A Three-Dimensional Model of Shallow-Sea Fronts

Abstract

Fronts, that is, sharp horizontal gradients of temperature and/or salinity, are widespread in the shelf seas. The evidence for this comes not only from ship surveys but in recent years from infra-red satellite images, which have provided a much needed synoptic view of the sea surface (on the occasions, rare for north-west Europe, when it is not obscured by cloud). Many of the shelf-sea fronts may be identified with regions where there is a transition between water which is stratified in the summer and water which is well-mixed throughout the year. The position of these fronts is controlled by variations in tidal mixing, and follows closely a critical contour of H/U³, where H is depth and U the tidal current amplitude. More strictly, the non-dimensional parameter NH/U³ may be considered, where N is buoyancy flux into the surface, but N varies slowly with position over the shelf (on a seasonal time scale) — this parameter is equivalent to the depth divided by the Monin-Obukhov length scale. Simpson and Hunter (1974) suggested the importance of the H/U³ parameter, and Pingree and Griffiths (1978) produced a map showing positions of frontal boundaries on the north-west European shelf predicted from a numerical tidal model, most of which are confirmed by observations. The seasonal cycle model of James (1977) suggested that the feedback effect (whereby the presence of stratification reduces the efficiency of tidal mixing) means that the frontal boundaries move little between spring and neap tides though there is a large change in U³. This was confirmed by Simpson and Bowers (1979, 1981).