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SPRI Review 2000: SPRI Review 2000

SPRI Review 2000

Sea Ice and Polar Oceanography Group

Dr P. Wadhams
Dr N.R. Davis, M. Doble, N. Hughes, A. Kaletzky, J. Wilkinson
Y. Aksenov, L. Brigham, D. Flocco, R. Hall, I. Jonsdottir
E. Harker, S. Vaughan, Professor D. Barber, Professor S. Sakai

The Sea Ice and Polar Oceanography Group is concerned with physical processes occurring in the ice-covered seas of the Arctic and Antarctic, with an emphasis on their relevance to climate and climate change. We are deeply involved in two areas of major current interest: the radical thinning of Arctic sea ice, which in summer has now reached 40% relative to the 1970s; and the shutdown of convection in the Greenland Sea associated with a decline in local ice production, which may be a process leading ultimately to a serious cooling of the climate of northwest Europe, including the UK. In addition we are involved in the study of Antarctic sea-ice dynamics and took on new responsibility as co-ordinators for the International Programme for Antarctic Buoys (IPAB).

During 1999-2000, research was undertaken on projects funded by the Natural Environment Research Council, European Commission, and US National Science Foundation. Two successful field programmes were carried out, in the Greenland Sea as part of a study of winter convection, and in the Antarctic to launch an array of buoys to study winter ice dynamics.

A highlight of 1999 was a major data release to us from the United Kingdom Hydrographic Office, Taunton, comprising declassified submarine ice-thickness profiles from eight cruises carried out between 1987 and 1994. The analysis of these, in conjunction with our existing datasets and satellite imagery, will be vital in determining the rate of change of mean ice thickness in the Arctic during the past decade.

NERC funding permitted the analysis and interpretation of ice thickness and along-track oceanographic data collected by Dr Peter Wadhams aboard HMS Trafalgar in the Arctic during August-September 1996, together with an initial assault on the older datasets. Dr Norman Davis was the senior research associate working on this project. [Dr Davis passed away, from cancer, on 8 May 2001]. The datasets available comprise upward-looking sonar profiles of ice thickness, sidescan sonar imagery of the ice underside, and along-track temperature and salinity data. The interpretation of the data showed that during the summer of 1996 there was a reduction of 43% in the ice thickness in the Eurasian Basin of the Arctic Ocean and the Greenland Sea, relative to data collected in 1976 in the same regions; this is a result that agrees with recently published US results from the Canada Basin. The thinning reveals itself through larger amounts of open water, a low mean draft of undeformed ice, and a shortage of deep pressure ridges. The deficit of deep ridges is particularly startling: there were fewer than a quarter as many ridges of draft 9 m or greater in 1996 than in 1976, which will have a major impact on the area-averaged strength properties of the ice cover. A paper on these results was published in Geophysical Research Letters and another is in press with Annals of Glaciology, the Proceedings of the 2000 Fairbanks IGS conference on sea ice.

The US National Science Foundation supported a project to compare the submarine data from the British cruise in 1996 and other years with complementary data collected by a US submarine during the same period in the SCICEX Programme, a five-year US programme whereby one civilian research cruise is carried out each year in the Arctic by a US submarine. The US boat in 1996 covered the western Arctic and the North Pole, and thus slightly overlapped the British boat, which covered the Greenland Sea and Eurasian Basin up to the Pole. Our statistics were compared with similar statistics generated for the US cruise by Dr Terry Tucker of the US Army Cold Regions Research and Engineering Laboratory (CRREL), Hanover, New Hampshire. The product was a mutually validated joint dataset covering the entire Arctic. Nick Hughes was supported as a research associate for this project.

During 1999-2000, work was completed on the 1996 joint datasets and begun on datasets from the 1985-94 series. The analysed data are being included in an international data management system on Arctic Basin ice thickness set up at NSIDC (National Snow and Ice Data Center), Boulder. Common formats for statistical analysis techniques and data layouts were agreed with our CRREL collaborators, and a suite of programs written and tested for the correction, interpolation, and statistical analysis of the processed sonar data. In addition, agreements were negotiated with British MOD/Naval security for the handling and presentation of restricted data from the UKHO release.

