An Investigation of Abyssal Mixing and Interior Transports in the North Atlantic

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

Ongoing efforts to develop a quantitative picture of the Atlantic meridional overturning circulation (AMOC) have largely focused on the western boundary current system and transports of water masses formed by buoyancy loss in the northern seas. By contrast, the structure and strength of the interior abyssal circulation and its buoyancy gain have been poorly sampled and are not known in much detail. These are, however, fundamentally important to the global ocean circulation and to ocean budgets of mass, heat and tracers. A process study will be conducted to provide quantitative estimates of abyssal mixing and circulation in order to assess the strength of, and linkages between, the horizontal and vertical components of the North Atlantic interior abyssal circulation. Analysis of climatological property fields suggests that a logical region to accomplish this lies along the eastern flank of Bermuda Rise and in the topographic basin to its southeast. Several lines of evidence suggest that these interior flows are highly structured and remarkably vigorous, and by implication, that the diapycnal mixing feeding them is particularly strong. That source region is situated on the western flank of the mid Atlantic Ridge (MAR) where rugged topography provides conditions favorable for mixing. The project is comprised of two elements: 1) An array of moored profilers designed to measure the geostrophic transports along the southeastern flank of Bermuda Rise, a primary pathway for the interior circulation, and 2) A shipboard survey to obtain microstructure and velocity profiles, plus detailed bathymetry. These will be used to construct estimates of turbulent buoyancy fluxes across a range of topographic features and a characterization of the basin-wide diapycnal forcing field. Intellectual merit Each component of the proposed experiment will provide direct measurements of a poorly quantified portion of the interior abyssal ocean, i.e., the structure and strength of horizontal flows along Bermuda Rise, and the diapycnal forcing that feeds those flows. Together, they will provide a basis for evaluating consistency between the observed circulations and the underlying dynamical theory. This attention to the interior basin will both complement and supplement the emerging quantitative picture of the AMOC. Specifically the process study will accomplish the following: 1 Quantify the structure, strength, and variability of the intermediate-to-abyssal interior flow field along Bermuda Rise. Diagnose the relative contributions of northern (high tracer concentration and low nutrients) and southern (low tracer concentrations and high nutrients) sources to these flows. 2 Assess the regional structure and strength of turbulent mixing that feeds the interior flow field. Apply these to improve dynamically-based parameterizations of diapycnal forcing. 3 Evaluate consistency between the magnitudes of the observed vertical and horizontal fluxes. 4 Synthesize transports from various measurement programs in temperature and density classes, to construct volume flux budgets for cold to warm limb transformation and abyssal circulation in the western North Atlantic basins. Broader impacts The results of this process study and synthesis will provide benchmarks for evaluation of ocean assimilation and reanalysis products. None of the current ocean models and analysis products exhibits much structure below 4000 m. Abyssal geostrophic flow fields in general circulation models have been shown to improve significantly when realistic representations of diapycnal mixing are incorporated. This effort will provide regional constraints for a dynamical parameterization of diapycnal forcing designed for testing and use in ocean models. Finally, the project will provide opportunities for student participation in the field and in the analysis of observational data.