Surface wave dynamics effects at multiple scales in the mediterranean sea

Abstract

Wind generated waves are crucial to transfer energy and momentum from the atmosphere to the sea surface, redistributing and transporting such energy to remote areas of the ocean. Waves induce ventilation in the ocean upper layer, enhancing vertical mixing and producing vertical transport of biogeochemical tracers. When waves reach coastal areas they dissipate energy through viscous damping at the bottom and eventually by breaking, resulting in morphological changes of the bathymetry, sediment transport and erosion. The general objective of this Thesis is to perform a characterization of spatiotemporal variability of surface ocean waves, and to study their effect on the dynamics at the upper layers and at a coastal system. In particular, we analyze the large scale variability of the extreme wave climate in the Mediterranean Sea and the North Atlantic Ocean. We compute the monthly extreme waves analyzing their interannual variability. Then, at regional scale, we study the regional impact of the wind and wave induced velocity on the total surface dynamics at different sub-regions of the Mediterranean Sea from the Eulerian and Lagrangian standpoints. Finally, at coastal scale, the effects of extreme waves from storm groups on the sediment transport is assessed based on a multi-system approach combining remote and in situ data with numerical techniques. Seasonal signal accounts for 50% of the extreme wave height variability in the North Atlantic Ocean and up to 70% in some areas of the Mediterranean Sea. For the winter season, the North Atlantic Oscillation and the Scandinavian modes are the dominant large-scale atmospheric modes of variability that modulate extreme waves in the North Atlantic Ocean; and to a lesser extent, the East Atlantic Oscillation also controls extreme waves in the central part of the basin. In the Mediterranean Sea, the negative phase of East Atlantic Oscillation dominates the variability of extreme waves during winter season. At regional scale, ageostrophic currents substantially modulates the total mesoscale dynamics by two non-exclusive mechanisms; by providing a vigorous input of momentum (e.g. where regional winds are stronger) and/or by opposing momentum to the main direction of the geostrophic component. To properly characterize the spatio-temporal variability of the mesoscale dynamics induced by wind and wave, we propose a regionalization of the Mediterranean Sea based on the homogeneous variability of the coupled geostrophic and agesotrophic velocity components, combining self-organizing maps (SOM) and wavelet coherence analyses. We study the impact of the wind and waves induced motions on the mixing and transport properties of the surface marine flow. Transport pathways unveiled by the geostrophic Lagrangian coherent structures are significantly modified by the ageostrophic currents, often leading to a decrease of the retention capacity of the eddies. The ageostrophic component induces an increase in mixing activity up to 36% in some regions of the Mediterranean basin, finding the largest values during autumn and winter seasons. The study of the anisotropy in the separation scales between pairs of trajectories reveals that the zonal component of the flow plays an important role, determining the properties of the relative dispersion. The characterization of time scales evolution of sandy coasts has been a topic of wide interest over the past decades, since sandy beaches and dune systems are the first natural lines of coastal defense against floods and erosion hazards. The results of this work show that sandy systems have two characteristic time scales: the eroding process associated with the extreme waves generated by the storm is of the order of hours, while the time scale of the transition to the equilibrium is of the order of months. This different behavior provides the basis for a more efficient beach management strategy.