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1. Cyclonic eddies in the West Greenland Boundary Current System

   https://doi.org/10.1175/JPO-D-20-0255.1  

   Pacini, Astrid ; Pickart, Robert S. ; Le Bras, Isabela A. ; Straneo, Fiammetta ; Holliday, N.P. ; Spall, M.A. ( April 2021 , Journal of Physical Oceanography)

   null (Ed.)

   Abstract The boundary current system in the Labrador Sea plays an integral role in modulating convection in the interior basin. Four years of mooring data from the eastern Labrador Sea reveal persistent mesoscale variability in the West Greenland boundary current. Between 2014 and 2018, 197 mid-depth intensified cyclones were identified that passed the array near the 2000 m isobath. In this study, we quantify these features and show that they are the downstream manifestation of Denmark Strait Overflow Water (DSOW) cyclones. A composite cyclone is constructed revealing an average radius of 9 km, maximum azimuthal speed of 24 cm/s, and a core propagation velocity of 27 cm/s. The core propagation velocity is significantly smaller than upstream near Denmark Strait, allowing them to trap more water. The cyclones transport a 200-m thick lens of dense water at the bottom of the water column, and increase the transport of DSOW in the West Greenland boundary current by 17% relative to the background flow. Only a portion of the features generated at Denmark Strait make it to the Labrador Sea, implying that the remainder are shed into the interior Irminger Sea, are retroflected at Cape Farewell, or dissipate. A synoptic shipboard survey east of Cape Farewell, conducted in summer 2020, captured two of these features which shed further light on their structure and timing. This is the first time DSOW cyclones have been observed in the Labrador Sea—a discovery that could have important implications for interior stratification. 

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2. Extending Residual-Mean Overturning Theory to the Topographically Localized Transport in the Southern Ocean

   https://doi.org/10.1175/JPO-D-22-0217.1  

   Youngs, Madeleine K. ; Flierl, Glenn R. ( August 2023 , Journal of Physical Oceanography)

   The Southern Ocean plays a major role in global air–sea carbon fluxes, with some estimates suggesting it contributes to up to 40% of the oceanic anthropogenic carbon dioxide uptake, despite only comprising about 20% of oceanic surface area. Thus, the Southern Ocean overturning, the circulation that transports tracers between the surface and deep ocean interior, is particularly important for climate. Recent studies show that vertical velocities and tracer transport are largest just downstream of bottom topography; these quantities are related to the overturning, but provide incomplete information about the net Lagrangian transport, usually described with the residual-mean theory in a zonally integrated sense. This study uses an idealized Southern Ocean–like channel model with particle tracking to visualize the thickness-weighted velocities that capture the net overturning transport of a parcel, connecting residual-mean overturning theory to the three-dimensional, localized nature of the overturning. From this, we split the flow into three main drivers of transport: a wind-driven Ekman pumping into or out of a density layer, and standing eddies and transient eddies, both of which are localized near the topography. In this framework, the three-dimensional overturning circulation is not a small residual between the eddy and Eulerian-mean transport. The existence of a ridge weakens the response of the overturning to changes in wind, especially in the lower cell. This local understanding of the overturning framework suggests that careful modeling and sampling of specific regions near topography in the Southern Ocean are vital to understand climate sensitivity, transport, carbon export, and connections with the oceans to the north.

    

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3. Structure and dynamics of mesoscale eddies over the Laptev Sea continental slope in the Arctic Ocean

   https://doi.org/10.5194/os-14-1329-2018  

   Pnyushkov, Andrey ; Polyakov, Igor V. ; Padman, Laurie ; Nguyen, An T. ( January 2018 , Ocean Science)

   Abstract. Heat fluxes steered by mesoscale eddies may be a significant, but still notquantified, source of heat to the surface mixed layer and sea ice cover inthe Arctic Ocean, as well as a source of nutrients for enhancing seasonalproductivity in the near-surface layers. Here we use 4 years (2007–2011)of velocity and hydrography records from a moored profiler over the LaptevSea slope and 15 months (2008–2009) of acoustic Doppler current profilerdata from a nearby mooring to investigate the structure and dynamics ofeddies at the continental margin of the eastern Eurasian Basin. Typical eddyscales are radii of the order of 10 km, heights of 600 m, andmaximum velocities of ∼0.1 m s−1. Eddies areapproximately equally divided between cyclonic and anticyclonicpolarizations, contrary to prior observations from the deep basins and alongthe Lomonosov Ridge. Eddies are present in the mooring records about 20 %–25 % of the time,taking about 1 week to pass through the mooring at anaverage frequency of about one eddy per month. We found that the eddies observed are formed in two distinct regions – near FramStrait, where the western branch of Atlantic Water (AW) enters the ArcticOcean, and near Severnaya Zemlya, where the Fram Strait and Barents Seabranches of the AW inflow merge. These eddies, embedded in the ArcticCircumpolar Boundary Current, carry anomalous water properties along theeastern Arctic continental slope. The enhanced diapycnal mixing that wefound within EB eddies suggests a potentially important role for eddies inthe vertical redistribution of heat in the Arctic Ocean interior. 

