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SPRI Review 1995: SPRI Review 1995

SPRI Review 1995

Research Overviews

Sea Ice and Polar Oceanography Group

Dr P Wadhams, Reader in Polar Studies
Dr N.R. Davis, E. Aldworth, M.A. Brandon, D.R. Crane, M. Huddleston, E. Prussen, D. Sheard, M.A. Tadross, S. Wells, M. Aitchison, F. Boud

The research work of the Group is concerned with air-sea-ice interaction processes in polar oceans and their role in climate, and also with the dynamics and mechanics of sea ice. Activities during 1994-95 focused on research projects in the Greenland Sea, especially in the central gyre region where winter convection occurs; in the Beaufort Sea with studies of ice deformation; and in the Weddell Sea in the Antarctic.

The largest research programme with which the group is involved is ESOP, the European Subpolar Ocean Programme. This is funded by the Commission for the European Communities under the MAST-II (Marine Science and Technology) Programme, and is the fourth largest of the 70 projects in MAST-II. The research involves 21 partners in seven countries (Denmark, France, Germany, Iceland, Norway, Sweden, and the UK) and the project is co-ordinated from SPRI by Dr Wadhams. The purpose is to study ice-ocean interaction in the Greenland Sea with special reference to the role of sea ice in winter convection. In the central Greenland Sea, the ice cover of the East Greenland pack frequently develops an eastward tongue in the latiutide range 72-76°N, where the Jan Mayen Polar Current diverts cold water to the east. The tongue is due to local ice growth, which occurs in the form of frazil and pancake ice owing to extreme sea states. Salt rejected from the growing sea ice contributes to the overturning of the surface water, leading to convective plumes that carry water down to the deep ocean. Evidence from tracers suggests that since 1982 convection has failed to reach the deep waters and is now terminating at depths of 1500 m and, latterly, less than 1000 m. This is having a profound effect on the Greenland Sea deep water, which is becoming warmer and more saline, and may also accelerate global warming, since the convection process acts as a sink for carbon dioxide carried down from the surface waters.

During 1994-95 the Group was involved in the analysis and interpretation of data from field experiments, in continued work with remote sensing imagery of the ice cover, in modelling studies, and in synthesis of the results with the findings of our European colleagues. Modelling work was carried out by Matthew Huddleston (Research Student), who is funded by a CASE studentship between NERC and the Meteorological Office (Dr Howard Cattle). His work on ice-ocean modelling of the Greenland Sea involves interaction with the Meteorological Office GCM (General Circulation Model) of the atmosphere, for which purpose a computer link was established between SPRI and the Met Office Cray computer. Remote sensing analysis involved interpretation of ERS-1 SAR (synthetic aperture radar) imagery and SSM/I (Special Sensor Microwave Imager) passive microwave data from the Odden ice tongue in relation to data from field observations. In October 1994 Dr Wadhams hosted a Mid-Term Science Symposium at SPRI, where ESOP participants presented results of the programme so far.

During the year the same group of European partners prepared a successful bid to continue this project for a further three years (1996-99) under the MAST-III programme, the new phase being known as ESOP-2. Another success for MAST-III was a joint proposal with Finnish and Norwegian colleagues called 'Ice State,' to study ice mechanics and their effect on icebreaker operations. Both new contracts will start in February 1996.

In the Greenland Sea, a second MAST-II project has been carried out by the group, a collaboration between SPRI and Dr Flavio Parmiggiani of IMGA-CNR, Modena, again with Dr Wadhams as coordinator. The aim of this project is to spectrally analyse SAR imagery of the Odden region to measure the wavelengths and propagation vectors of the dominant ocean wave species entering the frazil-pancake ice cover from the open sea. Theory developed by Dr Wadhams predicts that wavelength should decrease in ice (and thus refraction will occur if waves are incident obliquely) to an extent that depends on the thickness of the frazil-pancake layer. By examining successive subscenes it is possible to map the thickness of the ice in Odden and thus achieve a better insight into the likely flux of salt into the upper ocean. Others involved in the remote sensing analysis were Mark Tadross (Research Student) and Eileen Aldworth (Research Associate).

The main field work undertaken by the Group during 1994-95 was in the Weddell Sea, Antarctica, where a sea-ice research programme was run from HMS Endurance during February 1995. The project was designed to match surface observations of sea ice properties with aerial photographic swathes and satellite SAR imagery, in order to elucidate the nature of the Antarctic ice edge in late summer and its signature on SAR. The ship sailed from Punta Arenas, with the SPRI participants being Dr Wadhams, Huddleston, and Stephen Wells (Research Assistant). After a call at O'Higgins Base on the Antarctic Peninsula in order to co-ordinate data acquisition with the German SAR receiving station there, the ship entered the northwest Weddell Sea through Antarctic Sound and carried out a series of experiments during a seven-day period. On each day the ship would run into the ice along a line that was being covered by an ERS-1 SAR orbit, using her Lynx helicopters to land the SPRI team on a succession of ice floes. On each floe the party took an ice core, measured snow and ice physical properties, recorded strain and accelerations due to ocean waves, and took CTD (conductivity-temperature-depth) profiles through the uppermost 80 m of the water column to measure melt water effects. Meanwhile the ship was carrying out CTD profiles at 5-10 nml intervals. As near as possible to the exact time of the satellite overflight a helicopter would carry out a 100 nml aerial photography flight along the line of the orbit. By matching these datasets from three levels (satellite, air_borne, surface) we have been able to gain new insight into the radar reponse of Antarctic sea ice, also making the discovery that late summer sea ice in the Weddell Sea is covered with melt pools, a phenomenon previously thought to be confiend to the Arctic. Strain and acceleration measurements were also carried out on two tabular icebergs. The team disembarked at Port Stanley.

In the Beaufort Sea the Group was involved in SIMI, the Sea Ice Mechanics Initiative of the US Office of Naval Research (ONR). The purpose is to understand the mechanics of sea ice deformation in the Arctic on scales ranging from tens of km down to single cracks and individual pressure ridges, in order to be able to improve the parameterisation of these processes in large-scale ice-ocean models. Having taken part in a series of ice camp experiments during 1993-94, the group spent 1994-95 on data analysis, leading to participation in the ONR Sea Ice Mechanics and Arctic Modeling Workshop in Anchorage, Alaska, during April 25-28 1995, in which results were discussed and compared.

During the year Mark Brandon (Research Student) submitted his PhD thesis, entitled Winter surface water mass modification in the Greenland Sea, which was approved; he moved to a research position with British Antarctic Survey. Two Research Associates also left the group after several years of service: David Crane (to join Wimpey Offshore) and Eleanor Prussen (to return with her husband to the USA). Jeremy Wilkinson joined the group as a Research Associate from IASOS (Institute of Antarctic and Southern Ocean Studies) in Hobart, Tasmania.