Antarctic Circumpolar Current: Implications for the Antarctic Continental Shelves

OceTHE OFFICIALa MAGAZINEn ogOF THE OCEANOGRAPHYra SOCIETYphy CITATION Gille, S.T., D.C. McKee, and D.G. Martinson. 2016. Temporal changes in the Antarctic Circumpolar Current: Implications for the Antarctic continental shelves. Oceanography 29(4):96–105, https://doi.org/10.5670/oceanog.2016.102. DOI https://doi.org/10.5670/oceanog.2016.102 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 Temporal Changes in the Antarctic Circumpolar Current IMPLICATIONS FOR THE ANTARCTIC CONTINENTAL SHELVES By Sarah T. Gille, Darren C. McKee, and Douglas G. Martinson 96 Oceanography | Vol.29, No.4 One of the major sources for climate change in the Southern Hemisphere comes from the middle atmosphere, approximately “ 10–50 km above Earth’s surface. ” ABSTRACT. Some of the most rapid melting of ice sheets and ice shelves around serve as a conduit that connects atmo- Antarctica has occurred where the Antarctic Circumpolar Current (ACC) is in close spheric forcing to the ocean interior, proximity to the Antarctic continent. Several mechanisms have been hypothesized by allowing the Southern Ocean to change which warming trends in the ACC could lead to warmer temperatures on the Antarctic in response to a changing atmosphere. continental shelves and corresponding thinning of ice shelves. One possibility is that Profiling float observations collected a southward shift in the dominant westerly winds has led to a southward shift in the since 2004 indicate that the Southern ACC, bringing comparatively warm (1°C–3°C) Circumpolar Deep Water (CDW) in Hemisphere oceans, poleward of 20°S, closer contact with Antarctica; however, satellite altimetry does not provide strong show a more rapid increase in heat con- evidence for this option. A second possibility is that stronger winds have led to stronger tent than any other sector of the global poleward eddy heat transport, bringing more CDW southward. In addition, submarine ocean (Roemmich et al., 2015). Over canyons and winds are hypothesized to be critical for transporting CDW across the multidecadal time periods, observa- continental shelves. The specific mechanisms and the relative roles of westerly winds, tions indicate that the Southern Ocean easterly winds, and wind-stress curl remain areas of active research. has experienced some of the most sig- nificant long-term warming in the global INTRODUCTION difference between the north and south ocean (e.g., Gille, 2002, 2008; Böning et al The Antarctic Circumpolar Current of the Polar Front is about 3°C–4°C 2008). This warming extends through the (ACC) is the major current system of the (e.g., Dong et al., 2006). full water column: warming of bottom Southern Ocean, transporting approx- Physical oceanographers often hypoth- water in recent decades is more evident imately 150 × 106 m3 s–1 of water east- esize that the ACC fronts isolate the in the Southern Ocean than in bottom ward around Antarctica (e.g., Ganachaud Antarctic marginal seas from the mid- waters elsewhere in the world (Purkey and Wunsch, 2000; Rintoul and Sokolov, latitude gyres that transport heat pole- and Johnson, 2010, 2013). 2001; Mazloff et al., 2010; Griesel et al., ward within the ocean. This occurs not Because of the ACC’s role as a bar- 2012). The ACC consists of multiple only because of the water mass prop- rier between the mid-latitudes and the fronts, each corresponding to jet-like erty contrasts across the fronts, but Antarctic margin, we might expect that currents. From north to south, the fronts also because the fronts coincide with changes in the region of the ACC would are referred to as the Subantarctic Front, steeply tilted isopycnal surfaces that rise have little impact on the Antarctic con- the Polar Front, and the Southern ACC from 1,000 m to 2,000 m depth in mid- tinental shelves. However, evidence sug- Front (Figure 1; Orsi et al., 1995), though latitudes to outcrop at the surface within gests otherwise. Figure 2 shows regions of each of them may have multiple quasi- the ACC (Martinson, 2012). Water par- rapid ice mass loss from the continent (red stable manifestations (Sokolov and cels preferentially mix along isopycnals dots) determined by Rignot et al. (2008) Rintoul, 2009a). The fronts, which are rather than across them, bringing water and the mean ACC position (shaded). steered by bathymetry, are top-to-bottom from mid-depth into contact with the The latitude of the ACC varies along its features and serve as major water mass atmosphere but preventing it from reach- circumpolar path, and the latitude of the transitions, separating waters with