A fleet of surface drifters (SWIFTs) equipped with down-looking high-resolution ADCPs were used to estimate the turbulent kinetic energy (TKE) dissipation rate (ε) within highly stratified diurnal warm layers (DWLs) in the Southern California bight. Over a 10-day period, five instances of DWLs were observed with strong surface temperature anomalies up to 3 °C and velocity anomalies up to 0.3 m s-1. Profiles of ε in the upper 5 meters suggest turbulence is strongly modulated by the DWL stratification. Burst-averaged (8.5 minutes) ε is stronger than predicted by law-of-the-wall boundary layer scaling within the DWL and suppressed below. Predictions for ε within the DWL are improved by a shear-production scaling using observed shear and linearly decaying turbulent stress. However, ε is still under-predicted. Examination of the un-averaged acoustic backscatter data suggests elevated ε is related to the presence of turbulent structures in the DWL which span the layer height and strongly modulate TKE. Evolution in the bulk Richardson number each day suggests the DWLs become unstable to layer-scale overturning and entrainment each afternoon, thus the turbulent structures may result from shear-driven instability. This interpretation is supported by a conditional average of the data during a burst characterized by strongly periodic structures. The structures resemble high-frequency internal waves with strong asymmetry in the along-flow direction (steepening) which suggests they are unstable. Coincident asymmetric patterns in upwelling/downwelling and corresponding regions of strong vertical convergence/divergence suggest both vertical transport and local TKE generation are plausible sources of elevated ε in the DWL.

Wind, wave, and acoustic observations are used to test a scaling for ambient sound levels in the ocean that is based on the relative penetration depth of active bubbles during surface wave breaking. The focus is on acoustic frequencies in the range 1-10 kHz, which are typically scaled by wind speed alone. Wind and wave information are combined in a parametric form to describe the depth of the active bubble layer (which produces sound) relative to the depth of the passive bubble layer (which attenuates sound). The relative depth scaling has a primary dependence on wind speed and a secondary dependence on any departure of significant wave height from fully-developed, open-ocean conditions. The scaling is tested with long time-series observations of winds and waves at Ocean Station Papa (North Pacific Ocean), as well as with a case study with fetch limitation near the island of Jan Mayen (Norwegian Sea). When waves are less developed (e.g., limited by fetch) at a given wind speed, the attenuating layer is relatively thin and the sound levels are higher. The scaling is a plausible explanation for the observed reduction in sound levels during high wind events (winds greater than 15 m/s).