Journal of Physical Oceanography. 30:805-832.
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 (gamma = (N) over bar(2)/N-2, where N is the buoyancy frequency), 6.4-m gradient Richardson number Ri - N-2/S-2, and 2-m "effective strain rate" partial derivative (w) over cap/partial derivative z = 1 partial derivative gamma/gamma partial derivative t (the depth derivative of CTD-inferred vertical velocity (w) over cap) are obtained at 4 minute intervals over a 9-day, 100-400 m domain. The occurrence of overturns, static instabilities of vertical scale greater than or equal to 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: gamma greater than or equal to 2 in 60% of all overturning events, and \partial derivative (w) over cap/partial derivative 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 partial derivative (w) over cap/partial derivative z variance (expressed as a dissipation rate), and depth-averaged dissipation rate epsilon(T), estimated from observed overturning scales are highly correlated. Time-mean <(epsilon(T))over bar> = 8.6 x 10(-9) W kg(-1), implying an eddy diffusivity <(K-rho)over bar> = 0.89 x 10(-4) m(2) s(-1). Mean dissipation rate and diffusivity computed from 10-m shear display comparable magnitude (<(epsilon(IW))over bar> = 6.8 x 10(-9) W kg(-1), <(K-rho)over bar> = 0.70 x 10(-4) m(2) 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 partial derivative (w) over cap/partial derivative z waves.