Observations of Overturning in the Thermocline: The Context of Ocean Mixing

Abstract

Abstract The time sequence of events that lead to internal wave breaking and ocean turbulence is investigated. Data are obtained from depths 100–400 m with a repeat profiling CTD and a coded-pulse Doppler sonar. The instruments were deployed from R/P FLIP during February–March 1995 while stationed 30 km west of Point Arguello, California, as an aspect of the Marine Boundary Layer Experiment. Although the water depth at the site is 1500 m, both rms shear and diapycnal diffusivity, as inferred from the average rate and size of overturning events, increase with depth below 250 m. A deep source of wave energy is implied. Depth–time series of 6.4-m shear S, 2-m strain (γ ≡ N2/N2, where N is the buoyancy frequency), 6.4-m gradient Richardson number Ri ≡ N2/S2, and 2-m “effective strain rate” (the depth derivative of CTD-inferred vertical velocity ŵ) are obtained at 4 minute intervals over a 9-day, 100–400 m domain. The occurrence of overturns, static instabilities of vertical scale ≥2 m in the observed density ...