Abstract

A tidal mixing front in the southern North Sea is analyzed for its spatial and temporal distribution of the near-surface horizontal divergence and vertical velocity to understand the evolution of a tidal mixing front in a highly dynamic, semidiurnal tidal-influenced shallow shelf sea. A multi-sensor synoptic data set with a focus on the upper 10 m consisting of surface drifters, a drifting sensor chain, and an ADCP resolving length scales of , as well as subsequent CTD measurements, revealed a tidal mixing front that consistedof two separate tidal mixing density filaments that differed in 3D structure. In all observations, divergence developed with the accelerating tidal current and convergence evolved out of the deceleration of the tidal current. Vertical velocities ranged between , with divergence coupled to upwelling and convergence to downwelling. For the first tidal mixing density filament, convergence was limited to the upper 4 meters and was suppressed by the uplift of dense water. For the second filament, convergence was built from the bottom, propagating throughout the water column. These observations demonstrate that tidal mixing fronts play a significant role in promoting vertical transport within the water column.