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.
The Dynamics of Abyssal Mixing and Interior Transports Experiment (DynAMITE) PIs: Ruth Curry and Kurt Polzin Woods Hole Oceanographic Institution www.whoi.edu/science/PO/dynamite/ Program Objectives The principal goal of this project was to refine our knowledge of the deep circulation through the interior basins of the western North Atlantic Ocean with a suite of observations designed to measure horizontal and vertical flows in key locations. The underlying hypothesis was that relatively strong, deep currents flow through the interior basin in a clockwise direction around Bermuda Rise, and that these are driven by vertical mixing processes in the deep Gulf Stream and along the western flank of the Mid Atlantic Ridge. The deep and abyssal currents are part of the plumbing system at the core of global climate – the meridional overturning circulation (MOC) – which, in the Atlantic, moves ocean heat poleward in the upper layers toward the Arctic (the warm limb of the MOC) and returns cold, deep waters equatorward (the cold limb) from the seas around Greenland. Along that route, the ocean releases heat to the atmosphere -- a vital process that determines many characteristics of northern hemisphere climate (for example, storm tracks, surface temperatures and ice volume). Determining the pathways and magnitudes of flow is an important step toward deciphering the mechanics and dynamics of Earth’s changing climate. The general framework of AMOC cold limb flows (i.e. the deep Gulf Stream, deep western boundary current (DWBC) and lateral recirculating gyres) is depicted by black curves/arrows in Figure 1. The sources and sinks of the flows measured by the DynAMITE moorings (stars) are schematically delineated by blue and green curves. Blue lines reflect pathways for the strong flows measured by the upslope moorings, the green curve depicts the flows emanating from the Mid Atlantic Ridge (out of the map area). The dashed blue line represents flows below 3800 m, whereas the solid blue line reflects flow sources and sinks for layers between 1000 – 3800 m. Principal Results: A strong, bottom-intensified southwesterly flow feature, transporting an order 10 Sv of cold limb water masses, was measured on the upslope part of Bermuda Rise (between the 4500 – 5000 m isobaths, blue curves, Fig. 1). Flows in the downslope region (5000-6000 m isobaths) were generally weak and insignificant (green curve, Fig. 1)s. The strong flows were tagged with northern property characteristics (high CFCs and oxygen, low nutrients), and their sources were traced to regions on the southern side of the deep Gulf Stream where lateral recirculation systems govern the circulation. Recent studies have implicated these eddy-driven recirculation gyres as an alternative pathway (relative to the deep western boundary current) for AMOC cold limb water masses to travel from the subpolar seas to tropical latitudes. Those studies were based on observed and simulated float trajectories which traced these interior pathways in a Lagrangian sense (i.e. tracking the trajectory of individual water parcels). The DynAMITE measurements confirmed the existence of these interior flows with direct velocity measurements of flow past a fixed location, which suggests that these interior flows reflect a significant portion (~30%) of the total cold limb (e.g. ~30 Sv) measured near the Bahamas. The New England Seamounts to the north of Bermuda Rise, present a barrier to the deep Gulf Stream, particularly below 4000 m depth. The strong flows on Bermuda Rise below 3800 m (dashed blue line, Fig. 1) were traced to the western side of the seamount chain, while at shallower depths (1500-3800m), the flows arrived from the eastern side (solid blue line). Weak flows on the downslope side had distinctly different water properties and sources (low CFCs and oxygen, high nutrients). These emanated from the Mid Atlantic Ridge, where turbulent mixing, generated by currents interacting with steep and rugged topography, causes the waters to upwell into overlying layers. This process is repeated in localized regions of enhanced mixing around the basin, collectively transforming cold, dense layers into warmer, lighter water masses, in a system we have labeled the Abyssal Upwelling Cell (AUC). All of the interior flows measured by the DynAMITE moorings -- those associated with northern source cold limb water masses and those transformed through the AUC – continue to flow westward through the interior basin and entrain into the deep western boundary current between Cape Hatteras and the Bahamas. This approximately doubles the volume transports measured at the deep western boundary at 26N, compared to upstream in the boundary offshore of New England.
