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.

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Abstract. Lake Onega is located in the southern part of Karelia, in the north-west of Russia. We report data taken by various limnigraphs and current meters and isolate, by spectral analysis, the most conspicuous barotropic periods of these signals. The analysis that follows is based on the linearized shallow-water equations, that are solved for the free oscillations and thus identify the eigenperiods and corresponding mode structures for this lake. Computational results are presented for a finite difference representation for these surface-seiche equations applied to the entire Lake Onega including its bays. The grid consists of quadratic elements of 1000 m side length corresponding to 9344 active cells. The emerging matrix eigenvalue problem exceeds the storage and compuational capacity of standard PCs or workstations and thus requires use of the approximate Lanczos procedure to isolate the first ten eigenperiods and corresponding mode structures of the barotropic seiches, lying between 12.1 and 2.2 h. Comparison of computational results and inferences from the water level and current-meter records, disclose satisfactory agreement between theory and observation.