Ocean Surface Boundary Layer Response to Abruptly Turning Winds

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. Using 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 in which winds are initially constant to equilibrate OSBL turbulence before abruptly turning 908 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 as a result of 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 because of inertial resonance and nonequilibrium turbulence. Turbulence relaxes to its equilibriumstate at stage 3, but LT still adjusts as a result of slowly developing waves. During stages 1 and 2, greatly misaligned wind and waves lead to Eulerian shear TKE production exceeding Stokes drift shear TKE production. A Reynolds stress budget analysis and Reynolds-averaged Navier-Stokes equation models indicate that Stokes drift shear 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.

Automated summary

The study investigates the impact of turbulence driven by wind and waves on the transport of heat, momentum, and matter in the ocean surface boundary layer (OSBL). The experiments reveal a three-stage response of the OSBL to abrupt changes in wind direction, with different characteristics such as decreased turbulent kinetic energy (TKE), increased TKE, and eventual relaxation to equilibrium, while Langmuir turbulence (LT) continues to adjust due to slowly developing waves.