SST observations of upwelling induced by tidal straining in the Rhine ROFI

Abstract

In this paper SST imagery and a three-dimensional numerical model of a river plume were employed to detect upwelling induced by tidal straining in the Rhine ROFI (region of fresh water influence). Previous studies have shown that the Rhine ROFI in the North Sea exhibits strong cross-shore density gradients that compete with tidal and wind mixing to establish stratification. During neap periods with low mixing energy an area measuring 30 km offshore by 100 km alongshore becomes stratified. When the ROFI is stratified strong cross-shore currents are observed, with surface currents rotating anti-cyclonically and bottom currents rotating cyclonically. The cross-shore currents interact with the cross-shore density gradients to produce a semi-diurnal cycle of stratification. Due to continuity requirements imposed by the proximity of the coast, the offshore-directed surface currents and onshore-directed bottom currents should lead to coastal upwelling. A 6-day unclouded series of SST-imagery was found in May 1990 with about two images per tide. The ROFI is much warmer than the surrounding water, exhibits a distinct diurnal solar heating response and is clearly delineated by large spatial temperature gradients at the edges. This indicates that the whole ROFI area is stratified. On three consecutive middays, a marked 5–10 km wide and 100 km alongshore band of cold upwelling is visible along the coastline, while in the morning and afternoon this upwelling band is not present. The timing of this upwelling indicates that it is caused by tidal straining. This conclusion was supported with a simulation with a three-dimensional numerical model of an idealized river plume showing the same semi-diurnal cold upwelling band. We believe these are the first SST images to show upwelling induced by tidal straining, as well as the large spatial scale of tidal straining in the Rhine ROFI.