Bert Viikmäe

Impact of horizontal eddy diffusivity on Lagrangian statistics for coastal pollution from a major marine fairway

Lagrangian trajectory methods are often applied as deterministic transport models, where transport is due strictly to advection without taking into account stochastic elements of particle dispersion, which raises questions about validity of the model results. The present work investigates the impact of horizontal eddy diffusivity for a case study of coastal pollution in the Gulf of Finland, where the pollutants are assumed to originate from a major fairway and are transported to the coast by surface currents. Lagrangian trajectories are calculated using the TRACMASS model from velocity fields calculated by the Rossby Centre circulation model for 1982 to 2001. Three cases are investigated: (1) trajectory calculation without eddy diffusivity, (2) stochastic modelling of eddy diffusivity with a constant diffusion coefficient and (3) stochastic modelling of eddy diffusivity with a time- and space-variable diffusion coefficient. It is found that the eddy diffusivity effect increases the spreading rate of initially closely packed trajectories and the number of trajectories that eventually reach the coast. The pattern of most frequently hit coastal sections, the probability of hit to each such section and the time the pollution spends offshore are virtually invariant with respect to inclusion of eddy diffusivity.

Quantification and characterization of mesoscale eddies with different automatic identification algorithms

ABSTRACT Viikmäe, B., Torsvik, T., 2013. Quantification and characterization of mesoscale eddies with different automatic identification algorithms Automatic methods for detection of mesoscale eddies are usually based on either physical (e.g. Okubo-Weiss parameter) or geometrical (e,g, streamline winding-angle) flow characteristics. In this paper, a hybrid method combining the strengths of the two different approaches is applied to the Eulerian velocity fields for two case studies: (i) the Gulf of Finland (the Baltic Sea) and (ii) the Raunefjord and Vatlestraumen area south-west of Bergen, Norway. Velocity fields are investigated with a hybrid winding-angle method (HWA), where the Okubo-Weiss parameter is first used to detect potential eddies, and the winding-angle method is used locally within these regions to test the Okubo-Weiss result. In the Gulf of Finland, the HWA method results in a substantially reduced number of detected eddies compared with the Okubo-Weiss result, indicating that the Okubo-Weiss parameter severely overestimates the number of eddies. In Vatlestraumen, there was a better correspondence between results obtained by the HWA and the Okubo-Weiss methods. The HWA method requires careful analysis since more than one streamline may identify the same eddy structure.

Analysis of the structure of currents in the Gulf of Finland using the Okubo-Weiss parameter

We analyze transport properties of surface water flow in the Gulf of Finland, the Baltic Sea, using spatial distributions of the Okubo-Weiss parameter, which reveals the spatial pattern of strain and relative vorticity at a specific time. The calculations are based on surface velocities calculated using the OAAS model with a spatial resolution of 1 nautical mile for 1987 in the framework of BONUS+ BalticWay cooperation. The currents are, on average, strain-dominated, with typical OW parameter values about 0.1. During short instances the OW parameter can abruptly increase by a factor 10. These events are not correlated with wind speed but occur slightly more frequently during the windy autumn season. Substantial areas of strong strain and relative vorticity regularly occur along the coast (due to topographic effects and coastal current fluctuations) and at two offshore areas; (i) in the eastern part near the mouth of the Neva river and (ii) in the north-western part of the gulf. The patterns of strong strain in the western part of the gulf originate from the northern nearshore but at times span down to the southern coast.

Temporal scales for nearshore hits of current-driven pollution in the Gulf of Finland.

Lagrangian trajectories of water parcels reconstructed using the TRACMASS model from three-dimensional velocity fields by the RCO model for 1965-2004 are used to analyse the temporal scales and the probability for the hits to the nearshore by pollution originating from a major fairway in the Gulf of Finland and transported by surface currents. Increasing the simulation length from 10 to 20days induces a linear increase in particle age, but the pattern of nearshore hits remains the same. A reasonable benefit can be reached by relatively small shifts of certain parts of the present fairway in a few locations. The overall probabilities do not reveal any trend for 1965-2004. The largest changes in the nearshore hits are revealed for the proportion of hits to the opposite nearshore areas. This feature probably reflects an abrupt turn of the geostrophic air-flow over the southern Baltic Sea by ~40° since 1987.

Towards a New Generation of Techniques for the Environmental Management of Maritime Activities

An overview is presented of a new preventive method for minimizing environmental risks based on the optimization of the location of potentially dangerous activities. The starting point of the relevant technology is the frequent presence of semi-persistent surface current patterns in many water bodies. Due to these patterns the probability of transport of dangerous substances (for example, oil pollution) from different open sea areas to vulnerable regions often becomes highly variable. For certain offshore areas this probability is relatively small and (re)directing activities to these areas would involve very limited additional costs. Principles, key components and applications of a prototype method for the identification of such areas and for their use in environmental management of shipping, offshore and coastal engineering activities are described. The core idea is to identify and quantify the potential of different offshore domains to serve as a source of danger to the vulnerable areas through pollution transport by various met-ocean drivers. An approximate solution to this inverse problem of pollution propagation is obtained by means of statistical analysis of a large number of solutions to the direct problem of propagation of tracers in terms of so-called Lagrangian trajectories. The offshore domains are quantified in terms of the probability of the current-driven adverse impact reaching the near-shore after an accident has happened or, alternatively, in terms of time until this impact (for example, an oil spill) reaches the coast. Variations of this method can be used, for example, for estimates of risks of the offshore activities in the open ocean, for fairway design and for the prediction of the most frequently hit near-shore domains.