Alexander D. Fraser

East Antarctic Landfast Sea Ice Distribution and Variability, 2000–08

AbstractThis study presents the first continuous, high spatiotemporal resolution time series of landfast sea ice extent along the East Antarctic coast for the period March 2000–December 2008. The time series was derived from consecutive 20-day cloud-free Moderate Resolution Imaging Spectroradiometer (MODIS) composite images. Fast ice extent across the East Antarctic coast shows a statistically significant (1.43% ±0.30% yr−1) increase. Regionally, there is a strong increase in the Indian Ocean sector (20°–90°E, 4.07% ±0.42% yr−1), and a nonsignificant decrease in the western Pacific Ocean sector (90°–160°E, −0.40% ±0.37% yr−1). An apparent shift from a negative to a positive extent trend is observed in the Indian Ocean sector from 2004. This shift also coincides with a greater amount of interannual variability. No such shift in apparent trend is observed in the western Pacific Ocean sector, where fast ice extent is typically higher and variability lower than the Indian Ocean sector. The limit to the maximu...

Change and Variability in East Antarctic Sea Ice Seasonality, 1979/80–2009/10

Recent analyses have shown that significant changes have occurred in patterns of sea ice seasonality in West Antarctica since 1979, with wide-ranging climatic, biological and biogeochemical consequences. Here, we provide the first detailed report on long-term change and variability in annual timings of sea ice advance, retreat and resultant ice season duration in East Antarctica. These were calculated from satellite-derived ice concentration data for the period 1979/80 to 2009/10. The pattern of change in sea ice seasonality off East Antarctica comprises mixed signals on regional to local scales, with pockets of strongly positive and negative trends occurring in near juxtaposition in certain regions e.g., Prydz Bay. This pattern strongly reflects change and variability in different elements of the marine “icescape”, including fast ice, polynyas and the marginal ice zone. A trend towards shorter sea-ice duration (of 1 to 3 days per annum) occurs in fairly isolated pockets in the outer pack from∼95–110°E, and in various near-coastal areas that include an area of particularly strong and persistent change near Australias Davis Station and between the Amery and West Ice Shelves. These areas are largely associated with coastal polynyas that are important as sites of enhanced sea ice production/melt. Areas of positive trend in ice season duration are more extensive, and include an extensive zone from 160–170°E (i.e., the western Ross Sea sector) and the near-coastal zone between 40–100°E. The East Antarctic pattern is considerably more complex than the well-documented trends in West Antarctica e.g., in the Antarctic Peninsula-Bellingshausen Sea and western Ross Sea sectors.

Examining the interaction between multi‐year landfast sea ice and the Mertz Glacier Tongue, East Antarctica: Another factor in ice sheet stability?

The Mertz Glacier tongue (MGT), East Antarctica, has a large area of multi-year fast sea ice (MYFI) attached to its eastern edge. We use various satellite data sets to study the extent, age, and thickness of the MYFI and how it interacts with the MGT. We estimate its age to be at least 25 years and its thickness to be 10-55 m; this is an order of magnitude thicker than the average regional sea-ice thickness and too thick to be formed through sea-ice growth alone. We speculate that the most plausible process for its growth after initial formation is marine (frazil) ice accretion. The satellite data provide two types of evidence for strong mechanical coupling between the two types of ice: The MYFI moves with the MGT, and persistent rifts that originate in the MGT continue to propagate for large distances into the MYFI. The area of MYFI decreased by 50% following the departure of two large tabular icebergs that acted as pinning points and protective barriers. Future MYFI extent will be affected by subsequent icebergs from the Ninnis Glacier and the imminent calving of the MGT. Fast ice is vulnerable to changing atmospheric and oceanic conditions, and its disappearance may have an influence on ice tongue/ice shelf stability. Understanding the influence of thick MYFI on floating ice tongues/ice shelves may be significant to understanding the processes that control their evolution and how these respond to climate change, and thus to predicting the future of the Antarctic Ice Sheet.

