Experimental verification of wave-induced Stokes drift measurement by High Frequency radars

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. This question is important for practical applications, such as the use of HF radars in search and rescue operations and oil spill mitigation and for assimilating HF radar currents into numerical ocean models. However, the literature is inconsistent on the existence and the expression of the quantity related to the wave-induced Stokes drift. Three different expressions have been proposed in the literature: (1) the weighted depth-averaged Stokes drift, (2) the filtered surface Stokes drift, and (3) half of the surface Stokes drift. We evaluate these expressions for wave spectra measured by a moored buoy and an Acoustic Wave and Current profiler in the Lower Saint-Lawrence estuary, and for wave spectra predicted by a regional WaveWatch III simulation. Typical values are generally less than 5 cm/s, but can sometimes reach over 20 cm/s, and the three expressions typically differ from each other by less than 2 cm/s, but can sometimes reach over 4 cm/s. These predicted wave-induced contributions are then compared to detided radial currents measured by four HF radars.