Turbulence driven by wind and waves controls the transport of heat, momentum, and matter in the ocean surface boundary layer (OSBL). For realistic ocean conditions, winds and waves are often neither aligned nor constant, for example, when winds turn rapidly. Based on a Large Eddy Simulation (LES) method, which captures shear-driven turbulence (ST) and Langmuir turbulence (LT) driven by the Craik-Leibovich vortex force, we investigate the OSBL response to abruptly turning winds. We design idealized LES experiments, whose winds are initially constant to equilibrate OSBL turbulence before abruptly turning 90° either cyclonically or anticyclonically. The transient Stokes drift for LT is estimated from a spectral wave model. The OSBL response includes three successive stages that follow the change in direction. During stage 1, turbulent kinetic energy (TKE) decreases due to reduced TKE production. Stage 2 is characterized by TKE increasing with TKE shear production recovering and exceeding TKE dissipation. Transient TKE levels may exceed their stationary values due to inertial resonance and non-equilibrium turbulence. Turbulence relaxes to its equilibrium state at stage 3, but LT still adjusts due to slowly developing waves. During stages 1 and 2, greatly misaligned wind and waves lead to Eulerian TKE production exceeding Stokes TKE production. A Reynolds stress budget analysis and Reynolds-averaged Navier-Stokes equation models indicate that Stokes production furthermore drives the OSBL response. The Coriolis effects result in asymmetrical OSBL responses to wind turning directions. Our results suggest that transient wind conditions play a key role in understanding realistic OSBL dynamics.

Langmuir turbulence (LT) is an important turbulent process in the ocean surface boundary layer (OSBL), significantly affecting the transports of heat, salt, momentum, and suspended and dissolved matter. LT is formed by the Craik-Leibovich (CL) vortex force (Craik & Leibovich, 1976) that results in counterrotating vortices approximately aligned with the wind, so called Langmuir circulations. Those coherent vortex pairs of LT generate strong surface convergent regions and downwelling jets, enhancing the OSBL mixing (Grant & Belcher, 2009;Kukulka et al., 2009;McWilliams et al., 1997). Turbulence-resolving large eddy simulation (LES) models based on the filtered CL equation (McWilliams et al., 1997;Skyllingstad & Denbo, 1995) succeed in reproducing important LT characteristics that have been observed earlier, such as nearly wind-aligned coherent surface convergent regions (

A shallow and stratified diurnal warm layer (DWL) often forms near the ocean surface during daytime solar heating and erodes again during convective nighttime cooling, resulting in a diurnal cycle of the ocean surface boundary layer (OSBL) depth. For the fully turbulent OSBL, turbulent mixing distributes the injected heat, forming a DWL with a thermocline beneath. Although under very weak winds, stratification may be strongest at the sea surface, here we investigate cases with wind speeds of 4-7 m/s where the maximum stratification is found at the diurnal thermocline. The heat and momentum in the DWL are decoupled from deeper water to first order, leading to diurnal anomalies of sea surface temperature (SST) of about 0.1-1°C and relatively strong near-surface velocity, so-called diurnal jets with speeds up to 0.1-0.2 m/s (