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How the ocean melts Antarctic ice

The floating ice shelves around Antarctica are key to buttressing land-based ice. Observations, simulations and analyses from around Antarctica now identify mechanisms that lead to basal melting of these vulnerable shelves.

What happens in Antarctica does not stay in Antarctica: sea level rise and changed ocean properties caused by melting ice sheets will influence the globe. Changes occurring around the Antarctic margins, in particular, have profound impacts. Processes occurring on the Antarctic shelf control ice discharge into the ocean from the Antarctic ice sheet and thus global sea level rise1, the global ocean overturning circulation2 and global surface climate3. Yet, because the environment is so remote and extreme, the Antarctic margins remain sparsely observed. Further, simulating the range of processes important on the shelf is highly challenging. Three studies in Communications Earth & Environment, by Friedrichs and colleagues4, Aoki and colleagues5 and Verfaillie and colleagues6, investigate local, regional and hemispheric conditions and processes that influence how the ocean melts ice shelves.

Ice shelf melting from below, known as basal melting, is the dominant ice-loss process in Antarctica7. Because ice shelves float on the ocean (Fig. 1), sea level does not change when they melt. However their presence slows down the flow of the grounded Antarctic ice sheet from land into the ocean, which does affect sea level. Without buttressing ice shelves, the large, land-based ice sheets can flow into the ocean more quickly. There, they melt and raise sea level. Observations show this is happening now and the process is expected to accelerate over the coming century1. It is crucial that we understand the mechanisms involved in ice shelf melting to constrain future projections of ice sheet melt.

Fig. 1: The vulnerable beauty of Antarctic ice shelves.
figure 1

Three articles4,5,6 in Communications Earth & Environment identify mechanisms that could lead to melting from underneath. Credit: Richard Selwyn Jones, taken at Vanderford Glacier ice shelf in February 2022.

The three studies in Communications Earth & Environment4,5,6 focus on the role of ocean warming in ice shelf melt from different perspectives and scales. They consider mesoscale near-surface transport of heat by an ocean eddy4, regional-scale surface warming associated with a sea-ice albedo feedback5 and hemispheric-scale wind-driven subsurface warming6. All these processes influence basal melting of ice shelves.

Observations are critical to understanding how the ocean melts ice shelves. Friedrichs and colleagues4 present ship-based and autonomous underwater glider observations taken during the austral summer of 2018/19 in front of and beneath the Nansen Ice Shelf in Terra Nova Bay in the western Ross Sea (75°S, 163°E). They observed a 10 km wide eddy at the calving front of the ice shelf and investigate the vertical heat transport processes driven by the eddy. Seeking to gain insight into how these eddies affect heat transport beneath ice shelves, and ultimately drive basal melting, they find that the eddy promoted upwelling of cold subsurface water as well as a deepening of the warm surface layer, resulting in a substantial and complex pattern of vertical heat transport. Substantial basal melting was not observed during their observational campaign, but Friedrichs and colleagues4 note the vulnerability of the Nansen Ice Shelf to recurring summertime eddies that could drive basal melt and trigger fracturing and calving.

Considerable research emphasis has been placed on understanding the processes that cause melting of warm-cavity ice shelves such as those in the Amundsen Sea, because of their potential to retreat rapidly8. By contrast, relatively little is known about melting in cold-cavity ice shelves. Friedrichs and colleagues4 show how heat can be transported beneath a cold-cavity ice shelf and their findings are applicable to many Antarctic ice shelf systems.

Further west in East Antarctica, Aoki and colleagues5 combine observational and model analyses of conditions in Prydz Bay (north of 70°S, 73°E) to understand melting of the Amery Ice Shelf and its implications during the unusually warm austral summer of 2016/17. They seek to untangle the complexity of feedbacks between different components of the climate system.

