External gravity oscillations in Lake Onega
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.