On 6-8 September 2000 Dr Wadhams attended a workshop for submarine sonar data providers, which took place at the National Snow and Ice Data Center (NSIDC), CIRES, University of Colorado, Boulder, and in which the future course of joint research on submarine sonar profiling and analysis was determined. Participating in the meeting were representatives from NSIDC (holds public data set of sea-ice draft from submarine upward looking sonar (ULS)); the Arctic Submarine Laboratory (source of all US data and partner in processing and release of data); Scott Polar Research Institute; CRREL (led effort to process and release US data); Bronson Hills Associates (developed procedure and software for ULS data processing), and the University of Washington Applied Physics Laboratory (funded to digitize and process historical US analogue data). The workshop also included discussion of downward looking sonar (DLS), or sounding data, in terms of possible improved equipment for future cruises. Representatives from Lamont-Doherty Earth Observatory of Columbia University (lead in the development of new SCICEX downward looking sonar technology) and the NOAA NESDIS National Geophysical Data Center (source for sounding data and for the International Bathymetric Chart of the Arctic Ocean, or IBCAO) provided a new perspective on issues of error analysis and data declassification. The purpose of the workshop was to tackle immediate, practical data processing issues, based on the common goal of UK and US workers to release all draft and sounding data that meet US and UK navy declassification guidelines, after processing these data in a consistent way. It was resolved to:

  1. document the errors in ULS draft measurements more fully, including errors due to beam width and sensor geometry, and errors due to inability to distinguish open water from thin ice;
  2. include as much additional unclassified information for each cruise as possible in the public (NSIDC) data set;
  3. publish a comprehensive list of all US and UK cruises, with processing status and projected date of release, if available;
  4. urge that declassification guidelines be relaxed, so that the spatial extent of declassified ULS data at least matches that of DLS data released for IBCAO; and
  5. publicize the IBCAO bathymetry chart.

The chart significantly revises the location and depth of many Arctic bathymetric features, and is available on www.ngdc.noaa.gov/mgg/bathymetry/arctic/arctic.html.

Much of the US analogue ULS record of more than 1800 rolls will be digitized under the direction of APL, and released to NSIDC over the coming years. Digitization will be done using a strip chart scanner and special software. Further processing of UK data will be done in the same way but awaits new funding. NSIDC's existing submarine data set can be found in the NSIDC data catalogue.

The Greenland Sea is a region of enormous current interest for the study of climate change, because in recent years a major climatic switch appears to have taken place there. In a 'normal' winter the central Greenland Sea develops a local cover of pancake ice, which occupies a tongue-shaped region stretching northeast from the East Greenland Current (EGC) at 72-75°N. The tongue, called 'Odden' from the Norwegian word for headland, covers the area influenced by the cold Jan Mayen Current, which diverts eastwards from the EGC at these latitudes. Also in a 'normal' winter, the surface waters at the centre of the Greenland Sea gyre overturn to produce convection to the bottom, renewing and ventilating the deep waters. This is one of only four areas in the world ocean where ventilation occurs through open-ocean convective renewal of intermediate and deep waters in winter (the others being the Labrador Sea, western Mediterranean, and Antarctic). Since the early 1970s, convection has failed to reach the deep waters, and both the volume of descending water and the depth attained have diminished until, in recent years, convection has seldom exceeded 2000 m, with 1000 m being typical. At the same time, the Odden has shrunk in area, and during five of the last seven years (including 2000) it has failed to form properly during the winter.

There is a link between these two phenomena, since ice formation involves an immediate salt flux into the surface water (since less than a third of the salt in the freezing water is initially retained in brine cells within the ice cover), with a continuing smaller salt flux as brine drains further from the pancakes. Since the pancakes tend to form in the northwest part of Odden and are blown by cold northwest winds towards the southeastern edge of the ice tongue where they melt, the salt flux over a winter is positive on one side of Odden and negative on the other, a kind of 'salt refining' process. It is on the positive northwest side of Odden that convection is observed to take place, with the salt from ice formation providing a large part of the necessary density increase.

