An account of fieldwork in the Arctic Ocean

Transcription

1 An account of fieldwork in the Arctic Ocean Katharine Giles Centre for Polar Observation and Modelling, University College London 2007 marked the start of the 4th International Polar Year (IPY ), a global effort to increase our understanding of the Polar Regions in a wide number of research disciplines, including meteorology. The polar research, conducted over this period, will build on the 125-year history of international collaboration that began with the IPY of , and continued with the IPY of and the International Geophysical Year (Allison et al., 2007). In April 2007 I was privileged to take part in one of the first field experiments of IPY , the Sea-ice Experiment: Dynamic Nature of the Arctic (SEDNA), to study sea ice in the Beaufort Sea, Arctic Ocean, having received a grant from the Rupert Ford Memorial Fund of the Royal Meteorological Society. The importance of sea ice to the climate system, and the recent changes occurring in the Arctic, have been described in the special issues of Weather, published in March and April More recently, Weather news (October 2007) described how 2007 marked the record Arctic sea-ice extent minimum. This article briefly describes the importance of sea ice in the climate system, before describing the field experiment, future work and the challenges of conducting research in the polar environment. Sea ice and its importance in the climate system Sea ice plays an important part in the global climate system and is a potential indicator of climate change. Knowledge of the Arctic sea-ice mass balance is critical to our estimates of sea-ice reduction and consequently increased fresh water input into the Greenland, Iceland and Norwegian seas, a factor that may ultimately affect the thermohaline circulation (Aagaard and Carmack, 1989). Sea ice has a high albedo compared to the ocean (Curry et al., 1995), a decrease in sea-ice area exposes more water, which results in an increase in heat absorption and hence warming. This effect creates a positive feedback loop as the warming causes a further reduction in the sea-ice area and results in a further warming of the atmosphere. Sea ice also inhibits the transfer of heat, moisture and momentum between the atmosphere and the ocean (Ledley, 1993) and is therefore an important parameter in climate models. Sea-ice mass balance is controlled by thermodynamics (e.g. a net warming over time would cause the ice to thin and a net cooling would cause the ice to thicken), deformation (e.g. in areas where there is net ridging the ice will become thicker and in areas where there is net opening of leads the ice will be thinner) and transport. The aim of SEDNA was to study the spatio-temporal variability of sea-ice deformation, and its impact on the sea-ice mass balance by combining modelling, remote sensing and field experiments. The ice camp The Beaufort Sea, named after Sir Francis Beaufort, lies on the edge of the Arctic Ocean, to the north of Alaska, the Yukon and the Canadian Arctic Archipelago, covering an area of approximately km 2. For most of the year, the sea is almost permanently covered by a layer of frozen sea water, known as sea ice, with the area close to the coast opening up only during the summer. Ice that survives the summer melt is known as multi-year ice, whereas ice that has formed in the previous summer s open water areas is known as first-year ice. The ice camp was built 190 miles north of Prudhoe Bay (also known as Dead Horse), Alaska, at the edge of the perennial ice pack, with the camp itself situated on the thick (~3 m) multi-year floes and the runway spanning 2 km of flat first-year ice (Figures 1 and 2). Originally built for the US Navy to use during 148 Figure 1. The location of the ice camp. a) The camp drifted during the experiment and the black line shows its course (data from edu/index.php/public_data). b) is from msn maps at

