Ocean surface boundary layer response to abruptly changing wind direction

Idealized ocean surface boundary layer (OSBL) process studies often assume constant wind forcing. However, typical winds are rarely constant. Based on a large eddy simulation (LES) method, we investigate the OSBL response to abruptly changing wind direction. The LES captures shear-driven turbulence (ST) due to surface wind stress and Langmuir turbulence (LT) that is driven by the Craik-Leibovich vortex force (cross-product between Stokes drift of surface gravity waves and the vorticity vectors). We design idealized LES experiments for which the wind and wave forcing is initially constant, so that OSBL turbulence equilibrates with the forcing. Then the wind stress abruptly rotates by 90° either clockwise or anticlockwise. The transient Stokes drift, which slowly turns into the wind direction, is obtained from a spectral wave model. The associated OSBL response is described by three stages. During stage 1, turbulent kinetic energy (TKE) decreases because of relatively weak TKE production and excess TKE dissipation. Stage 2 is characterized by a TKE increase when TKE shear production recovers and exceeds TKE dissipation. During this stage, relatively large TKE levels (exceeding those of equilibrated turbulence) occur for the clockwise wind rotation due to resonant forcing. During stage 3, turbulence relaxes to the equilibrium state. This adjustment is much slower for LT than for ST because of the relatively slow wave response. Because waves are greatly misaligned during stages 1 and 2, LT is weak and TKE shear production significantly exceeds Stokes drift shear production. Our results suggest that transient wind conditions play a key role in understanding realistic OSBL dynamics.