In the spring of 2016 and continuing through to the summer of 2016/17, Antarctic sea ice declined dramatically to then-record low levels9. Aoki and colleagues find that in Prydz Bay, the very sparse sea ice cover during summer 2016/17 was associated with warm sea surface temperatures: these two conditions probably related through the ice-albedo feedback. In mooring and ship-based measurements they also detected a record high glacial meltwater fraction in the surface ocean, indicative of higher than usual ice-shelf melting. Numerical simulations suggest that this melt was caused by the unusually warm surface ocean.

The extra meltwater observed by Aoki and colleagues caused the warm surface waters to freshen. Usually, when sea ice growth begins in autumn, surface waters become denser and sink over the continental shelf. Eventually, this dense water sinks further off the shelf, and moves slowly across the bottom of the global ocean. These density-driven processes on the Antarctic shelf comprise an important component of the global meridional overturning circulation that controls the ocean’s uptake of heat and carbon from the atmosphere2. Importantly, Aoki and colleagues find that in the autumn of 2017, the unusually warm and fresh surface waters delayed the formation of dense water.

Aoki and colleagues demonstrate that it is not only ocean warming at depth that has the potential to melt ice shelves: their observations suggest that warm surface waters also play an important role in ice shelf melt. The summer of 2016/17 provided a pertinent example of the warm surface conditions expected in the future: initial low sea ice and a warm surface ocean were reinforced by the ice-albedo feedback and resulted in ice shelf melting and reduced dense water formation. Over time, these processes are likely to accelerate ice sheet melting and sea level rise, and reduce the meridional overturning and ocean uptake of heat and carbon.

Moving up in spatial scale, Verfaillie and colleagues6 present circumpolar simulations with an ocean-sea ice model that resolves the cavities beneath the ice shelves, to understand the influence of the Southern Annular Mode—the hemispheric fluctuation of westerly winds—on ocean circulation on the Antarctic continental shelf. A shift towards the positive phase of the Southern Annular Mode is one of the most prominent changes in the high latitude Southern Hemisphere over the past century10. Observed changes in the Southern Annular Mode are attributed to human influences through both stratospheric ozone depletion and increasing greenhouse gas concentrations, with the latter expected to dominate and continue driving positive trends in the Southern Annular Mode over the coming century11.

Averaged around the continent, Verfaillie and colleagues find that the positive phase of the Southern Annular Mode causes increased upwelling of warm subsurface waters towards ice shelves. Upwelling, in turn, leads to increases in the basal melting of ice shelves. These findings are consistent with our mechanistic understanding of the wind-driven ocean circulation12; it is encouraging to see the effect also in simulations with a circumpolar ice-shelf cavity resolving model. Interestingly, the circumpolar average masks strong regional differences in how simulated shelf temperatures and basal melting respond to wind changes, emphasising that the Southern Annular Mode, often thought simplistically to be a zonally symmetric feature of the climate, can have strong regional impacts. The Amundsen Sea response in the circumpolar simulations6 diverges from another recent regional simulation13. These differences highlight the value in multiple modelling approaches to understand ocean processes around the Antarctic margins, as well as the need to consider the combined influences of various factors on Antarctic basal melt.

The trio of studies enhances our understanding of the various ways in which ocean warming melts ice shelves from below and provide hints as to what processes may control melting in the future: mesoscale features of the ocean circulation can control the transport of heat underneath ice shelves4, warm surface waters can melt ice shelves and affect dense water formation5, and hemispheric-scale winds affect different ice shelves in divergent ways6.

Taken together, the studies highlight the cascade in scale in the processes that drive ice shelf melt. Continued observations, improved modelling, and dedication by the scientific community will be needed to refine our understanding of the ocean’s role in melting Antarctic ice shelves.

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Acknowledgements

This work was supported by the Australian Research Council Special Research Initiative for Securing Antarctica’s Environmental Future (SR200100005). The author is grateful for helpful discussions with Julie Arblaster and suggestions from Heike Langenberg.

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Purich, A. How the ocean melts Antarctic ice. Commun Earth Environ 3, 141 (2022). https://doi.org/10.1038/s43247-022-00471-0

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