OceTHE OFFICIALa MAGAZINEn ogOF THE OCEANOGRAPHYra SOCIETYphy CITATION Heywood, K.J., L.C. Biddle, L. Boehme, P. Dutrieux, M. Fedak, A. Jenkins, R.W. Jones, J. Kaiser, H. Mallett, A.C. Naveira Garabato, I.A. Renfrew, D.P. Stevens, and B.G.M. Webber. 2016. Between the devil and the deep blue sea: The role of the Amundsen Sea continental shelf in exchanges between ocean and ice shelves. Oceanography 29(4):118–129, https://doi.org/10.5670/oceanog.2016.104. DOI https://doi.org/10.5670/oceanog.2016.104 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 Between the Devil and the Deep Blue Sea THE ROLE OF THE AMUNDSEN SEA CONTINENTAL SHELF IN EXCHANGES BETWEEN OCEAN AND ICE SHELVES By Karen J. Heywood, Louise C. Biddle, Lars Boehme, Pierre Dutrieux, Michael Fedak, Adrian Jenkins, Richard W. Jones, Jan Kaiser, Helen Mallett, Alberto C. Naveira Garabato, Ian A. Renfrew, David P. Stevens, and Benjamin G.M. Webber Release of a meteorolog- ical radiosonde balloon from RRS James Clark Ross in February 2014 to col- lect a profile of atmospheric properties adjacent to the Pine Island Ice Shelf. Photo credit: Karen Heywood 118 Oceanography | Vol.29, No.4 Processes on the continental shelf and slope all around Antarctica are crucially important for determining future sea level rise, for setting the “ properties and volume of exported dense bottom water, and for regulating the carbon cycle. Yet our ability to model and predict these processes over future decades is still rudimentary.”. ABSTRACT. The Amundsen Sea is a key region of Antarctica where ocean, atmosphere, from the grounded ice sheet. There is sea ice, and ice sheet interact. For much of Antarctica, the relatively warm water of growing evidence that the widespread the open Southern Ocean (a few degrees above freezing) does not reach the Antarctic thinning of ice shelves observed over continental shelf in large volumes under current climate conditions. However, in the recent decades (Paolo et al., 2015) has Amundsen Sea, warm water penetrates onto the continental shelf and provides heat been responsible for the acceleration and that can melt the underside of the area’s floating ice shelves, thinning them. Here, we thinning of Antarctic outlet glaciers and discuss how the ocean’s role in melting has come under increased scrutiny, present 2014 the associated retreat of their ground- observations from the Amundsen Sea, and discuss their implications, highlighting ing lines (e.g., Park et al., 2013; Mouginot aspects where understanding is still incomplete. et al., 2014; Rignot et al., 2014). The Intergovernmental Panel on BACKGROUND grounded ice sheet upstream from the Climate Change (IPCC) Fifth Assessment The Antarctic Ice Sheet, which holds floating ice shelf downstream, is a sen- Report chapter on sea level rise (Church a vast reservoir of water—enough to sitive function of the ice thickness there et al., 2013) was the first such report to be increase global sea level by 58 m (Fretwell (Schoof, 2007). Thus, if thinning of the able to quantify a likely range of sea level et al., 2013)—has been a focus of atten- ice sheet initiates an inland retreat of the rise during the twenty-first century. This tion in recent decades because it has been grounding line that takes it into deeper was made possible by recent improve- losing mass and thereby making a posi- water, the discharge across the grounding ments in our understanding and model- tive contribution to sea level (Shepherd line tends to increase, exacerbating the ing of ice sheet dynamics, ice-ocean inter- et al., 2012). The mass discharged to the mass imbalance, and the associated pos- actions, and climate dynamics. However, ocean by the gravity-driven flow of ice itive feedback can lead to rapid and irre- it was not possible to provide a very from the interior of the continent has versible retreat. The ice shelves play a crit- likely range or upper bound of global sea been greater than the mass added by pre- ical role in regulating the retreat, because level rise because of the unknown risk of cipitation over the ice sheet. The ice sheet as they contact seabed shoals and the sides potential collapse of Antarctic ice shelves. in West Antarctica has received most of confining embayments, the resulting There is an increasing rate of ice loss in attention because, although consider- resistance to their flow buttresses the ice the Amundsen Sea Embayment in West ably smaller than its counterpart in East upstream. As a result of ice shelf buttress- Antarctica (Paolo et al., 2015). Here, we Antarctica, it rests predominantly on land ing, the flow of ice across the grounding focus on two vulnerable ice shelves