Mixing and Upwelling Dynamics along the Continental Slope off Peru inferred from Tracer Release, Hydrographic and Microstructure Measurements

Martin Visbeck1, Marcus Dengler2, Toste S Tanhua3 and Madeleine Freund1, (1)GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany, (2)GEOMAR, Kiel, Germany, (3)Helmholtz Centre for Ocean Research Kiel, GEOMAR, Kiel, Germany
In the framework of the Peruvian Oxygen minimum zone System Tracer Release Experiment (POSTRE) about 70 kg of trifluoromethyl sulfur pentafluoride (SF5CF3) was injected into the bottom boundary layer of the upper Peruvian continental slope at 250m depth in October 2015. Three different injection sites, at 10°45’S, 12°20’S and 14°S (25kg, 28kg and 16kg, respectively) were selected. At the tracer release cites and due to tide-topography interaction, mixing above the upper continental slope of Peru in bottom intensified. Turbulent dissipation rates increase by about an order of magnitude in lower 50 to 100m above the bottom.
During previous tracer release experiments, where tracer was injected into the stratified mixing layer above the bottom boundary layer, a change of the center of mass toward higher densities resulted. Newer theories suggest that this diapycnal downwelling is balanced by a diapycnal upwelling within the bottom boundary layer.
Indeed, during the tracer survey, it was found that the density of tracer’s center of mass had decreased by about 0.12 kgm-3, which corresponds to an upward displacement of 100-150m. Using hydrographic and microstructure shear data from eight cruises, we obtain a diapycnal velocity of 1 m/day to 2 m/day within the bottom boundary layer.
Overall, our tracer study provides intriguing observational evidence of diapycnal upwelling occurring within the bottom boundary layer of a bottom enhanced mixing environment and supports recent ideas of a complex dynamics and embedded mixing.