USAP-DC

Overview: In order to close the global overturning circulation, high-density deep- and bottom waters produced at high latitudes must be made less dense and upwell to shallower depths. Available observations from the subtropical South Atlantic indicate that the bulk of the mixing in the deep ocean there takes place over the topographically rough Mid-Atlantic Ridge, in particular in the quasi-regularly spaced "fracture zone canyons" corrugating the ridge flanks. There, dense water is advected toward the ridge crest (i.e. upwelled) by persistent along-valley currents that flow down the unidirectional density gradients, which are maintained by strong turbulence (diapycnal mixing). Most of the data on which these inferences are based were collected during the Brazil Basin Tracer Release Experiment (BBTRE) along a single ridge-flank canyon in the western South Atlantic near 22S where previous analyses have shown that both tidal mixing and overflow processes are important. Therefore, it is likely that both processes must be considered in order to understand and parameterize the effects of turbulence and mixing in the canyons corrugating the flanks of all slow-spreading ridges, which make up large fractions of the sea floor, in particular in the Atlantic, Indian and Southern Oceans. The primary aim of this follow-on project is to improve our understanding of the dynamics over the corrugated flanks of slow-spreading mid-ocean ridges. Due to the coarse sampling resolution and choice of station locations it is not possible to answer important questions, such as the relative importance of tidal and sill mixing, from the BBTRE data. Therefore, high-resolution surveys of hydrography, three-dimensional flow, turbulence and mixing will be carried out in two neighboring canyons and over the intervening topographic spur in the BBTRE region to determine the relative contributions of tidal and sill-related mixing. Furthermore, profiling moorings deployed on two nearby sill regions will be used to derive time series of spatially integrated mixing related buoyancy fluxes and to investigate the strong but unexplained sub-inertial variability of the along-canyon flow recorded previously. Additionally, three small moorings will be deployed in saddles between the two canyons to investigate inter-canyon exchange. The data analysis will include available data from previous experiments, including a set of tracer profiles that has not been analyzed before. Intellectual Merit: The corrugated flanks of slow-spreading ridges cover large areas of the sea floor of several major ocean basins. Therefore, understanding the dynamics in the ~100 km of ridge-flank canyons and its effects on the buoyancy and upwelling budget of the abyssal ocean is of global significance. In addition to determining the relative importance of tidal mixing and cross-sill flows in two canyons, the temporal variability of turbulence and mixing from tidal to yearly time scales will be investigated to gain insights into the forcing of the along-canyon flows, the exchange between neighboring canyons, and the eventual fate of the canyon waters. Broader Impacts: It is anticipated that insights gained during this project will improve our understanding of abyssal mixing in many different regions with similar bottom topography and provide the basis for better parameterizations of the effects of turbulence and mixing in large-scale circulation and climate models that cannot resolve these small-scale processes. As part of the project, a graduate student and a post-doctoral researcher will be trained in all aspects of observational physical oceanography, from data acquisition to interpretation.

Person	Role
Thurnherr, Andreas	Investigator and contact

Unknown Program

Award # 1235094

NBP1508

Deployment	Type
NBP1406	ship expedition
NBP1508	ship expedition

None in the Database

Repository	Title (link)	Format(s)	Status
R2R	Expedition Data	None	exist
R2R	NBP1406 Expedition data	None	exists
R2R	NBP1508 Expedition data	None	exists
USAP-DC	Processed Current Measurement, Pressure, Salinity and Temperature Data from the Southern Mid-Atlantic Ridge Spreading acquired during R/V Nathaniel B. Palmer expedition NBP1508	None	exists
USAP-DC	Processed Current Measurement, Pressure and Temperature Data from the Southern Mid-Atlantic Ridge Spreading acquired during R/V Nathaniel B. Palmer expedition NBP1508 (2015)	None	exists
USAP-DC	Processed Current Measurement Data from the Southern Mid-Atlantic Ridge Spreading acquired during R/V Nathaniel B. Palmer expedition NBP1508	None	exists

1. Clément, L., Thurnherr, A. M., & St. Laurent, L. C. (2017). Turbulent Mixing in a Deep Fracture Zone on the Mid-Atlantic Ridge. Journal of Physical Oceanography, 47(8), 1873–1896. (doi:10.1175/jpo-d-16-0264.1)
2. Clément, L., & Thurnherr, A. M. (2018). Abyssal Upwelling in Mid‐Ocean Ridge Fracture Zones. Geophysical Research Letters, 45(5), 2424–2432. (doi:10.1002/2017gl075872)
3. Thurnherr, A. M., Clément, L., St. Laurent, L., Ferrari, R., & Ijichi, T. (2020). Transformation and Upwelling of Bottom Water in Fracture Zone Valleys. Journal of Physical Oceanography, 50(3), 715–726. (doi:10.1175/jpo-d-19-0021.1)
4. Zhao, J., & Thurnherr, A. M. (2018). Changes in Bottom Water Physical Properties Above the Mid‐Atlantic Ridge Flank in the Brazil Basin. Journal of Geophysical Research: Oceans, 123(1), 708–719. (doi:10.1002/2017jc013375)