The importance of salt fingers in diapycnal mixing parametrization in Eastern Boundary Systems: Study Case in the Oxygen Minimum Zone off Central Chile

Recent studies have highlighted the importance of double diffusion instabilities in the diapycnal transport of dissolved substances in large regions of the oceans. Off central Chile (30-38°S), waters with very low dissolved oxygen are present at intermediate depths (between 50 m and 400 m depth) in a region where double diffusion instabilities can take place. This oxygen minimum layer (OML) is closely related to Equatorial Subsurface Waters (ESSW), a relatively salty and warm water mass that is transported poleward along the continental slope by the Peru-Chile Undercurrent (PCUC). Thus, this water mass –and so the OML– is delimited between two water masses of southern origin that are well-ventilated, relatively fresh, and cold, namely: the Eastern South Pacific Intermediate Water (ESPIW) and the Antarctic Intermediate Water (AAIW). In this study, we analyzed the role of diapycnal mixing in the dissolved oxygen fluxes in the upper and lower oxyclines that delimit the OMZ in the water column off central Chile (~36.5°S). Special emphasis is given to the evaluation of the contribution of salt fingers to these fluxes. We use a set of observations of fine structure (1-10 m) and microstructure (<1m) using CTD casts and a vertical microstructure turbulence profiler (VMP-250), respectively, along with current profiles obtained with ADCPs. The net diapycnal mixing is estimated using mixing models that allow us to separate the relative contributions of turbulent shear processes and instabilities associated with salt fingers. The thermohaline contrast in the ESSW-AAIW transition conditions the region for the development of double diffusion instabilities by salt fingers, which significantly contribute to the oxygen transport from the lower oxycline, thus favoring the ventilation of the OMZ from the AAIW.