THE COASTAL CURRENT CIRCULATION DURING THE ORANGE RIVER FLOOD 1988

SUMMARY

The flow of the Orange River into the Atlantic ocean increased dramatically at the end of February 1988 in response to high rainfall in the catchment areas upstream. This coastal flow is discussed in the light of both barotropic and baroclinic models so as to be able to set up the appropriate dynamical framework for the observations. The surface signal of the warm, fresh river water was deduced from NOAA 9 and LANDSAT satellite imagery, and is shown to form a large anticyclonic eddy (40 km in diameter) at the mouth of the river, and subsequently to flow southwards along the coast in a narrow (15 km wide) plume at 20 ms⁻¹ for some 120 km. An unusual aspect of the satellite observations was the very clear thermal signal seen on 8 and 11 March 1988, which can be ascribed to the fact that very light winds prevailed at this time. Such a flow is to be expected from the theoretical dynamics in the absence of local wind. Later observations showed that after the south-easterly wind increased on 16/17 March, the surface river plume responded by flowing to the north. Measurements of surface salinity were made from the RS Benguela on 12 March and showed very fresh water (22 × 10⁻³) at the surface in the region of the eddy at the river mouth. Lower than usual salinities (28–34 × 10⁻³) were measured as far south as Hondeklip Bay, about 200 km south of the Orange River.