Fronts on the continental shelf

Abstract

A well‐defined class of fronts occurring in the shelf seas around the United Kingdom during the summer months marks the boundary between stratified and vertically mixed regimes. The occurrence of these fronts may be interpreted in terms of the distribution of available turbulent kinetic energy from the tidal currents and wind stress and the buoyancy flux input at the surface. The principal parameter controlling stratification is the ratio of the water depth to a Monin‐Obhukov length determined by the tidal velocity. A corresponding parameter based on the wind stress is also found to contribute significantly in a combined model of tide and wind mixing. Vertical sections perpendicular to the front by undulating CTD indicate a strongly baroclinic region with horizontal temperature gradients of ∼1°C/km, which imply strong flows parallel to the front. Drogue observations show that nontidal velocities of ∼10 cm/s occur in the vicinity of the fronts but the flow regime is apparently complicated by large‐scale (∼25 km) instabilities which are clearly manifest in satellite infrared imagery and airborne radiation thermometer surveys. There is also indirect evidence for vertical motions suggesting that both upwelling and down‐welling occur in the frontal zone.