Turbulent Large-Eddy Momentum Flux Divergence during High Wind Events

Abstract

omentum transport by energy-containing turbulent eddies in the oceanic mixed layer were investigated during high wind events in the northern Gulf of Alaska off Kayak Island. Sixteen high-wind events with magnitudes ranging from 7 to 22 ms-1 were examined. Winds from the southeast prevailed from one to several days with significant wave heights of 5 to 9 m and turbulent Langmuir numbers of about 0.2-0.4. Surface buoyancy forcing was much weaker than the wind stress forcing. The water column was well-mixed to the bottom depth of about 73m. Spectral analyses indicate that a major part of the turbulent momentum flux was concentrated on 10 to 30 minute time scales. The ratio of horizontal scale to mixed layer depth was 2-8. Turbulent shear stresses in the mixed layer were horizontally asymmetric. The downwind turbulent stress at 10-20 m below the surface was approximately 40% of the averaged wind stress, and reduced to 5%-10% of the wind stress near the bottom. Turbulent kinetic energy in the crosswind direction was 30% larger than in the downwind direction, and an order of magnitude larger than the vertical component. The averaged eddy viscosity between 10m and 30m depth was ~0.1 m2s-1, decreased with depth rapidly below 50 m, and was ~5?10-3 m2s-1 at 5 m above the bottom. The divergence of turbulent shear-stress accelerated the flow during the early stages of wind events before Coriolis and pressure gradient forces became important.