in that lies below sea level and often deep- line is not a monotonic function of the ice the eastern Amundsen Sea, Pine Island ens toward the interior of the ice sheet. thickness there (Gudmundsson, 2013), so and Thwaites Glaciers, where ice shelf Ice at the margins floats free of the bed to the positive feedback can be prevented. thinning and glacier acceleration have form ice shelves, and the flow of ice across However, any changes in buttressing will been observed throughout the past four the grounding line, which separates the inevitably lead to changes in discharge decades (Mouginot et al., 2014; Rignot Oceanography | December 2016 119 et al., 2014). Church et al. (2013) had “high circulating around Antarctica within shelf break and may turn onto the shelf confidence” that the retreat of Pine Island the Antarctic Circumpolar Current, has when it encounters crosscutting troughs Glacier (if it occurs) would lead to a sea core temperatures of approximately 2°C (Walker et al., 2013). Once on the con- level rise of several centimeters by 2100, and is ubiquitous beneath the cold sur- tinental shelf, CDW is thought to follow but that models were not yet good enough face layer and broad thermocline that the glacially carved bathymetric troughs to predict when this might happen. They typically occupy the upper few hundred to the ice shelves (Thoma et al., 2008; suggested that Thwaites Glacier may meters of the water column (Schmidtko Jacobs et al., 2011; Nakayama et al., 2013). be less prone to undergo ocean-driven et al., 2014). The main pycnocline sepa- Although the ocean has been impli- grounding line retreat than its neigh- rating CDW from surface waters typically cated in the thinning and acceleration of ice shelves, it is too simplistic to state that the ocean is warming globally and that the ice shelf is melting like an ice cube in a warm bath; rather, ocean dynam- Atmospheric forcing is important for ics have an important role to play. For determining ocean conditions and dynamics, for example, Antarctic sea ice is not under- going the decline that is seen in the Arctic sea ice formation and melt processes, and for (Turner et al., 2015). Although the pre- “ surface processes on the Antarctic continent that cise combination of atmospheric, oce- anic, and cryospheric processes responsi- determine ice mass balance. Despite this, in situ ble is still debated, the observed increase meteorological observations in the Amundsen Sea in total Antarctic sea ice extent is con- embayment remain very sparse. sistent with the predictions of coupled climate model experiments when per- turbed with an addition of meltwater from the Antarctic Ice Sheet (Richardson et al., 2005). Nonetheless, there is evi- dence that the waters of the Antarctic bor in the twenty-first century, but that deepens” over the Antarctic continental Circumpolar Current are warming (Gille, this remains uncertain. Modeling results slope to form the Antarctic Slope Front, 2008). Available data, though sparse, sug- (Joughin et al., 2014) suggest that unstable associated with westward flow around gest statistically significant warming and retreat is already underway for Thwaites Antarctica in the slope current. The shoaling of the CDW core, particularly at Glacier, but that eventual collapse is not depth of the pycnocline, a key control on the West Antarctic Peninsula and on the likely to occur for hundreds of years. exchange across the shelf break, may be Bellingshausen and Amundsen Sea conti- Why are the ice shelves thinning and strongly dependent on the surface wind nental slopes (Schmidtko et al., 2014); fur- losing mass? For some Antarctic ice stress (Thoma et al., 2008; Spence et al., ther long-term observations are required shelves, such as the Larsen Ice Shelf to 2013; Jenkins et al., 2016, in this issue). to clearly distinguish trends from decadal the east of the Antarctic Peninsula, it is In the eastern Weddell Sea upstream of variability. On the Antarctic continental suggested that warmer surface winds the vast Filchner-Ronne Ice Shelf system, shelf, the temperature of the water at the are causing the ice shelf to disinte- the pycnocline characteristically inter- seabed (often the warmest location in the grate (Doake et al., 1998; Elvidge et al., sects the continental slope well below water column) shows a decadal warming 2016). However, in most other locations the shelf break, and the Antarctic Slope trend in the Bellingshausen Sea and fresh- around the continent, and especially in Undercurrent flows eastward on the slope ening in the Ross Sea (Jacobs et al., 2002; the Amundsen, Bellingshausen, and West (Chavanne et al., 2010).