Surface currents and transport processes in the Strait of Gibraltar: Implications for modeling and management of pollutant spills

The main features of the surface currents in the Strait of Gibraltar and Algeciras Bay are analyzed from the numerical results of three-dimensional hydrodynamic modeling, complemented and validated through observational data. The relative contributions of the main processes affecting the surface current patterns (i.e. mean background flows, barotropic tide, baroclinic internal waves, and wind regime) are described and interpreted, focusing on their effects on surface transport-dispersion processes, with special attention paid to the simulation and prediction of the evolution of oil spills and other marine pollutants in this sensitive environment in the context of coastal management. In general, the mean background surface flow promotes eastward transport in the Strait and re-circulation patterns within Algeciras Bay, with inflows and outflows mainly near the western and eastern coasts, respectively. The barotropic tide produces periodic reversals of water transport (and hence higher persistence and re-entrance processes) on the western side of the Strait and within the Bay, while its effects are generally weaker than the mean eastward flow in the eastern region. Baroclinic, tide-induced, short-period internal waves are responsible for occasional local high-divergence of the current field, with effects on the surface transport-dispersion processes. Finally, the wind regimes commonly present in this environment (mainly easterlies and westerlies) generally modulate the transport intensity in the Strait, while affecting more strongly the surface transport patterns within Algeciras Bay. Outputs from the developed metocean modeling system fed a Lagrangian particle-tracking model, validated by drifting-buoy data, to reproduce past oil spill events: the tanker Sea Spirit in the western Strait of Gibraltar (1990), and the ships Fedra and Tawe in Algeciras Bay (2008), each well documented and characterized by different metocean conditions and spill quantities. The predicted time-spatial distribution of the oil slicks and affected coasts were compared with available reports and photographs, showing that the strongly baroclinic oceanographic regime of the Strait of Gibraltar and Algeciras Bay demands a fully 3-D approach to simulate properly the behavior of surface pollutant spills in these environments, in order to achieve an efficient management by supporting risk assessment, contingency planning and palliation activities.