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

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 Argüello, 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 (? ≡ 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 ?) 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 profiles, is monitored. Data are examined in both Eulerian and isopycnal-following (semi-Lagrangian) reference frames. Over two thousand overturns are detected and mapped relative to the background shear, strain, Ri, and strain rate fields. Correspondence between low values of 6.4-m Ri and overturns is indeed significant. However, 2-m strain and effective strain rate appear to be better indices of overturning: ? ≥ 2 in 60% of all overturning events, and |??/?z| is greater than average in 80% of overturns. Depth?time maps of effective strain rate show wavelike features on vertical scales 5?20 m, which extend coherently across isopycnal surfaces. Time series of depth-averaged ??/?z variance (expressed as a dissipation rate), and depth-averaged dissipation rate εT, estimated from observed overturning scales are highly correlated. Time-mean = 8.6 ? 10?9 W kg?1, implying an eddy diffusivity K? = 0.89 ? 10?4 m2 s?1. Mean dissipation rate and diffusivity computed from 10-m shear display comparable magnitude ( = 6.8 ? 10?9 W kg?1, K? = 0.70 ? 10?4 m2 s?1) and similar dependence upon mean stratification and shear to the overturn-inferred quantities. It is suggested that the overturns seen at this site result from breaking of these high ??/?z waves.