Persistent, Depth‐Intensified Mixing During The Western Mediterranean Transition's Initial Stages

Major deep‐convection activity in the northwestern Mediterranean during winter 2005 triggered the formation of a complex anomalous deep‐water structure that substantially modified the properties of the Western Mediterranean deep layers. Since then, evolution of this thermohaline structure, the so‐called Western Mediterranean Transition (WMT), has been traced through a regularly sampled hydrographic deep station located on the outer continental slope of Minorca Island. A rapid erosion of the WMT's near‐bottom thermohaline signal was observed during 2005–2007. The plausible interpretation of this as local bottom‐intensified mixing motivates this study. Here, the evolution of the WMT structure through 2005–2007 is reproduced by means of a one‐dimensional diffusion model including double‐diffusive mixing that allows vertical variation of the background mixing coefficient and includes a source term to represent the lateral advection of deep‐water injections from the convection area. Using an optimization algorithm, a best guess for the depth‐dependent background mixing coefficient is obtained for the study period. WMT evolution during its initial stages is satisfactorily reproduced using this simple conceptual model, indicating that strong depth‐intensified mixing (K∞(z) ≈ 22 × 10−4m2s−1; z⪆ 1,400 dbar) is a valid explanation for the observations. Extensive hydrographic and current observations gathered over the continental slope of Minorca during winter 2018, the first deep‐convective winter intensively sampled in the region, provide evidence of topographically localized enhanced mixing concurrent with newly formed dense waters flowing along‐slope toward the Algerian sub‐basin. This transport‐related boundary mixing mechanism is suggested to be a plausible source of the water‐mass transformations observed during the initial stages of the WMT off Minorca. In winter 2005, an exceptional production of deep waters with anomalous temperature and salinity induced a series of drastic changes in the deep waters of the Western Mediterranean. The evolution of these new deep waters was traced up to the present day through hydrographic measurements in the outer continental slope of Minorca. During 2005–2007, a rapid erosion of the bottom water characteristics was observed. This may be indicative of enhanced mixing near the seafloor. By means of an idealized numerical model, the evolution of the deep waters off Minorca during that period is satisfactorily reproduced, indicating that the deep waters experienced persistent, strongly depth‐intensified mixing. More than a decade later, extensive hydrographic and current observations gathered above the continental slope of Minorca evidence enhanced bottom mixing associated with the along‐slope flow of newly formed deep waters during winter of 2018, a much less relevant winter in terms of deep‐water formation albeit intensively sampled. This boundary mixing mechanism is suggested to be a plausible source of the strong mixing necessary to reproduce the observed evolution of the deep waters during the 2005–2007 period, as indicated by our modeling approach. A rapid erosion of bottom water thermohaline signatures was observed off Minorca Island during the Western Mediterranean Transition's (WMT) initial stagesLocal evolution of the WMT deep hydrographic structure during 2005–2007 is consistent with persistent intense mixingEvidence that intense mixing occurs at the continental slope and mixed waters are exported offshore is found A rapid erosion of bottom water thermohaline signatures was observed off Minorca Island during the Western Mediterranean Transition's (WMT) initial stages Local evolution of the WMT deep hydrographic structure during 2005–2007 is consistent with persistent intense mixing Evidence that intense mixing occurs at the continental slope and mixed waters are exported offshore is found