EXTERNAL GRAVITY OSCILLATIONS IN LAKE ONEGA

Lake Onega is located in the southern part of Karelia, in the north-we st of Russia. We report data taken by various limnigraphs and current meters and isolate, by spectral analysis, the most conspicuous barotro pic periods of these signals. The analysis that follows is based on th e linearized shallow-water equations, that are solved for the free osc illations and thus identify the eigenperiods and corresponding mode st ructures for this lake. Computational results are presented for a fini te difference representation for these surface-seiche equations applie d to the entire Lake Onega including its bays. The grid consists of qu adratic elements of 1000 m side length corresponding to 9344 active ce lls. The emerging matrix eigenvalue problem exceeds the storage and co mpuational capacity of standard PCs or workstations and thus requires use of the approximate Lanczos procedure to isolate the first ten eige nperiods and corresponding modes 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 s atisfactory agreement between theory and observation.