2 Figure 2. Photograph of the ice camp taken from the runway. ( Katharine Giles.) submarine trials, my temporary residence consisted of a number of plywood huts, a large mess tent and well-stocked kitchen, all heated by stoves (with the exception of the out-houses) and with electricity provided by a generator. We arrived at the ice camp at the beginning of April and stayed for two weeks. At this time of the year the temperature is consistently below 0 C, typically between 10 C and 25 C, with wind chill down to 30 C, and there are only a few hours of darkness. The purpose of my trip to the ice camp was to take part in SEDNA. SEDNA was designed by Jenny Hutchings (Principal Investigator (PI)), from the International Arctic Research Center (IARC), University of Alaska Fairbanks, along with co-pis: Cathy Geiger, University of Delaware; Jackie Richter-Menge, Cold Regions Research and Engineering Laboratory (CRREL); Andrew Roberts (IARC) and Chandra Kambhamettu, University of Delaware, and funded by the US National Science Foundation. In total, approximately 25 scientists worked at the ice camp, from many countries including the UK, USA and Germany. The Discovery Channel and a filmmaker from Polar Palooza ( documented the work we were doing at the camp, along with the practicalities of living in such a remote and cold area. Figure 3. Drilling through an ice ridge to measure its depth and porosity. ( Katharine Giles.) The field experiments The field experiments were designed to measure the ice deformation at nested scales ranging from a single ridge to 1 km, 10 km, 100 km and regional scales. Sea ice deforms as it is subjected to stress and the amount of deformation the ice undergoes compared to its original shape is called the strain. The strain rate (the rate of change of the deformation with time) of the ice was measured by placing buoys at a 10 km and 70 km radius from the camp in concentric polygons, and tracking their motion using Global Positioning System (GPS). Buoys to measure the stress transmitted through the ice pack were placed within the 10 km array of strain rate buoys. In addition an Ice Mass Balance buoy, which provided information about the thermodynamic evolution of the ice, was placed at the centre of the buoy array. On a smaller scale, the ridge survey provided the most detailed study of a relatively small area of ice. The shape of the ridge sail and keel, the volume of ice incorporated into the ridge and the size of the voids between the ridge blocks were measured by a number of methods. These included drilling through the ridge (Figure 3) and deploying divers and a multi-beam sonar mounted on an autonomous underwater vehicle called GAVIA ( to survey the ridge keel. In tandem with the deformation studies, we also made observations of ice thickness 149

3 150 Figure 4. The EM-bird towed by the helicopter, measuring ice plus snow thickness. ( Katharine Giles.) Figure 5. Examples of the freeboard estimates derived from RA-2 data during the SEDNA ice camp. The red triangle marks the position of the ice camp. In a) the centre of the nearest RA-2 freeboard estimate to ice camp is 324 km away. In b) the centre of the nearest RA-2 freeboard estimate to the ice camp is 25 km away. In c) the centre of the nearest RA-2 freeboard estimate to the ice camp is 26 km away. and snow properties. Snow depth and density are important parameters for climate modelling and for converting measurements of ice freeboard from satellites into estimates of ice thickness (Giles et al., 2007). Measurements of sea-ice thickness are important for validating satellite estimates and also determining its distribution. From the centre of the buoy arrays six 1-km survey lines were marked out, each line running in the direction of a buoy as if it were the spoke of a wheel, with the buoys marking the edge of the wheel. Along these survey lines, ice thickness was measured using an electromagnetic probe (EM-31). The EM field transmitted from the probe induces a secondary EM field in the water beneath the ice, which is detected at the receiver allowing the distance between the probe and the water to be estimated. As the probe is carried at a constant height above the snow and the snow depth was measured at each EM-31 measurement point, the ice thickness could be calculated. On a much larger scale, an EM-bird was towed by a helicopter (Figure 4) to measure the ice, plus snow, thickness, over the 1 km survey lines and over the 10 km, 70 km and regional scales. The EM-bird operates using the same principle as the EM-31, by inducing a secondary EM field in the water; using that to measure the distance between the bird and the water, it then uses a laser altimeter to measure the distance to the top of the snow, therefore giving an estimate of the snow plus ice thickness. In addition to the ground and air observations, near realtime satellite data were sent to the camp showing measurements of sea ice freeboard (the elevation of the ice above the water) from the radar altimeter (RA-2) onboard the European Space Agency (ESA) satellite Envisat (Figure 5). Snow depth and density were measured along each survey line (Figure 6) and we also conducted detailed snow pits at regular intervals. Figure 7 shows the bulk snow density measurements from the six survey lines. A meteorological observation station (Figure 8) was set up adjacent to the camp to measure temperature at 1 m, 2 m and 3.1 m, and barometric pressure, dew-point temperature, relative humidity, wind speed and wind direction at 2 m and 3.1 m. Measuring the wind speed and the ice motion can provide information about the surface roughness of the ice. The constant of proportionality of the wind stress and the surface roughness is known as the drag coefficient, and it is a function of the surface roughness. The temperature profile also affects the wind stress on the ice as it affects the development of the small-scale turbulence above the ice surface, which generates the drag. The information recorded by the meteorological station, during the ice camp, will be used to investigate these relationships, as well as being a valuable resource for the other experiments.

