Internal seiche climatology in a small lake where transversal and second vertical modes are usually observed

From the analysis of thermistor chain data recorded in a small clongatcd lake, compared with the results of numerical simulations, second vertical modes of the internal scichcs are shown to be active during more than twothirds of the whole stratifcd period, although no rcsonancc with the wind is observed. On the other hand, due to the wind pattern, transversal seiches (mainly the first vertical mode) are also usually excited and they somctimcs dominate. At the end of the stratified period, however, when the thermocline is very sharp, first vertical modeslongitudinal and transversal-are the only ones that are excited. Finally, due to the wind, a forced oscillation of the whole lake is excited. After Mortimer (1952), who brought up to date both the earlier descriptions of internal seiches in Loch Ness (Watson 1904) and their further interpretations (Schmidt 1908; Wedderburn 1907; Wedderburn and Young 1915), internal seiches have been recognized as an everyday feature of movement in stratified water systems subject to wind action (Marmorino and Mortimer 1978; Hutter 1993; Korgen 1995). Now, the relevance of the internal seiches is widely documented: the structure of the horizontal velocities in the water column may determine the regions where the shear is strong enough to create instabilities leading to turbulence that might facilitate the resuspension of bottom sediments (Wiegand and Chamberlain 1987); the periodic vertical displacements of the internal seiches can advect the suspended algal cells, forcing them to be exposed to variations in light intensity (Webster 1990; Gaedke and Schimmele 1990); and internal scichcs greatly support the flux path of the transference of energy from the wind to the hypolimnion (Imberger 1994). The importance of internal seiches is greater due to the fact that once they are set up they persist, even if the wind stops, until the momentum is diffused. Knowledge of the nodal structure of the energy spectra, in the frequency bandwidth where the energy arises mainly from stationary waves directly established by the wind, is relevant to understanding the global behavior of a lake. However, although it is well known that the response of a lake to the wind Forcing may be different to first vertical modes, higher vertical modes are poorly documented apart from some examples (LaZerte 1980; Wiegand and Chamberlain 1987; Mtinnich et al. 1992). With the idea of illustrating the relative importance of the second vertical modes and their horizontal structure in a We arc grateful to two anonymous reviewers. WC also thank the Catalan Research Commission (CIRIT) for having supported this work by ARM, AR89, and AR90 (E.R.). The English was reviewed by V. Greatorex-Davies. small lake, we ran different simulations in order to plan some field campaigns covering the whole stratification period. Although emphasis was put on higher vertical modes, the regularity and the horizontal structure of first vertical modes were also studied, together with the wind pattern.