An Examination of Halocline Eddies Across the Arctic Ocean
[1]
Zhao ,
[1]
Timmermans ,
[2]
Toole ,
[2]
Proshutinsky ,
[2]
Krishfield
Mengnan
Mary-Louise
John
Andrey
Rick
[1]Yale university, Department of Geology and Geophysics [2] Woods Hole Oceanographic Institution
Abstract
Eddy Types
Ice-Tethered Profilers (ITPs), deployed in the Arctic’s Eurasian and
Canadian basins between 2004 and 2013, have provided detailed
temperature and salinity measurements of an extensive collection of
halocline eddies. Both anticylcones and cyclones are sampled in the
upper 300 m of the water column; their distribution and properties can
shed light on the dynamics of the water masses from which they
originate. Approximately 120 eddies have been detected, with
anticyclones comprising 90% of these. The majority of anticyclones
have anomalously cold cores. Horizontal scales are on the order of the
Rossby deformation radius (around 10 km). Maximum azimuthal
speeds are between 10 and 30 cm/s. In general, the upper waters of the
Canadian Basin have a more prominent eddy field than the upper
Eurasian Basin waters.
Warm-core anticyclonic eddy (ITP64, profile 55)
B
A
Cyclonic eddy (ITP14, profile 49)
A
http://www.whoi.edu/website/itp/
return profiles of temperature, salinity,
and in some cases velocity, from just under the
supporting ice floe to about 750 m depth.
Horizontal and vertical data resolution are of the
order of a few kilometers and 25 cm, respectively.
Number of ITP profiles per year
returned from the Canadian Basin (CB)
and the Eurasian Basin (EB).
7000
6000
5000
4000
CB
EB
3000
2000
1000
0
2004 2005 2006 2007 2008 2009 2010 2011 2012 2013
Eddy Detection and Characterization
thickness
diameter
core position
Cold-core anticyclonic eddy (ITP41, profile 37)
𝑣2
1 𝜕𝑝
+ 𝑓𝑣 −
=0
𝑟
𝜌0 𝜕𝑟
Ice-Tethered Profiler Measurements
ITPs[1][2]
Eddy Azimuthal Velocity
core
depth
Eddy Detection
Eddies are detected in ITP profiles based on the presence of anomalously
low buoyancy frequency (in the case of anticyclones) and the
characteristic convex-shaped isopycnals. The presence of an eddy is only
confirmed if the ITP drift does not reverse direction as it transits an
apparent eddy core.
Eddy Characterization
Eddy core positions are determined to be the center of convex-shaped
isopycnal displacements (left panel). Eddy core depths are defined to be
the depth of the minimum potential temperature through the core (middle
panel). Eddy thickness is defined to be the difference between the two
buoyancy frequency maxima above and below the core. Eddy diameters
are defined as the distance between the two maximum azimuthal
velocities on either side of the core (right panel).
B
Cyclogeostrophic velocity calculated through a typical eddy
and measured velocity[3] through an eddy (left panels) indicates
that the cyclogeostrophic balance is an appropriate
representation. Eddy velocity through the eddy core (right
panel) suggests that the eddy approximates a Rankine Vortex
model[4].
C
Eddy Properties
PDFs of primary eddy parameters
C
Sections of potential temperature, salinity and buoyancy frequency showing the
range of eddy types in the upper 300 m transected by ITPs. Cold-core
anticyclonic eddies are the most prevalent type. The eddies shown here are
marked by A, B, and C in the map.
Distribution of Eddies
Eddy Core Depth Distribution
Canadian Basin:
Core depth < 60m
60m < Core depth < 120m
Core depth > 120m
Eurasian Basin:
Core depth < 100m
Core depth > 100m
104 cold-core anticyclonic
eddies were detected from
2004 to 2013, 85% are in the
Canadian Basin. Eddies are
most prevalent in the
Beaufort Gyre region and in
the vicinity of the Transpolar
Drift Stream, although there
is some bias due to the higher
ITP sampling in these regions.
In the Canadian Basin, eddies
are observed in all halocline
layers – the near surface
temperature max. & the
remnant winter mixed layer (above ~60m), the Pacific summer water layer
(from ~60m to ~120m), and the Pacific winter water layer (below ~120m). The
Eurasian Basin does not display the same eddy-rich halocline, and most eddies
are observed to lie at the top boundary of the inflowing Atlantic water layer.
TS diagram of eddy
core properties
(black: Canadian
Basin, red: Eurasian
Basin); most eddy
cores have a nearfreezing temperature.
Depth: Depth shows a tri-modal distribution, centering around
50m, 80m and 140m, representing eddies in the 3 water layers.
It is of note that deeper eddies are thicker, consistent with a
decrease in stratification with depth.
Radius: Approximating the Arctic Ocean as a two-layer system,
𝟐
the Rossby deformation radius 𝑹𝒅 =
𝒈′ 𝒉
𝒇
≈ 𝟏𝟎𝒌𝒎. The radius
of eddies detected is on the order of Rossby deformation radius.
Velocity: Typical eddy maximum azimuthal velocities range
from 0.1m/s to 0.3m/s.
Rossby Number: The median Rossby number of eddies is 0.42;
the cyclogeostrophic balance is appropriate.
Summary
Several eddy-types were detected in ITP data, with cold-core
anticyclonic eddies being the most prevalent type; statistics are
shown here for the 104 cold-core anticyclones detected from
2004 to 2013. Eddies are observed in all layers of the halocline
in the Canadian Basin, which shows a much richer eddy field
than the Eurasian Basin. Future work will examine the
formation mechanisms of eddies in both basins as well as their
implications to circulation and dynamics in the Arctic Ocean.
References
[1] Toole, J.M. et al., 2011. The Ice-Tethered Profiler: Argo of the Arctic.
Oceanography.
[2] Krishfield, R. et al., 2008. Automated Ice-Tethered Profilers for seawater
observations under pack ice in all seasons. JAOTech.
[3] Cole, S. et al., 2014. Ekman veering, internal waves, and turbulence
observed under Arctic sea-ice. JPO
[4] Timmermans, M.-L., et al., 2008. Eddies in the Canada Basin, Arctic
Ocean, observed from Ice-Tethered Profilers. JPO