated over wind-driven motions (fig. S2C) ( 36 ). Differential cooling occurred continuously and led to lateral surface temperature gradients of ∂ T /∂ x ≈ 0.5°C km −1 with diel cycles (fig.…”

“…These anomalies would, however, not persist during warm periods without convective circulation and would not occur in spring when surface cooling is weak. Wind-driven flows may suppress or enhance lateral transport by convective circulation ( 36 ) and may also contribute to advective gas transport. This could be the case in Rotsee in spring and early summer when wind-driven flows dominate lateral exchange ( 29 ).…”

Basin-scale hydrodynamics and physical connectivity in a Great Patagonian Lake

“…The increasing awareness of how lake littoral waters are becoming strongly stressed areas and collectors of their surrounding watershed processes (Jenny et al, 2020) is motivating new studies to better understand and characterize the mechanisms enhancing the exchange between nearshore and offshore regions, including wind-driven (Ulloa et al, 2018;Ramón et al, 2022) and buoyancy-driven flows (Forrest et al, 2008;Ulloa et al, 2022;Doda et al, 2022). However, little has been investigated on littoral transport in lakes.…”

Adaptation of Wind Drag Coefficient Parameterization: Improvement of Hydrodynamic Modeling by a Wave‐Dependent Cd in Large Shallow Lakes

“…In lake ecosystems, wind is not only an important driving force for hydrodynamic processes but also the key factor affecting water mixing and nutrient cycling in the epilimnion, littoral, and pelagic zones (Bhowmik & Stall, 1978;Faller, 1978;Ramón et al, 2022). For large shallow lakes, wind-driven turbulence causes resuspension of sediments and the release of nutrients at the water-sediment interface (Chung et al, 2009;Reardon et al, 2014;Roberts et al, 2019;Zhang & Chen, 2023b).…”