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East Antarctic landfast sea-ice distribution and variability

Fraser, AD 2011 , 'East Antarctic landfast sea-ice distribution and variability', PhD thesis, University of Tasmania.

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Landfast sea ice (sea ice which is held fast to the coast or grounded icebergs, also
known as fast ice) is a pre-eminent feature of the Antarctic coastal zone, where it
forms an important interface between the ice sheet and pack ice/ocean to exert a major
influence on high-latitude atmosphere-ocean interaction and biological processes.
It is highly vulnerable to climate variability and change, given that its formation
and breakup are intimately associated with oceanic and atmospheric forcing, yet is
not currently represented in global climate models or coupled atmosphere-ocean-ice
models. Fast ice forms a key breeding habitat for a number of iconic species, including
Weddell seals and Emperor penguins, and plays a crucial role in the breeding
success and foraging behaviour of Ad´elie penguins. Recent work further suggests
that fast ice may stabilize floating glacier tongues and ice shelves, to affect iceberg
calving and ultimately the mass balance of the ice sheet plus the drift rates of icebergs.
Moreover, fast ice has a major impact on the logistics of the resupply of
Antarctic bases.
While Antarctic sea ice extent and variability has been the focus of considerable recent
research, fast-ice extent and variability are currently poorly understood. This
is in large part due to the difficulty associated with discriminating fast ice from
pack ice on a large scale in satellite data. “Snapshot” analyses are unable to discriminate
between the ice types (i.e., fast ice has a non-unique signature), and ice
motion techniques have various problems, including persistent cloud cover at visible
and infrared wavelengths and low spatial resolution for passive microwave sensors.
Furthermore, Synthetic Apterture Radar (SAR) imagery is inherently difficult to
interpret over the sea-ice zone.
This thesis, presented in a “thesis-by-publication” style, overcomes the problems
associated with remotely sensing fast ice at visible and infrared wavelengths to produce
cloud-free time-averaged images of the surface from March 2000 to December
2008, enabling discrimination between pack and fast ice. From these, the first East
Antarctic (10◦ W - 172◦ E) high spatio-temporal resolution (2-km, 20-day) maps
of fast-ice extent are created. This allows the first detailed time series analysis of
the seasonal to inter-annual variability of East Antarctic fast ice. Fast-ice growth
and breakout events are then related to large scale and local atmospheric forcing
In addition to presenting the first near decade-long, high spatio-temporal resolution
time series of fast-ice maps in East Antarctica, the main findings of this thesis are
as follows. Using MODerate resolution Imaging Spectroradiometer (MODIS) visible
and thermal infrared data, quality cloud-free composite images of the high-latitude
surface can generally be constructed using 20 days’ raw imagery. The compositing
technique developed here involves using a modified MODIS cloud mask to select cloud-free pixels, which are then composited together over number of days. The
MOD35 MODIS cloud mask performs well during daytime, when shortwave tests
could be included into the cloud masking algorithm. However, during night-time,
cloud mask performance is insufficient to create quality cloud-free composite images.
Spatial filtering on the cloud mask is required to produce high-quality composite
images. With a sufficiently long compositing interval (i.e., 20 days), pack ice motion
acts to “blur” the pack ice zone in the composite imagery, while the fast/pack ice
shear zone remains sharply defined. This property of the compositing process is
very useful for discriminating between pack and fast ice.
Despite the success of the compositing technique, persistent cloud cover and inaccurate
cloud masking are found to lower composite image quality at times. Thus, a
technique is developed to augment lower quality composite images with Advanced
Microwave Scanning Radiometer - EOS (AMSR-E) and additional MODIS data.
Fast-ice maps are generated from the resulting imagery. An error analysis shows
that fast-ice extent can be retrieved to within ∼ ±3% for over 80% of the 159 consecutive
fast-ice maps comprising the 8.8-year time series, with the remainder retrieved
to within ∼ ±9%.
Analysis of the 8.8-year East Antarctic fast-ice time series shows a statistically significant
increase in fast-ice extent (1.43 ± 0.30% yr−1), albeit based upon a
relatively short period. Regionally, there is a stronger increase observed in the
Indian Ocean sector (20 - 90◦ E) of 4.07 ± 0.42% yr−1, compared to a slight (nonsignificant)
decrease in the Western Pacific Ocean sector (90 - 160◦ E) of -0.40 ±
0.37% yr−1. In the Indian Ocean sector, a slightly decreasing trend in fast-ice extent
changes to a strongly increasing trend from 2004 - 2008. An analysis of the timing
of maximum and minimum fast-ice extent shows high variability compared to that
of overall sea ice. Analysis of the shape of the mean annual fast-ice extent cycle
reveals a limit to maximum fast-ice extent, apparently related to the locations of
grounded icebergs. Ten fast-ice regimes were identified across the coast, relating
to bathymetry, coastal configuration, and prevailing atmospheric conditions. The
percentage of fast ice comprising overall sea-ice extent varies seasonally from ∼19%
during the summer minimum extent to ∼3.8% during the winter maximum extent.
Nine case studies of anomalous fast-ice breakout or growth are conducted in four
regions around the East Antarctic coast. These anomalous fast-ice extents are observed
in conjunction with locally anomalous wind speeds and directions, surface
air temperatures, and pack ice conditions. On a hemisphere-wide scale, a reasonably
strong correlation is found between the Southern Oscillation Index (SOI) and
fast-ice extent in the Indian Ocean sector (R=+0.45). No strong correlation is observed
in the Western Pacific Ocean sector, and, in contrast to previous work, the
correlation between SAM and fast-ice extent is weak in both sectors.
This work has greatly improved our knowledge of fast-ice distribution and variability
around the East Antarctic coast, and provides benefits for many areas of current
research. It has also provided an important new climatic dataset that is directly
comparable to, and complements, the widely-used passive microwave-derived time
series of overall sea-ice extent.

Item Type: Thesis - PhD
Authors/Creators:Fraser, AD
Keywords: glaciology, remote-sensing, landfast sea ice, fast ice, Antartica
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Copyright 2011 the Author

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