Mesoscale Ocean Surface Current Structure Detected by High-Frequency Radar

Abstract

The quality and vertical correlation scales of high-frequency (HF) radar-derived ocean surface current measurements from an ocean surface current radar (OSCR) are assessed by comparing surface to subsurface current observations from 11 June to 8 July 1993 at directional discus buoys DW and DE, each instrumented with a three-axis ultrasonic current meter at the 13.8- and 9.5-m depths, respectively. A dual-station OSCR mapped the current fields at 20-min intervals at a horizontal resolution of 1.2 km over a 30 km ? 44 km domain inshore of the Gulf Stream using the HF (25.4 MHz) mode. Over a 27-day experimental period, surface current observations were acquired 97% of the time extending to the maximum theoretical range of 44 km. Linear regression analyses indicated a bias of 2?4 cm s?1 and slopes of O(1). While there were periods when the daily averaged complex correlation coefficients were highly correlated (>0.8), periods of low correlation (<0.3) are explained in 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), near-inertial (20.7 h), and high-frequency (4.5 h) bands. Tidal analyses, based on the semidiurnal (K2, M2, L2, S2) and diurnal (K1, O1, P1, Q1) constituents, indicated maximum amplitudes of 5 cm s?1 at DW, whereas these amplitudes increased offshore to a maximum of 13 cm s?1 at DE. Net differences between the surface and subsurface tidal currents ranged between 2 and 5 cm s?1 with the largest difference of 7.7 cm s?1 for the K1, constituent at DE. The tidal currents were removed from the surface and subsurface current time series and low-pass filtered at 48 h, bandpass filtered between 18 and 23 h, and high-pass filtered at 8 h. The mean current components were highly correlated (>0.9) over most of the record with small phase differences. Intrusions of the mean flow at 3?5-day intervals were correlated with bursts of near-inertial motions having amplitudes of 20 cm s?1 at DE and 15 cm s?1 at DW. 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 oscillated at periods of 4.3?4.7 h but were directly out of phase with the subsurface currents, which caused the correlations to decrease below 0.3. Thus, temporal decorrelations appeared to be a result of high-frequency motions in the internal wave band between the inertial and Nyquist (1.5 cph) frequencies.