The general implications of a convection shutoff may well be dire for the UK, according to climate models. Greenland Sea convection forms part of the thermohaline circulation, a worldwide 'conveyor belt' driven by heat and salt differences, and a reduction in its strength would lead to a serious cooling of the climate of northwest Europe. This regional cooling has been predicted to take over from the current warming trend in about 100 years' time.

From 1993-99 the Sea Ice Group studied this process through two phases of the European Subpolar Ocean Programme (ESOP), funded by the EU, in which Dr Wadhams was co-ordinator for phase 1 and a principal investigator for phase 2. The programme included winter fieldwork in 1993 and 1997, with results from the ice physics studies, which enabled us to construct and test a physical model of the convection process. This depends on the density enhancement and overturning of surface water through salt rejection from ice formation within the western part of Odden and Nordbukta, with the frazil and pancake ice thus formed being transported eastward by cold westerly winds, and subsequently melting on the outer, eastern fringes of Odden.

In winter 2000 we were able to carry out a further cruise into the Odden region with NERC support under the ARCICE thematic programme. The project is joint with the James Rennell Division of Southampton Oceanography Centre (SOC), with Jeremy Wilkinson working as the Institute researcher. The winter cruise, using the ice-strengthened RV Jan Mayen, chartered from the University of Tromsø, involved 17 scientists led by the Institute, Hamburg University (Dr Jan Backhaus) and the University of Tromsø (Dr Else Hedgpeth), with Dr Wadhams as chief scientist. The staff also included Sheldon Bacon and Robin Pascal (SOC) to deploy a set of pancake ice motion monitoring (PIMMS) buoys in the ice and water, and Leif Toudal (Danish Technical University) to handle on-board satellite data acquisition (AVHRR and SSM/I data) via an Iridium link, which provided near real-time ice edge information.

The ship spent the period from 16 February to 10 March in the central Greenland Sea, mapping ocean circulation, deep and shallow convective intensity, winter ice formation and melt, salt flux associated with ice formation, physical properties of ice, and ice dynamics. The first work was a CTD survey at 75°N from 4°E to 10°W at 1° intervals. This crosses the zone in the centre of the gyre, where convection normally occurs, but at this time there was warm water at the surface (-0.9°C) and a stable upper layer structure. At 8°W on this line we crossed the East Greenland Polar Front and entered the east Greenland ice cover. The next phase of the voyage was a series of transects into and out of the ice cover at decreasing latitudes (75°N, 74°10'N, 73°10'N, 72°30'N, 72°N) interspersed by CTD transects parallel to the ice edge. The aim was to determine the trajectory of the Jan Mayen Polar Current as it separated from the EGC and to examine the along-ice edge variability of ice conditions due to this eastward diversion of cold water. We found that the Jan Mayen Current was actually undetectable, the cold wedge of the EGC being continuous from 75 to 71°N, and this was clearly one reason why no ice cover had formed in the Odden. On account of the absence of this eastward flow, the entire central Greenland Sea was about a degree warmer at the surface than is normal in winter.

The character of the ice edge changed with penetration and with downstream distance. The outer ice edge zone was one of melting, with ice edge bands occurring when the wind was off-ice, no local ice growth, and floes broken by waves into brash fragments. Deeper into the ice the freezing zone began, with a dense mush of frazil and pancake ice growing in the interstices between floes, producing ice very rapidly as the slush was pumped to and fro by wave action and spilled over neighbouring floes and brash lumps. Only in the south were there discrete fields of pure pancake ice, normally seen over the entire central Greenland Sea.

