On the parameterization of interleaving and turbulent mixing using CTD data from the Azores Frontal Zone

CTD-data obtained in the Azores Frontal Zone using a towed undulating vehic le are analyzed to study the relationship between characteristics of intrus ions and mean parameters of the thermohaline field, A self-similar dependen ce between intrusion intensity and hydrological parameters is obtained. The most well-founded interpretation of the empirical dependence is as follows : (a) the main source supporting intrusive layering is the salt finger conv ection; (b) the abrupt decrease of intrusion intensity with the reduction o f geostrophic Richardson number obtained from the analysis is explained by the beginning of turbulence when salt fingers do not work any longer, so th e "driving force" for intrusive motion disappears. These results are consis tent with the conclusions of the paper [Kuzmina N.P., Rodionov V.B., 1992. About the influence of baroclinicity upon generation of the thermohaline in trusions in the oceanic frontal zones. Izvestiya Akad. Nauk SSSR, Atmosperi c and Oceanic Physics 28 (10-11), 1077-1086]. These conclusions imply that there are three main mechanisms of intrusive layering at oceanic fronts, na mely the 2D baroclinic instability of geostrophic flow, the vertical sheer instability and the thermohaline instability where the driving source of in trusive motion is double diffusive convection. The baroclinic and thermohal ine instabilities can generate intrusions of large vertical scale, while ve rtical shear instability usually gives rise to thin turbulent layers. Turbu lence in these thin layers can prevent salt finger convection and thus dest roy the energy source of the intrusive motion conditioned by thermoclinicit y. Therefore, the baroclinicity plays two parts in the processes of the int rusive layering: (1) it prevents double-diffusion interleaving by means of turbulence, and (2) it generates intrusions due to the 2D baroclinic instab ility of geostrophic current. Using features of thermohaline interleaving a s a specific tracer of turbulent mixing, we have estimated turbulent mixing coefficient as k(t) similar to Ri(-0.8) (Ri > 1), where Ri is the geostrop hic Richardson number. Application of the proposed approach to other fronta l zones is discussed. (C) 2000 Elsevier Science B.V. All rights reserved.