Lagrangian coherent structures in the Rijeka Bay current field

Abstract

      Some of the major European oil supply routes pass through the Adriatic Sea representing a potential to endanger the whole basin with an oil spill. Particularly high ecological vulnerability of Rijeka Bay is due to its geospatial characteristics as semi-enclosed basin. The simulated onemonth sea surface velocity field of Rijeka Bay was analyzed using Lagrangian coherent structures (LCSs) to assess the diffusion and chaotic advection of passive pollutants (dye). LCSs were extracted by the Finite-Time Lyapunov Exponents (FTLE), hypergraphs, Lagrangian advection alone and advection-diffusion methods. Results show relatively complex nonstationary flow dynamics in Rijeka Bay. Areas of ridges and valleys of FTLE fields spread in north-south and northeast-southwest directions, and they move toward east. Similarly, the hypergraphs method shows areas of attraction and areas of mixing that can be observed in stripes that stretch in directions north-south and northeast-southwest. In addition, the stripes of mesohyperbolic area spread toward east. To assess the potential of passive pollutant spreading, Gaussian pollutant source was simulated at the middle of the bay. The pollutant spreads in the same direction (north-south and northeast-southwest) with lateral diffusion of material proportional to diffusion coefficient.