NEAR-SURFACE CURRENT MEASUREMENT FROM A SURFACE FOLLOWING DATA BUOY (DB1) .2. HARMONIC AND RESIDUAL ANALYSIS OF CURRENT-METER RECORDS

This part of the paper examines near-surface current data from different periods during the test mooring of the data buoy, DB1. Harmonic analyses show that variations in the major tidal constituents between each period are not significantly greater than the standard error of the measurement: the differences observed in the principal lunar semi-diurnal tide are explained partly in terms of a modulation by adjacent constituents. Unexpected forms for the quarter diurnal tidal ellipses derived from the DB1 data have been found also in independent measurements, in which a different mooring technique had been used. These are therefore unlikely to be of instrumental origin. Non-tidal energy at periods exceeding three days is shown to correlate with changes in wind and surface elevation, the correspondence between wind and current being greatest during a period of strong winds, when current at 3 m depth was ∼0.9% of wind speed at 8 m. The data are interpreted in favour of an Eulerian current rather than Lagrangian Stokes transport due to waves. Rectification of wave orbital velocities due to buoy motion is not detectable. In view of the overall quality of the data it is concluded that this combination of surface following buoy and long-path acoustic current meter can contribute usefully to the determination of mean near-surface currents in the open sea. © 1979.