Collaborative Research: Bottom Boundary Layer Turbulent and Abyssal Recipes

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The classic view of the deep overturning circulation of the ocean is one in which densest waters formed at high latitudes sink and spread along the abyssal basins. Small-scale mixing, such as is caused by breaking internal waves, drives upwelling of these densest waters slowly back toward the surface over the interior of the basins. However, turbulence measurements over the last 20 years have shown that mixing becomes more vigorous toward the ocean bottom, which should result in the sinking of the water masses formed by mixing. Recent work, combining theory, numerical models and turbulence measurements have suggested that the upwelling necessary to bring the water back toward the surface to close the loop happens in thin boundary layers very close to the ocean bottom. This is a region typically avoided in turbulence measurements to prevent the instruments from hitting the bottom. This US-UK joint project will seek the first direct evidence that turbulent mixing drives sinking in the stratified interior and upwelling along thin boundary layers. It has potentially wide impact because it explores the importance of boundary layer upwelling in the overturning circulation, a process that has received little attention to date. Should this experiment succeed in finding evidence for large upwelling confined to deep boundary layers, it will reinvigorate studies of boundary layer turbulence. The field program will compare different approaches to measure turbulent buoyancy fluxes in the ocean, and help settle the ongoing debate on which ones are most accurate. The result of this experiment will have important implications for climate studies, because the ocean uptake of carbon and heat is regulated by the pathways of deep water masses. Finally, the project has a strong educational component through the training of two postdocs at WHOI and SIO, who will lead the analysis of the observations, and one graduate student at MIT, who will run numerical simulations to put the observations in the overall context of the regional circulation and the global overturningStarting with Munk (1966), it is generally understood that small-scale mixing, such as is caused by breaking internal waves, drives upwelling of the densest waters that sink to the ocean bottom at high latitudes. However, turbulence measurements over the last 20 years have shown that mixing becomes more vigorous toward the ocean bottom, and thus converts light waters into denser ones and not vice versa. Using a combination of theoretical ideas, numerical models, and turbulence measurements, Ferrari et al. (2016), de Lavergne et al. (2016) and McDougall and Ferrari (2017) have argued that abyssal waters are converted from dense to light along weakly stratified bottom boundary layers, where small-scale turbulent buoyancy fluxes decrease to zero to satisfy the no-density flux condition at the ocean bottom. In this view, the lower branch of the meridional overturning circulation is the residual of a large diapycnal sinking, driven by convection at high latitudes and small-scale mixing in the stratified ocean interior, balanced by an even larger diapycnal upwelling along the ocean boundary layers. Callies and Ferrari (2017) illustrate that the confinement of upwelling along boundary layers results in a different abyssal circulation from the classical view pioneered by Stommel (1958) and Munk (1966), with important implications for ocean carbon and heat uptake. Observational support for this emerging view of the overturning circulation is lacking, because tracers are advected rapidly in and out of the boundary layers and thus reflect some average of the diapycnal sinking in the stratified interior and diapycnal upwelling along the boundaries. Vertical profiles of turbulence in the deep ocean generally stop above the boundary layer to avoid hitting the seafloor, and thus miss the crucial decrease of turbulent buoyancy flux through the bottom boundary layer. This US-UK collaborative project will use the Rockall Trough in the Northeast Atlantic as a natural laboratory to study diapycnal upwelling along sloping boundaries. This basin is characterized by rough topography and strong topographic mixing, and is an important conduit of abyssal waters in the North Atlantic. Tracers will be released along the Trough's eastern boundary to see whether their movement is consistent with these new ideas and with inferences in prior work that deep waters enter the Rockall Trough from the south and upwell in the basin. The temporal evolution of the tracers will be compared with diapycnal velocities estimated from buoyancy flux measurements from vertical profilers in the stratified interior and moored sensors across the boundary layer. Diapycnal velocities are expected to be strong and upward in the boundary layer, and downward in the stratified interior. Successful completion of the field program will return the first direct observation of the role played by deep boundary layers in the oceanic overturning circulation.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.