The oscillatory nature of the flows in the system of straits and small semienclosed basins of the Baltic Sea

The Gulf of Riga and the Väinameri are small semi-enclosed sub-basins connected to each other and to the open Baltic Sea by straits. Hydrographical features and configuration of the study area give an opportunity to build a forced oscillation model and to describe the system as a combination of five simple Helmholtz oscillators, each composed of a basin and a strait. The response to the wind stress and the open sea level is modelled using motion equations with a quadratic bottom friction for the narrow channel (Coriolis’ effect is neglected). The flows in the straits and the sea levels within the sub-basins are calculated. The model was calibrated and verified against field measurements with a total duration of 6 months. The model behaviour has been analysed in both stationary and nonstationary conditions, and in both idealistic and realistic forcings. Using HIRLAM winds and open sea level data as input, the oscillations of currents in all the straits were calculated for 1995. It appeared that the system could be looked upon as a superposition of two Helmholtz oscillators with resonance periods of about 24 and 13 h. 24 h is the only resonance period for the flows in the Irbe Strait and for the sea level of the Gulf of Riga basin, the Suur, Hari and Soela straits have two roughly equal maxima at 24 and 13 h while the sea level in the Väinameri basin has the primary maximum at 13 h and the secondary one at 24 h. These oscillations appear as a response of the system both to rapid changes in the wind conditions and to the sea-level changes in the boundaries of the study area. The role of the tides is relatively small. The most effective forcing function for the system is the wind stress above the straits, having a considerable role in motions with time scale between 1 yr and 1–2 d. The seasonally pulsating component in the water exchange processes has major ecological consequences for the study area. This seasonality is governed by seasonal changes in the large-scale atmospheric circulation scheme above the North Atlantic. The open sea level has the main impact on the flow spectra around the resonance frequencies. The spectra of the sea levels of sub-basins are produced mainly by the open sea level. The characteristic motions in the Väinameri have a small influence on the Gulf of Riga sub-system, while the (nearly diurnal) motions in the Gulf and in the Irbe Strait also affect the Väinameri.