The ice physics work included sampling pancake and brash ice by lifting specimens onto the deck using an 'ice lifter' of two tons capacity designed by Wilkinson and built by the Cambridge Engineering Department. The pancakes were cut into small samples for salinity and temperature profiling and thin section analysis to determine crystal fabric. More than 100 pancakes were analysed in this way. Frazil ice was collected in sampling tubes and weighed to give mass per unit area of sea surface; the samples were analysed for salinity. On larger floes we carried out ice stations involving drilling a core from the ice cover, which was sectioned to give salinity, temperature and ice structure; sampling snow properties; and deploying a hand-held CTD under the ice to measure near-surface water structure. A wave buoy was deployed to measure wave spectra within the ice for comparison with spectra extracted from ERS-2 SAR images; this is a technique developed by ourselves and Italian colleagues as a method of remotely monitoring pancake ice thickness from the change in wavelength of ocean waves in ice. PIMMS buoys were deployed by Bacon and Pascal on and between ice floes and in the open ocean; those deployed on ice floes survived best, mapping the downstream flow rate of the ice in the East Greenland Current.

The last part of the cruise comprised further CTD transects in the region 71-75°N, 18°W to 2°E, covering the central part of the gyre. The region near the gyre centre was found on 7-8 March to be exhibiting thermal convection down to 950 m, having had stratified surface waters during our previous transect on 19 February, a consequence of the succession of cold air outbreaks associated with northerly storms experienced during the intervening period. This is typical of the shallow depth of convection attained when there is no assistance from sea-ice production.

The physics of convection in the Greenland Sea is the subject of a new EU research programme, CONVECTION, with 10 partners from France, Germany, Denmark, Iceland, Italy, Norway, and the UK (two partners: Dr Wadhams as co-ordinator, and Dr Barry Uscinski, DAMTP, Cambridge). This will begin in December 2000 and will run for three years.

A second successful field programme was carried out under a NERC-supported project called STiMPI (Short Timescale Motion of Pancake Ice), a three-year joint project between the Institute and the Dunstaffnage Marine Laboratory, Oban (David Meldrum). The objective is to improve understanding of the mechanisms of Antarctic sea-ice formation and deformation in winter, using a novel design of drifting buoy to clarify the processes involved in deformation, seasonal variability, and dynamics of sea ice in the advancing winter marginal ice zone. A particular interest is the transition between the initial pancake ice form and its later consolidation into first-year ice. Martin Doble is the Institute research associate on the project.

An array of six buoys was designed and constructed for deployment in the highly mobile ice formation zone. The body design, which mimics the shape, draft, and freeboard of the pancake floes among which the buoys were deployed, was developed from design studies of Yannice Faugere, a 1997-98 exchange student from Ecole Supérieure d'Ingenieurs de Marseille. Further development of the buoy and its instrument package was carried out by Doble in collaboration with the Marine Technology Group at Dunstaffnage, and the buoy shared many systems with the pancake drifters designed for the ARCICE cruise by Wilkinson. Global positioning system (GPS) and low Earth orbit (LEO) satellite technology allow location and environmental data to be provided at a frequency high enough to resolve fine detail of the ice motion. The array is mapping the dynamics of first-year ice from its formation in early winter to its decay the following summer. The buoys were launched from FS Polarstern during her ANT-XVII/3 cruise tothe Weddell Sea in April-June 2000, and measure sea surface temperature over wide and narrow ranges, wind speed and direction, vertical acceleration (hence wave height) with spectral moments transmitted, and differential GPS position. A differential GPS base was set up at Neumayer station. Doble represented the Institute on board the 60-day cruise, and was joined by Oli Peppe (DML) and Max Coon (North West Research Associates). The team also carried out a programme of ice physics measurements in the pancake zone of the Weddell Sea, sampling pancake and frazil ice from the ship. They were also able to deploy a remotely operated vehicle under the ice, and flew aerial photography transects using the ship's helicopters. The buoys have emailed their data including GPS position, wave spectra, and meteorological measurements back to the UK via the new low-Earth-orbit Orbcomm system. All buoys survived the transition of the ice cover from pancake ice to pack, with three surviving the whole winter to melt out into the open water of the Antarctic Circumpolar Current.

A paper on the buoy system was presented at Oceanology International 2000 in Brighton in March 2000 by Meldrum, the conference also being attended by Doble. Doble gave a poster paper with initial results from the cruise at the IGS conference on sea ice in Fairbanks in June 2000.

