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1. Wind-Driven Seasonal Cycle of the Atlantic Meridional Overturning Circulation

   https://doi.org/10.1175/JPO-D-13-0144.1  

   Zhao, Jian; Johns, William (June 2014, Journal of Physical Oceanography)

   null (Ed.)

   Abstract The dynamical processes governing the seasonal cycle of the Atlantic meridional overturning circulation (AMOC) are studied using a variety of models, ranging from a simple forced Rossby wave model to an eddy-resolving ocean general circulation model. The AMOC variability is decomposed into Ekman and geostrophic transport components, which reveal that the seasonality of the AMOC is determined by both components in the extratropics and dominated by the Ekman transport in the tropics. The physics governing the seasonal fluctuations of the AMOC are explored in detail at three latitudes (26.5°N, 6°N, and 34.5°S). While the Ekman transport is directly related to zonal wind stress seasonality, the comparison between different numerical models shows that the geostrophic transport involves a complex oceanic adjustment to the wind forcing. The oceanic adjustment is further evaluated by separating the zonally integrated geostrophic transport into eastern and western boundary currents and interior flows. The results indicate that the seasonal AMOC cycle in the extratropics is controlled mainly by local boundary effects, where either the western or eastern boundary can be dominant at different latitudes, while in the northern tropics it is the interior flow and its lagged compensation by the western boundary current that determine the seasonal AMOC variability. 

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2. Structure and Variability of Iceland Scotland Overflow Water Transport in the Western Iceland Basin

   https://doi.org/10.1029/2023JC020107  

   Devana, M.; Johns, W. (August 2024, Journal of Geophysical Research: Oceans)

   Abstract The Iceland Scotland Overflow Water (ISOW) plume supplies approximately a third of the production of North Atlantic Deep Water and is a key component of the meridional overturning circulation (MOC). The Overturning in the Subpolar North Atlantic Program (OSNAP) mooring array in the Iceland Basin has provided high‐resolution observations of ISOW from 2014 to 2020. The ISOW plume forms a deep western boundary current along the eastern flank of Reykjanes Ridge, and its total transport varies by greater than a factor of two on intra‐seasonal timescales. EOF analysis of moored current meter records reveal two dominant modes of velocity variance. The first mode explains roughly 20% of the variance and shows a bottom intensified structure concentrated in the rift valley that runs parallel to the ridge axis. The transport anomaly reconstructed from the first mode explains nearly 80% of the total ISOW plume transport variance. The second mode accounts for 15% of velocity variance, but only 5% of the transport variance. The geostrophically estimated transport (2.9 Sv) recovers only 70% of the total ISOW transport along the ridge flank estimated from the direct current meter observations (4.2 Sv), implying a significant ageostrophic component of ISOW mean transport and variability. Ageostrophic flow is strongly linked to the leading mode of velocity variability within the rift valley. The ISOW transport variability along the upper and middle part of the ridge is further shown to correlate with changes in the strength of deep MOC limb across the basin‐wide OSNAP array. 

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3. An idealized modeling study of the mid-latitude variability of the wind-driven meridional overturning circulation

   https://doi.org/10.1175/JPO-D-20-0317.1  

   Spall, Michael A. (May 2021, Journal of Physical Oceanography)

   Abstract The frequency and latitudinal dependence of the mid-latitude wind-driven meridional overturning circulation (MOC) is studied using theory and linear and nonlinear applications of a quasi-geostrophic numerical model. Wind-forcing is varied by either changing the strength of the wind or by shifting the meridional location of the wind stress curl pattern. At forcing periods less than the first mode baroclinic Rossby wave basin crossing time scale the linear response in the mid-depth and deep ocean is in phase and opposite to the Ekman transport. For forcing periods close to the Rossby wave basin crossing time scale, the upper and deep MOC are enhanced, and the mid-depth MOC becomes phase shifted, relative to the Ekman transport. At longer forcing periods the deep MOC weakens and the mid-depth MOC increases, but eventually for long enough forcing periods (decadal) the entire wind-driven MOC spins down. Nonlinearities and mesoscale eddies are found to be important in two ways. First, baroclinic instability causes the mid-depth MOC to weaken, lose correlation with the Ekman transport, and lose correlation with the MOC in the opposite gyre. Second, eddy thickness fluxes extend the MOC beyond the latitudes of direct wind forcing. These results are consistent with several recent studies describing the four-dimensional structure of the MOC in the North Atlantic. 

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4. How is New England Coastal Sea Level Related to the Atlantic Meridional Overturning Circulation at 26° N?

   https://doi.org/10.1029/2019GL083073  

   Piecuch, Christopher G.; Dangendorf, Sönke; Gawarkiewicz, Glen G.; Little, Christopher M.; Ponte, Rui M.; Yang, Jiayan (May 2019, Geophysical Research Letters)

   Abstract Monthly observations are used to study the relationship between the Atlantic meridional overturning circulation (AMOC) at 26° N and sea level (ζ) on the New England coast (northeastern United States) over nonseasonal timescales during 2004–2017. Variability inζis anticorrelated with AMOC on intraseasonal and interannual timescales. This anticorrelation reflects the stronger underlying antiphase relationship between ageostrophic Ekman‐related AMOC transports due to local zonal winds across 26° N andζchanges arising from local wind and pressure forcing along the coast. These distinct local atmospheric variations across 26° N and along coastal New England are temporally correlated with one another on account of large‐scale atmospheric teleconnection patterns. Geostrophic AMOC contributions from the Gulf Stream through the Florida Straits and upper‐mid‐ocean transport across the basin are together uncorrelated withζ. This interpretation contrasts with past studies that understoodζand AMOC as being in geostrophic balance with one another. 

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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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