The observation of seiches in the Baltic Sea using a multi data set of water levels

This investigation addresses the difficult problem of using a multi data set of water levels to study seich oscillations in the Baltic Sea. The Baltic Sea is a semi-enclosed sea area and is connected with the North Atlantic Ocean through the North Sea. The applied method is based on the analysis of environmentally corrected radar altimeter (RA) data in conjunction with simulated water levels obtained by an operational circulation model of the Federal Maritime and Hydrographic Agency (Bundesamt für Seeschiffahrt und Hydrographie, BSH) and tide gauge data. RA data of the First European Remote Sensing Satellite (ERS-1) have been analysed. The RA data were collected during the three-day repeat cycle from August 12 to December 9, 1991. An advantage of the analysis is the coverage and the geographical location of one and the same frequently repeated descending sub-satellite pass over the Bothnian Bay and the central Baltic Sea. Due to the assumed linear dynamics of this sea area the observation of sea level changes in the sense of a standing wave between the Bothnian Bay and the Baltic Proper is investigated. Amplitudes of water level changes up to 1 m have been recorded at tide gauge stations in the northern and southern parts of the Baltic Sea, respectively, during August and December 1991. Water level changes of less than 20 cm were calculated from the corrected altimeter data during the overflights of ERS-1. Tide gauge data and water level differences of tide gauge data measured at Kemi (Bothnian Bay), Finland, and Kolobrzeg at the Polish coast were used for frequency analyses. The analysed time series have been limited to five months due to the duration of the three-day repeat cycle of ERS-1. For frequency analysis the given sampling interval of 4 h is not appropriate, but a period of 12.4 h was clearly identified. This period corresponds to the fourth mode of theoretical estimated ones. The differences of water levels of the simulated data between Kemi and Kolobrzeg of about 25 cm agree fairly well with the results obtained by the RA data. Due to these results and the synoptic view of the along-track subsatellite passes it turned out that seiches can be observed by the ERS-1 radar altimeter.