Differential cooling and near-shore dynamics in a large lake (Lake Geneva)

Density currents generated by differential cooling in lakes with shallow shore regions or side basins are associated with increased vertical mixing, deep water renewal and cross-shore transport of nutrients and other substances. Previous field experiments in Lake Geneva, the largest lake in Western Europe, showed that such cold-water density currents are a common phenomenon during winter, frequently reaching the thermocline at O(100 m) depth. As part of a project investigating large-scale interbasin exchange in Lake Geneva, we examined near-shore wintertime density currents in order to better understand cold-water formation. The study is based on field observations taken on the northern shore of Lake Geneva near Buchillon, 20 km west of Lausanne, in the winters of 2016-2018. Bottom temperatures are inferred from a fiber-optic Distributed Temperature Sensing (DTS) system, laid down on the lateral slope starting at the shore, with a spatial and temporal resolution of O(1 m) and O(0.1°C), respectively. Temperature and current velocities in the water column were recorded by ADCP and thermistor chain moorings. The time-series data are complemented by several CTD profiles taken along transects normal to the shoreline near the moorings. Data from a nearby meteorological mast were used to select calm days favoring convective cooling. Earlier studies focused on the steady-state dynamics along one transect. Our goal is to investigate the generation of density currents and the effect of different shore geometries. During periods of surface heat loss, i.e., positive surface buoyancy flux B0, DTS bottom temperatures show cold fronts progressing downslope with the coldest temperatures in the near-shore part, illustrating differential cooling. However, our observations indicate that even during periods with low wind and strong cooling, currents on the shallow shelf have a dominant alongshore component. This suggests that local wind stress and possibly the large-scale circulation might have to be taken into account when investigating nearshore cold-water formation and cooling dynamics in Lake Geneva. The wide and narrow parts of the shelf show significantly different responses to surface cooling. Starting with similar temperatures the wide shelf cooled down by 0.5°C whereas only minor temperature fluctuations were observed at the narrow shelf. At the wide shelf a cold bottom layer of up to 10-m thickness was visible in the CTD transect until the thermocline at 100-m depth. To further elucidate the effect of the shallow shelf width on the generation of cold-water density currents as well as the contribution of both surface heat loss and wind stress a numerical modeling study is planned.