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1. Redrawing the Iceland−Scotland Overflow Water pathways in the North Atlantic

   https://doi.org/10.1038/s41467-020-15513-4  

   Zou, Sijia ; Bower, Amy ; Furey, Heather ; Susan Lozier, M. ; Xu, Xiaobiao ( April 2020 , Nature Communications)

   Iceland-Scotland Overflow Water (ISOW) is a primary deep water mass exported from the Norwegian Sea into the North Atlantic as part of the global Meridional Overturning Circulation. ISOW has historically been depicted as flowing counter-clockwise in a deep boundary current around the subpolar North Atlantic, but this single-boundary-following pathway is being challenged by new Lagrangian observations and model simulations. We show here that ISOW leaves the boundary and spreads into the interior towards the central Labrador and Irminger basins after flowing through the Charlie-Gibbs Fracture Zone. We also describe a newly observed southward pathway of ISOW along the western flank of the Mid-Atlantic Ridge. The partitioning of these pathways is shown to be influenced by deep-reaching eddies and meanders of the North Atlantic Current. Our results, in tandem with previous studies, call for a revision in the historical depiction of ISOW pathways throughout the North Atlantic.

    

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2. Transition paths of North Atlantic Deep Water

   https://doi.org/10.1175/JTECH-D-22-0022.1  

   Miron, P. ; Beron-Vera, F. J. ; Olascoaga, M. J. ( May 2022 , Journal of Atmospheric and Oceanic Technology)

   Abstract Recently introduced in oceanography to interpret the near surface circulation, Transition Path Theory ( TPT ) is a methodology that rigorously characterizes ensembles of trajectory pieces flowing out from a source last and into a target next, i.e., those that most productively contribute to transport. Here we use TPT to frame, in a statistically more robust fashion than earlier analysis, equatorward routes of North Atlantic Deep Water (NADW) in the subpolar North Atlantic. TPT is applied on all available RAFOS and Argo floats in the area by means of a discretization of the Lagrangian dynamics described by their trajectories. By considering floats at different depths, we investigate transition paths of NADW in its upper (UNADW) and lower (LNADW) layers. We find that the majority of UNADW transition paths sourced in the Labrador and southwestern Irminger Seas reach the western side of a target arranged zonally along the southern edge of the subpolar North Atlantic domain visited by the floats. This is accomplished in the form of a well-organized deep boundary current (DBC). LNADW transition paths sourced west of the Reykjanes Ridge reveal a similar pattern, while those sourced east of the ridge are found to hit the western side of the target via a DBC and also several other places along it in a less organized fashion, indicating southward flow along the eastern and western flanks of the Mid-Atlantic Ridge. Naked-eye inspection of trajectories suggest generally much more diffusive equatorward NADW routes. A source-independent dynamical decomposition of the flow domain into analogous backward-time basins of attraction, beyond the reach of direct inspection of trajectories, reveals a much wider influence of the western side of the target for UNADW than for LNADW. For UNADW, the average expected duration of the pathways from the Labrador and Irminger Seas was found to be of 2 to 3 years. For LNADW, the duration was found to be influenced by the Reykjanes Ridge, being as long as 8 years from the western side of the ridge and of about 3 years on average from its eastern side. 

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3. Interior Pathways of Labrador Sea Water in the North Atlantic From the Argo Perspective

   https://doi.org/10.1029/2018GL081439  

   Biló, T. C. ; Johns, W. E. ( March 2019 , Geophysical Research Letters)

   The pathways and transports of Labrador Sea Water (LSW) within the southward‐flowing lower limb of the Atlantic Meridional Overturning Circulation are studied using 12 years of Argo profiles and subsurface Argo drift data. Consistent with previous studies, the results show clear evidence for interior pathways of LSW that separate from the western boundary near the Grand Banks and flow eastward and then southward around a large‐scale deep anticyclonic gyre in the northern subtropical Atlantic. Most of the LSW exported into the interior recirculates in the Newfoundland Basin (9.3 ± 3.5 Sv). However, approximately 3.2 ± 0.4 Sv cross the Mid‐Atlantic Ridge and flow southward east of the Azores. This branch feeds a westward quasi‐zonal pathway that recrosses the Ridge and returns to the western boundary around 30°N.

    

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4. A Deep Eastern Boundary Current Carrying Indian Deep Water South of Australia

   https://doi.org/10.1029/2018JC014569  

   Tamsitt, V. ; Talley, L. D. ; Mazloff, M. R. ( March 2019 , Journal of Geophysical Research: Oceans)

   In the Southern Hemisphere, the ocean's deep waters are predominantly transported from low to high latitudes via boundary currents. In addition to the Deep Western Boundary Currents, pathways along the eastern boundaries of the southern Atlantic, Indian, and Pacific transport deep water poleward into the Southern Ocean where these waters upwell to the sea surface. These deep eastern boundary currents and their physical drivers are not well characterized, particularly those carrying carbon and nutrient‐rich deep waters from the Indian and Pacific basins. Here we describe the poleward deep eastern boundary current that carries Indian Deep Water along the southern boundary of Australia to the Southern Ocean using a combination of hydrographic observations and Lagrangian experiments in an eddy‐permitting ocean state estimate. We find strong evidence for a deep boundary current carrying the low‐oxygen, carbon‐rich signature of Indian Deep Water extending between 1,500 and 3,000 m along the Australian continental slope, from 30°S to the Antarctic Circumpolar Current southwest of Tasmania. From the Lagrangian particles it is estimated that this pathway transports approximately 5.8 ± 1.3 Sv southward from 30°S to the northern boundary of the Antarctic Circumpolar Current. The volume transport of this pathway is highly variable and is closely correlated with the overlying westward volume transport of the Flinders Current.

    

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5. Labrador sea water spreading and the Atlantic meridional overturning circulation

   https://doi.org/10.1098/rsta.2022.0189  

   Le Bras, Isabela Alexander-Astiz ( December 2023 , Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences)

   In 1982, Talley and McCartney used the low potential vorticity signature of Labrador Sea Water (LSW) to make the first North Atlantic maps of its properties. Forty years later, our understanding of LSW variability, spreading time scales and importance has deepened. In this review and synthesis article, I showcase recent observational advances in our understanding of how LSW spreads from its formation regions into the Deep Western Boundary Current and southward into the subtropical North Atlantic. I reconcile the fact that decadal variability in LSW formation is reflected in the Deep Western Boundary Current with the fact that LSW formation does not control subpolar overturning strength and discuss hypothesized connections between LSW spreading and decadal Atlantic Meridional Overturning Circulation variability. Ultimately, LSW spreading is of fundamental interest because it is a significant pathway for dissolved gasses such as oxygen and carbon dioxide into the deep ocean. We should hence prioritize adding dissolved gas measurements to standard hydrographic and circulation observations, particularly at targeted western boundary locations.This article is part of a discussion meeting issue ‘Atlantic overturning: new observations and challenges’. 

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