To estimate the water‐renewal time of the Gulf of Finland, a numerical‐model simulation comprising the years 1987–1992 was carried out for realistic meteorological forcing. The results concerning water age as well as renewal index proved to reflect properties of the mean circulation system. The highest water ages (of around 2 yr) were found in the south‐eastern part of the Gulf. The water‐age distribution after the 5‐yr simulation proved to be almost identical to that obtained after 3 yr, indicating that the circulation in the Gulf may be rather stable. The analysis of the renewal‐index results showed that it takes around 5 yr to renew 98% of the water masses of the Gulf. When local maxima of the renewal index are found in conjunction with high water ages, this may indicate that the region is particularly vulnerable to eutrophication.

The water exchange of the Baltic coastal zone is characterized by its seasonally varying regimes. In the safety assessment of a potential repository for spent nuclear fuel, it is important to assess the consequences of a hypothetical leak of radionuclides through the seabed into a waterborne transport phase. In particular, estimates of the associated residence times in the near-shore coastal zone are of interest. There are several methods to quantify such measures, of which three are presented here. Using the coastal location of Forsmark (Sweden) as an example, methods based on passive tracers, particle trajectories, and the average age distribution of exogeneous water parcels are compared for a representative one-year cycle. Tracer-based methods can simulate diffusivity more realistically than the other methods. Trajectory-based methods can handle Lagrangian dispersion processes due to advection but neglect diffusion on the sub-grid scale. The method based on the concept of average age (AvA) of exogeneous water can include all such sources simultaneously not only boundary water bodies but also various (fresh)-water discharges. Due to the inclusion of sub-grid diffusion this method gives a smoother measure of the water renewal. It is shown that backward in time trajectories and AvA-times are basically equipollent methods, yielding correlated results within the limits set by the diffusivity.

No abstract

No abstract

We present preliminary results of the extension of the OAAS circulation model to a high-resolution bathymetry with a finest resolution of 0.25 nautical miles in the Gulf of Finland, the Baltic Sea. The models with a resolution of 1 mile or finer are capable of resolving typical mesoscale eddies in this basin where the internal Rossby radius is usually 2-4 km. An increase in the model resolution from 1 to 0.5 NM leads to a clear improvement of the representation of the key hydrophysical fields. A further increase in the resolution to 0.25 NM has a lesser impact on hydrophysical fields, but may lead to some changes in the instantaneous patterns of currents. The parameterization of the spreading effect of sub-grid-scale turbulence on the trajectories of initially closely located drifters is realized by means of accounting for the largely rotational character of the dynamics in this basin. The modelled average spreading rate for initially closely located particles for 1991 was 2 mm/s.