Rafael Sebastian Reiss
Water quality in lakes is closely linked to hydrodynamics and often dominated by stratification, limiting the exchange between the epi- and hypolimnion. Thus, the vertical redistribution of biogeochemical tracers such as dissolved oxygen and nutrients by convective overturning during winter is an important process in annual lake cycles. In deep, monomictic lakes convective cooling often does not reach the deepest layers. Studies have shown that climate change will further reduce the efficiency of this process, motivating a good understanding of alternative deepwater renewal mechanisms.Combining field observations, three-dimensional (3D) hydrodynamic modeling, and particle tracking, this thesis explores wind-induced transport processes contributing to vertical exchange in Lake Geneva during winter. A first focus is on Ekman-type coastal upwelling at the northern shore. Then, wind-induced interbasin exchange and hypolimnetic upwelling between the Petit Lac (depth 75 m) and Grand Lac (depth 309 m) basins of Lake Geneva are investigated. Finally, the effect of stratification in the Petit Lac on the latter phenomenon is revealed by model-based momentum budget analysis.Coastal upwelling at the northern shore of Lake Geneva was investigated in the winter 2017/2018. Following a strong alongshore wind, nearshore temperatures decreased and remained low for 10 days, with the lowest temperatures corresponding to values found at 200-m depth. Current measurements at 30-m depth showed dominant alongshore currents with episodic upslope transport of cold hypolimnetic water in the lowest 10 m. Model results suggest that upwelled waters spread over ~10% of the Grand Lac's surface area. Furthermore, particle tracking confirmed that upwelled waters originated from far below the thermocline and descended back to these layers after the wind ceased.Exchange and upwelling between the lake's two basins was investigated in the winter 2018/2019. Following a strong along-axis wind, a two-layer flow established, with the downwind surface drift into the Grand Lac being balanced by opposing bottom currents into the Petit Lac and the current reversal occurring around the thermocline depth. Bottom temperatures at the confluence gradually decreased to values found in the deep Grand Lac hypolimnion. Furthermore, particle tracking revealed a current loop, whereby hypolimnetic water from below 150-m depth first upwelled into the Petit Lac, intruding ~10 km, i.e., half its length, and then descended back into the Grand Lac hypolimnion. Model results indicated a temporary doubling of the Petit Lac hypolimnetic volume during the upwelling.The observed interbasin upwelling occurred only during early winter, with the bottom inflow of deep hypolimnetic water driven by baroclinic pressure gradients at the confluence, produced by upwind upwelling at the western end of the Grand Lac. Once the Petit Lac was fully mixed, the wind-induced net volume exchange between the basins was reduced by > 50% and deep hypolimnetic interbasin upwelling was suppressed altogether.This thesis shows that wind-induced coastal upwelling and hypolimnetic interbasin exchange are complex 3D processes, regularly occurring during winter and providing important mechanisms for deepwater renewal in Lake Geneva. They cannot be represented by one-dimensional models frequently employed to predict climate change induced shifts and merit further attention when assessing deepwater renewal in large, deep lakes.
EPFL2021