An EU project that continued during 1999-2000 is AMOC, the acoustic monitoring of ocean circulation in the Arctic. The aim is to model the propagation of under-ice acoustic energy across the Arctic Ocean as a way of monitoring the evolving temperature-salinity structure (due to global change) and the changing ice thickness. Both types of change affect the transoceanic travel time for such pulses, either through affecting sound speed or by changing the depth at which upwardly refracting sound is reflected off the bottom of the sea ice. Data used in the project include ice profile data collected during our submarine voyages in the Arctic. Our partners in the project are the Nansen Environmental and Remote Sensing Centre (Bergen, co-ordinators), the St Petersburg branch of the same centre, and the Max-Planck-Institut für Meteorologie (Hamburg). We are using a remote link between Cambridge and the University of Bergen supercomputer for travel time simulations, and also began using the Cambridge High Performance Computer Facility. Our work on the project during 1999-2000 focused on small-scale detailed simulations of acoustic reflection phenomena at the ice-ocean interface, and on simulations of sound propagation at higher frequencies and over shorter ranges than the earlier transoceanic simulations, to improve resolution of ice cover effects. Dr Wadhams and A. Kaletzky attended project meetings in Bergen during the year. Kaletzky was also involved in software design for the SPRI ice drifters, and began design work to assist Dr Uscinski in designing the acoustic shadowgraph moorings planned for use in the forthcoming EU CONVECTION project.

In 1997 the Sea Ice Group was funded by NERC to deploy in an Antarctic coastal polynya the OSCR-II (Ocean Surface Current Radar) system, a shore-based HF radar system that maps surface currents over a 1 km grid out to a range of 50 km using doppler shift. The plan was to mount the system near the Italian base at Terra Nova Bay, Ross Sea, and use it to map currents in the coastal polynya there in co-operation with the Italian CLIMA oceanographic programme in order to determine the physics of heat loss and ice production in the polynya. In 1997 NERC, which maintained the system at Southampton Oceanography Centre, was unable to reclaim the system from its previous borrowers in time to allow the experiment to be done. The next opportunity occurred in the 1999-2000 Antarctic summer season, but during the intervening period the radar system was transferred from NERC ownership to that of Saturn Solutions Ltd, Southampton. Two employees of the company therefore carried out the measurements during December 1999-January 2000 on our behalf, but with only modest success, finding that the two radar stations could be made to work together for less than two days out of the month's deployment.

An in-house project carried out by Nick Hughes was the development of IceCam, an integrated visual monitoring and environmental data logging package for use on ships of opportunity. Hughes took the system to sea on the February 2000 Jan Mayen cruise, its third successful deployment. The design has been taken up for promotion commercially by the Wolfson Cambridge Industrial Unit of Cambridge University, and an internet page providing details of the project can be found at www.spri.cam.ac.uk/people/neh25/IceCam.html. A presentation of results from the instrument, 'Results from the IceCam and its role in Arctic navigation,' was made by Hughes at the EurOCEAN 2000 conference in Hamburg on 29 August 2000. Development of the IceCam system continues, and work has also been carried out to develop a means of measuring ice concentrations from IceCam images. This work was done in conjunction with research student Richard Hall and will improve IceCam's ability to provide ground truth data for satellite sensors.

Captain Lawson Brigham completed his PhD thesis entitled 'Sea ice variability in Russian Arctic coastal seas: influences on the Northern Sea Route,' which was submitted in June 2000 and approved in October 2000. During the year Brigham gave lectures at several conferences.

Yevgeny Aksenov continued his PhD research with support from the EU Ice State programme on sea ice mechanics. He moved to a research associate position at Southampton Oceanography Centre, working on a NERC ARCICE project in ice-ocean modelling whilst completing his thesis.

Finlo Cottier, having been awarded his PhD in 1999 with a thesis on brine drainage channels, moved to a research position in physical oceanography at the Dunstaffnage Marine Laboratory, Oban.

Richard Hall, a research student on a NERC CASE studentship sponsored by Earth Observation Sciences Ltd, Farnham, continued his research on remote sensing and classification of ice types in the Greenland Sea and Kara Sea, concentrating on the signatures of the developing forms of young ice. He took part in the Scoresby cruise to the Greenland Sea in winter 2000.

