Wolfgang Fennel

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%.

A box model approach for a long-term assessment of estuarine eutrophication, Szczecin Lagoon, southern Baltic

Abstract We develop a layered “box model” to evaluate the major effects of estuarine eutrophication of the Szczecin lagoon which can be compared with integrating measures (chlorophyll a (Chl a ), sediment burial, sediment oxygen consumption (SOC), input and output of total nutrient loads) and use it to hindcast the period 1950–1996 (the years when major increase in nutrient discharges by the Oder River took place). The following state variables are used to describe the cycling of the limiting nutrients (nitrogen and phosphorus): phytoplankton (Phy), labile and refractory detritus (D N , D Nref , D P , D Pref ), dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and oxygen (O 2 ). The three layers of the model include two water layers and one sediment layer. Decrease of the carrying capacity with respect to the increased supply of organic matter of the system with advancing eutrophication over the period studied is parameterized by an exponential decrease of the sediment nitrogen fluxes with increasing burial, simulating changing properties from moderate to high accumulating sediments. The seasonal variation as well as the order of magnitude of nutrient concentrations and phytoplankton stocks in the water column remains in agreement with recent observations. Calculated annual mean values of nutrient burial of 193 mmol N m −2 a −1 and 23 mmol P m −2 a −1 are supported by observed values from geological sediment records. Estimated DIN remineralization in the sediments between 100 and 550 mmol N m −2 a −1 corresponds to SOC measurements. Simulated DIP release up to 60 mmol P m −2 a −1 corresponds to recent measurements. The conceptual framework presented here can be used for a sequential box model approach connecting small estuaries like the Szczecin lagoon and the open sea, and might also be connected with river box models.

Dynamics of the western Baltic

Abstract The western Baltic is the southernmost part of the transition area between North Sea and Baltic Proper. The relatively shallow, strongly stratified region is subject to three kinds of external forcing: (i) local winds, (ii) barotropic pressure gradient due to sea level differences between Baltic and North Sea and (iii) baroclinic pressure gradients sustained by freshwater outflow at the sea surface and inflow of saline water near the bottom. Owing to the time scale of the weather patterns (2 to 4 days) the circulation is mostly governed by transients rather than by permanent currents. In order to elucidate the dynamics of the area four expeditions have been carried out in summer 1983, 1987, 1989 and in autumn 1984 where dense grids of CTD stations were worked in the area of the Mecklenburg Bight and Darss sill. The experimental findings reveal that easterly winds force coastal jets off the german coast associated with coastal upwelling. This response can be amplified substantially by pressure gradient forcing. Westerly winds generate eddy patterns rather than coastal jets. This can be explained by the fact that the cross-circulation forced by westerly winds is diminished by the opposite cross-circulation driven by pressure gradients, implying only a weak resulting coastal up- or downwelling. There are indications that the eddies are generated by vortex shedding at topographical features and eddy detachment at coastal irregularities.

The mesoscale variability of nutrients and plankton as seen in a coupled model

The effect of the variability of mesoscale current patterns on the dynamics of nutrients and plankton is studied by means of a simple coupled model. A chemical-biological model in conjunction with a high resolution circulation model is applied to the southwestern Baltic. The biological model has four state variables; a limiting nutrient, phytoplankton, Zooplankton and detritus. The circulation model is based on an implementation of the GFDL-model in the modular version (MOM 1).

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.

Wind forced oceanic responses near ice edges revisited

Abstract The wind forced response of a flat bottomed, stratified, hydrostatic ocean on the f-plane near an ice edge is studied by means of an analytical theory. The properties of the ice are implicitly taken into account as a medium to communicate the wind stress to the water below the ice cover. The consistency of the ice (grease, floes of varying size, or pack ice etc.) as well as the stability of the atmospheric boundary layer causes variations of the effective drag coefficient. Various types of across-ice-edge profiles of the wind stress were recently discussed by Guest et al., which can be modelled by a piecewise linear profile with a maximum close to, but iceward of the edge. We consider fixed and moving ice edges with piecewise constant and piecewise linear cross-edge wind stress profiles. The response patterns are characterized by the along-edge jets as well as up- and downwelling near the ice edge. The detailed structures depend crucially on the wind stress profile and the movement of the ice edge. The spatial scales of the wind driven oceanic responses near the ice edge are set by the baroclinic Rossby radius, R 1 the distance the edge has travelled, and by the structure of the wind stress profile. The time scales are basically determined by the duration of the forcing. Comparisons with observations show that a relative simple analytical model is able to describe qualitatively the observed features.

Coupling Biology and Oceanography in Models

Abstract The dynamics of marine ecosystems, i.e. the changes of observable chemical-biological quantities in space and time, are driven by biological and physical processes. Predictions of future developments of marine systems need a theoretical framework, i.e. models, solidly based on research and understanding of the different processes involved. The natural way to describe marine systems theoretically seems to be the embedding of chemical-biological models into circulation models. However, while circulation models are relatively advanced the quantitative theoretical description of chemical-biological processes lags behind. This paper discusses some of the approaches and problems in the development of consistent theories and indicates the beneficial potential of the coupling of marine biology and oceanography in models.

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.

Inertial waves and inertial oscillations in channels

Abstract A theory is considered for the inertial response of a stratified flat-bottomed channel, with straight walls, to longshore wind. The discussion is focused particularly on transient variables, such as inertial oscillations and waves. The purely “slab-like” inertial oscillations, which occur only in the horizontal currents, are in direct response to the wind. Inertial waves are generated at coastlines and propagate offshore; in the vertical, they have the shape of the associated vertical modes. These waves, which can be expressed by infinite sums of Poincarecross-oscillations, are dispersive with locally varying frequencies and wavenumbers. Close behind the wave fronts, which move at the maximum group velocity of the associated vertical mode, the frequencies and wavenumbers are relatively high. Far behind the fronts, the frequency approaches the local inertial frequency, f , and the wavenumber becomes smaller. Contrary to a semi-infinite ocean, where inertial waves propagate away from a source and decay algebraically, inertial waves in a channel are being reflected at the opposing coastlines and may propagate back and forth. After reflection, their dispersive properties are conserved and the resulting patterns are characterized by locally varying frequencies and wavenumbers. Hence, seemingly stochastic patterns may sometimes occur; therefore, the properties of inertial waves in a channel are quite different from those of a semi-infinite ocean.

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.