4 The challenges of working in the polar environment As with any fieldwork, research in the Polar Regions has both its hazards and joys. In general the latter greatly outweigh the former, but of course there is the cold, which can make writing notes or drilling through ice ridges when your drill bits have frozen together, difficult. In addition there is the wildlife, you have to be aware that you are not on human territory when living in an ice camp and that there are polar bears in the area. And finally you are living on the ocean, which is easy to forget when you first arrive and see what looks like a huge expanse of solid ice. By the end of our two weeks on the ice our runway had broken in two, along with a number of our survey lines. Figure 10 shows a new lead opening up in the ice approximately 200 m from the ice camp. On the plus side, you have an unlimited supply of fresh water and it is very easy to keep food frozen, but most importantly, you are in a truly inspiring and beautiful location. Figure 6. Making snow depth and density measurements. ( Katharine Giles.) What happened next? The next stage was to take a Ground Penetrating Radar (GPR) onto Antarctic sea ice to investigate the penetration of the signal into the snow on top. The frequency of the GPR is the same as the radar altimeters onboard the ERS satellites, Envisat and the future radar altimetry mission CryoSat-II (Figure 9). This experiment was exciting as this is the first time a GPR of this frequency has been used over sea ice. The fieldwork was in conjunction with the Australian Antarctic Division and we travelled and lived on the Aurora Australis (Voyage 1) from the beginning of September to mid-october. The ship made two transects into the sea ice towards the coast at 118 E and 128 E, and took ice stations approximately every 50 km along the transect. Acknowledgements I would like to thank the Royal Meteorological Society for awarding me the Rupert Ford Travel Award, which enabled me to participate in SEDNA. In addition, I would like to thank Jenny Hutchings and others, for inviting me to participate in SEDNA and for their guidance and enthusiasm during the camp. I would also like to thank Andy Ridout and Seymour Laxon from CPOM for sending Envisat data to the camp and to NERC for my postdoctoral funding. Figure 7. Snow bulk density measurements with error bars from the 6 small-scale in-situ survey lines. Data were collected by C. Geiger, K. Giles, A. Turner, R. Harris, M. Osborne and N. Hughes. 151

5 Figure 8. The meteorological station. ( Katharine Giles.) References Aagaard K, Carmack EC The role of sea ice and other fresh-water in the Arctic circulation. J. Geophys Res. 94(C10): Allison I, Beland M, Alverson K, Bell R, Carlson D, Danell K, Ellis-Evans C, Fahrbach E, Fanta E, Fujii Y, Glaser G, Goldfarb L, Hovelsrud G, Huber J, Kotlyakov V, Krupnik I, Lopez-Martinez J, Mohr T, Qin D, Rachold V, Rapley C, Rogne O, Sarukhanian E, Summerhayes C, Xiao C The scope of science for the International Polar year ICSU/ WMO Joint Committee for IPY. Curry JA, Schramm JL, Ebert EE Sea ice albedo climate feedback mechanism. J. Climate 8: Giles K, Laxon SW, Wingham DJ, Wallis DW, Krabill WB, Leuschen CJ, McAdoo D, Manizade SS, Raney RK Combined airborne laser and radar altimeter measurements over the Fram Strait in May Remote Sens. Environ. 111: Ledley TS Variations in snow on sea ice: a mechanism for producing climate variations. J. Geophys Res. 98(D6): Correspondence to: Katharine Giles, Centre for Polar Observation and Modelling, University College London, Gower St., London, WC1E 6BT k.giles@cpom.ucl.ac.uk Royal Meteorological Society, 2008 DOI : /wea.233 Figure 9. CryoSat-II. ( ESA- P. Carril.) 152 Figure 10. A new lead opening up in the ice about 200 m from camp. ( Katharine Giles.)