Observational evidence 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 circulation (Wunsch & Ferrari 2004). However, the processes leading to mixing and the pathways through which this upwelling occurs remain insufficiently understood. Recent observational and theoretical work suggests that deep water upwelling may be focused in bottom boundary layers on the ocean’s sloping seafloor; however, direct evidence of this is lacking (Ledwell et al. 2000, St. Laurent et al. 2001, Ferrari et al. 2016, de Lavergne et al. 2016). Here, we present observations from a near-bottom dye release within a canyon on the North Atlantic continental slope showing upwelling across density surfaces at a rate of 250 +/- 75 m/day over three days, ∼10,000 times higher than the global average value required to account for ∼30 Sv of upwelling globally (Munk 1966). The vigourous upwelling is coupled with adiabatic exchange of near-boundary and interior fluid. These results provide direct evidence of strong, bottom-focused diapycnal upwelling in the deep ocean, supporting previous suggestions that mixing at topographic features, such as canyons, leads to upwelling.

Deep inflow transport and dispersion in the Gulf of St. Lawrence revealed by a tracer release experiment

“…Closer inspection of tracer profiles suggest local increases in vertical mixing rates near the basin slopes (Fig. 3a, c) 27,48 , likely due to turbulent boundary layer processes such as bottom shear stress and internal wave scattering 17,49,50 , in addition to possible isopycnal spreading through the boundary layer 26,51 . These findings are of the same magnitude as microstructure measurements from the LSLE, where depth-averaged diapycnal mixing rates of Oð10 À5 m 2 s À1 Þ were observed with a similar relative increase in effective diffusivity when comparing between boundary and interior regions 16 .…”

“…Given these findings, the basin slopes are identified as key upwelling regions that warrant close attention in future surveys. Recent evidence from near-bottom dye release experiments indicates that diapycnal upwelling rates can reach Oð100 m day À1 Þ within bottom boundary layers 51 , which would designate these processes as vital components of a regional overturning circulation. In light of the apparent asymmetrical nature of the tracer spreading, more complex models could build upon the 1D models presented here by incorporating space and time variations in spreading parameters to better characterize the asymmetry and improve our understanding of its regional significance to vertical mixing processes in the Gulf.…”

Breaking internal waves on sloping topography: connecting parcel displacements to overturn size, interior-boundary exchanges, and mixing

“…Internal tides impinging on both critical and off-critical topography can result in bottom-enhanced turbulent mixing, and diapycnal upwelling necessary for the closure of the abyssal circulation (Eriksen 1985;Polzin et al 1997;Slinn and Riley 1998;Kunze et al 2012;Cyr and van Haren 2016;Chalamalla et al 2013;van Haren and Gostiaux 2012b). Recent theoretical work and observations suggest upwelling near sloping bottom boundaries may be limited to turbulent Bottom Boundary Layers (BBLs) where the mixing profile allows for convergent turbulent buoyancy fluxes, (Ferrari et al 2016;Mashayek et al 2017;Wynne-Cattanach et al 2023). Exchanges between the stratified interior and the well-mixed BBL associated with breaking internal waves could be a pathway for the restratification of these boundary waters, necessary to maintain an efficient diapycnal process (Armi 1978;van Haren 2023).…”

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.…”