Patterns of Turbulent and Double-Diffusive Phenomena: Observations from a Rapid-Profiling Microconductivity Probe

Abstract

Throughout much of the ocean interior, the diapycnal buoyancy flux is maintained by both mechanical and double-diffusive processes. Assessing the relative roles of each is a challenge, particularly in complex coastal environments. During February?March 1995, a repeat-profiling CTD system, equipped with a dual-needle microconductivity probe, was deployed off the central California coast (35°N, 121°W) from the research platform FLIP. The probe?s vertical resolution (8 cm) appears sufficient to resolve the low wavenumbers of the turbulent inertial subrange. This paper presents depth?time maps, spanning 12 days and 100?400 m, of temperature dissipation rate ??, and Cox number ?. High ?? and ? values tend to occur in layers, on a variety of spatial scales. Simultaneously, finescale (6.4-m) Richardson number, effective strain rate, and Turner angle are measured. The occurrence of intense microstructure fluctuations is correlated with all three quantities, affirming that both mechanical turbulence and double diffusion are active at the site. Depth-averaged dissipation rate ε? is inferred from the ?? records under the assumption that a Batchelor spectrum for scalars obtains and that the buoyancy flux Jb and dissipation ε are related through a constant mixing efficiency Γ, Jb = Γε. Time series of ε? are highly correlated with dissipation rate computed from Thorpe scales (εT), estimated from large (2 m and greater) density overturns (except during periods when large portions of the water column are double-diffusively unstable: ε? ? εT in these regions, suggesting enhanced fluxes due to double diffusion.