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Lagrangian pathways and transformation of the Denmark Strait Overflow Water
in the Irminger Basin
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Inga Koszalka*, T. W. N. Haine* and M. G. Magaldi
*Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD, + ISMAR-CNR, Lerici, Italy
Abstract
Dense water pathways revealed by the Lagrangian study
Results
Our study extends the conceptual view of the DSO in the Irminger Basin, and urges observational campaigns to
verify these results and monitor the pathways of the different DSO components.
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t=1 DAY
t=8 DAY
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LATITUDE
LATITUDE
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DSO
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SILL
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Spill Jet
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Snapshots of particle positions at day 1, 8 and 22 projected onto a horizontal plane.
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SPILL JET SECTION
The Denmark Strait Overflow (DSO) supplies one third of the North Atlantic Deep Water and is a key component
of the global thermohaline circulation. Knowledge of the pathways of DSO through the Irminger Basin and its
transformation there is still incomplete however. The Lagrangian framework is ideal for addressing these questions, but no Lagrangian observations of DSO exist. Instead, its evolution has been derived solely from sparse
Eulerian (fixed-point) measurements. To fill this gap we use a high resolution ocean model and deploy over 10,000
Lagrangian particles in DSO at the Denmark Strait to study its pathways and transformation in the Irminger Basin.
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ANGMAGSSALIK SECTION
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Longitude
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HORIZONTAL EDDY DIFFUSIVITY [m 2/s]
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M(sill)=5d
M(shelf)=11d
M(kanger)=(multimodal)
Work in progress: Mixing processes
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KANGER
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M(sill)=13d
M(shelf)=18d
M(kanger)=(multimodal)
Longitude
SILL
SHELF
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NV
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NT
NA
The DSO pathways derived from hydrographic observations before our study (left) and the view emerging from
trajectories of the Lagrangian particles (right). The distribution of dense particles in the domain is shown in gray.
At the Denmark Strait, dense water is found in the sill (blue), on the adjacent shelf (cyan), and in the Kangerdlugssuaq Trough (KT; red). Over 60 days these different water masses spread according to the arrows. There is
cyclonic recirculation in the KT and anticyclonic recirculation on the Dohrn Bank (DB). Some of this recirculating
water spills over the continental shelf break.
Travel times
Motivation
DSO
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VERTICAL EDDY DIFFUSIVITY [m /s]
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Methods
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Numerical Lagrangian particles
• Trajectories integrated with model 3D velocities offline using MATLAB (ode23t) and dt=15min
• Boundary conditions - ’wall-sliding’
• Velocity & tracers linearly interpolated in space
and time on particle positions
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TRANSIT TIME [DAYS]
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60
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TRANSIT TIME [DAYS]
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Transit time distributions from the Denmark Strait to the Spill Jet and the Angmagssalik sections. The modal
transit times for SILL particles to these sections are 5-6 days and 2-3 weeks, respectively. The SHELF particles
recirculate before spilling over the shelf break and joining the sill overflow; their modal transit is longer by about a
week. The KANGER particles recirculate in the DS for several weeks, their transit time distributions are broad.
Ensemble-mean density evolution and comparison with the data
σθ (kg/m3)
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SJ
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ANGM
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Distance from DSE (km)
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Evolution of the mean particle density with distance from the DSE for the three deployments groups (SILL, SHELF
and KANGER) as well as dense particles recorded over a seabed deeper than 600 m (SLOPE). Superimposed
with circles are the observations from Girton & Sanford (2003) that should be compared to SLOPE particles.
DENSITY TRANSFORMATION LOCATIONS
POTENTIAL DENSITY AT THE SJ SECTION
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Dσθ/Dt <-0.025
Dσθ/Dt <-0.05
Dσθ/Dt <-0.1
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Depth [m]
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• The strongest transformation occurs: 1) upon the DSO cascading the sill (with IC/AW), and 2) at dense water
spilling locations along the shelf break. Following the transformation, the DSO particles exhibit a wide spectrum
of densities in the Irminger Basin.
• Mixing with PSW causes the transformation on the shelf and may be a mechanism to transmit the FW variability
to the Atlantic.
• KANGER and SHELF releases contribute to the pool of dense particles at Angmagssalik with 25%. The particles released on the shelf are separated from mixing with IC, thus preserving their densities and contributing to
the densest water at the SJ section.
• Planning an observational study to corroborate these model results.
