Torsten Seifert

Rossby radii and phase speeds in the Baltic Sea

On the basis of a data set recorded during seasonal cruises of the R.V.s A. v. Humboldt and Prof. A. Penck of the Academy of Sciences, G.D.R., in the Baltic Sea during 1977–1987, mean Brunt—Vaisalafrequency profiles were derived in order to compute vertical eigenvalues and internal, or baroclinic, Rossby radii for different seasons in various parts of the Baltic Sea. Due to regional variations of stratification and the depth of the different basins, these parameters vary considerably. Moreover, the Rossby radii show a seasonal cycle with minimum values during the winter and autumn, and maxima during summer. The largest Rossby radius, 7 km, was found in the Bornholm Basin and the smallest ones, 1.3 km, in the Belt Sea and in the Gulf of Finland during autumn. Approximated Rossby radii, as obtained by a WKB treatment, appear to be too small by about 10–30%.

On the dynamics of the Pomeranian Bight

Abstract CTD and ADCP measurements were obtained from the Pomeranian Bight, on cruises of 2–3 weeks duration together with time-series measurements of currents and salinity, from 1993 to 1996, in different seasons of the year. The observations were supported by high-resolution circulation model runs, representing the whole of the Baltic Sea. The model results and current/salinity observations in the Pomeranian Bight reveal the remote impact of both barotropic and baroclinic processes. The remote barotropic impact is driven by the exchange processes between the North Sea and the Baltic Sea. The process is controlled by a coherent band of intensive barotropic jets, which follow the north coast of Rugen Island, the northern edge of the Pomeranian Bight, and the central and easterly parts of the Pomeranian coast. This barotropic circulation pattern shelters the southern part of the Pomeranian Bight, from intensive currents. The remote baroclinic impact is due to the density difference between the surface waters of the Arkona Basin and the Pomeranian Bight. It drives a southward-directed current along the east coast of Rugen Island, as far south as Greifswalder Oie Island. This baroclinic current is essential for maintaining the salt balance of the Pomeranian Bight, but it hampers the wind-driven flow of water from moving along the northwestern rim of the Pomeranian Bight, along the eastern coast of Rugen Island in the direction of the Arkona Basin. The dynamical regime of the southern part of the Pomeranian Bight is governed by a locally wind-driven Ekman current and a compensating bottom current, as well as by coastal jets; these are weakest near the Swina river mouth and increase eastwards. Basic differences were observed in the development of the river plumes in response to westerly and easterly wind forcing. The lagoon waters were transported in a narrow band along the Pomeranian coast, during westerly winds. Easterly winds of more than a few days duration cause the band of lagoon waters to be dispersed, by upwelling filaments in the offshore direction into the Bornholm Basin. Simultaneously, outflowing lagoon waters form stable plumes off the Usedom coast, which are restricted by a topographical ridge to the west of the Greifswalder Oie Island and the southward-directed salt flow along the east coast of Rugen Island, from flowing directly northwards into the Arkona Basin. However, Ekman transport associated with long-lasting southeasterly wind transports the plume towards the western edge of the Oderbank, where a northward-directed edge current carries the lagoon water plume into the Arkona Basin.

Patterns of salt propagation in the Southwestern Baltic Sea

The paper gives a descriptive study of salt propagation in the area between Fehmarn Belt and Darσ Sill which is part of the transition region between the Baltic and the North Sea. A synthesis of observations and numerical modelling is used to elucidate the dynamics of currents and salinity patterns in response to the external forcing.

Modeling Transport of Fluff Layer Material in the Baltic Sea

To estimate impacts of nutrients and pollutants discharged by river inflow on the ecosystem it is essential to quantify the sedimentation, resuspension and transport processes of fine particulate matter. The present study aims at the modeling of basic transport processes of fine material in the western Baltic. A three-dimensional model of the Baltic Sea, that is based on the Modular Ocean Model (MOM-3.1) (Pacanowski & Griffies 2000), was applied to study maximum bottom shear velocities. Comparing the calculated shear velocities with the critical value for resuspension of fluff layers allows an identification of potential areas of deposition and accumulation. Further, model experiments were conducted to study transport paths of fluff layers in the southwestern Baltic. The model results give information about the resuspension and accumulation areas of fine material and could provide indications of potential accumulation areas of diapause eggs or cysts.

Sediment Properties in the Western Baltic Sea for Use in Sediment Transport Modelling

Abstract To simulate transport of clastic material in the Baltic a sediment transport module is linked to a Baltic Sea Model that is based on the Modular Ocean Model—MOM3. In order to describe the properties of the seabed sediment parameters as mean grain size, critical shear velocity and bed roughness length must be provided as input data to the numerical model system. To obtain maps of these quantities for the Baltic Sea area the proxy-target concept is applied. As proxy-variable the mean grain size of the sediment types is used. For different sediment samples the critical shear velocity was measured and serves as the target variable. Using the relation between the sediment classifications based on the mean grain size (proxy) and the measured critical shear velocity (target) a map of the critical shear velocity in the Baltic is derived. In January 1993 several extreme strong storm events occurred in the Baltic. Using this period for a model calculation maximum values of current and wave induced bottom shear velocities were obtained. Comparing these model results with the critical shear velocity distribution provided by the proxy-target concept we identify potential erosion areas. Further we show the transport path of material initially deposited in the Mecklenburgian Bight.