Convective regimes induced by surface cooling and topography in stratified waterbodies

Gravitational convection plays a significant role in the ventilation, heat and mass distribution of aquatic systems. This study investigates thermally driven convection resulting from heat loss at the air-water interface during cooling periods in freshwater environments. In the littoral zone, where the water depth increases from the shoreline to interior waters (pelagic zone), uniform heat loss at the surface generates differential cooling between shallow and deep regions. If the latter process occurs for a long enough time, the density-induced cross-shore pressure gradient may drive an overturning circulation across littoral waters, known as "thermal siphon". This paper examines the conditions under which a thermal siphon develops in natural water bodies and its associated convective regimes. For the above setting, we derive time and velocity scales associated with the transition from Rayleigh-Benard type convection to horizontal overturning circulation across sloping basin regions. The above transition in the convective regime is characterised by a three-way horizontal momentum balance between the cross-shore pressure gradient and inertia before reaching a quasi-steady regime. Our theoretical scaling expressions are supported by high-fidelity numerical simulations and field- scale experiments, and they provide a robust conceptual framework to characterise convective flows induced by night-time or seasonal surface cooling in nearshore aquatic systems, such as lakes, reservoirs, and coastal seas.