KANGER
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SHELF
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SILL
A publication with results presented here has been submitted to J. Phys. Oceanography.] The draft is available at
blaustein.eps.jhu.edu/∼koszalka/:
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The paper
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• The modal travel times for the sill-DSO are 5 days to the SJ section and 2-3 weeks to Angmagssalik. Particles
seeded and recirculating on the shelf arrive one week later, and the dense waters from KT spill near SJ section
at ∼ 25-day intervals.
Water property transformation
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• Pathways of dense waters on the shelf are mapped providing context for hydrographic observations. They reveal importance of the anticyclonic recirculation on the Dohrn Bank (DB) and the cyclonic flow in the Kangerdlugssuaq Trough (KT).
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LATITUDE
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Conclusions
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Left: Occurrence frequency of model dense waters (σθ ≥27.8) in the Denmark Strait/East Greenland Shelf
(EGS)/Irminger Basin (IB). Superimposed are mean dense water current vectors. The hydrographic sections
are: Denmark Strait Extended (DSE), DS Sill, Spill Jet (SJ) and the Angmagssalik array (ANGM). The Dohrn
Bank (DB) and the Kangerdlugssuaq Trough (KT) are marked. Right: A snapshot density along DSE with marked
particle deployments. The particles were released along the section in (σθ ≥27.8) layer in 10 sets separated by
12h, starting 7/1. They are classified according to the release site in 3 groups: SILL, SHELF and KANGER.
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Horizontal and vertical eddy diffusivities estimated from particle velocities using a recently proposed clustering
method (Koszalka & LaCasce, 2010). The horizontal diffusivities are elevated on the shelf and downstream off
the SJ section. The highest vertical diffusivities co-locate with the dense water crossing the sill and spilling off the
shelf break.
DOWNSTREAM EVOLUTION OF THE MEAN POTENTIAL DENSITY ON PARTICLES
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obs (GS2003)
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dense-KANGER
dense-SHELF
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dense-SILL
dense-SLOPE
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Model dense waters and the particle deployments
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LATITUDE
• Highest spatial resolution to date of Irminger
Basin: dx ∼ 2km, max dz=15m (210 layers)
• Simulation period: summer 2003 (7/1 - 9/1)
• 3rd tracer advection with implicit diffusion KPP for
vertical diffusion, Leith horizontal viscosity
• Forcing by NCEP fluxes & SeaWinds
• Model validated with observations (Magaldi etal
2011)
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Model - MITgcm
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KT
Spill Jet
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AFTER
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Latitude
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BEFORE
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Latitude
The DSO particles
LATITUDE
We deploy over 10,000 Lagrangian particles at the Denmark Strait in a high resolution ocean model to study
pathways and transformation of the Denmark Strait Overflow (DSO) in the Irminger Basin. Their trajectories show
that:
(–) The mean position and density of dense waters cascading over the Denmark Strait sill evolve consistently with
hydrographic observations. The sill particles transit the Irminger basin to the Spill Jet section (65.25N) in 5-7 days
and to the Angmagssalik section (63.5N) in 2-3 weeks.
(–) There are distinct dense water pathways on the continental shelf. Particles can circulate on the shelf for several
weeks before they spill off the shelf break, join the overflow from the sill and contribute significantly to the dense
water overflow downstream (∼ 25%).
(–) The density on particle trajectories decreases rapidly due to mixing with Atlantic waters along the continental
slope. Intense mixing with the Polar Water occurs on the shelf. As a result, the overflow particles exhibit a wide
range of densities downstream. Notably, the densest waters in the Irminger Basin originate from the shelf adjacent
to the sill, not from the sill itself.
(–) The horizontal eddy diffusivities are largest on the shelf and downstream off the Spill Jet section. The vertical
diffusivities are largest at the sill and at spilling locations along the shelf break.
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Distance from the shelf break (km)
Left: Particle positions when density transformation rate is high; Dσθ /Dt ≤ -0.025, -0.05 and -0.1 kg m−3 day −1.
Along the slope (spilling locations) the transformation is high due to mixing with warm and salty Atlantic Waters.
On the shelf the freshening suggests mixing mixing with Polar Waters. Right: Average σθ on particles at the SJ
section. The inset shows the dominant contributions from the deployment sets to the bins with hσθ i ≥ 27.8. Note
significant transformation to intermediate waters (gray bins) and that the SHELF particles dominate in bins with
densest & deepest waters.
Contact: Inga Koszalka ([email protected])
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