Wintertime Coastal Upwelling in Lake Geneva: An Efficient Transport Process for Deepwater Renewal in a Large, Deep Lake

Combining field measurements, 3-D numerical modeling, and Lagrangian particle tracking, we investigated wind-driven, Ekman-type coastal upwelling during the weakly stratified winter period 2017/2018 in Lake Geneva, Western Europe's largest lake (max. depth 309 m). Strong alongshore wind stress, persistent for more than 7 days, led to tilting and surfacing of the thermocline (initial depth 75-100 m). Observed nearshore temperatures dropped by 1 degrees C and remained low for 10 days, with the lowest temperatures corresponding to those of hypolimnetic waters originating from 200 m depth. Nearshore current measurements at 30 m depth revealed dominant alongshore currents in the entire water column (maximum current speed 25 cm s(-1)) with episodic upslope transport of cold hypolimnetic waters in the lowest 10 m mainly during the first 3 days. The observed upwelling dynamics were well reproduced by a 3-D hydrodynamic model (RMSE 0.2 degrees C), whose results indicated that upwelled waters spread over approximately 10% of the lake's main basin surface area. Model-based Lagrangian particle tracking confirmed that upwelled waters originated from far below the thermocline, that is, >150 m depth, and descended back to around 150-200 m depth over a wide area after wind stress ceased. Observational and particle tracking results suggest that wintertime coastal upwelling, which can occur several times during winter, is an overlooked transport process that is less sensitive to the effects of global warming than convective cooling. It can provide an effective but complex 3-D pathway for deepwater renewal in Lake Geneva, and other large, deep lakes with a sufficiently long wind fetch. Plain Language Summary Freshwater lakes are increasingly important as drinking water sources and for societal and economic activities. To maintain good water quality, deepwater renewal is essential for lake ecosystems. Traditionally, convectively driven vertical mixing during wintertime is considered as one of the main processes for deepwater renewal. In deep lakes, this mixing often cannot reach down to the deepest layers. Since this situation is expected to worsen due to climate change-induced warming, a good understanding of alternative deepwater renewal processes is crucial. We investigate wind-driven coastal upwelling of hypolimnetic water during the weakly stratified winter period. The study was carried out in monomictic Lake Geneva, a large, deep lake (depth 309 m), combining field observations, 3-D hydrodynamic modeling, and Lagrangian particle tracking, an effective combination for addressing the complex 3-D upwelling dynamics over a large area of the lake. We show that coastal upwelling can lift water masses from far below the thermocline up to the surface in the nearshore region, where they spend up to 5 days before descending back to the deeper layers. Our results demonstrate that wintertime coastal upwelling, an as yet overlooked transport process, can efficiently contribute to the renewal and aeration of the deepwater layer in deep lakes.