Flow dynamics during upwelling events in rotationally influenced lakes: A numerical study

Wind-driven lake flows under stratified conditions can drive impacts of ecological nature and exert changes in the water quality. Historically, conceptual models classify lake responses during the upwelling setup as following either a non-rotational (2D, closed-basin) or a rotational (coastal upwelling) model. Upwelling in lakes different from large lakes such as the Great Lakes and Lake Geneva is often assumed to follow the 2D closed-basin model and is parameterized using the Wedderburn number. The influence of the Coriolis force in the lake response is often considered negligible, or only analyzed during the relaxation phase of an upwelling event (e.g., Roberts et. al., 2021). However, during the upwelling setup, the combination of both conceptual models may constitute the complex lake response. Here, we present the results of fine-grid numerical simulations of a rotationally influenced lake (Lake Tahoe) and focus our analyses on classifying the flow structures and the influence of the Coriolis force on the fluid motions during the upwelling setup and relaxation stages. The model results were validated via an unprecedented field campaign. During the wind forcing, surface cooling was observed to be a combination of overturned regions predicted by both conceptual models (i.e., upwind boundary and coasts to the left of the wind direction). In addition, Ekman transport rotated the main flow of the thermal front in the pelagic zone, resulting in baroclinic pressure gradients being non-orthogonal to the wind direction. During wind forcing, alongshore currents limited to the densitys interface appeared as a response to the wind parallel to the coast. After the wind relaxation, rotational effects were observed on the internal wave field and on the return flow of the thermal front. Strong cyclonic alongshore currents accompanied the rotational response of the thermocline and were found to be in quasi-geostrophic balance. These currents exhibited similar characteristics to the flows that describe the coastal jets observed in large lakes. The results allow us to conclude that flow structures during upwelling setup, in rotationally influenced lakes (i.e., Burger number, S < 1), cannot be described by a single available conceptual model, and observed cyclonic currents are a purely rotational response to wind forcing.