Abstract
Abstract. The aim of this study was to determine the dispersion of passive pollutants associated with the Tiber discharge into the Tyrrhenian Sea using numerical marine dispersion models and satellite data. Numerical results obtained in the simulation of realistic discharge episodes were compared with the corresponding evolution of the spatial distributions of MODIS diffuse light attenuation coefficient at 490 nm (K490), and the results were discussed with reference to the local climate and the seasonal sub-regional circulation regime. The numerical model used for the simulation of the sub-tidal circulation was a Mediterranean sub-regional scale implementation of the Princeton Ocean Model (POM), nested in the large-scale Mediterranean Forecasting System. The nesting method enabled the model to be applied to almost every area in the Mediterranean Sea and also to be used in seasons for which imposing climatological boundary conditions would have been questionable. Dynamical effects on coastal circulation and on water density due to the Tiber discharge were additionally accounted for in the oceanographic model by implementing the river estuary as a point source of a buoyant jet. A Lagrangian particle dispersion model fed with the POM current fields was then run in order to reproduce the effect of the turbulent transport of passive tracers mixed in the plume with the coastal flow. Two significant episodes of river discharge in both winter and summer conditions were discussed in this paper. It was found that the winter regime was characterized by the presence of a strong coastal jet flowing with the ambient current. In summer the prevailing wind regime induced coastal downwelling conditions, which tended to confine the riverine waters close to the shore. In such conditions sudden wind reversals due to local weather perturbations, causing moderate local upwelling, proved to be the only effective way to disperse the tracers offshore, moving the plume from the coast and detaching large pools of freshwater.
Highlights
Most pollution of the coastal marine environment is associated with river runoff, which carries into the sea the effects of atmospheric pollution and human urban, agricultural and industrial wastes
The realistic representation of the mesoscale dynamics in the domain was important for determining the scale of dispersion of the river plume and assessing the importance of the interactions of processes at regional and coastal scales
A numerical experiment simulating the release of particles carried by the riverine waters by means of a Lagrangian model gave promising results when compared to satellite maps
Summary
Most pollution of the coastal marine environment is associated with river runoff, which carries into the sea the effects of atmospheric pollution and human urban, agricultural and industrial wastes. Important aspects of the problem on a local scale are the dynamics associated with the discharge of light freshwater in a denser ambient flow, the presence of currents and tides, the speed and direction of the wind, the concentration of pollutants in the river and the amount of volumetric discharge. K Rossby radiuses (in the “near field flow”, in stagnant uniform ambient), there are other length scales which define the aspect of the buoyant jet These are related to the relative importance of the discharge momentum flux M = u0Q0 (“jetlike” flow behavior close to the source where M is prevailing) with respect to the discharge buoyancy flux B = g Q0. 44oN cate the strengths and the weaknesses of both the analyses, providing information on how to make the best use of them in environmental monitoring (Sect. 5)
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