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Periodic Stratification in the Rhine ROFI in the North Sea

Oceanologica Acta, Vol 16, No 1, pp. 23...32, 1993.
Abstract - - The nature of the physical regime in the vicinity of the Rhine ROFI (Region Of Freshwater Influence) has been determined in a series of collaborative observations. Extensive surveys with shipboard CTD/rosette systems have been used to complement time series observations by an array of moorings instrumented with currentmeters, transmissometers and fluorimeters.

The observations reveal a highly variable system in which the influence of the freshwater input from the Rhine extends northeastwards from the source and out to 30 km from the coast. The mean flow within this region is generally parallel to the coast (northeastwards) and with surface speeds, determined by the HF radar, of 15-20 cm/s. The residual current at sub-tidal frequencies was strongly correlated with windstress-forcing with a transfer factor of ca. 1%.

Water column structure exhibits marked periodic variations particulary on semidiurnal and semi-monthly time scales, the latter highlighted by contrasting post-springs and post-neaps surveys of the ROFI region. Springs tidal stirring was reinforced by strong wind (and wave) mixing which brought about complete vertical homogeneity everywhere except at the Rhine mouth. After the following neaps, and a period of light winds, the water column was observed to have re-stratified over the whole inshore region through the relaxation of the horizontal gradients under gravity and with the influence of rotation as in the model of Ou (1983).

The switching of the water column regime between stratified and mixed conditions was observed to markedly change the coupling between low frequency surface and bottom currents and is also reflected in the suspended sediment variations. Generally high levels of seston throughout the coastal boundary layer in the post-springs period were followed by a dramatic reduction especially in the region which re-stratified. An interesting exception was combined occurrence of high turbidity and low salinity in surface waters due to the immediate influence of the Rhine outflow near the source.
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