Observation and estimation of Lagrangian, Stokes and Eulerian currents induced by wind and waves at the sea surface

TL;DR

This study analyzes sea surface currents induced by wind and waves using radar data and wave modeling, decomposing the currents into Eulerian and Stokes components, revealing their relative magnitudes and directions.

Contribution

It introduces a novel parameterization of wave dissipation for estimating Stokes drift and decomposes observed currents into Eulerian and wave-induced parts using combined radar and wave model data.

Findings

Measured currents are 1-1.8% of wind speed, deflected 10-40 degrees to the right.

Stokes drift increases quadratically with wind speed and significant wave height.

The Eulerian current is weaker and deflected more, indicating strong near-surface mixing.

Abstract

The surface current response to winds is analyzed in a two-year time series of a 12 MHz (HF) Wellen Radar (WERA) off the West coast of France. The measured currents, with tides filtered out, are of the order of 1.0 to 1.8% of the wind speed, in a direction 10 to 40 degrees to the right of the wind. This Lagrangian current can be decomposed as the vector sum of a quasi-Eulerian current U_E, representative of the top 1 m of the water column, and a part of the wave-induced Stokes drift Uss at the sea surface. Here Uss is estimated with an accurate numerical wave model, thanks to a novel parameterization of wave dissipation processes. Using both observed and modelled 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 estimated contribution of Uss to the radar measurement has a magnitude of 0.6 to 1.3% of the wind speed, in the wind direction, a fraction that increases with wind speed. The difference U_E of Lagrangian and Stokes contributions is found to be of the order of 0.4 to 0.8% of the wind speed, and 45 to 70 degrees to the right of the wind. This elatively weak quasi-Eulerian current with a large deflection angle is interpreted as evidence of strong near-surface mixing, likely related to breaking waves.