Presenter: Cedric Chavanne (ISMER-UQAR)

Description:
High-frequency (HF) radars remotely measure ocean surface currents based on the Doppler shift of electromagnetic waves backscattered by surface gravity waves with half the electromagnetic wavelength, called Bragg waves. Since their phase velocity is affected not only by their interaction with the mean Eulerian currents (wave-current interaction), but also by interactions with all the other waves present at the sea surface (wave-wave interaction), 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—both theoretically and experimentally—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: the filtered surface Stokes drift, half of the surface Stokes drift, and the weighted depth-averaged Stokes drift. Here, we evaluate these expressions and the surface Stokes drift for directional wave spectra measured by bottom-mounted Acoustic Wave and Current (AWAC) profilers in the Lower St-Lawrence Estuary (Québec, Canada). We then compare the Eulerian surface currents measured by the AWACs with the radial currents measured by four HF radars: two Wellen Radars (WERA) and two Coastal Ocean Dynamics Applications Radars (CODAR). During Summer 2013, when winds are weak, correlations between the AWAC and HFR currents are stronger (0.93) than during Winter 2016-2017 (0.42-0.62), when winds are high. But after adding the different wave-induced quantities to the Eulerian currents measured by the AWACs, correlations during Winter 2016-2017 significantly increase. Among the different expressions tested, the highest correlations (0.80-0.96) are obtained using half of the surface Stokes drift, suggesting that HF radars measure the latter in addition to mean Eulerian currents.