Turbulence Intensity Measurements in a Wind-Driven Mixed Layer

Abstract

Neutrally buoyant, high-drag floats were used to measure vertical velocity in the upper-ocean mixed layer during a period of rapid mixed layer deepening resulting from a storm. Salinity and temperature profiles, air–sea fluxes, and surface wave spectra were also measured. The location, Georgia Strait, British Columbia, is coastal with strong horizontal variability and may not be representative of the open ocean. The floats moved freely within the deepening mixed layer; the envelope of their motion corresponded closely to the extent of the mixed layer. The maximum vertical velocity was 0.12 m s−1; the rms vertical velocity was about (0.02 m s−1)2. The mean square vertical velocity, excluding surface waves, was 1.5–3.0 u2∗, clearly higher than the upper bound of u2∗ found in solid-wall turbulent boundary layers. The authors speculate that these anomalously high vertical velocities were due to Langmuir circulations generated by surface waves. These cannot occur in solid-wall boundary layers.