Detection of Mesoscale Sea Ice Vortices and Sea
Ice Model Validation
Mani V. Thomas, Chandra Kambhamettu, Andrew Roberts, Jennifer K. Hutchings, Cathleen A. Geiger
Sea Ice Motion Tracking System
Abstract
From 30 March to April 5, 2007, during the Applied Physics Laboratory Ice
Station (APLIS), in the Beaufort Sea, a high spatial resolution (400m) sea ice
motion tracking system was used to detect and track a rotational feature with
a 60 km radius in the field of sea ice motion relative to APLIS. At the start
of the time period, the ice pack motion field contained weak discontinuities,
and showed an isotropic rotational motion. Around April 5, synoptic changes
led to a change in direction of the rotation relative to APLIS. We ground
truth against in-situ GPS buoy drift, finding qualitative agreement of the
rotational feature in the buoy motion. A simulation of ice motion in the
Beaufort Sea with an ice-ocean model, which displayed coherence between
wind and ice drift, shows similar large scale drift patterns to those observed.
This model is used to investigate the relationship between ice drift vorticity
and wind forcing. Two possible explanations of the rotational features are:
That they are either small scale solid body rotation of individual sea ice floes
or regional sea ice vorticity. Our results suggest rotation may be locally driven
by deformation or regionally driven by wind. The emergent mechanism, at
a given time, depends on an interaction between wind stress and boundary
conditions.
Synoptic Activity During APLIS 2007
Fig. 1: [top] Time series of surface pressure at APLIS07. [middle
panels] Strain rate invariants estimated from the buoy triad shown in
fig. 2, following Hutchings & Rigor
(2008). [bottom] Area of this buoy
array. Dashed lines are approximate
times of RADARSat overpasses in fig.
5.
Fig. 5: Global and local motion time series, derived from four RADARSat ScanSAR B images (as Thomas et al 2008). Upper four panels are time series from 28 March to 7 April
2007 of sea ice motion during APLIS07 from an Eulerian frame of reference. Lower panels cover the same time period but in Lagrangian reference frame relative to the drifting ice
camp. Green circles denote GPS buoys and red dots/lines denote discontinuities in the flow field.
Motion in East−West direction (u)
4
1
5
A high pressure system developed over APLIS07
on March 30th. A day later a large shear event
occurred (fig. 1), opening the lead system in fig.
2, 3 and 5 (panel 3). After April 1 this weather
system moves eastward, which led to opening of
the leads. On April 5 a low system moves into the
region, with northly winds over the camp, leading
to a reorganization of leads and aggregate plates
between the leads (fig 5 (panel 4)). During March
30 to April 5 a vortex of relative ice motion is
apparent around the ice camp. Note in fig. 2 differential buoy drift agrees with SAR Lagrangian
velocity estimates, indicating vortex is not an artifact of SAR processing.
Fig. 3: MODIS image showing leads that opened on March
31. The discontinuity that formed in our buoy triad is
associated with this event
r: 0.898, (p < 0.001)
Data points: 324
Estimated Motion (cm/s)
Fig. 2: Buoy triad, used in fig.
shown with yellow lines. See fig.
for further explanation.
Estimated Motion (cm/s)
3
6
2
1
0
−1
−2
−3
−4
−4
−3
−2
−1
0
1
GPS Motion (cm/s)
2
3
4
4
Motion in North−South direction (v)
r: 0.957, (p < 0.001)
Data points: 324
Ground Truth
2
0
−2
−4
−6
−6
−4
−2
0
2
GPS Motion (cm/s)
4
6
Fig. 6: Scatter plots between estimated motion and ground truth.
Due to good agreement between
SAR estimated motion and buoy
drift (Thomas 2008) we have
confidence that the motion product in fig. 5 represents synoptic
scale ice drift.
• Ice motion is organised as rigid plates seperated by linear regions of
deformation.
• The plate the ice camp is located on experiences anti-clockwise rigid
body rotation.
• Cyclonic circulation is experienced throughout the entire region plotted after April 4th.
