MESOSCALE OCEAN SURFACE CURRENT STRUCTURE DETECTED BY HIGH-FREQUENCY RADAR

The quality and vertical correlation scales of high-frequency (HF) rad ar-derived ocean surface current measurements from an ocean surface cu rrent radar (OSCR) are assessed by comparing surface to subsurface cur rent observations from 11 June to 8 July 1993 at directional discus bu oys D-W and D-E, each instrumented with a three-axis ultrasonic curren t meter at the 13.8- and 9.5-m depths, respectively. A dual-station OS CR mapped the current fields at 20-min intervals at a horizontal resol ution of 1.2 km over a 30 km X 44 km domain inshore of the Gulf Stream using the HF (25.4 MHz) mode. Over a 27-day experimental period, surf ace current observations were acquired 97% of the time extending to th e maximum theoretical range of 44 km. Linear regression analyses indic ated a bias of 2-4 cm s(-1) and slopes of O(1). While there were perio ds when the daily averaged complex correlation coefficients were highl y correlated (>0.8), periods of low correlation (<0.3) are explained i n terms of vertical phase differences and a decoupling between surface and subsurface records. Surface and subsurface current time series at the two mooring sites were decomposed into the tidal, mean (>48 h), n ear-inertial (20.7 h), and high-frequency (4.5 h) bands. Tidal analyse s, based on the semidiurnal (K-2, M(2), L(2), S-2) and diurnal (K-1, O -1, P-1, Q(1)) constituents, indicated maximum amplitudes of 5 cm s(-1 ) at D-W, whereas these amplitudes increased offshore to a maximum of 13 cm s(-1) at D-E. Net differences between the surface and subsurface tidal currents ranged between 2 and 5 cm s(-1) with the largest diffe rence of 7.7 cm s(-1) for the K-1 constituent at D-E. The tidal curren ts were removed from the surface and subsurface current time series an d low-pass filtered at 48 h, bandpass filtered between 18 and 23 h, an d high-pass filtered at 8 h. The mean current components were highly c orrelated (>0.9) over most of the record with small phase differences. Intrusions of the mean Bow at 3-5-day intervals were correlated with bursts of near-inertial motions having amplitudes of 20 cm s(-1) at D- E and 15 cm s(-1) at D-W. The frequency of these motions was shifted 5 %-10% above and below fduring these episodes of mean flow intrusions. The higher-frequency surface motions with amplitudes of 5-8 cm s(-1) o scillated at periods df 4.3-4.7 h but were directly out of phase with the subsurface currents, which caused the correlations to decrease bel ow 0.3. Thus, temporal decorrelations appeared to be a result of high- frequency motions in the internal wave band between the inertial and N yquist (1.5 cph) frequencies.