BAS Polar oceans

Lead Research Organisation: NERC British Antarctic Survey

Abstract

Global ocean circulation is one of the few mechanisms by which polar processes can directly influence the whole Earth System, including the UK, and possibly on timescales as short as decades. Its importance results from the enormous capacity of the ocean to store and redistribute heat, fresh water, carbon dioxide and other climatically-important substances. The polar regions are disproportionately important in determining the strength and shape of global ocean circulation. The Polar Oceans programme investigates the role of processes and changes both in the shelf sea and in open-ocean environments, and will further our understanding of polar control of the Earth System. Programme Goals are: a) to explain the processes that drive and close the overturning circulation in the Southern Ocean; b) to determine the impact of, and the feedback between, the ocean and ice shelves; c) to understand the physical drivers of changes in the marine environment, and the likely implications for climate; d) to determine the impacts that changes in the polar regions have on the Earth System via ocean circulation; and e) to measure and understand the changes in properties in key water masses of polar origin.

Publications

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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 oil and gas industry uses data collected from marine survey vessels to analyse the rock layers and hydrocarbons beneath the sea. The vessels tow an array of multiple cables ("streamers"), up to 10 km long, containing hundreds of hydrophones recording data from a seismic source. If the ocean currents are known, the streamers can be actively steered to achieve even spacing and more efficient operations with the potential for reduced environmental impact. But routine ocean current measurements, e.g. from satellite data, are not of sufficiently high resolution. A research project, created and funded collaboratively with Schlumberger Gould Research, has developed an inverse method for inferring the required ocean currents using information from the streamers themselves, paving the way for more efficient and economic surveys. This information on the small-scale ocean currents is also of interest to oceanographers precisely because it is finer than the resolution of satellite data. These small-scale currents are poorly represented in climate models and improvements in our understanding may directly contribute to improved climate model projections. To date, the key impacts of the research have been: (i) Derivation of more accurate vertical streamer profiles and prediction of horizontal streamer movement, which can be used in techniques to develop improved spatio-temporal reservoir monitoring. (ii) Prediction of horizontal streamer movement within local ocean currents, which is valuable input to streamer control mechanisms that minimise seismic noise and produce operational efficiencies.
First Year Of Impact 2012
Sector Energy
Impact Types Economic

 
Description Energy & Climate Change Select Committee inquiry into IPCC
Geographic Reach National 
Policy Influence Type Gave evidence to a government review
Impact Emily Shuckburgh was a witness for the Energy and Climate Change Select Committee's inquiry into the Intergovernmental Panel on Climate Change Fifth Assessment Report, appearing in person and submitting written evidence.
URL http://www.parliament.uk/business/committees/committees-a-z/commons-select/energy-and-climate-change...
 
Description Government advice on climate science
Geographic Reach Multiple continents/international 
Policy Influence Type Membership of a guidance committee