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Sijia Zou
,
Amy S. Bower
,
Heather Furey
,
Robert S. Pickart
,
Loïc Houpert
, and
N. Penny Holliday

Abstract

Recent mooring measurements from the Overturning in the Subpolar North Atlantic Program have revealed abundant cyclonic eddies at both sides of Cape Farewell, the southern tip of Greenland. In this study, we present further observational evidence, from both Eulerian and Lagrangian perspectives, of deep cyclonic eddies with intense rotation (ζ/f > 1) around southern Greenland and into the Labrador Sea. Most of the observed cyclones exhibit strongest rotation below the surface at 700–1000 dbar, where maximum azimuthal velocities are ~30 cm s−1 at radii of ~10 km, with rotational periods of 2–3 days. The cyclonic rotation can extend to the deep overflow water layer (below 1800 dbar), albeit with weaker azimuthal velocities (~10 cm s−1) and longer rotational periods of about one week. Within the middepth rotation cores, the cyclones are in near solid-body rotation and have the potential to trap and transport water. The first high-resolution hydrographic transect across such a cyclone indicates that it is characterized by a local (both vertically and horizontally) potential vorticity maximum in its middepth core and cold, fresh anomalies in the deep overflow water layer, suggesting its source as the Denmark Strait outflow. Additionally, the propagation and evolution of the cyclonic eddies are illustrated with deep Lagrangian floats, including their detachments from the boundary currents to the basin interior. Taken together, the combined Eulerian and Lagrangian observations have provided new insights on the boundary current variability and boundary–interior exchange over a geographically large scale near southern Greenland, calling for further investigations on the (sub)mesoscale dynamics in the region.

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Hartmut Peters
,
William E. Johns
,
Amy S. Bower
, and
David M. Fratantoni

Abstract

When the salty and heavy water of the Red Sea exits from the Strait of Bab el Mandeb, it continues downslope into the Gulf of Aden mainly along two channels. The 130-km-long “Northern Channel” (NC) is topographically confined and is typically only 5 km wide. In it, the Red Sea plume shows unanticipated patterns of vertical structure, turbulent mixing, and entrainment. Above the seafloor a 25–120-m-thick weakly stratified layer shows little dilution along the channel. Hence this bottom layer undergoes only weak entrainment. In contrast, a 35–285-m-thick interfacial layer shows stronger entrainment and is shown in a companion paper to undergo vigorous turbulent mixing. It is thus the interface that exhibits the bulk of entrainment of the Red Sea plume in the NC. The interfacial layer also carries most of the overall plume transport, increasingly so with downstream distance. The “Southern Channel” (SC) is wider than the NC and is accessed from the latter by a sill about 33 m above the floor of the NC. Entrainment into the bottom layer of the SC is diagnosed to be strong near the entry into the SC such that the near-bottom density and salinity are smaller in the SC than in the NC at the same distance from Bab el Mandeb. In comparison with winter conditions, the authors encountered weaker outflow with shallower equilibration depths during the summer cruise. Bulk Froude numbers computed for the whole plume varied within the range 0.2–1. Local maxima occurred in relatively steep channel sections and coincided with locations of significant entrainment.

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Amy S. Bower
,
William E. Johns
,
David M. Fratantoni
, and
Hartmut Peters

Abstract

Hydrographic, direct velocity, and subsurface float observations from the 2001 Red Sea Outflow Experiment (REDSOX) are analyzed to investigate the gravitational and dynamical adjustment of the Red Sea Outflow Water (RSOW) where it is injected into the open ocean in the western Gulf of Aden. During the winter REDSOX cruise, when outflow transport was large, several intermediate-depth salinity maxima (product waters) were formed from various bathymetrically confined branches of the outflow plume, ranging in depth from 400 to 800 m and in potential density from 27.0 to 27.5 σθ , a result of different mixing intensity along each branch. The outflow product waters were not dense enough to sink to the seafloor during either the summer or winter REDSOX cruises, but analysis of previous hydrographic and mooring data and results from a one-dimensional plume model suggest that they may be so during wintertime surges of strong outflow currents, or about 20% of the time during winter. Once vertically equilibrated in the Gulf of Aden, the shallowest RSOW was strongly influenced by mesoscale eddies that swept it farther into the gulf. The deeper RSOW was initially more confined by the walls of the Tadjura Rift, but eventually it escaped from the rift and was advected mainly southward along the continental slope. There was no evidence of a continuous boundary undercurrent of RSOW similar to the Mediterranean Undercurrent in the Gulf of Cadiz. This is explained by considering 1) the variability in outflow transport and 2) several different criteria for separation of a jet at a sharp corner, which indicate that the outflow currents should separate from the boundary where they are injected into the gulf.

