Observations of turbulent structures at the base of the ocean mixed layer

Abstract

Direct observations of coherent turbulent structures at the base of the ocean surface boundary layer are rare. Here, we present a unique dataset from a drifting thermistor chain that captured temperature structures associated with mixed layer and transition layer turbulence. Following a storm with wind speeds up to 15 m s −1 , the mixed layer deepened from 35 to 45 m depth over 6 days. Within the transition layer we observe temperature fluctuations indicative of shear-driven turbulence, including features consistent with Kelvin-Helmholtz and Holmboe instabilities. We examine the size and frequency of these structures in the context of the surface forcing, identifying four distinct regimes based on wind, wave, and surface heat flux magnitude. Thorpe scale–based estimates of entrainment velocity align with the observed mixed layer deepening when averaged over the upper portion of the transition layer. This rare observational dataset offers new insight into the mechanisms controlling mixing at the base of the mixed layer and provides a valuable benchmark for future numerical studies of stratified turbulence that resolve turbulent overturns.