The shelf-sea fronts: implications of their existence and behaviour

Abstract

This paper reviews the results of recent observations of the shelf-sea fronts and discusses the relation of these features to what is known of the mean circulation. Evidence from satellite infrared imagery and ship data are presented to illustrate the movements and structure of the fronts. Many of the data can be rationalized in terms of models of tidal mixing based on vertical exchange only, with a feedback element, which is needed to account for the small observed adjustment to the springs-neaps stirring cycle. The density gradients within the front imply strong geostrophic currents along the isobars but the available evidence does not support the idea of a frontal jet. The complicated velocity field observed may be the result of large-scale (20-40 km) eddies that develop from instability of the flow. The relative success of local energy models implies that horizontal advection normal to the fronts is weak. A simple model of depth-uniform advection indicates that frontal positions and gradients should be sensitive to flows as small as 1 cm s$^{-1}$, and two examples of such influence are tentatively identified.