Double-diffusive layering and mixing in Patagonian fjords

Abstract

Double-diffusive layering was quantified for the first time in the Chilean Patagonian fjords region (41.5–56°S). Approximately 600 temperature and salinity profiles collected during 1995–2012 were used to study water masses, quantify diffusive layering and compute the vertical diffusivity of heat. Development of “diffusive-layering” or simply “layering” was favored by relatively fresh–cold waters overlying salty–warm waters. Fresh waters are frequently derived from glacial melting that influences the fjord either directly or through rivers. Salty waters are associated with Modified Subantarctic (MSAAW) and Subantarctic Water (SAAW). Double-diffusive convection occurred as layering in 40% of the year-round data and as salt fingering in <1% of the time. The most vigorous layering, was found at depths between 20 and 70m, as quantified by (a) Turner angles, (b) density ratios, and (c) heat diffusivity (with maximum values of 5×10−5m2s−1). Diffusive-layering events presented a meridional gradient with less layering within the 41–47°S northern region, relative to the southern region between 47° and 56°S. Layering occupied, on average, 27% and 56% of the water column in the northern and southern regions, respectively. Thermohaline staircases were detected with microprofile measurements in Martinez and Baker channels (48°S), showing homogeneous layers (2–4m thick) below the pycnocline (10–40m). Also in this area, increased vertical mixing coincided with the increased layering events. High values of Thorpe scale (LT∼7m), dissipation rate of TKE (ε=10−5–10−3Wkg−1) and diapycnal eddy diffusivity (Kρ=10−6–10−3m−2s−1) were associated with diffusive layering. Implications of these results are that diffusive layering should be taken into account, together with other mixing processes such as shear instabilities and wind-driven flows, in biological and geochemical studies.