Ocean Circulation and Ice Shelf Melting on the Amundsen Sea Continental Shelf

Lead Research Organisation: NERC British Antarctic Survey
Department Name: Science Programmes

Abstract

Sea levels around the world are currently rising by about 2 mm every year. That may not sound very much, but people living in areas such as Holland or East Anglia are already threatened by coastal erosion. If we are to say how that threat might change in the future we must learn how to forecast changes in sea level. To do this we must understand what is happening to the Earth's great reservoirs of freshwater, and whether or not they are slowly draining into the ocean. The largest of these reservoirs by far is the Antarctic Ice Sheet, which contains 70% of all the freshwater on the planet. At present we do not know whether the ice sheet is growing or shrinking overall, but we do know that some parts of it are getting smaller. The fastest changes are happening at the edge of the ice sheet, where it flows into the sea, in a place called Pine Island Bay. Nobody yet knows what is causing these changes, and their speed has taken scientists by surprise. Pine Island Bay is geographically the far south of the Pacific Ocean, and the image of warmth that this conjures up is not entirely misplaced. The air temperatures never rise above freezing and the coast is battered by storms, but beneath the cold surface of the sea, water temperatures rise as high as 1 degree Celcius. This may seem cold by our standards (sea temperatures around Britain rarely drop into single figures, even in winter), but it is warm enough to melt the ice. Pine Island Glacier is a vast river of ice that flows out into Pine Island Bay. It carries as much water as the River Rhine, but in frozen form. The last 75 km of the Glacier floats on the waters of Pine Island Bay, and the bottom melts so intensely that half of the ice carried in the glacier is lost within the space of 30 years. The other half breaks off the end of the glacier as icebergs, which drift away to melt elsewhere. It is not hard to understand that warm water causes rapid melting, but what do 'warm' and 'rapid' really mean? If we change the water temperature by a small amount, by how much will the melt rate change? To find the answers to those questions we must make measurements of the water temperature beneath the glacier, but to do so is enormously challenging. The glacier is between 300 m and 1 km thick, so we cannot get instruments through from above, while the drifting Antarctic pack ice bars access to the front of the glacier to all but the most powerful ships. Engineers working at the Southampton Oceanography Centre have, over many years, designed and built a solution to this problem in the form of a robotic submarine that they can programme to dive beneath the ice, make measurements along a pre-defined track, then return to the surface with the vital data. By teaming up with American scientists, who can make use of a powerful icebreaker, we hope to take the submarine right up to Pine Island Glacier and launch it on its mission beneath the ice.The underwater cavern beneath the glacier is completely unknown and the submarine must find its own way in and out, avoiding any obstacles that it finds along its path. The Antarctic pack ice is notoriously unpredictable and could prove a huge challenge to the ship. But the potential return makes the risks worthwhile. Armed with our new knowledge we will build a computer model that describes the flow of water within the remote cavern beneath the glacier and in the sea to the north of it. Using this model we will determine if there have been any changes in the water temperature in Pine Island Bay over the past 20 years and how such changes would have affected melting of the glacier base. Other scientists can then use our results to establish if changes in the glacier's melt rate could have caused the ice sheet to thin in the way that has been observed, and together we will be able to say with greater certainty what impact the glaciers of Pine Island Bay will have on the future coastlines of Holland and East Anglia.

Publications

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De Rydt J (2014) Geometric and oceanographic controls on melting beneath Pine Island Glacier in Journal of Geophysical Research: Oceans

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Dutrieux P (2014) Basal terraces on melting ice shelves BASAL TERRACES ON MELTING ICE SHELVES in Geophysical Research Letters

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Jacobs S (2013) Getz Ice Shelf melting response to changes in ocean forcing GETZ ICE SHELF MELTING in Journal of Geophysical Research: Oceans

