Upwelling and Mixing in a Supercritical Submarine Canyon

Abstract Rhodamine dye was released on the seafloor near the head of a supercritically sloped submarine canyon in La Jolla, California. Fluorometers on a moored profiler and a ship‐towed CTD simultaneously tracked dispersion of the dye patch along and across isopycnals. The dye spread quickly, spanning a few hundred meters along the bottom within the first few hours after release. Over the next day, the dye separated from the bottom boundary layer and extended 3 km along the canyon axis. The dye spread across isopycnals with an effective diapycnal diffusivity of and upwelled at a rate of 6 m . Microstructure‐ and dye‐based diffusivity estimates are broadly consistent: the dye‐derived value falls within the log‐normal distribution of microstructure‐based estimates within the dye layer. These results provide direct tracer‐based estimates of near‐bottom diapycnal mixing, upwelling and isopycnal spreading in a supercritically sloped submarine canyon.