River plume fronts off NW Iberia from satellite observations and model data

River plume fronts off NW Iberia during autumn 2002 are examined based on the gradient of the mixed layer depth (MLD) in threedimensional model data and on sea surface temperature gradients in Advanced Very High Resolution Radiometer (AVHRR) imagery. The method reveals new aspects of the dynamics of fronts in the area. The strong gradient of the MLD between the plume and the adjacent ocean serves as a physical indicator of the maximum extension of the river plume and takes into account its threedimensional structure as well as its surface signal. Differences in the position of river plume fronts are associated with local upwelling/downwelling conditions and their relaxation. During expansions of the plume induced by upwelling in late autumn, the shallower plume waters become colder and the thermal gradient with offshore waters increases. The model simulation also illustrates the existence of instabilities at the plume front and cross-shore variations in plume width that could induce the appearance of thermal gradients which can be resolved with AVHRR. Our simulations also show regional differences in front location, structure, and probability north (Galicia) and south (north Portugal) on the shelf, where bathymetry differs in shape and depth.