Influence of shelf topography upon along shelf flow and across shelf exchange in the region of the Ebro Delta

The influence of along shelf topography upon shelf edge flow, and cross shelf exchange in the Ebro Region of the Catalan shelf, is studied using a non-linear hydrodynamic model. This region is of particular interest as it is an area of transition from a narrow to a broad shelf. Calculations are performed using both idealized and physically realistic topography, for a number of inflows at the shelf edge on the narrow shelf. ations using idealized topography show that the position of the inflow relative to the shelf edge, and the horizontal shear within the inflow are the dominant factors influencing the along shelf flow and the formation of eddies on the narrow shelf. For a wide jet, or a narrow jet inflow in shallow water, the along shelf flow is more likely to be steered along the topography with minimum cross shelf exchange of water. However, in the case of a narrow jet discharged on the deep water side of the shelf edge, small-scale eddies (having a length scale comparable with the width of the narrow shelf and period corresponding to the first shelf wave) are formed on the narrow shelf and instabilities occur in the shelf edge flow. The intensity of the instability in the inflow is related to the degree of across shelf shear in the inflow current. In essence the onset of the instability in the along shelf flow is governed by barotropic stability theory. Hence, it is controlled by the potential vorticity gradient across the shelf edge at the location of the inflow. Similar effects are found in the model when realistic topography is used, although in this case some eddies appear on the wide shelf associated with topographic features. fluence of bottom friction and the form and location of the inflow upon the dynamics of the flow in the region is investigated using an empirical orthogonal function (EOF) approach. This shows that in the case of weak barotropic instability a large proportion of the energy is contained within the first two modes. Coefficients of these modes have a period close to the resonant period of the shelf wave of order 48 h with a spatial pattern reflecting that of the shelf wave. On the wide shelf, a larger scale circulation is present with a period of the order of 5 days. In the case of strong barotropic instability, the EOF analysis shows that the shelf modes and associated eddies grow with time and non-linear interaction can take place due to non-linearities in the model. In this case higher modes are important and the coefficients of the EOFs show the presence of a range of periods associated with some non-linear effects. ations clearly show that the magnitude of the circulations associated with these flows, and the intensity of meanders of the along shelf flow are critically dependent upon the inflow conditions and bottom friction.