Presenter: Bethan Wynne-Cattanach (Scripps Institution of Oceanography)

Description:
Observational evidence of bottom intensified mixing together with recent theoretical work suggests that upwelling of deep water is confined to a bottom boundary layer on the ocean's sloping boundaries. The transformation of dense water to lighter water is a vital part of the global overturning circulation. The Boundary Layer Turbulence and Abyssal Recipes (BLT Recipes) Program set out to observe diapycnal mixing close to the seafloor to investigate this view of upwelling. We present data from the fluorescein dye release experiment undertaken as part of BLT Recipes in July 2021. The experiment focused on one of many submarine canyons on the boundary of the Rockall Trough in the North Atlantic. Dye was released on the 3.76°C isotherm, approximately 1850m deep and 10m above the bottom at the center of the canyon. Fluorometer measurements of the dye concentration were accompanied by CTD data and estimates of the turbulence from microconductivity; all collected using the FastCTD profiler. The rapid cycling FastCTD allowed for high spatial and temporal resolution of the dye as it evolved. With the altimeter attached to the instrument, we were able to get within 10m of the seafloor. The dye was sampled for three days before concentrations dropped below the detection limit. We tracked the dye patch as it was advected up and down the canyon by tidal sloshing. Our observations provide the first direct evidence of very rapid diapycnal upwelling along the canyon at rates of O(100m/day). In this study, we examine the time evolution of the dye in the bottom boundary layer and investigate the mixing associated with the translation of the dye's center of mass to warmer water.