A model study of sea level variations in the Gulf of Riga and the Väinameri Sea

A high-resolution (of order 1 km grid) 2D hydrodynamic model is used to examine the water level in two nearly tideless semi-enclosed sub-basins of the Baltic Sea. The model is forced by the spatially homogeneous wind stress above the study area, the sea level fluctuations along the open boundaries and the river runoff into the basin. Based on the input and verification data from 1999, the water level and flow dynamics are reproduced, also, the dependence of the sea level variations from different forcing and morphometrical factors is investigated using idealistic forcing schemes. Comparison with the field data proved that the model is applicable for the development of an operational system for predicting extreme sea levels, focusing on low-level events in the Väinameri Sea and storm surges in the Pärnu Bay. While the low-level events associated with continuous eastern winds are mainly determined by the regional wind field over the Baltic Sea and Scandinavia, the storm surges are short-term and local. Very high sea levels (up to 253 cm in Pärnu) appear in combination with several factors, including a very strong (25−30 m s−1) SW or W wind as the most important one. The level grows due to the initial volume increase in the Baltic Sea, then, due to an additional volume increase in the Gulf of Riga, and finally, due to the local level slope and long wave rise effect in the narrow and tapering bays or channels. In the Pärnu Bay an amplitude growth due to the resonance in accordance with the barotropic 5 h seiche period of the Gulf of Riga is possible.