Sea ice production variability in Antarctic coastal polynyas

Enhanced sea ice production (SIP) in Antarctic coastal polynyas forms dense shelf water (DSW), leading to Antarctic Bottom Water (AABW) formation that ultimately drives the lower limb of the meridional overturning circulation. Some studies suggest that the variability of SIP in Antarctic coastal polynyas is driven by the influence of atmospheric forcing, i.e., surface winds and air temperature. Our previous mapping of SIP in 13 major Antarctic coastal polynyas from 1992 to 2007, using a heat flux calculation with ice thickness data derived from satellite data, is extended here to examine the interannual and seasonal variability of SIP from 1992 to 2013. The interannual variability of total ice production correlates more strongly with polynya extent than with atmospheric forcing, with the exception of the Shackleton Polynya, which correlates well with wind. There is no coherent signal in the interannual variability between the major Antarctic coastal polynyas. We find that stochastic changes to the coastal “icescape,” i.e., ice shelves, floating glaciers, fast ice, together with offshore first-year ice, are also important factors driving SIP variability on multiyear time scales. Both the Ross Ice Shelf Polynya and Mertz Glacier Polynya experienced a significant reduction in SIP due to calving events and the repositioning of icebergs and fast ice. Our results also show opposing trends between polynya-based SIP and sea ice extent in key regions of Antarctic sea ice change. Close monitoring of coastal icescape dynamics and change is essential to better understand the long-term impact of coastal polynya variability and its influence on regional AABW production.

The suppression of Antarctic bottom water formation by melting ice shelves in Prydz Bay

A fourth production region for the globally important Antarctic bottom water has been attributed to dense shelf water formation in the Cape Darnley Polynya, adjoining Prydz Bay in East Antarctica. Here we show new observations from CTD-instrumented elephant seals in 2011–2013 that provide the first complete assessment of dense shelf water formation in Prydz Bay. After a complex evolution involving opposing contributions from three polynyas (positive) and two ice shelves (negative), dense shelf water (salinity 34.65–34.7) is exported through Prydz Channel. This provides a distinct, relatively fresh contribution to Cape Darnley bottom water. Elsewhere, dense water formation is hindered by the freshwater input from the Amery and West Ice Shelves into the Prydz Bay Gyre. This study highlights the susceptibility of Antarctic bottom water to increased freshwater input from the enhanced melting of ice shelves, and ultimately the potential collapse of Antarctic bottom water formation in a warming climate.

A Method for Compositing Polar MODIS Satellite Images to Remove Cloud Cover for Landfast Sea-Ice Detection

This paper presents details of techniques for generating thermal infrared and visible composite images from the cloud-free portions of temporally closely spaced MODerate resolution Imaging Spectroradiometer (MODIS) images, with a focus on studies of landfast sea ice along the East Antarctic coast. Composite image inclusion criteria are based on modified MODIS Earth Observing System cloud mask product results. The compositing process presented places emphasis on retaining maximum spatial resolution while minimizing storage space requirements. Composite images can be produced either as a regular product (e.g., on a ten-day grid), or dynamically (whenever enough information is acquired to produce a new output image). The techniques presented are applicable at any latitude, are available for all MODIS channels at their native resolution, can combine Aqua and Terra images, and can produce maps in any output projection. However, due to the polar orbit of NASAs Terra and Aqua satellites which host the MODIS instrument, more frequent coverage is produced at higher latitudes. Thus, the techniques presented are particularly applicable to polar research. Examples of composite image generation of the landfast sea ice around the Mertz Glacier region, East Antarctica, are included for both winter and summer.

