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Ocean-Ice Shelf Interaction in East Antarctica OceTHE OFFICIALa MAGAZINEn ogOF THE OCEANOGRAPHYra SOCIETYphy CITATION Silvano, A., S.R. Rintoul, and L. Herraiz-Borreguero. 2016. Ocean-ice shelf interaction in East Antarctica. Oceanography 29(4):130–143, https://doi.org/10.5670/ oceanog.2016.105. DOI https://doi.org/10.5670/oceanog.2016.105 COPYRIGHT This article has been published in Oceanography, Volume 29, Number 4, a quarterly journal of The Oceanography Society. Copyright 2016 by The Oceanography Society. All rights reserved. USAGE Permission is granted to copy this article for use in teaching and research. Republication, systematic reproduction, or collective redistribution of any portion of this article by photocopy machine, reposting, or other means is permitted only with the approval of The Oceanography Society. Send all correspondence to: [email protected] or The Oceanography Society, PO Box 1931, Rockville, MD 20849-1931, USA. DOWNLOADED FROM HTTP://TOS.ORG/OCEANOGRAPHY SPECIAL ISSUE ON OCEAN-ICE INTERACTION Ocean-Ice Shelf Interaction in East Antarctica By Alessandro Silvano, Stephen R. Rintoul, and Laura Herraiz-Borreguero Photo credit: Esmee van Wijk (CSIRO/ACE CRC) 130 Oceanography | Vol.29, No.4 Given the large volume of marine-based ice in East Antarctica, it is critical to develop a better understanding of the potential “ vulnerability of the [East Antarctic Ice Sheet] to changes in the ocean.”. ABSTRACT. Assessments of the Antarctic contribution to future sea level rise twenty-first century (Church et al., 2013). have generally focused on ice loss in West Antarctica. This focus was motivated by Melt of even a small fraction of the water glaciological and oceanographic observations that showed ocean warming was driving stored in the ice sheets would therefore loss of ice mass from the West Antarctic Ice Sheet (WAIS). Paleoclimate studies have a substantial impact on future sea confirmed that ice discharge from West Antarctica contributed several meters to sea level rise, with widespread consequences level during past warm periods. On the other hand, the much larger East Antarctic Ice for society, particularly in coastal regions. Sheet (EAIS) was generally considered to be relatively stable because of being largely Satellite measurements show that the grounded above sea level and therefore protected from ocean heat flux. However, Greenland and Antarctic Ice Sheets have recent studies suggest that a large part of the EAIS is grounded well below sea level and made positive and growing contributions that the EAIS also retreated and contributed several meters to sea level rise during past to sea level rise in the last two decades warm periods. We use ocean observations from three ice shelf systems to illustrate the (Rignot et al., 2011), and the ice sheets are variety of ocean-ice shelf interactions taking place in East Antarctica and to discuss the expected to make the largest contribution potential vulnerability of East Antarctic ice shelves to ocean heat flux. The Amery and in centuries to come (Rignot et al., 2011; the Mertz are “cold cavity” ice shelves that exhibit relatively low area-averaged basal Dutton et al., 2015). The response of the melt rates, although substantial melting and refreezing occurs beneath the large and ice sheets to continued warming of the deep Amery Ice Shelf. In contrast, new oceanographic measurements near the Totten climate remains the largest single source Ice Shelf show that warm water enters the sub-ice-shelf cavity and drives rapid basal of uncertainty in projections of long-term melting, as is seen in West Antarctica. Totten Glacier is of particular interest because sea level rise. it holds a marine-based ice volume equivalent to at least 3.5 m of global sea level rise, Ice sheet stability depends on the bal- an amount comparable to the entire marine-based WAIS, and recent glaciological ance between gains (from snowfall) and measurements show the grounded portion of Totten Glacier is thinning and the losses (from iceberg calving and melt- grounding line is retreating. Multiple lines of evidence support the hypothesis that ing). The presence of floating ice shelves parts of the EAIS are more dynamic than once thought. Given that the EAIS contains a around the margin of Antarctica contrib- volume of marine-based ice equivalent to 19 m of global sea level rise, the potential for utes to the stability of the Antarctic Ice ocean-driven melt to destabilize the marine-based ice sheet needs to be accounted for Sheet. Ice shelves form where ice streams in assessments of future sea level rise. flow off the continent into the ocean and start to float. Back stresses generated BACKGROUND 2013). The amount of ice stored in ice when a flowing ice shelf interacts with Global mean sea level rose by 0.19 sheets, equivalent to ~7 m of global sea the seafloor or side walls restrains the ± 0.02 m between 1901 and 2010 in level rise for Greenland (Dowdeswell, drainage of glacial ice into the ocean and response to global warming (Church 2006) and ~58 m for Antarctica (Fretwell thus “buttresses” the ice sheet (Dupont et al., 2013). Ocean thermal expansion et al., 2013), is vast compared to pro- and Alley, 2005). Thinning or collapse and ice loss from glaciers have been the jected sea level contributions from gla- of an ice shelf reduces the buttressing dominant contributors to sea level rise ciers (0.04–0.23 m) and thermal expan- effect and increases the discharge of ice over the past century (Church et al., sion (0.10–0.33 m) by the end of the into the ocean. Ice shelves thin when the Oceanography | December 2016 131 inflow of ice from the continent is insuf- et al., 2012). Oceanographic evidence Totten Glacier are similar to those seen ficient to balance the loss of ice to melt confirms that the most rapid mass loss, in West Antarctica, including thinning and iceberg calving. Melting from above grounded ice thinning, and grounding of the grounded ice (e.g., Pritchard et al., by a warm atmosphere and melting from line retreat have occurred where rela- 2009; Flament and Remy, 2012; Harig below by a warm ocean can both contrib- tively warm ocean waters reach sub-ice- and Simons, 2015; Li et al., 2015) and ute to thinning of ice shelves. For most shelf cavities, driving rapid basal melt- retreat of the grounding line (Li et al., of Antarctica, air temperatures remain ing (e.g., Jenkins and Jacobs, 2008; Jacobs 2015). While the rates of change at Totten below freezing year-round at present et al., 2011). The rate of ice loss and thin- Glacier are not as large as those seen at (Picard and Fily, 2006), so basal melt- ning of floating ice shelves has increased Pine Island Glacier and other glaciers in ing by the ocean makes the dominant over the past two decades (Rignot et al., West Antarctica, these observations come contribution. This means the future of 2011; Paolo et al., 2015). Consistent with as a surprise, given that little warm water Antarctic ice shelves, and the grounded recent observations of dynamic behav- was thought to reach the continental ice sheets buttressed by the ice shelves, is ior in response to ocean forcing, studies shelf in this region, which lies well south strongly tied to the surrounding ocean. of past climate suggest the West Antarctic of the core of the Antarctic Circumpolar Ice shelves whose grounding lines (the Ice Sheet has waxed and waned many Current (ACC). Until recently, no ocean- boundaries between the floating ice and times in the past (Scherer, 1991; Naish ographic data from the Totten ice front the grounded ice) are located well below et al., 2009; Pollard and DeConto, 2009). were available to test the hypothesis that sea level are potentially more sensitive to East Antarctica, on the other hand, has ocean-ice shelf interaction could explain ocean forcing. Because the freezing tem- long been thought to be more stable. Most the dynamic behavior of this sector of perature is lowered by pressure, the ther- of the East Antarctic Ice Sheet (EAIS) East Antarctica. mal forcing available for melting is higher was understood to be grounded on bed- Given the large volume of marine- for deeper grounding lines for a given rock well above sea level, and the marine- based ice in East Antarctica, it is criti- water temperature. Moreover, ice sheets based parts of the EAIS were believed to cal to develop a better understanding of grounded on bedrock that slopes upward be isolated from warm Southern Ocean the potential vulnerability of the EAIS toward the sea are particularly vulnera- waters. But recent studies have over- to changes in the ocean. New modeling ble (Weertman, 1974; Schoof, 2007). In turned these assumptions. For example, studies of the response of the Antarctic this geometry, a retreat of the ground- inferences of past sea level from proxy Ice Sheet to continued high emissions of ing line increases the thickness at the data and models conclude that the EAIS greenhouse gases underscore the urgency ice front and therefore the ice discharge, retreated and made substantial contri- of this task, showing that mass loss from producing a self-sustaining retreat that butions to sea level during past warm Antarctica may be larger and more rapid is hard to reverse (Joughin and Alley, periods in Earth’s history, suggesting the than previously thought, with a large con- 2011). This process is called marine ice EAIS is more dynamic than previously tribution from the EAIS (Golledge et al., sheet instability. Several studies suggest thought (Williams et al., 2010b; Young 2015; DeConto and Pollard, 2016). that marine ice sheet instability is already et al., 2011; Cook et al., 2013; Patterson In this review, we summarize what is underway in West Antarctica (Favier et al., 2014; Pollard et al., 2015; Aitken known about ocean-ice shelf interaction et al., 2014; Joughin et al., 2014; Rignot et al., 2016).
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