Abstract

Water quality in lakes is closely linked to hydrodynamics and is often dominated by thermal stratification which limits the exchange between the upper layers (called the epilimnion) and the deeper layers (called the hypolimnion). Consequently, the vertical redistribution of biogeochemical tracers such as dissolved oxygen and nutrients by convective overturning during winter is a key process in annual lake cycles. In deep lakes, convective cooling often does not reach the deepest layers. Furthermore, convective cooling is weakening due to climate change, motivating a good understanding of (i) alternative deepwater renewal mechanisms, and (ii) deepwater dynamics in large deep lakes in general. Understanding deepwater dynamics is crucial because of the role deepwater currents play in mediating water-sediment exchanges, hypolimnetic mixing, and horizontal and vertical transport.

In this talk, I will present results from several studies conducted in Lake Geneva, Western Europe’s largest lake (max. depth 300 m), combining field observations, 3D numerical modelling, and particle tracking. The first part of the talk will cover the dynamics and ecological implications of wintertime coastal upwelling and interbasin exchange and upwelling, highlighting their role in deepwater renewal. The second part of the talk will present recent findings on the importance of different vertical modes of rotationally-modified standing internal waves (i.e., Kelvin and Poincaré waves) on the deepwater dynamics in Lake Geneva, highlighting the impact of seemingly negligible but ever-present weak stratification in the deep hypolimnion on the vertical structure of higher vertical-mode Poincaré waves.