Wave-Induced Stokes Drift Measurement by High-Frequency Radars: Preliminary Results

Abstract

High frequency (HF) radars measure ocean surface currents by sending electromagnetic (EM) waves in the HF radio band (3–30 MHz) and recording the EM waves backscattered by ocean surface gravity waves. The recorded signal is dominated by EM waves backscattered from ocean surface waves with half the EM wavelength, called Bragg waves. Since their phase velocity is affected not only by wave-current interactions with mean Eulerian currents, but also by wave-wave interactions with all the other waves present at the sea surface, the question arises as to whether HF radars measure a quantity related to the wave-induced Stokes drift in addition to mean Eulerian currents. However, the literature is inconsistent on the expression and even on the existence of the contribution of the wave-induced Stokes drift to the HF radar measurements. Three different expressions have been proposed in the literature: (1) the filtered surface Stokes drift, (2) half of the surface Stokes drift, and (3) the weighted depth-averaged Stokes drift. We evaluate these expressions for directional wave spectra measured by a bottom-mounted Acoustic Wave and Current (AWAC) profiler in the Lower St. Lawrence estuary, Canada. We then compare the Eulerian surface currents measured by the AWAC with the radial currents measured by two HF radars: one Wellen Radar (WERA) and one Coastal Ocean Dynamics Applications Radar (CODAR). The AWAC and HF-radar currents are surprisingly not correlated, but when subtracting the wave-induced Stokes drift contributions from the radar measurements, moderate but significant correlations are obtained. The highest correlations are obtained when subtracting the filtered surface Stokes drift, suggesting that HF radars measure the latter in addition to mean Eulerian currents.