 
Description The Antarctic ice sheet appears to be losing mass at a rate that has accelerated over recent decades. Some of the most significant changes have been observed on Pine Island Glacier, where thinning, acceleration and grounding line retreat have all been observed. Even during the relatively short satellite record, rates of change have been observed to increase. In January 2009 the NERC Autosub3 autonomous underwater vehicle was deployed on six sorties, totalling over 500 km, into the ocean cavity beneath Pine Island Glacier. Data revealed an apparently continuous ridge with an undulating crest that extends across the cavity about 30km in from the current ice front. This topographic feature currently blocks the warmest ocean water from the inner cavity. Features observed on the ridge crest indicate that the glacier was once grounded on it, and satellite imagery from the early 1970's hints that the glacier may still have been in contact with the ridge at that time. These findings suggest that the changes observed by satellite over the past two decades are the continuation of a longer period of retreat that started as the ice began to float free of the ridge. A related modelling study showed how the widening of the gap over the ridge crest, as the ice has thinned, has played a role in driving that thinning by allowing progressively warmer water into the inner cavity to drive an increasing melt rate there. These results also suggested the existence of a threshold beyond which the inner cavity temperature was much less sensitive to further widening of the gap. It appears that the cavity is currently in this latter regime where the temperature in the inner cavity is now controlled by climate forcing from beyond the ice front that determines the thickness of the warm water layer at the seabed and how much of that layer can spill over the ridge. Observations made at the ice front in 2009 were compared with similar data collected in 1994 and analysis of both datasets revealed 50% higher melting caused by the slightly thicker layer of warm water found in 2009. Subsequent thinning of that layer in 2012 halved the melt rate. The flow of the warm water layer onto the continental shelf from the deep ocean surrounding Antarctica is focussed in seabed troughs that cut the continental shelf edge. Analysis of historical data from the shelf edge revealed that the inflows are supplied by an eastward-flowing undercurrent that follows the shelf edge beneath the westward-flowing surface waters. An earlier modelling study suggested the strength of such an undercurrent, and the resulting inflows, should be sensitive to wind-forcing over the shelf edge. A new analysis of the West Antarctic atmospheric response to tropical forcing suggested that inflow of warm water to the continental shelf would be enhanced during a central Pacific El Nino event. Thus our findings indicate how remote climate forcing could drive ice sheet change and why Pine Island Glacier is particularly sensitive to such forcing.
Exploitation Route The new data have transformed our understanding of what might be driving the current changes in Pine Island Glacier. Our discovery of a submarine ridge on which the glacier was once grounded suggests that the changes we see now are part of a longer-term process that started many decades ago at least. This implies longer-term climate-driven change in Antarctica, and the search for forcing is now underway. This was the first use of autonomous underwater vehicle technology to obtain such an extensive dataset from beneath an ice shelf. Many other groups worldwide are now trying to emulate the success of NERC in the application of this technology. Work will soon start within NERC to adapt the new Autosub Long Range to undertake even more extensive sub-ice-shelf missions.
Sectors Environment

 
Description The results of this work have been cited in Chapters 4 (Observations: Cryosphere) and 13 (Sea Level Change) of the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (Climate Change 2013: The Physical Science Basis). The results have also been incorporated into lectures presented at seven international summer schools aimed at postgraduate students from across the globe. The success of the under-ice autonomous underwater vehicle missions has proved a stimulus to the development of such technology internationally. In particular a "Mobile Underwater System Tools" programme has now been established in Sweden and the PI has been invited to join the Advisory Board.
First Year Of Impact 2013
Sector Aerospace, Defence and Marine,Education,Environment
Impact Types Policy & public services

 
Description Cruise NBP0901 
Organisation Columbia University
Department Lamont Doherty Earth Observatory
Country United States 
Sector Academic/University 
PI Contribution Deployment of Autosub3 beneath Pine Island Glacier, West Antarctica
Collaborator Contribution Fifty day cruise on Nathaniel B Palmer to Pine Island Glacier to deploy Autosub3
Impact All the outcomes of NERC grant NE/G001367/1 resulted from this cruise, without which the project could not have been undertaken.
Start Year 2009