Quantifying Sea Ice in the Southern Ocean Using ArcGIS
Tracy L. DeLiberty, Cathleen A. Geiger and Mary D. Lemcke
Abstract: A National Science Foundation (NSF) project is underway with researchers at the
University of Delaware, the Australian Antarctic Division, the National Ice Center (NIC) and
Clarkson University to evaluate two datasets - in situ (point) sea-ice thickness observations and
weekly ice charts (polygon). The goal is to ascertain their quality for use in monitoring sea-ice
thickness and mass balance changes in the Southern Ocean. Sea-ice thickness calculations from
both datasets are temporally joined with spatially averaged in situ observations matching their
respective NIC ice chart using ArcGIS's field calculator, attribute query, spatial join and dissolve
tools. The uncertainties of total ice thickness for both in situ observations and NIC ice charts are
propagated through individual calculations and GIS tools. A composite product of the two
datasets and their error estimates is being developed for monitoring sea-ice thickness, mass
balance and validation fields for climate modeling. ArcGIS is used for the analysis of sea-ice
conditions over the 1995-2000 period of study by visually and quantitatively examining the
spatial extent of sea-ice and the variability of sea ice thickness for selected weeks during 1995
and 1998. The 3D Analyst extension also provides a means for displaying sea-ice thickness
fields by draping the errors over the thickness estimates.
Introduction
Sea-ice covers approximately 7% of the earth's oceans at any time (Parkinson and
Washington 1979), greatly affecting the exchange of energy between the ocean and the
atmosphere and increasing the albedo of the polar regions. Antarctic sea ice is an important
feature of the global climate system because it is a sensitive indicator of climate change and
therefore plays a complex role in global climatic and oceanographic processes. For these reasons,
sea ice is also referred to as a climatological canary or early warning detector of change.
Comprised of mostly thin, first-year ice, Antarctic sea ice extends to a latitude of about 60 S
except within the Weddell Sea sector where it may extend further northward (King and Turner
1997). At its maximum extent, Antarctic sea ice covers 20% more area than its Arctic
counterpart (Comiso et al. 1992) and has a much larger seasonal variation, about 80%, or 16X106
km2, than that observed in the Arctic (King and Turner 1997).
Large-scale general circulation modeling studies indicate that Antarctic sea ice will
exhibit changes in extent, thickness, and compactness in response to long-term changes in global
climate as well as short-term climate variations (Ledley 1991, Rind et al. 1995, Jacobs and
Comiso 1997, Geiger et al. 1997). Most models agree that atmospheric warming will result in a
reduction in northern hemispheric ice cover. On the contrary, the southern ice cover modeling
results produce conflicting responses in global warming scenarios. Moreover, the variability in
sea-ice conditions are poorly understood due to the current lack of long-term and large-scale
thickness observations within the Southern Ocean. At this time, there is no comprehensive
climatology of Antarctic sea-ice conditions to validate such large-scale models or to study past
trends (Worby and Ackley 2000, Geiger et al. 2000).
A National Science Foundation project is underway to ascertain the quality of the two
largest operational data products that currently exist for monitoring sea-ice thickness and mass
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