Vigorous lateral export of the meltwater outflow from beneath an Antarctic ice shelf

• Published in: Nature (London) Vol. 542; no. 7640; pp. 219 - 222
• Main Authors: Garabato, Alberto C. Naveira, Forryan, Alexander, Dutrieux, Pierre, Brannigan, Liam, Biddle, Louise C., Heywood, Karen J., Jenkins, Adrian, Firing, Yvonne L., Kimura, Satoshi
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 09.02.2017; Nature Publishing Group
• Subjects: 704/106/829/2737; 704/829/2737; Antarctic Regions; Carbon Cycle; Climate; Climate Change; Ecosystem; Freezing; Fresh Water - analysis; Global climate; Humanities and Social Sciences; Hydraulic flow; Ice; Ice Cover - chemistry; Ice shelves; Icebergs; Kinetic energy; letter; Marine ecosystems; Melting; Meltwater; Models, Theoretical; multidisciplinary; Natural history; Ocean circulation; Oceans and Seas; Scale models; Science; Sea ice; Sea level; Seawater - analysis; Seawater - chemistry; Surface water; Surface-ice melting; Vertical distribution; Water circulation; Water flow; Water Movements; Antarctica; Southern Ocean
• ISSN: 0028-0836; 1476-4687; 1476-4687
• DOI: 10.1038/nature20825

The mechanism producing Antarctic meltwater at depth is elucidated and modelled. Ice-shelf meltwater gets deep As ice shelves melt, they deliver fresh water to the ocean. The resulting mixing process affects not only the salinity of the ocean, but also the upwelling of warmer deep waters. Climate models use an extremely simplified approximation with ice-shelf meltwater being delivered near the surface, but observations show lateral export at depth instead. This shortcoming in the models is understandable, however, as the underlying physical mechanisms have remained unclear. Alberto Naveira Garabato et al . now present a series of detailed observations from Pine Island Bay, Antarctica. They show that the meltwater plume coming from the base of the ice shelf is diverted by lateral shear forces in a process called centrifugal instability. Idealized modelling suggests that the process is likely to be important for Antarctic ice shelves. The instability and accelerated melting of the Antarctic Ice Sheet are among the foremost elements of contemporary global climate change 1 , 2 . The increased freshwater output from Antarctica is important in determining sea level rise 1 , the fate of Antarctic sea ice and its effect on the Earth’s albedo 4 , 5 , ongoing changes in global deep-ocean ventilation 6 , and the evolution of Southern Ocean ecosystems and carbon cycling 7 , 8 . A key uncertainty in assessing and predicting the impacts of Antarctic Ice Sheet melting concerns the vertical distribution of the exported meltwater. This is usually represented by climate-scale models 3 , 4 , 5 , 6 , 7 , 8 , 9 as a near-surface freshwater input to the ocean, yet measurements around Antarctica reveal the meltwater to be concentrated at deeper levels 10 , 11 , 12 , 13 , 14 . Here we use observations of the turbulent properties of the meltwater outflows from beneath a rapidly melting Antarctic ice shelf to identify the mechanism responsible for the depth of the meltwater. We show that the initial ascent of the meltwater outflow from the ice shelf cavity triggers a centrifugal overturning instability that grows by extracting kinetic energy from the lateral shear of the background oceanic flow. The instability promotes vigorous lateral export, rapid dilution by turbulent mixing, and finally settling of meltwater at depth. We use an idealized ocean circulation model to show that this mechanism is relevant to a broad spectrum of Antarctic ice shelves. Our findings demonstrate that the mechanism producing meltwater at depth is a dynamically robust feature of Antarctic melting that should be incorporated into climate-scale models.