YEAR-LONG MEASUREMENTS OF FLOW-THROUGH THE DOVER STRAIT BY HF RADAR AND ACOUSTIC DOPPLER CURRENT PROFILERS (ADCP)

Contaminants from the Channel flow through the Dover Strait into the N orth Sea where they represent a significant fraction of the enhanced c oncentrations observed along the continental coast. Despite numerous p revious investigations, the magnitude of this net flow and its depende ncy on various forcing factors remain uncertain. The new UK H.F. Radar system, OSCR (Ocean Surface Current Radar) developed for measuring ne arshore surface currents offers a clear opportunity of establishing th e magnitude and nature of this flow. Starting in July 1990, H.F. Radar observations were made in the Dover Strait for five months from the F rench coast overlapping (for two months) with five months from the Eng lish coast. These measurements involved fully-automised monitoring of surface currents at 700 locations every 20 minutes. A bottom-mounted A DCP was moored continuously for die same period in the middle of the S trait. Tidal current atlases were produced from the radar measurements showing ellipses for seven major constituents at up to 160 locations in the strait, examples for the two major constituents M2 and S2, are shown in Figures 2 and 3. A net tidal (M2) residual flow of 36,000 m3 s-1 into the North Sea was calculated from a combination of the M2 tid al ellipse data with the M2 tidal elevation distribution. The localise d response of surface currents to wind forcing is shown to follow the classical pattern of Ekman veering, with angles of up to 45-degrees to die wind direction in deep water. In addition to this localised surfa ce response, wind-forced residual currents, persistent through depth a nd coherent spatially, flow through the strait amounting, on average, to 45,000 m3 s-1. In addition a steady (non-tidal, non wind-driven) co mponent was observed of approximately 6000 m2 s-1. Thus net longterm f low into the North Sea was estimated as 87,000 m3 s-1. i.e. 40% of ear liest estimates and 60% of the value assumed in many present-day North -Sea models. The time-averaged residual currents revealed a gyre of ap proximately 20 km diameter off the westerly edge of Cap Gris Nez rotat ing anticlockwise with a period of approximately seven days. These res idual flow time-series obtained from the radar measurements are correl ated against corresponding time-series obtained from: a) wind measurem ents; b) the bottom-mounted ADCP; c) flows through the Dover Strait co mputed by the UK numerical model used operationally to predict storm s urges Significant correlations are found in all cases with a maximum v alue of 0.8, between the depth-averaged ADCP residual current and the flow through the Strait calculated by the model. These correlations in dicate that the low-frequency flows through the Dover Strait determine d from the radar measurements on the French and English coasts are gen erally in opposing directions. Additional preliminary comparisons with observed sea-surface slopes were restricted by the availability of ti de gauge data.