Studying the Baltic Sea Circulation with Eulerian Tracers

As shipping of environmentally hazardous cargo, like oil, has increased considerably in the Baltic in recent years, methods are needed to calculate the fairways between two harbours such that hazardous substances from a hypothetical accident will stay as long as possible away from ecologically sensitive areas like the coastal zone. For this purpose an ensemble approach based upon Eulerian tracer simulations is presented which has the potential to be further developed to become operational for the optimization of fairways. First, we introduce and compare Eulerian and Lagrangian descriptions of any fluid in general. Second, a three-dimensional circulation model of the Baltic Sea is presented from which currents are used to calculate the evolution of the Eulerian tracers in time that obey traditional advection-diffusion equations. The model set-up is presented in detail to illustrate the potential of ocean circulation models for our purposes but also their shortcomings. Third, examples of studies using Eulerian tracers are presented that analyse the characteristics of the circulation, like ventilation time scales and age of water masses. Finally, we focus on three selected examples of oil spill modelling using Eulerian methods. Although oil spill modelling very often utilizes a Lagrangian particle approach, we show that even Eulerian methods can be used that might under certain circumstances have some advantages compared to the Lagrangian approach.