Pathways of suspended particles released in the bottom boundary layer of the Bornholm Deep, Baltic Sea (numerical simulations)

A model system consisting of a circulation model and a random-walk model is developed to simulate suspended particulate matter transport in the bottom boundary layer (BBL) of the southern Baltic Sea. The circulation model is based on POM, the Princeton Ocean Model, in which the vertical grid size is refined towards the bottom in order to resolve BBL properly. 3D fields of velocity, vertical and lateral apparent diffusivities generated by the circulation model are used as an input for the random walk model to simulate transport and dispersion of particles with prescribed settling velocity. The random-walk scheme allows for non-uniform vertical profiles of the vertical apparent diffusivity, and test runs have been done to make sure that the model does not display unrealistic removal of particles from highly turbulent BBL and further accumulation in low-diffusivity above-lying layers. A number of numerical experiments have been performed to study pathways of suspended particles released in the BBL in the centre of the Bornholm Deep at different wind conditions. At northerly and easterly winds the particles initially move westward and then get involved into either northern or southern detours around the Deep. The particles from the northern detour are finally absorbed into the Slupsk Furrow while those of the southern detour do not enter the Furrow keeping on cyclonic rotation within the Bornholm Basin. To the contrast, for the westerly and southerly wind conditions the particles move to the northeast for some 20 km and then get involved into the cyclonic rotation. The cyclonic rotation implies the convergence of currents in the BBL due the Ekman transport and, in view of continuity, the upwelling, so that the particles will remain trapped within the Bornholm Deep if the settling velocity is large enough to overcome the upwelling. Since the westerly wind conditions dominate in the climatic sense, the trapping effect may be considered as an important factor that controls dispersion of chemical warfare agents dumped in the Bornholm Deep after the World War II.