PERIODIC STRATIFICATION IN THE RHINE ROFI IN THE NORTH-SEA

The nature of the physical regime in the vicinity of the Rhine ROFI (R egion Of Freshwater Influence) has been determined in a series of coll aborative observations. Extensive surveys with shipboard CTD/rosette s ystems have been used to complement time series observations by an arr ay of moorings instrumented with currentmeters, transmissometers and f luorimeters. The observations reveal a highly variable system in which the influence of the freshwater input from the Rhine extends northeas twards 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-1. The residual current at sub-tidal frequencies was strongly correlated with windstress-forcing with a transfer factor of approximately 1 %. Water column structure exhibits marked periodic variations particularl y on semi-diurnal 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) mixi ng 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 wh ole 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 a nd mixed conditions was observed to markedly change the coupling betwe en low frequency surface and bottom currents and is also reflected in the suspended sediment variations. Generally high levels of seston thr oughout the coastal boundary layer in the post-springs period were fol lowed by a dramatic reduction especially in the region which re-strati fied. An interesting exception was the combined occurrence of high tur bidity and low salinity in surface waters due to the immediate influen ce of the Rhine outflow near the source.