Ingibjorg Jonsdottir continued her PhD research on the historical statistics of sea-ice distribution around Iceland, moving during the year to a tenured research position at the Science Institute of the University of Iceland whilst completing her thesis. She will be a partner in the forthcoming EU research programme CONVECTION of which Dr Wadhams is co-ordinator.

Daniela Flocco had spent much of 1999 with the group, on a Leonardo da Vinci studentship from the Oceanography Department of the Istituto Universitario Navale, Napoli, during which time she worked on a theoretical model of coastal polynya dynamics for her Master's thesis. Having been awarded this degree she was accepted onto an NERC studentship to begin PhD research with the Group in October 2000, working on a similar topic.

During the year two overseas professors made extended visits to the Group. Professor David Barber, Department of Geographical Sciences, University of Manitoba, visited for three months from August to October 2000, to develop collaboration with Canadian sea ice projects. Professor Shigeki Sakai, Department of Civil Engineering, Iwate University, Morioka, visited for six months starting September 2000, to develop collaboration on current Japanese projects, including wave-ice interaction and oil-in-ice problems in the Sea of Okhotsk.

Dr Wadhams, the Reader in Polar Studies and leader of the Group, had his book Ice in the ocean published by Gordon and Breach Science Publishers. This is the first comprehensive textbook dealing with the physics of sea ice and icebergs and their interaction with the ocean and with climate. He was also co-editor of the book The Arctic Ocean Freshwater Budget, which arose from a NATO Advanced Research Workshop on the Fresh Water Budget of the Arctic Ocean, Tallinn, Estonia, 26 April-1 May 1998, at which Dr Wadhams was a rapporteur.

As the 1990 winner of the Italgas Prize for Environmental Sciences, Dr Wadhams was a committee member of Club Premio Italgas, which is organising a series of conferences on sustainable development. Planning meetings were held in Turin in October 1999 and 2000.

As a UK delegate to the European Union COST Action 714 programme on Directional Spectra of Ocean Waves, he attended meetings in Bologna in April 2000. Under the COST Action he made a short term visit to ISAO-CNR, Bologna, in May in order to complete work with Dr F. Parmiggiani and Dr G. de Carolis on the analysis of wave data from SAR images of frazil-pancake icefields.

A major conference on sea ice during 2000 was the International Glaciological Society's conference on sea ice and its interactions with the atmosphere and ocean, held in Fairbanks, Alaska, in June 2000. Dr Wadhams attended and gave a paper on sea-ice thinning. The conference was also attended by Doble and Hall, who gave poster presentations. As is usual on such occasions, other research business was conducted around the conference. Before the conference, Doble represented the Institute at a meeting of ASPeCt (Antarctic Sea Ice Processes and Climate), the SCAR project on Antarctic sea ice monitoring. As incoming co-ordinator of the WCRP International Programme for Antarctic Buoys, Dr Wadhams attended a joint IPAB/IABP (International Arctic Buoy Programme) meeting just after the conference and began to implement a strategy devised for data transfer from Hobart (previous data centre for the programme) to Cambridge.

A new intergovernmental initiative involving collaborative research between the UK and Norway on rapid climatic cooling in the European Arctic seas began during 1999. Dr Wadhams was invited by NERC to be on the steering group for this programme, and, together with Keith Nicholls (BAS), compiled a bilateral science plan that emerged from a meeting in Bergen in September 1999. The Norwegian Research Council funded a series of research projects in Norway on the basis of this document, but the UK part of the collaboration was subsumed by NERC into a thematic programme that has not yet received funding.

Dr Wadhams also attended the meeting of the European Geophysical Society, Nice, in April. He lectured on global warming issues at a meeting of the Parliamentary Maritime Group at the Royal Society on 27 January, and he gave an invited lecture at the Universitario Navale, Naples, on 12 April. He also gave several radio interviews, and made television appearances on Channel 4 News and BBC Breakfast News.