Current shear and turbulence during a near-inertial wave

Abstract

Surface currents and turbulent mixing were observed during a near-inertial wave (NIW) using an accousting doppler current profiler (ADCP) and satellite-tracked drifters. Drifter trajectories sampled at three depth levels show characteristics of an Ekman solution superposed with the NIW. Velocity and dissipation estimates from the ADCP reveal strong shear with a distinct constant flux layer in between the roughness length and a critical depth at 4m. Below, a shear free slab layer performing an inertial oscillation is observed. Dissipation, as estimated from the vertical beam of the ADCP, peaks in the wave-enhanced friction layer when the current opposes the wind and wave direction. Below the constant flux layer, maximum turbulence is observed when the NIW is in a phase that is in opposite direction to the time-averaged current. During this phase, currents at various depths rapidly realign in the entire boundary layer.

Drifter observations of surface currents in the Cyprus Gyre

“…NIOs at the ocean surface occur ubiquitously as small loops in the tracks of Lagrangian drifters (Poulain, 1990;Poulain et al, 1992;D'Asaro et al, 1995;Niiler and Paduan, 1995;van Meurs, 1998;Elipot and Lumpkin, 2008;Rohrs et al, 2023) which can be up to ~10 km in diameter. Several statistical methods have been used to extract the near-inertial signals from drifter velocity time series: complex demodulation (Poulain, 1990;Poulain et al, 1992;Chaigneau et al, 2008), scale spectral moment (Poulain et al, 1992), spectral method (Elipot et al, 2010) and wavelet analysis (Lilly and Gascard, 2006;Lilly et al, 2011;Lilly and Perez-Brunius, 2021).…”