Internal seiche dynamics in Lake Geneva

We analyzed season-long water level records at 12 stations around the Lake of Geneva (local name Léman) for evidence of internal seiches modified by Coriolis force and compared the results with predictions from a two-layer numerical model with real bottom topography for typical wind situations. Results are also compared with those obtained from current and temperature measurements in the lake. Agreement was satisfactory in all cases. Model predictions and measurements both indicated that only three internal seiche modes are excited: the 1st mode and the 3rd mode, which are Kelvin-seiche oscillations, and the 12th mode, which is a Poincare ́ seiche. The model, driven by winds from different directions, demonstrates that the wind field, constrained by the local topography, determines which of the modes is generated. Long internal waves in lakes commonly take the form of standing waves (seiches) of frequency and structure deter-mined by basin shape and density (temperature) stratification in the water column (Wedderburn 1912; Mortimer 1953). The history of research on such waves is reviewed by Mor-timer (1993); and their rotation-affected roles in a large lake (Lake Michigan) are described in Mortimer (2004). A controlling length scale is the Rossby radius of defor-mation, a 5 ci/f, in which f is the latitude-dependent Coriolis parameter and ci is the speed of long, internal waves in the absence of rotation. When the basin rotates and its width exceeds a, the waves take the form of amphidromic internal seiche modes. As described below, the first horizontal mode resembles a shore-hugging wave of a form similar to a Kelvin wave (Platzman 1971) traveling counterclockwise (cyclonically) around the lake basin in northern hemisphere lakes, as was first demonstrated in the Lake of Geneva b