Residual and tidal flow at a tidal mixing front in the North Sea

Abstract

A ship-mounted acoustic Doppler current profiler (ADCP) has been used to study the circulation in a frontal region of the North Sea, at the boundary between mixed and stratified water. The ship steamed backwards and forwards across a front on two sections for at least two tidal cycles to enable tidal and residual flows to be resolved. The residual flows parallel to the front are generally larger (by a factor of ∼3) than those perpendicular to it and exhibit significant structure. On a section near the Yorkshire coast, observations at neaps show that the flow is to the southeast with two regions where the flow is up to ∼6 cm s −1 separated by a zone of low flow. This structure is in qualitative agreement with the geostrophic flow based on the density field. At a section near the Dogger Bank there is evidence of a frontal jet, with currents reaching 15 cm s −1. However, the jet is not present all the time, and there are indications that it is stronger at springs and weaker at neaps. The cross-frontal residual current also is better defined at spring tides, with convergence at the surface and divergence near the bottom. The ADCP has enabled details of the tidal current profile to be elucidated. There are clear indications that the profile is influenced by the density structure. The tidal currents near the bottom lead those near the surface on both sides of the front, but on the stratified side this phase change is concentrated in a narrow band at the thermocline depth. On the mixed side, the phase change is spread more evenly over the water column. The tidal current vector rotates clockwise in the top 20 m and then anticlockwise in the rest of the water column, an effect known to be caused by bottom friction. A more subtle feature is the observed clockwise veering of the semi-major axis with depth from the surface.