Drivers of Oceanic Change in the Amundsen Sea (DeCAdeS)

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Sea level rise is one of the most disruptive consequences of global warming, threatening coastal populations and infrastructure worldwide. If we are to develop strategies to either adapt to, or mitigate against, that threat, we need to know what to expect in the future. The biggest uncertainty in estimates of future sea level is the contribution of the vast Antarctic Ice Sheet. Observations of thinning in some parts of the ice sheet have led to suggestions that an irreversible change may already be underway that could add over a metre to sea level over the coming centuries.Thinning is most prominent in the Amundsen Sea sector of the ice sheet, where it has been observed to spread inland from the coast, and to affect neighbouring outflow glaciers in a similar way. Those facts demonstrate that some change in ocean-driven melting of the glacier termini has been the trigger of change, leading to a widespread belief that warming of the ocean waters, driven ultimately by global warming, is responsible. However, observations of ocean temperature in the Amundsen Sea suggest a more complex history. The records start in 1994, and include only a few observations prior to 2009, but suggest cycles between warm and cool conditions occurring over decadal periods. That motivates a major rethink of how the ocean interacts with the ice sheet to produce the observed thinning.In this project we plan to exploit new techniques to fill the gaps in the record of ocean temperature change in the Amundsen Sea. We will modify a robotic submarine so that it can over-winter beneath the pack ice, periodically measuring the properties and strength of the currents carrying warm water towards the ice. Those measurements will be complemented by fixed instruments that record continuously at selected locations and seal-borne sensors that will record the depth of the warm water wherever and whenever the seals dive below the surface to feed. We will relate these detailed observations of what is happening below the sea surface to changes in the height of the sea surface that can be detected by satellites. That will enable us to exploit the satellite records collected over three decades to infer past changes in the sub-surface ocean. The results will allow us to confirm the timing and magnitude of recent warm and cool cycles and relate them directly to the records of ice sheet thinning.To extend our knowledge of Amundsen Sea temperatures beyond the satellite era we will use a numerical model of the ocean circulation in the region to identify the patterns of atmospheric forcing that were responsible for the changes in temperature that we have observed. We will then examine reconstructions of the past atmospheric circulation to generate a history of the key atmospheric changes. Finally, we will investigate how those changes in the regional atmospheric circulation relate to global scale atmospheric change, providing us with the longer-term perspective that is needed to address the questions of what past conditions initiated the current thinning and what the future might hold.Should we find that the most recent decade is typical, and that earlier decades have been characterised by similar cycles in ocean forcing, we will have shown that predicting the future of the ice sheet requires an understanding of its response to extremes. Much like the coastal engineer planning for the impacts of climate change, who must construct sea defences to protect against the extreme levels generated when storm surges coincide with high tides, we need to understand how long and severe the warm extremes in the Amundsen need to be in order to trigger episodes of ice sheet thinning. We also need to know what combination of larger-scale modes of atmospheric variability produces the "perfect storm" in the Amundsen that can push the ice sheet out of balance. Our project will deliver the knowledge needed to address those critical questions.