Periodic Stratification in the Rhine ROFI in the North Sea
J.H. SIMPSON, W.G. BOS, F. SCHIRMER, A.J. SOUZA,
T.P. RIPPETH, S.E. JONES and D. HYDES
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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