A Lagrangian trajectory model, TRACMASS with the use of velocity fields calculated by the Rossby Centre (Swedish Hydrological and Meteorological Institute) circulation model, is employed to analyse trajectories of current-driven surface transport in the Gulf of Finland, the Baltic Sea, for the period of 1987-1991. Statistical analysis of trajectories is performed to calculate a map of probabilities for adverse impacts released in different sea areas to hit the coast. There is a clearly defined curve (equiprobability line) in the western part of the gulf from which the chances of the propagation of adverse impacts to either of the coasts are equal. The current-driven propagation of tracers from a wide area (of reduced risk) to the coast in the central and eastern parts of the gulf is unlikely within about three weeks. A safe fairway in terms of coastal protection goes over the equiprobability line and the area of reduced risk.
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.
We address possibilities of minimising environmental risks using statistical features of current-driven propagation of adverse impacts to the coast. The recently introduced method for finding the optimum locations of potentially dangerous activities (Soomere et al. in Proc Estonian Acad Sci 59:156-165, 2010) is expanded towards accounting for the spatial distributions of probabilities and times for reaching the coast for passively advecting particles released in different sea areas. These distributions are calculated using large sets of Lagrangian trajectories found from Eulerian velocity fields provided by the Rossby Centre Ocean Model with a horizontal resolution of 2 nautical miles for 1987-1991. The test area is the Gulf of Finland in the northeastern Baltic Sea. The potential gain using the optimum fairways from the Baltic Proper to the eastern part of the gulf is an up to 44% decrease in the probability of coastal pollution and a similar increase in the average time for reaching the coast. The optimum fairways are mostly located to the north of the gulf axis (by 2-8 km on average) and meander substantially in some sections. The robustness of this approach is quantified as the typical root mean square deviation (6-16 km) between the optimum fairways specified from different criteria. Drastic variations in the width of the 'corridors' for almost optimal fairways (2-30 km for the average width of 15 km) signifies that the sensitivity of the results with respect to small changes in the environmental criteria largely varies in different parts of the gulf.
The basic time scales for current-induced net transport of surface water and associated time scales of reaching the nearshore in the Gulf of Finland, the Baltic Sea, are analysed based on Lagrangian trajectories of water particles reconstructed from three-dimensional velocity fields by the Rossby Centre circulation model for 1987-1991. The number of particles reaching the nearshore exhibits substantial temporal variability whereas the rate of leaving the gulf is almost steady. It is recommended to use an about 3 grid cells wide nearshore area as a substitute to the coastal zone and about 10-15 day long trajectories for calculations of the probability of reaching the nearshore. An appropriate time window for estimates of the properties of net transport patterns is 4-10 days.
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