Anisotropic response of surface circulation to wind forcing, as inferred from high‐frequency radar currents in the southeastern Bay of Biscay

Abstract

The short-term (less than 20 days) response of surface circulation to wind has been determined in waters of the southeastern Bay of Biscay, using wind impulse response (time domain) and transfer (frequency domain) functions relating high-frequency radar currents and reanalysis winds. The response of surface currents is amplified at the near-inertial frequency and the low-frequency and it varies spatially. The analysis indicates that the response of the ocean to the wind is slightly anisotropic, likely due to pressure gradients and friction induced by the bottom and coastline boundaries in this region. Thus, the transfer function at the near-inertial frequency decreases onshore due to the coastline inhibition of circularly polarized near-inertial motion. In contrast, the low-frequency transfer function is enhanced toward the coast as a result of the geostrophic balance between the cross-shore pressure gradient and the Coriolis forces. The transfer functions also vary with season. In summer, the current response to wind is expected to be stronger but shallower due to stratification; in winter, the larger mixed layer depth results in a weaker but deeper response. The results obtained are consistent with the theoretical description of wind-driven circulation and can be used to develop a statistical model with a broad range of applications including accurate oceanic forecasting and understanding of the coupled atmosphere-ocean influence on marine ecosystems.

Drift in the uppermost part of the ocean

“…Values for the leeway were roughly the same, but the wind deflection angle was 3° larger for iSphere drifters and 10° larger for the CODE drifters. Using high‐frequency radar observations, Fontán and Cornuelle [] show how the wind response of surface currents is modified near coastalines by polarizing inertial oscillations.…”

Observations of Near-Inertial Surface Currents at Palau

“…Additionally, near-inertial internal waves radiate away from the boundary due to a geostrophic adjustment process. Similar to the analytical solution by Kundu et al (1983), many subsequent observational studies of wind-generated NIOs near coasts have shown this decay of near-inertial energy near topography (Schahinger, 1988;Tintoré et al, 1995;Shearman, 2005;Fontán and Cornuelle, 2015). The behavior of wind-generated NIOs near islands has received relatively little attention.…”

Poleward along-shore current pulses on the inner shelf of the Bay of Biscay

“…As bottom stress nearly vanishes, this could favor the persistence and extension of downstream poleward jets. fluences of cross-and along-shore wind stress components is described in [18], elaborating on the work of [33]. We present here a first concept to extend the approach to the study of cases in which the influence of wind at different loca-450 tions and not just different instants, is to be analyzed.…”