sub- ing the Antarctic continental margins Antarctic continental margin also var- stantially different densities and cor- (e.g., Marshall and Speer, 2012). ies with longitude, meaning that the dis- respondingly different temperature While the tilted isopycnals of the ACC tance between the ACC and the conti- and salinity properties. For exam- may help to confine warmer, less-dense nent can be thousands of kilometers or ple, in surface waters, the temperature water to the north of the ACC, they also just a few hundreds of kilometers. Rapid Oceanography | December 2016 97 ice melt is concentrated in the Amundsen changes. We follow with an examina- originates in the ACC (e.g., Orsi et al., and Bellingshausen Seas, just upstream of tion of the roles that wind and submarine 1995), has temperatures of 1°C–3°C Drake Passage, and in the region south canyons play in bringing water from the (e.g., Santoso et al., 2006; Schmidtko et al., of Australia. Strikingly, these regions are ACC onto the Antarctic continental shelf. 2014) and has warmed along with the also places where the ACC has its closest The article concludes with a summary of Southern Ocean as a whole. CDW can be approach to the Antarctic continent. the current understanding of ACC influ- transported onto the Antarctic continen- The goal of this paper is to review the ences on Antarctic ice shelves. tal shelf, and because it is dense relative possible mechanisms by which changes to shelf waters, it can circulate through in the ACC might influence water prop- BACKGROUND: LINKING THE the cavities beneath the ice shelves. In erties on the Antarctic continental ATMOSPHERE AND OCEAN TO addition, CDW is comparatively warm, shelves and correspondingly control ice THE CRYOSPHERE which can lead to basal melting of the ice melt. First, we review the oceanic pro- Recent studies (e.g., Jacobs et al., 2011; shelves and eventually ice shelf collapse. cesses that are hypothesized to contrib- Pritchard et al., 2012) emphasize the Ice sheet thinning or collapse can, in ute to glacial melt rates and also the role possibility that the ocean plays a critical turn, lead to thinning of the continental that changing atmospheric processes are role in determining how Antarctic con- ice sheets because their buttressing has thought to play in the forcing that drives tinental ice sheets melt. These ice sheets been weakened or removed. Evidence the ACC. We then evaluate observed are buttressed in part by the floating ice from satellites and aircraft surveys indi- changes in the ACC and discuss whether shelves that extend out over the ocean. cates that the ice shelves have thinned in they are consistent with observed forcing Circumpolar Deep Water (CDW), which recent years (Rignot et al., 2013; Paolo et al., 2015), predominantly as a result 0° of basal melting (Rignot et al., 2013; see Dinniman et al., 2016, in this issue, for a review basal melting mechanisms). 40°E Figure 2 shows that regions of signifi- 40°W cant ice shelf thinning largely coincide with regions of substantial ice loss from the Antarctic continent where the ACC is in close proximity to the continent. These observations indicate the possibility that the system could be highly sen- 80°E sitive to modifications in ocean circu- 80°W lation (e.g., Hellmer et al., 2012). Thus, a critical goal has been to understand the chain of climate-related events that allow CDW to enhance basal melting of ice shelves. One of the major sources for climate change in the Southern Hemisphere 120°W comes from the middle atmosphere, 120°E approximately 10–50 km above Earth’s surface. Anthropogenic emissions of chlorofluorocarbons during the twen- tieth century resulted in a stratospheric ozone hole over the South Pole, start- 160°W 160°E ing in the 1980s. The ozone hole resulted meters in exceptionally cold temperatures over the pole in spring and summer and cor- –6,000 –5,000 –4,000 –3,000 –2,000 –1,000 0 respondingly intensified temperature FIGURE 1. Major frontal features that define the Antarctic Circumpolar Current (ACC) gradients between the pole and mid- superimposed over bathymetry: the Subantarctic Front (magenta), the Polar Front (dark latitudes, as well as a stronger polar vortex blue), and the Southern ACC Front (black). The figure also shows the Subtropical Front (red dotted line) and the Continental Water Boundary (black dotted line). Frontal positions (e.g., Baldwin et al., 2003). These strato- from Orsi et al. (1995) and bathymetry by Smith and Sandwell (1997) spheric effects can propagate downward 98 Oceanography | Vol.29, No.4 through the troposphere (D.

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