Observations of diapycnal upwelling within a sloping submarine canyon

Abstract

Small-scale turbulent mixing drives the upwelling of deep water masses in the abyssal ocean as part of the global overturning circulation1. However, the processes leading to mixing and the pathways through which this upwelling occurs remain insufficiently understood. Recent observational and theoretical work2–5 has suggested that deep-water upwelling may occur along the ocean’s sloping seafloor; however, evidence has, so far, been indirect. Here we show vigorous near-bottom upwelling across isopycnals at a rate of the order of 100 metres per day, coupled with adiabatic exchange of near-boundary and interior fluid. These observations were made using a dye released close to the seafloor within a sloping submarine canyon, and they provide direct evidence of strong, bottom-focused diapycnal upwelling in the deep ocean. This supports previous suggestions that mixing at topographic features, such as canyons, leads to globally significant upwelling3,6–8. The upwelling rates observed were approximately 10,000 times higher than the global average value required for approximately 30 × 106 m3 s−1 of net upwelling globally9.

Spatial and Temporal Variability of Turbulent Mixing in the Deep Northwestern Pacific

“…In-situ observations have reported that this enhanced mixing is concentrated near rough bottom topography such as ridges (Finnigan et al, 2002;Ledwell et al, 2000;Polzin et al, 1997;St Laurent & Thurnherr, 2007;Xiao et al, 2023), seamounts (Carter et al, 2006;Lavelle et al, 2004;Lueck & Mudge, 1997;Toole et al, 1997;Ye et al, 2022), canyons (Carter & Gregg, 2002;Laurent et al, 2001;Lee et al, 2009) and passages (Alford et al, 2013;MacKinnon et al, 2008;Polzin et al, 1996;Tian et al, 2009). More recently, evolving theories, numerical simulations and observations impressively established the potential mechanism of abyssal upwelling, namely that it is more likely to occur in narrow, turbulent, bottom boundary layer (BBL) rather than in the stratified ocean interior ( de Lavergne et al, 2017;Dell & Pratt, 2015;Holmes & McDougall, 2020;McDougall & Ferrari, 2017;Wynne-Cattanach et al, 2024). The counteraction between the diapycnal upwelling within the BBL and downwelling in the stratified mixing layer above the BBL results in a net diapycnal transport.…”