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Heather H. Furey
,
M. Femke de Jong
,
James R. Valdes
, and
Amy S. Bower

Abstract

Two submerged autonomous launch platforms (SALPs) were deployed at 500-m depth on a deep-water mooring in the northeastern Labrador Sea from 2007 to 2009 to automatically release profiling floats into passing warm-core anticyclonic Irminger Rings (IRs). The objective was to investigate the rings’ vertical structure and evolution as they drifted from their formation site near the western coast of Greenland to the area of deep convection in the south-central part of the basin. Mechanically and electronically, the SALP worked well: 10 out of 11 floats were successfully released from the mooring over 2 years. However, getting floats trapped in eddy cores using a preprogrammed release algorithm based on temperature and pressure (a proxy for current speed) measured by the SALPs met with limited success mainly because 1) the floats settled at a park pressure that was initially too deep, below the volume of water trapped in the eddy core; 2) the eddies translated past the mooring much more quickly than anticipated; and 3) there is a seasonal cycle in both background and eddy core temperature that was not known a priori and therefore not accounted for in the release algorithm. The other mooring instruments (100–3000 m) revealed that 12 anticyclones passed by the mooring in the 2-yr monitoring period. Using this independent information, the authors assessed and improved the release algorithm, still based on ocean conditions measured at one depth, and found that much better performance could have been achieved with an algorithm that allowed for faster eddy translation rates and the seasonal temperature cycle.

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M. Susan Lozier
,
Sheldon Bacon
,
Amy S. Bower
,
Stuart A. Cunningham
,
M. Femke de Jong
,
Laura de Steur
,
Brad deYoung
,
Jürgen Fischer
,
Stefan F. Gary
,
Blair J. W. Greenan
,
Patrick Heimbach
,
Naomi P. Holliday
,
Loïc Houpert
,
Mark E. Inall
,
William E. Johns
,
Helen L. Johnson
,
Johannes Karstensen
,
Feili Li
,
Xiaopei Lin
,
Neill Mackay
,
David P. Marshall
,
Herlé Mercier
,
Paul G. Myers
,
Robert S. Pickart
,
Helen R. Pillar
,
Fiammetta Straneo
,
Virginie Thierry
,
Robert A. Weller
,
Richard G. Williams
,
Chris Wilson
,
Jiayan Yang
,
Jian Zhao
, and
Jan D. Zika

Abstract

For decades oceanographers have understood the Atlantic meridional overturning circulation (AMOC) to be primarily driven by changes in the production of deep-water formation in the subpolar and subarctic North Atlantic. Indeed, current Intergovernmental Panel on Climate Change (IPCC) projections of an AMOC slowdown in the twenty-first century based on climate models are attributed to the inhibition of deep convection in the North Atlantic. However, observational evidence for this linkage has been elusive: there has been no clear demonstration of AMOC variability in response to changes in deep-water formation. The motivation for understanding this linkage is compelling, since the overturning circulation has been shown to sequester heat and anthropogenic carbon in the deep ocean. Furthermore, AMOC variability is expected to impact this sequestration as well as have consequences for regional and global climates through its effect on the poleward transport of warm water. Motivated by the need for a mechanistic understanding of the AMOC, an international community has assembled an observing system, Overturning in the Subpolar North Atlantic Program (OSNAP), to provide a continuous record of the transbasin fluxes of heat, mass, and freshwater, and to link that record to convective activity and water mass transformation at high latitudes. OSNAP, in conjunction with the Rapid Climate Change–Meridional Overturning Circulation and Heatflux Array (RAPID–MOCHA) at 26°N and other observational elements, will provide a comprehensive measure of the three-dimensional AMOC and an understanding of what drives its variability. The OSNAP observing system was fully deployed in the summer of 2014, and the first OSNAP data products are expected in the fall of 2017.

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