Sea Ice Drift Simulation
Fig. 4: Motion, relative to ice camp, of buoy triad. Note the rigid behavior preceding the
shear event on March 31, suggesting vortex is deformation driven.
Acknowledgements Funded by the National Science Foundation.
The U.S. Navy’s Arctic Submarine Laboratory provided access to APLIS07.
Many thanks to Fred Karig and the APL team who ran APLIS07. The Alaska
Satellite Facility and the National Ice Center facilitated near-real-time transfer of RADARSat 1 ScanSAR-B imagery provided by the Canadian Space
Agency. Many thanks to Melanie Engram and Pablo Clemente-Colon for their
assistance. NASA and ASF provided a full year of SAR covering the entire Beaufort Sea. The International Arctic Buoy Program archives SEDNA
buoy data. Nick Hughes (Norsk Polar Institute) provided the MODIS image. Tim Wen (APL) provided surface pressure data. NCEP Reanalysis
2 data provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA,
http://www.cdc.noaa.gov
References
Hibler, III, W. D., A. Roberts, P. Heil, A. Proshutinsky, H. Simmons, J.
Lovick and J. Hutchings (in prep) Modeling tidal and inertial variability in
sea-Ice drift and deformation.
Hutchings, J. K. and I. G. Rigor (2008), Mechanisms explaining anomalous
ice conditions in the Beaufort Sea during 2006 and 2007. Submitted to J.
Geophys. Res.
M. V. Thomas (2008), Analysis of Large Magnitude Discontinuous Non-rigid
Motion, Ph.D. Dissertation, University of Delaware.
M. V. Thomas, C. A. Geiger and C. Kambhamettu (2008), High resolution (400 m) motion characterization of sea ice using ERS-1 SAR
imagery, Cold Regions Science and Technology, 52(2), 207–223, DOI:
10.1016/j.coldregions.2007.06.006
Fig. 7: [top] The ice-ocean model is a parallel version of the Hibler et al (in prep) ice-tide model. The model has been forced with geostrophic surface winds derived from NCEP2
mean sea level pressure. The resultant wind field has been anti-aliased with a pass band accepting waves greater than 650km, and a transition band extending to 200km. The data has
also been low-pass filtered-in-time to remove a resonant ice-ocean boundary layer frequency with the pass band starting at 18.5 hours, and a transition band that includes waves down
to 13.5 hours. This filtering ensures that the synoptic response of the model has minimal noise amplifications which can occur in high frequency sea ice models. This filtering also
provides good coherence between the model and SEDNA drifters in the synoptic band (see Fig. 8). The top panels are NCEP2 Mean Sea Level Pressure overlaid on mean model ice
velocities for the time periods in Figure 5 with the SEDNA buoy array tracks for March 28 to April 15 (box). The lower panel provides the corresponding vorticity of the ice velocity
and the wind field used to drive the model.
Summary
fig.
8: Rotary coherence of
the model ice velocity verses a
SEDNA-deployed buoy for late
March to mid July, 2007. High
inner- and low outer-coherence
in the synoptic band (-0.25 to
0.25 cycles/day) indicate modeled ice vorticity is useful for interpreting broad pack rotations
such as those seen in the final
panel of Figure 5, which matches
a zone of relatively high anticlockwise vorticity (Figure 8(h))
over the buoy array that preceeded passage of a trough over
the SEDNA field area on April
8.
• We find agreement between SAR and buoy drift.
• Rotation in relative ice drift is bounded by discontinuities in the
velocity field. Aggregate plates display rigid motion.
• Model results indicate anti-clockwise rotation, March 30 - April 4 is
locally driven by deformation. After April 5, cyclonic motion is wind
driven.
• Coherence between model and buoy drift indicate the model is showing the correct rotational sense.
• i.e. whether rotation is confined to local plates or across larger scales
depends on the ice interaction, wind stress and boundary conditions.
Further work needs to be done to verify if motion within plates is resolved by
the motion tracking system.