Variability of coastal water hydrodynamics in the Southern Baltic - hindcast modelling of an upwelling event along the Polish coast

This paper presents the results of an attempt to reproduce, with the aid of a numerical circulation model, the hydrological conditions observed in the coastal area of the southern Baltic in September 1989. A large fall in surface layer seawater temperature was recorded in September 1989 at two coastal stations in the vicinity of Kołobrzeg and Władysławowo. This upwelling-like phenomenon was assumed to be related to the specific anemobaric situation in September 1989, however typical of this phenomenon to occur along the Polish Baltic coast (Malicki & Miętus 1994). A three-dimensional (3-D) σ-coordinate baroclinic model of the Baltic Sea, with a horizontal resolution of ∼5 km and 24 sigma-levels in the vertical, was applied to investigate water circulation and thermohaline variability. Hindcast numerical simulation showed that the model provided a good reproduction of the temporal history of the surface seawater temperature and the duration of the upwelling-like fall, but that the model results were underestimated. The maxima of this large fall in the surface layer temperature at both coastal stations are closely related to the phase of change of the upwelling-favourable wind direction to the opposite one. The results of simulation runs showed details of upwelling development due to wind field fluctuations in time and differences in shaping the temperature and current patterns in conjunction with the variations in topography and coastline features in some areas along the Polish coast. Two different hydrodynamic regimes of water movements along the coast resulting from topographical features (the Słupsk Bank) can be distinguished. From the model simulation the specific conditions for the occurrence and development of upwelling at the eastern end of the Polish coast (in the vicinity of Władysławowo) can be deduced.