Turbulent Large-Eddy Momentum Flux Divergence during High-Wind Events

Abstract

AbstractMomentum 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 m s−1 were examined. Winds from the southeast prevailed from one to several days with significant wave heights of 5–9 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 73 m. Spectral analyses indicate that a major part of the turbulent momentum flux was concentrated on 10–30-min time scales. The ratio of horizontal scale to mixed layer depth was from 2 to 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 was 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 10- and 30-m depth was ~0.1 m2 s−1, decreased with depth rapidly below 50 m, and was ~5 × 10−3 m2 s−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.