Remote sensing of shelf sea currents using a high‐frequency ocean surface current radar system

Abstract

In a deployment of an ocean surface current radar system made during May 1985, currents were measured in a region of sea to the northeast of Anglesey, Wales, and compared with data obtained from three current meters. The uppermost current meter was fixed at 5 m from the surface, in a water column of 40‐m depth with tidal range of 9 m. In the east–west direction, close to the orientation of the major axis of the tidal ellipse, the radar and top current meter show good agreement to within an rms value of 0.16 m s−1, consistent with earlier comparative studies. For the weaker north–south components of the flow, however, differences between the radar results and those from the current meters are apparent. A larger‐amplitude north‐south signal is recorded by the radar. In terms of the M2 rotary (clockwise and anticlockwise) components of the flow, the radar data contain amplitude and phase anomalies with respect to a predicted surface behavior deduced from the current meter records and theory. In particular, anticlockwise phase at the surface is advanced considerably (by ≃ 20°). Further analysis shows that the flow field recorded by the radar has two components: the predicted tidal response, and an additional elliptical term of lower eccentricity than the main east–west tidal signal and with major axis directed close to north. Hourly synoptic maps of tidal circulation patterns derived from the radar show a smooth transition from near‐westward flows on the ebb to near‐eastward flows on the flood. The variation of north–south residual currents with height can be interpreted in terms of a “mean state” consistent with geostrophic shear and associated with the roughly east–west density gradient.