Atmospheric and Oceanic forcing of sea ice drift
and deformation during the SEDNA field campaign
Andrew Roberts*1,2, Jennifer Hutchings2, W. D. Hibler III2 and Mark Seefeldt3
Introduction
2
1
Figure 3
3
This is a synopsis of the surface atmospheric and oceanic forcing of
sea ice drift and deformation during the Sea ice Experiment:
Dynamical Nature of the Arctic (SEDNA) Beaufort Sea campaign in
April 2007. During the campaign two hexagonal arrays of GPS
buoys were deployed with diameters of 10km and 70km,
respectively, providing an excellent record of sea ice drift and
deformation (Figure 1). The record was analyzed in light of in situ
surface wind and current measurements, Prudhoe Bay sea level, an
ice-tide model, high resolution Weather Research and Forecast
Model (WRF) simulations focused on the Beaufort Sea, and NCEP
atmospheric analyses (Figures 2, 3 and 4). Analysis of the drift
track in the time domain indicates tidally-synchronous ice drift,
however rotary spectra of the drift indicate semi-diurnal oscillations
are clockwise (Figure 5), and therefore mostly inertial, because tidal
power is concentrated in the counter clockwise direction at this
location. Given that the SEDNA field camp was positioned between
73.19 and 73.37oN (close to the coincidence of inertial and M2 tidal
frequencies at ~74.5oN), this result suggests semi-diurnal
oscillations in sea ice drift are tidally amplified inertial peaks. Sea
ice deformation is analyzed with additional information from buoys
deployed in August 2006 as part of the Sea Ice Tide-Inertial
Interaction (SITII) project. Results from this array of buoys, in
addition to the SEDNA arrays suggest spatially scaled deformation
in response to both semi-diurnal forcing and the passage of a storm
over the pack field camp between April 7 and 9, 2007.
AGU 2007 C11B-0435
0000Z NCEP Mean Sea Level Pressure
reanalysis and WRF 10m winds during
passage of a cyclonic system over the ice
camp (marked in red) causing spatially scaled
sea ice deformation (see ice shear and
divergence in Figure 2, April 8-9). One vector
represents two grid points on the WRF limited
area domain.
Figure 4
4
5
Rotary spectra of sea ice drift from the Hibler et
al (2006) ice-tide model at the location of the
ice camp for February to April, 2007. Negative
(positive) frequencies indicate clockwise
(counter clockwise) motion. The main M2 tidal
wave at the ice camp position has most power
in the counter clockwise direction, with strong
clockwise power stemming from inertial
oscillations.
Figure 5
Rotary Spectra for March to June, 2007 of two
SEDNA buoys and one SITII buoy. The strong
semi-diurnal clockwise signal shown here,
together with synchronized sea ice drift with the
M2 tide (Figure 2) and model spectra (Figure 4)
suggest tidal amplification of of inertial
oscillations in sea ice drift
Figure 1 (left)
Tracks of the hexagonal buoy
arrays deployed for SITII and
SEDNA in the Beaufort Gyre.
Figure 6
6
7
Figure 2 (right)
Time series of observed and
modeled sea ice drift and
deformation during SEDNA
including output from a 50km
resolution version of WRF over the
ice camp, and an ice-tide model of
the Arctic Ocean. Prudhoe Bay
has an M2 tide of similar phase to
the ice camp location (Kowalik and
Proshutinsky, 1994).
Spectra of sea ice deformation (divergence and
maximum shear) from the the SEDNA 10km
and 70km buoy arrays. Of particular note is the
existence of a semi-diurnal signal in the 10km
array that is not apparent in the 70km array,
suggesting a cascade of energy from both fine
temporal and spatial scales to longer time and
distance scales in sea ice drift and deformation.
Figure 7
Spectra of sea ice deformation from the the
SITII buoy array. The semi-diurnal peak shown
here does not appear in the mid winter (20062007) time series.
First-look simulations
.
Deformation
Signal
Conclusions
Initial simulations of the WRF model and the Hibler et al (2006) icetide model have been conducted to shed light on the forcing of sea
ice deformation, and to serve as a benchmark for future model
improvements for downscaling to SEDNA observations. WRF
provides a close approximation of winds over the ice camp (Figure
2), with prescribed surface boundary conditions most likely causing
the disparity in surface temperature. The initial ice-tide simulation
was run constant-strength sea ice in a bid to determine the likely
tidal amplification of inertial oscillations in sea ice drift. A
comparison of Figure 4 and Figure 5 suggest considerably more
inertial power in sea ice drift than the model predicts, the shortfall
attributable to the rigid approximation.
Spectra of sea ice deformation from both SEDNA and SITII in
Figures 6 and 7, in light of the deformation time series in Figure 2,
suggest a scaled response to atmospheric and oceanic forcing. Of
note in Figure 2 is the ‘removal’ of semi-diurnal drift oscillations
(upper panel) during passage of the storm over the SEDNA ice
camp during deformation. While the 10km array indicates
significant semi-diurnal power (Figure 6), the same spectral peak for
the SITII array (Figure 7) could not be found during the 2006-2007
winter passage of that array (black tracks in Figure 1).
This work integrates in-situ observations with atmospheric and iceocean model output to provide an overview of sea ice drift during
and following the SEDNA field campaign. There are strong
indications of tidal amplification of inertial oscillations by virtue of
synchronous ice drift speeds with sea surface height and dominant
clockwise drift in a region where counter clockwise tidal motion is
more powerful than clockwise tidal motion. Deformation results
hint at spatial scaling of sea ice deformation in the semi-diurnal
band. This scaling property will be investigated in a planned
modeling study using a high resolution ice-tide model, driven by
high resolution winds from WRF.
References
Hibler, W. D. III, A. Roberts, P. Heil, A. Y. Proshutinsky, H. L. Simmons and J. Lovick, 2006. Modeling M2
tidal variability in Arctic sea-ice drift and deformation. Ann. Glaciol., 44, 418-428.
Kowalik, Z., and A. Y. Proshutinsky, 1994: The Arctic Ocean Tides. The polar oceans and their role in
shaping the global environment: The Nansen centennial volume, O. M. Johannessen, R. D. Muench, and J.
E. Overland, Eds., American Geophysical Union, 137-157.
Acknowledgements
The SEDNA field campaign was funded by the National Science Foundation,OPP0612527. The eastern
Beaufort Sea buoy array, and ice-tide modeling was supported by NSF OPP0520574. Reanalysis data
courtesy of the Environmental Modeling Center. Tidal data provided by the National Oceanic and
Atmospheric Administration.
Affiliations
* Correspondence to: aroberts@arsc.edu
1 Arctic Region Supercomputing Center, University of Alaska Fairbanks
2 International Arctic Research Center, University of Alaska Fairbanks
3 Cooperative Institute for Research in Environmental Sciences, University of Colorado