| dc.description.abstract | Water quality in lakes is determined by concentrations in various dissolved substances. Deep lakes in mild climates, such as Lake Geneva, are thermally stratified water bodies, which means they present a vertical structure with layers of water of different density and temperature, that each present different concentrations in substances such as oxygen or nutrients. Water quality is dependent on the mechanisms that govern water exchanges and transport between the different layers. It is therefore important to understand mixing in the lake interior to devise water management strategies. Climate change is expected to perturb lake ecosystems, and a good comprehension of the latter is necessary to predict their behaviour in the future. Deep layer dynamics of Lake Geneva still lack studying and the effect of climate change on the bottom layer of the lake is not well understood. Recent measurements suggest dynamic processes are occurring near the lakebed, in particular, currents have been recorded in the deepest part, however, their origin remains to be determined. The goal of this study is to provide insight on the dynamics of Lake Geneva’s deep layers. We aim at characterising the currents in that region and at explaining the underlying mechanisms. For that purpose, a measurement campaign was conducted over the year 2021. Numerical modelling was also performed to obtain additional data. The obtained current and temperature fields are analysed to characterise the dynamics of the currents in the deep layer over a whole annual cycle of the lake. The spatial structure of the currents is studied to derive the processes that generate them. We show the deep layer dynamics are strongly affected by the passage of internal waves, oscillating at near-inertial frequencies, that are excited by wind events. The waves occur throughout the whole year, and cause clockwise rotating currents in the entire water column that have significant magnitudes. We demonstrate that the frequencies of the oscillations depend on stratification conditions and we show different wave structures in winter and in summer. A Poincare wave vertical and horizontal mode 1 is identified in both seasons, propagating at the thermocline (150 m depth in winter, 60 m depth in summer), and generating currents down to the lakebed, at 300 m depth. The period of oscillation is of 14-15 hrs in winter and around 9 hrs in summer. Moreover, we highlight that in summer, the deep layer dynamics are dominated by another wave mode. We show evidence of an internal wave, generating clockwise rotating currents with a period of 14 hrs, that propagates within the deep layer of the lake, well below the thermocline level. Such a mode has not been reported yet to our knowledge. The wave structure presents three layers of currents oscillating out of phase, with reversals in current direction occurring at the thermocline level and at 150 m depth, within the deep layer. Results suggest this wave is a mode linked to the basin bathymetry and to the weak stratification that remains present in the deep layers of Lake Geneva in summer. Our results thus provide insight on the deep layer dynamics that will be important to further assess mixing in the lake interior. The evidenced mechanisms are relevant for other large deep lakes in mild climates, e.g., for many Alpine lakes. | |