|Observations on the horizontal and vertical structure of currents at sub-tidal frequencies in the central North Sea|
van Haren, J.J.M. (1990). Observations on the horizontal and vertical structure of currents at sub-tidal frequencies in the central North Sea. Neth. J. Sea Res. 27(1): 1-23
In: Netherlands Journal of Sea Research. Netherlands Institute for Sea Research (NIOZ): Groningen; Den Burg. ISSN 0077-7579, more
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During the spring of 1986 and the summer of 1987 current, bottom pressure and CTD data were gathered in the North Sea 60 km north of the Netherlands to determine the contribution of three mechanisms to the local dynamics: wind stress, density differences and rectification due to non-linear tide-topography interaction. This was achieved by comparing the observations with simplified mathematical models each describing one of the processes. Although a 'global' wind-driven model disagreed rather strongly with the data if the total period of measurements is considered, reasonable agreement has been obtained for small subperiods of constant wind action. During a selected period of SW-wind stress of 0.2 N·m-2, 6±2 cm·s-1 of the dominating along-isobath current was wind-driven. Near the bottom above the steepest bottom slope additionally 5.5±1.7 cm·s-1 was due to the frontal zone. Above this slope additionally 1.8 cm·s-1 was attributed to the rectifying mechanism. It is noted, however, that the latter value was smaller than the sum of the uncertainties in the currents resulting from the other models. The observed near-bottom along-isobath current velocity was 12.3 cm·s -1. The density-induced motion was not explained by the thermal wind relation when integrated from the bottom upwards. The absence of sea-level adjustment and, to a lesser extent, internal friction had to be considered. At the same position the vertical shear of the cross-isobath current component generally was largest compared with other moorings due to the enhanced cross-isobath circulation by the density gradient and the rectifying mechanism.