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SPRI Review 2001: Sea Ice and Polar Oceanography Group

Sea Ice and Polar Oceanography Group

Professor P. Wadhams

Dr N.R. Davis (to 8 May), Dr B. Denby, Y. Aksenov, M. Doble, D. Flocco, R. Hall, N. Hughes, I. Jonsdottir, A. Kaletzky, S. Nowottny, J. Wilkinson

Sea ice covers 7% of the surface of our planet. It is one of the most important and variable components of the planetary surface and is the key to understanding many basic questions about the energy balance of the Earth. The ice-covered seas represent the cold end of the enormous heat engine that enables the Earth to have temperatures suitable for human life over most of its surface. Solar radiation, ab-sorbed by the ocean at equatorial latitudes, is transported poleward and lost through the sea ice to the atmosphere at a rate determined by the extent, thickness, and consistency of the ice cover. Sea ice also helps drive the oceanic thermohaline circulation through salt rejected by ice formation in critical regions, and directly affects climate through its high albedo, which causes sea ice retreat to have a positive feedback effect on climatic warming. These global effects are due to a material that itself experiences a huge annual cycle of growth and decay, especially in the Antarctic where almost all of the ice disappears in summer while in winter its area reaches 20 million km-2. The role of the Sea Ice and Polar Ocean-ography Group at the Institute is to study the mechanisms by which physical processes occurring in the polar seas affect the global climate and global climatic change, and the nature and magnitude of the changes that are taking place.

The most apparent change at the moment is that the Arctic sea ice cover is both shrinking and diminishing in thickness. The shrinking, measured from satellite imagery, amounts to about 4% per decade. However, the thinning, at least when summer datasets are compared, has been more drastic. In 1999 Rothrock and colleagues found by comparing ice-thickness data measured from US submarines during the 1958-76 and 1993-97 periods that a thinning of 42% had occurred between these two epochs. In the December 2000 issue of Geophysical Research Letters, Wadhams and Davis showed by comparing upward looking sonar data from two British submarine operations in 1976 and 1996, that a very similar thinning of 43% had occurred in a different region of the Arctic: between Fram Strait and the North Pole. This is an example of the results that are being obtained from the Sea Ice and Polar Ocean-ography Group's long-term collaboration with the Royal Navy in collecting data from submarines in the Arctic. Until recently this involved direct participation in Arctic cruises (most recently in 1996), but as a result of a decision by the UK Hydrographic Office, a large amount of additional data from hitherto-classified submarine cruises was released to us, covering the critical period from 1985 to 1995, when thinning may have accelerated due to warming of the Atlantic water layer in the Arctic Ocean itself. It is our current task to process and analyse this large dataset. The tragic death from cancer of Dr Norman Davis, who has worked on these problems since 1987 and was an acknowledged expert on sea ice thickness distributions, has been a severe blow to this work, as well as being a keenly felt personal loss for us all. Dr Bruce Denby, formerly of the University of Utrecht, joined the group to help continue this project, which is funded by NERC and by the EU as part of AICSEX (Arctic Ice Cover Simulation Experiment). The archived data, as processed, is being made publicly available on the Institute website and, in combination with similar US submarine data, on the NSIDC (National Snow and Ice Data Center) website in Boulder, Colorado. The results will be used to build a historical record of sea ice thickness and to verify modelling efforts incorporated in the AICSEX project. Recently a digital scanner has been acquired that will facilitate a speedy and more accurate interpretation of the data, which is for the most part stored on paper rolls.

Another change in the polar seas that may have major implications for our climate is the fact that winter convection in the Greenland Sea has been diminishing both in volume and depth. Open-ocean deep convection, which feeds the global thermohaline circulation, occurs at only three known northern hemisphere sites in the Greenland, Labrador, and Mediterranean seas as well as in the Weddell Sea in Antarctica. These sites, of small geographical extent, are of great importance for climate, since a shutting-off of convection would lead to a decline in the strength of the thermohaline circulation, resulting in a markedly colder climate for Britain and the rest of northwest Europe. During the period since about 1970, convection to the bottom in the Greenland Sea was thought to have ceased. Causes of the change have been hypothesised to be global warming, giving an increase in air temperature and thus a reduction in thermal convection; a reduction in the occurrence and growth of frazil-pancake ice in the Odden ice tongue, which used to form over the region every winter and produce a positive salt flux through ice formation followed by advection (Wadhams and Wilkinson 1999); and an increase in the occurrence of warm easterly winds over the region due to a change in the atmospheric circu-lation associated with a positive North Atlantic Oscillation Index.

