Observations of coherent structures in the mixed layer

The U.S. Naval Research Laboratory conducted a three-week field experiment to investigate turbulence variability in the upper ocean during wintertime under variable wind and surface-wave conditions. Atmospheric flux, ocean current, and hydrographic observations were collected in February 2017 on the outer shelf (depths from 80 to 100 m) in the northern Gulf of Mexico. Wintertime weather in the northern Gulf is dominated by passages of atmospheric cold fronts characterized by strong rotating winds and high destabilizing buoyancy fluxes followed by low-wind/daytime stabilizing buoyancy flux periods. When cold fronts affected the study area, well-defined coherent structures were observed in the mixed layer (ML). Observations indicated that they extended from the sea surface to the base of the ML with along-wind, cross-wind, and vertical velocities as high as 10, 10, and 5 cm/s, respectively. When the wind direction was nearly constant and the destabilizing buoyancy flux was persistent the coherent structures became very energetic, whereas rotating winds and/or stabilizing buoyancy fluxes caused them to vanish. Estimated along-wind turbulent shear stress normalized by the water frictional velocity squared was positive and decreased nearly linearly from 0.6 at 10 m to near zero at the base of the ML, while the cross-wind turbulent stress was small and very variable in the ML. The estimated mean eddy viscosity was 0.06 m²/s at 10 m and decreased to about 0.006 m²/s at the base of the ML. The turbulent Langmuir number (0.3-0.4) and the Langmuir stability length (0.2-1.17) indicate that the coherent structures might have been a combination of both Langmuir and convective cells.