Internal solitary waves in Upper Lake Constance

The conceptual hypothesis that internal solitary waves (ISWs) in lakes are generated at fronts of basin-scale internal waves, propagate until they break at the terminal boundary and extend over the lake width in cross-propagation direction is tested using temperature and current profiles measured in Upper Lake Constance. Trains of ISWs of depression propagate in close association with the front of a Kelvin wave at almost constant phase speed from the main basin of Upper Lake Constance to the western shore of a sub-basin, Lake Überlingen. The trough of the ISWs extends over the entire basin width in narrow Lake Überlingen but not in the wider main basin. A substantial part of the ISWs propagating towards the western terminal boundary did not return, suggesting wave-energy loss due to wave breaking. Close to the shorelines parallel to their propagation direction the amplitudes of the ISWs are diminished. Temperature inversions suggest that wave breaking is occurring at these boundaries, but also in the open water where breaking events extend along the width of the ISW front. The heterogeneity of ISW breaking suggests that the associated mixing and vertical fluxes of plankton and nutrients across the thermocline are highly variable in space and time