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4. Rotating horizontal convection

   https://doi.org/10.1017/jfm.2013.136  

   Barkan, Roy ; Winters, Kraig B. ; Llewellyn Smith, Stefan G. ( May 2013 , Journal of Fluid Mechanics)

   null (Ed.)

   Abstract ‘Horizontal convection’ (HC) is the generic name for the flow resulting from a buoyancy variation imposed along a horizontal boundary of a fluid. We study the effects of rotation on three-dimensional HC numerically in two stages: first, when baroclinic instability is suppressed and, second, when it ensues and baroclinic eddies are formed. We concentrate on changes to the thickness of the near-surface boundary layer, the stratification at depth, the overturning circulation and the flow energetics during each of these stages. Our results show that, for moderate flux Rayleigh numbers ( $O(1{0}^{11} )$ ), rapid rotation greatly alters the steady-state solution of HC. When the flow is constrained to be uniform in the transverse direction, rapidly rotating solutions do not support a boundary layer, exhibit weaker overturning circulation and greater stratification at all depths. In this case, diffusion is the dominant mechanism for lateral buoyancy flux and the consequent buildup of available potential energy leads to baroclinically unstable solutions. When these rapidly rotating flows are perturbed, baroclinic instability develops and baroclinic eddies dominate both the lateral and vertical buoyancy fluxes. The resulting statistically steady solution supports a boundary layer, larger values of deep stratification and multiple overturning cells compared with non-rotating HC. A transformed Eulerian-mean approach shows that the residual circulation is dominated by the quasi-geostrophic eddy streamfunction and that the eddy buoyancy flux has a non-negligible interior diabatic component. The kinetic and available potential energies are greater than in the non-rotating case and the mixing efficiency drops from ${\sim }0. 7$ to ${\sim }0. 17$ . The eddies play an important role in the formation of the thermal boundary layer and, together with the negatively buoyant plume, help establish deep stratification. These baroclinically active solutions have characteristics of geostrophic turbulence. 

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5. The Deep Western Boundary Current and Adjacent Interior Circulation at 24°–30°N: Mean Structure and Mesoscale Variability

   https://doi.org/10.1175/JPO-D-20-0094.1  

   Biló, Tiago Carrilho ; Johns, William E. ( September 2020 , Journal of Physical Oceanography)

   null (Ed.)

   Abstract The mean North Atlantic Deep Water (NADW, 1000 < z < 5000 m) circulation and deep western boundary current (DWBC) variability offshore of Abaco, Bahamas, at 26.5°N are investigated from nearly two decades of velocity and hydrographic observations, and outputs from a 30-yr-long eddy-resolving global simulation. Observations at 26.5°N and Argo-derived geostrophic velocities show the presence of a mean Abaco Gyre spanning the NADW layer, consisting of a closed cyclonic circulation between approximately 24° and 30°N and 72° and 77°W. The southward-flowing portion of this gyre (the DWBC) is constrained to within ~150 km of the western boundary with a mean transport of ~30 Sv (1 Sv ≡ 10 6 m 3 s −1 ). Offshore of the DWBC, the data show a consistent northward recirculation with net transports varying from 6.5 to 16 Sv. Current meter records spanning 2008–17 supported by the numerical simulation indicate that the DWBC transport variability is dominated by two distinct types of fluctuations: 1) periods of 250–280 days that occur regularly throughout the time series and 2) energetic oscillations with periods between 400 and 700 days that occur sporadically every 5–6 years and force the DWBC to meander far offshore for several months. The shorter-period variations are related to DWBC meandering caused by eddies propagating southward along the continental slope at 24°–30°N, while the longer-period oscillations appear to be related to large anticyclonic eddies that slowly propagate northwestward counter to the DWBC flow between ~20° and 26.5°N. Observational and theoretical evidence suggest that these two types of variability might be generated, respectively, by DWBC instability processes and Rossby waves reflecting from the western boundary. 

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