Patterns of turbulent and double-diffusive phenomena: Observations from a rapid-profiling microconductivity probe

Throughout much of the ocean interior, the diapycnal buoyancy Bur is mainta ined by both mechanical and double-diffusive processes. Assessing the relat ive roles of each is a challenge, particularly in complex coastal environme nts. During February-March 1995, a repeat-profiling CTD system, equipped wi th a dual-needle microconductivity probe, was deployed off the central Cali fornia coast (35 degrees N, 121 degrees W) from the research platform FLIP. The probe's vertical resolution (8 cm) appears sufficient to resolve the l ow wavenumbers of the turbulent inertial subrange. This paper presents dept h-time maps, spanning 12 days and 100-400 m, of temperature dissipation rat e <(chi)over cap>, and Cox number (C) over cap: High <(chi)over cap> and (C ) over cap 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 fl uctuations is correlated with all three quantities, affirming that both mec hanical turbulence and double diffusion are active at the site. Depth-averaged dissipation rate epsilon(mu) is inferred from the <(chi)over cap> records under the assumption that a Batchelor spectrum for scalars ob tains and that the buoyancy flux J(b) and dissipation epsilon are related t hrough a constant mixing efficiency Gamma, J(b) = Gamma epsilon. Time serie s of epsilon(mu) are highly correlated with dissipation rate computed from Thorpe scales (epsilon(r)). estimated from large (2 m and greater) density overturns (except during periods when large portions of the water column ar e double-diffusively unstable: epsilon(mu) >> epsilon(T) in these regions, suggesting enhanced fluxes due to double diffusion.