Poleward shift of Circumpolar Deep Water threatens the East Antarctic Ice Sheet

Abstract

Future sea-level rise (SLR) projections carry large uncertainties, mainly driven by the unknown response of the Antarctic Ice Sheet to climate change. During the past four decades, the contribution of the East Antarctic Ice Sheet to SLR has increased. However, unlike for West Antarctica, the causes of East Antarctic ice-mass loss remain largely unexplored. Here, using oceanographic observations off East Antarctica (80º-160ºE) we show that mid-depth Circumpolar Deep Water has warmed by 0.8-2.0°C along the continental slope between 1930-1990 and 2010-2018. Our results indicate that this warming may be implicated in East Antarctic ice-mass loss and coastal water-mass reorganisation. Further, it is associated with an interdecadal, summer-focused poleward shift of the westerlies over the Southern Ocean. As this shift is predicted to persist into the 21st century, the oceanic heat supply to East Antarctica may continue to intensify, threatening the ice sheet's future stability. Main textThe Antarctic Ice Sheet has been losing mass at an accelerated rate since the 1970s 1 , and now accounts for ~15% of global SLR 5 . In West Antarctica, the ice-mass loss has been attributed to a wind-driven increase in the transfer of warm CDW toward the floating ice shelves 2,3 . As the ice shelves melt, their mechanical buttressing of tributary glaciers is reduced, leading to faster glacier flow toward the coast and greater freshwater input to the ocean 6 . Consistent with this view, observed interdecadal trends in West Antarctic ice-mass loss have been linked to warming of the continental shelf's deeper oceanic layers, with either the evolving decadalmean temperature 7 or amplitude of interannual temperature variability 2,8 posited to exert the key melting-controlling role. A singular feature of West Antarctic change is that the measured ocean warming occurs on the continental shelf 2 , rather than in the Antarctic Circumpolar Current (ACC) offshore 2 , pointing to a perturbed rate of CDW transport across the shelf break as the warming's underpinning factor.The ice-mass loss in East Antarctica is dominated by changes in the Wilkes Basin 9 (100º-142ºE), in the eastern Indian Ocean sector. Its causes remain both undetermined and little probed. The presence of modified CDW in proximity to several ice shelves 10,11,12 hints at an increased delivery of heat from the ACC plausibly being implicated in regional ice-mass loss. However, evidence of such an increase, and knowledge of its possible forcings, are currently scarce. While investigations of the interdecadal evolution of CDW temperature reveal a widespread, persistent warming to the north of and within the ACC, equatorward of ~60ºS 7,13,14 , there has been little indication of robust long-term temperature changes further to the south. Only recently, two studies 15,16 have put forward the suggestion that a CDW warming might be underway poleward of ~60ºS, possibly linked to a southward shift of the ACC's southern boundary in the eastern Indian region (115-150ºE) 15 or a CDW shoaling so...

Vulnerability of Denman Glacier to Ocean Heat Flux Revealed by Profiling Float Observations

“…The changes in glacial flow may reflect an increase in ocean heat transport to the base of the glacier and/or ice dynamics (i.e., ice tongue thinning, changes in ice tongue structure following calving, or release from pinning points) (Miles et al., 2021; Rignot et al., 2019). While changes in ocean heat transport cannot be assessed with available observations, recent studies have documented warming of waters off East Antarctica that may have increased ocean heat transport to the continental shelf (e.g., Herraiz‐Borreguero & Naveira Garabato, 2022; Yamazaki et al., 2021).…”

Glaciological history and structural evolution of the Shackleton Ice Shelf system, East Antarctica, over the past 60 years

“…The high basal melt rates close to the grounding line in this region of Antarctica have been linked to a warming of up to 0.5 • C over the last 40 years that occurred in the open ocean off East Antarctica, concurrent with an even more pronounced warming of 0.8-2 • C observed over the continental slope (Herraiz-Borreguero and Naveira Garabato, 2022). This Circumpolar Deep Water (CDW) warming is linked to a poleward shift of the Antarctic Circumpolar Current's southern extent into the Indian Ocean sector of the East Antarctic continental slope, in which the Shackleton system is located.…”

Extensive and anomalous grounding line retreat at Vanderford Glacier, Vincennes Bay, Wilkes Land, East Antarctica

“…If grounding line retreat continues at the present rate, this retrograde slope will be reached within 100 years, raising the potential for MISI. With basal melt predicted to further inhibit the formation of DSW within the Vanderford polynya (Ribeiro et al, 2021;Herraiz-Borreguero & Garabato, 2022), mCDW intrusion may be enhanced, thereby providing the oceanic forcing required to drive further grounding line retreat. Although the dynamic response of Vanderford Glacier has been limited thus far, a consistent thinning trend was seen over the observational period, measured between -0.07 and -0.12 m yr -1 (2003-2017).…”

“…Ribeiro et al (2021) therefore suggested that a positive feedback may be initiated, whereby continued freshwater input from basal melt could hinder the formation of Dense Shelf Water (DSW) in the Vanderford polynya, thereby strengthening water column stratification and enabling the enhanced intrusion of warm mCDW at depth. Analysis by Herraiz-Borreguero & Garabato (2022) also suggests that this potential feedback may already be underway, with an observed decline in DSW concurrent with consistent sea ice production indicative of increased penetration of mCDW within the Vincennes Bay region. Whilst Vanderford Glacier is currently grounded on a prograde bedrock slope (Rignot et al, 2019), it overlies the Vanderford Trench, a deep subglacial trench characterised by an inland retrograde slope (Figure 1b) (Davis et al, 1986;Chen et al, 2011;Sun et al, 2016).…”

“…2022). With climatic models predicting that the southern annular mode will continue trending towards its positive phase under high-emission scenarios (Zheng et al, 2013;Lim et al, 2016;Lee et al, 2021), further CDW warming may be expected off East Antarctica, enabling enhanced basal melt rates (Herraiz-Borreguero & Garabato, 2022). Subsequent increased outflow of glacial meltwater will likely further hinder the formation of DSW in the Vanderford polynya, thereby facilitating the enhanced incursion of warm mCDW at depth (Ribeiro et al, 2021).…”