Abstract
Abstract The surface current response to winds is analyzed in a 2-yr time series of a 12-MHz (HF) Wellen Radar (WERA) off the west coast of France. Consistent with previous observations, the measured currents, after filtering tides, are on the order of 1.0%–1.8% of the wind speed, in a direction 10°–40° to the right of the wind, and with systematic trends as a function of wind speed. This Lagrangian current can be decomposed as the vector sum of a quasi-Eulerian current UE, representative of the top 1 m of the water column and part of the wave-induced Stokes drift Uss at the sea surface. Here, Uss is estimated with an accurate numerical wave model using a novel parameterization of wave dissipation processes. Using both observed and modeled wave spectra, Uss is found to be very well approximated by a simple function of the wind speed and significant wave height, generally increasing quadratically with the wind speed. Focusing on a site located 100 km from the mainland, the wave-induced contribution of Uss to the radar measurement has an estimated magnitude of 0.6%–1.3% of the wind speed, in the wind direction—a percentage that increases with wind speed. The difference UE of Lagrangian and Stokes contributions is found to be on the order of 0.4%–0.8% of the wind speed and 45°–70° to the right of the wind. This relatively weak, quasi-Eulerian current with a large deflection angle is interpreted as evidence of strong near-surface mixing, likely related to breaking waves and/or Langmuir circulations. Summer stratification tends to increase the UE response by up to a factor of 2 on average, and further increase the deflection angle of UE by 5°–10°. At locations closer to the coast, Uss is smaller and UE is larger with a smaller deflection angle. These results would be transposable to the World Ocean if the relative part of geostrophic currents in UE was weak, which is expected. This decomposition into Stokes drift and quasi-Eulerian current is most important for the estimation of energy fluxes to the Ekman layer.
Highlights
Surface drift constitutes one of the most important applications of the emerging operational oceanography systems (e.g. Hackett et al 2006), as it plays an important role in the fate of oil pollutions and larvae recruitment
In the present paper we extend the previous analyses of High Frequency (HF) radar data by independently estimating the Stokes drift, using an accurate wave model
Using a 2 year time series of HF radar data, and a novel numerical wave model that is shown to reproduce the observed variability of the surface Stokes drift with wind speed and wave height, we have analyzed the wind-driven surface current
Summary
Surface drift constitutes one of the most important applications of the emerging operational oceanography systems (e.g. Hackett et al 2006), as it plays an important role in the fate of oil pollutions and larvae recruitment. We chose to use this numerical wave model for the estimation of Uss and USf. We can propose an error buget for our estimate of the wind-driven quasi-Eulerian current in which the measurement error is dominated by USf with a bias of 5% at most and a standard deviation less than 20% overall. We estimate that the root mean square error of the modelled quasi-Eulerian current UE at 3 hour intervals is of the order of 0.2% of U10 On this time scale, it is difficult to rule out contributions from horizontal pressure gradients in the momentum balance, and this current may not be purely wind-driven. Gereralizing this result to the entire radar time series, the averaged values of UE can be interpreted as a wind-driven current with an accuracy to within 0.3% of U10
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