Abstract: Lagrangian Coherent Structures and mixing properties of a tidal driven ow in the Ria de Arousa (Galicia, Spain)

Authors: Florian Huhn and Vicente Pérez-Muñuzuri and Pedro Montero

Florian Huhn and Vicente Pérez-Muñuzuri
Group of Nonlinear Physics, Faculty of Physics,
University of Santiago de Compostela, E-15782 Santiago de Compostela, Spain

We study the two-dimensional time dependent surface currents at the western coast of Galicia, Spain, including estuarines (Rias Baixas) and o-shore regions (Costa da Morte). Turbulence in ocean currents plays an important role for the mixing of water volumes with dierent scalar properties. The spatial structures of the current indicate, where turbulence-induced mixing can be expected. The mixing spreads available nutrients and has therefore a strong influence on the biological production. The currents also drive the temporal evolution of oil spills and other contaminations on the surface. Lagrangian Coherent Structures (LCS) of the flow are detected by the analysis of Finite Time Lyapunov Exponents (FTLE) of the velocity eld, see [1],[2],[3]. This approach is based on the theory that the distance of two particle trajectories grows exponentially in time for turbulent flows and reveals the flow structures that are relevant for mixing. The analyzed velocity data is obtained from a 3-dim ocean model (MOHID), which was run for dierent typical meteorological situations at the coast of Galicia.
Concerning aquaculture, the region Rias Baixas is one of the most productive areas in the world.
The presence of harmful algae blooms (HAB) is a severe problem for this industry. Therefore the
knowledge of areas of retention of primary production and HABs inside the Rias is very important. In
order to monitor the distribution of these retention zones, several drifter experiments are performed
in the framework of the DRIFTER project in an estuarine (Ria de Arousa), where the flow is mainly
forced by the tides and the wind. The predicted mixing properties and LCSs are compared to data
from these experiments.

[1] G. Haller and G. Huan. Langrangian coherent structure and mixing in two-dimensional turbulence. Physica D, 147:352{370,
2000.
[2] F. Ovidio, V. Fernáandez, E. Hernandez-Garcia, and C. Lopez. Mixing structures in the Mediterranean Sea from nite-size
Lyapunov exponents. Geophys. Res. Letters, 31, 2004.
[3] S. Shadden, F. Lekien, J. Paduan, F. Chavez, and J. Marsden. The correlation between surface drifters and coherent
structures based on high-frequency radar data in Monterey Bay. Deep Sea Research Part II: Topical Studies in Oceanography,
2008.