Dynamics of tidal mixing fronts in the North Sea

Abstract

Twenty years since the discovery of tidal mixing fronts there are still few convincing observations of the velocity field associated with these structures. Simple models of shelf sea fronts predict strong along-front jets, weaker convergent circulations and instabilities. During the North Sea Project a series of studies of the Flamborough frontal system has used a new approach based upon novel combinations of modern instrumentation (HF radar, acoustic Doppler current profiler, Decca-Argos drifting buoys and towed undulating CTD) and have provided one of the first directly observed pictures of shelf sea frontal circulation. Observational confirmation of jetlike along-front flow has been found together with evidence of cross-frontal convergence. A new generation of eddy-resolving models will help to focus the next phase of frontal circulation studies in relation to questions concerning baroclinic instability and eddy generation.

Bubble momentum plume as a possible mechanism for an early breakdown of the seasonal stratification in the northern North Sea

“…It is about twice as large as previous observations of the seasonal frontal jet in the North Sea (Brown et al, 1999;Hill et al, 1993;Pingree and Griffiths, 1978;Simpson and Pingree, 1978;van Aken et al, 1987). Moreover, the passage of the frontal jet is not restricted to one single episode, but the frontal jet appears to move across the 22/4b site at certain intervals, although it had weakened with every passage.…”

“…Within this period the maximum (absolute) residual velocity (u0, v0) = (0.7,-0.25) m/s is observed at ~75 m above the seafloor, whereas the time-mean residual velocity at this depth is (u0,v0) = (0.3,+0.15) m/s. This phenomenon likely is related to a frontal jet-like structure -similar ones have been observed in the southern part of the North Sea, albeit with a much smaller maximum along (cross) frontal velocities of only 0.15 (0.05) m/s (Hill et al, 1993). In our study, additional (continuously weaker) jets appear around 5, 13, 26, and 31 November.…”

“…At the location of the tidal mixing fronts, sub-surface, along-frontal jets have been observed with speeds of 0.02 to 0.16 m/s (Brown et al, 1999;Simpson and Pingree, 1978;van Aken et al, 1987) and cross-frontal velocities of 0.04 m/s (Hill et al, 1993;Lwiza et al, 1991;Matthews et al, 1993). The alongfrontal jet is in near geostrophic balance; deviations from geostrophy lead to a secondary circulation with upwelling at the mixed side of the front (Simpson and and surface convergence (Hill et al, 1993). The typical width of the tidal mixing front is fairly narrow, only 2 km.…”

“…The timing of the breakdown also may be derived from the temporal behavior of the horizontal velocities, because they could signify the passage of a frontal jet (Brown et al, 1999;Hill et al, 1993;Simpson and Pingree, 1978;van Aken et al, 1987). To remove variations related to the tides, a running window LSHA was applied separately to the time-series of the horizontal velocities, u and v at each depth.…”

Sources, Fate and Transformation of Organic Matter in Wetlands and Estuaries

“…Apart of the mixing of OM sources (mangrove, terrestrial and marine; , living vs. nonliving sources of POM may vary with vegetation type, estuarine geomorphology, and hydrology, among others. In addition, POC dynamics can be influenced by diverse factors such as water discharge (Chao, 1988;Czitrom, 1986), turbulence (West and Sangodoyin, 1991) and tidal mixing (Hill et al, 1993). OM from different sources is not only transported by coastal rivers through estuaries but it also undergoes transformations before reaching the ocean (Markager et al, 2011;Bristow et al, 2013).…”