Modeling Ocean–Cryosphere Interactions off Adélie and George V Land, East Antarctica

AbstractOcean–cryosphere interactions along the Adelie and George V Land (AGVL) coast are investigated using a coupled ocean–sea ice–ice shelf model. The dominant feature of the Mertz Glacier Tongue (MGT), located at approximately 145°E, was a highly productive winter coastal polynya system, until its calving in February 2010 dramatically changed the regional “icescape.” This study examines the annual mean, seasonal, and interannual variabilities of sea ice production; basal melting of the MGT; ice shelves, large icebergs, and fast ice; Dense Shelf Water (DSW) export; and bottom water properties on the continental slope and rise, and assesses the impacts of the calving event. The interannual variability of the winter coastal polynya regime is dominated by the regional offshore winds and air temperature, which are linked to activity of the Amundsen Sea low pressure system. This is the main driver of the interannual variability of DSW exported from the AGVL region. The calving event led to a decrease in sea...

Comparison of Microwave Backscatter Anisotropy Parameterizations of the Antarctic Ice Sheet Using ASCAT

The C-band EUMETSAT Advanced Scatterometer (ASCAT) was launched in 2007, and provides backscatter measurements with excellent azimuth and incidence angle diversity in polar regions. C-band backscatter measurements can be used to retrieve near-surface snow/firn properties such as grain size and accumulation rate, however the Antarctic Ice Sheet (AIS) exhibits strong microwave backscatter anisotropy as a function of both incidence and azimuth angles, and this anisotropy must be well understood before such physical parameter retrieval can be accurately performed. This paper presents a detailed comparison of several different parameterizations of both the azimuth and the incidence anisotropy. Parameterizations used here result in residuals similar to the radiometric performance of ASCAT, indicating accurate characterization of the anisotropy. The analysis of these parameterizations provides an insight into the physical mechanisms taking place near the surface of the AIS. 30-day composite maps of several parameters are presented, and discussed in the context of near-surface glaciology. This anisotropy characterization will form the basis of a technique for extraction of physical parameters of the AIS.

Environmental drivers of benthic communities and habitat heterogeneity on an East Antarctic shelf

Abstract This study presents the first analysis of benthic megafauna and habitats from the Sabrina Coast shelf, encompassing a proposed Marine Protected Area. Sea bed imagery indicated an abundant benthic fauna compared to other parts of the Antarctic shelf, dominated by brittle stars, polychaete tubeworms, and a range of other sessile and mobile taxa. The distribution of taxa was related (ρ=0.592, P<0.001) to variations in water depth, latitude, substrate type and phytodetritus. High phytodetritus cover was associated with muddy/sandy sediments and abundant holothurians and amphipods, while harder substrates hosted abundant brachiopods, hard bryozoans, polychaete tubeworms, massive and encrusting sponges, and sea whips. Brittle stars, irregular urchins and anemones were ubiquitous. Variations in substrate largely reflected the distribution of dropstones, creating fine-scale habitat heterogeneity. Several taxa were found only on hard substrates, and their broad regional distribution indicated that the density of dropstones was sufficient for most sessile invertebrates to disperse across the region. The hexactinellid sponge Anoxycalyx joubini and branching hydrocorals exhibited a more restricted distribution, probably related to water depth and limited dispersal capability, respectively. Dropstones were associated with significant increases in taxa diversity, abundance and biological cover, enhancing the overall diversity and biomass of this ecosystem.

Variability in sea ice cover and climate elicit sex specific responses in an Antarctic predator

Contrasting regional changes in Southern Ocean sea ice have occurred over the last 30 years with distinct regional effects on ecosystem structure and function. Quantifying how Antarctic predators respond to such changes provides the context for predicting how climate variability/change will affect these assemblages into the future. Over an 11-year time-series, we examine how inter-annual variability in sea ice concentration and advance affect the foraging behaviour of a top Antarctic predator, the southern elephant seal. Females foraged longer in pack ice in years with greatest sea ice concentration and earliest sea ice advance, while males foraged longer in polynyas in years of lowest sea ice concentration. There was a positive relationship between near-surface meridional wind anomalies and female foraging effort, but not for males. This study reveals the complexities of foraging responses to climate forcing by a poleward migratory predator through varying sea ice property and dynamic anomalies.