The Sea Ice and Polar Oceanography Group has been studying this region for many years, and in 2001 began a new EU-funded project, CONVECTION, in which we are co-coordinating a group of 10 laboratories from seven countries. In winter 2001 two cruises took place to the central Greenland Sea gyre under this project. The first, in which our collaborator was Institut für Meereskunde, University of Hamburg, used RV Jan Mayen during 12-26 March. The second, used RV Lance of Norsk Polarinstitutt (see cover photograph) for a resurvey of the same region one month later (11-24 April). During the first cruise, at 75°N, 0°E, a feature was discovered and surveyed that was found again, with the same structure and in an identical position, during the second cruise. The feature is a rotating convective 'chimney,' the first to be mapped in the Greenland Sea, in which water of uniform properties extends from the surface to a depth exceeding 2400 m within a narrow cylindrical structure only 10 km in diameter. The chimney has extraordinary stability. In a later cruise by FS Polarstern in October 2001, it was found to still be in the same location and with the same structure, although capped by a surface fresh water layer arising from summer ice melt in the Greenland Sea. We will be investigating its continued existence during a further winter cruise in February-March 2002 using Lance. At a time when deep convection was thought to have ceased in the Greenland Sea, the existence and stability of this feature are a challenge to our understanding of the causes and impacts of open-ocean winter convection.

Within this large programme we also carried out a series of tank experiments on the growth and properties of frazil-pancake ice in an ocean wave field, using the ice tank at HSVA, Hamburg (Hamburgische Schiffbau-Versuch-sanstalt), an EU Large Scale Facility. This also constituted the third annual phase of the INTERICE pro-ject, an effort by a group of European laboratories to study physical and biological properties of sea ice by in situ testing. In addition we are working on modelling the way in which brine rejection from growing frazil-pancake ice may be a mechanism for increasing the density of surface water to induce winter convection. People working on the CONVECTION project and its co-ordination include Jeremy Wilkinson (research associate), Nick Hughes (technical manager), and Sandra Nowottny (administrator).

In the Antarctic the most interesting feature of the sea ice is that it does not seem to be changing in extent or mean thickness, despite the predictions of climate models that a retreat should be beginning. Part of the problem may be the shortage of observational data. During the year we continued work on the NERC-supported project Short Timescale Motion of Pancake Ice (STiMPI), analysing data from the field experiment of April 2000 and from the six drifting buoys deployed then. These buoys, placed in the marginal ice zone of the advancing winter ice cover, mapped the motion and properties of the ice as it advanced and transformed itself from pancake ice into a continuous ice sheet. Data from the buoys have proved to be of high quality, with the differential global positioning system (DGPS) location data proving especially valuable in delimiting the relative movement of buoys in the array, and thus of the sea ice cover itself. These 'differential kinematic parameters' have previously suffered from a very low signal-to-noise ratio, arising from the relatively inaccurate Argos positioning system conventionally used. The short-interval and highly accurate fixes of the STiMPI buoys have finally allowed the cyclic convergence/divergence signal of the young ice to be determined, with all the implications for increased ocean-atmosphere heat flux that this holds. The buoys also transmitted information on the changing nature of the wave field as the ice consolidated, allowing the effects of wave motion on the young ice field to be determined. Martin Doble (research associate) worked on this project, which is collaborative with the Dunstaffnage Marine Laboratory, Oban (David Meldrum).

Doble performed an additional Antarctic deployment, this time in the Bellingshausen Sea aboard the Alfred Wegener Institute ship FS Polarstern. Four MetOcean Argos drifting buoys were used, supplied by both the UK Meteorological Office and by Dr John King of the British Antarctic Survey (BAS). The buoys were deployed in a quadrilateral array using the ship's helicopters during extremely bad weather conditions. On an international scale, Professor Wadhams took over as co-ordinator of the WCRP International Programme for Antarctic Buoys (IPAB), which co-ordinates the deployment of ice buoys by nations working in the Antarctic to achieve maximum symbiosis in circumpolar data gathering. The existing databank was transferred from the Australian Antarctic Division, and work is under way on integrating it into a web-accessible Oracle database for maximum ease of use by the geophysical community.

Further international collaboration includes the development of a UK-Japan agreement for collaboration in Arctic marine science, negotiated by Professor Wadhams under a grant from the Royal Society. First fruits of this agreement included participation by the National Institute of Polar Research, Tokyo, in the Lance cruise, and a six-month visit to the Institute by Professor Sakai of Iwate University, Morioka, with collaboration in the Hamburg ice-tank experiments.

A new PhD student began work in October 2000, Daniela Flocco from the Naval University Partenope in Naples, working on modelling the dynamics of Antarctic coastal polynyas. As a related new project, we have been funded to participate in the NERC AUV-Under-Ice project, using the Autosub autonomous underwater vehicle to map polynyas and sea ice near the Filchner-Ronne Ice Shelf and off northeast Greenland. Other students nearing completion of their PhDs have moved to new posts, at Southampton Oceanography Centre (Yevgeni Aksenov), the University of Iceland (Ingibjorg Jonsdottir), and the German Remote Sensing Centre, Darmstadt (Richard Hall).