River plume development in semi-enclosed Mediterranean regions: North Adriatic Sea and Northwestern Aegean Sea

Abstract

The Mediterranean Sea has many coastal regions with complex topography. The study concentrates on semi-enclosed regions that receive freshwater input from rivers. The development of the river plumes under the influence of the important circulation forcing mechanisms and under the guidance of the topographic controls is examined. The study rivers are the Po (North Adriatic) and the Axios (North Aegean). They both provide important inputs of low-salinity waters and land-drained materials for their respected shelf regimes. The rivers are parameterised as sources of zero salinity in the three-dimensional, primitive equation Princeton Ocean Model (POM). The river plumes are studied in the presence of realistic topography and idealised winds. The influence of the general basin circulation on the plume structure is also examined. The development and evolution of the two river plumes exhibit the following common characteristics. The buoyancy-driven flow is characterised by an anticyclonic bulge seaward of the river mouth and a coastal current south of the discharge site contained within a narrow strip along the coast. The coastal current has a southward direction, as both rivers discharge at the west side of the basin. The “meandering” along this dynamic region is due to a baroclinic instability process. Downwelling-favorable winds enhance the buoyancy-driven coastal current and restrict offshore removal of low-salinity waters, while the opposite holds for upwelling-favorable winds. The circulation is influenced by the presence of three coastal boundaries. Differences in the modeled phenomena for the two study regions arise from (a) interaction with ambient flow, which may substantially influence the behavior of the plumes, and (b) the topography and geometry of each basin. For instance, the presence of strong neighboring rivers influences the Po River plume. On the other hand, the narrowness of the basin near the discharge site of the Axios River allows for offshore expansion of the plume all the way to the east coast and the establishment of anticyclonic flow over a large part of the shelf, regardless of wind conditions.