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Teil des Forschungsschwerpunktes

Development and calibration of a sea-ice - climate - glacier model for Northern Iceland

Gljúfurárjökull
Funding agency Austrian Science Fund (Project P14093-N06)
Duration 04/2000 - 03/2004
Project leader Prof. Dr. Johann Stötter
Project team Dr. Thomas Geist, Dr. Clemens Geitner, Mag. Maria Grießer, Mag. Edith Hessenberger, Mag. Stephan Jenewein, Michael Kasper, Dipl.-Geogr. Jörn Lippert (contact for further information), Dr. Gertraud Meißl, Dr. Thomas Mölg, Mag. Andrew Moran, Mag. Bernd Öggl, Mag. Ronald Schmidt, Mag. Markus Tusch, Elvira Waltle-Spiegl, Dr. Maria Wastl
Cooperation Dr. Helgi Björnsson, Science Institute, University of Iceland, Reykjavík
Dr. Þór Jakobsson, The Icelandic Meteorological Office, Reykjavík
Dr. Trausti Jónsson, The Icelandic Meteorological Office, Reykjavík
Dr. Oddur Sigurðsson, National Energy Authority, Reykjavík
Prof. Dr. Jörg-Friedhelm Venzke, Institute of Geography, University of Bremen
Project aim
Situated just south of the Arctic Circle, Iceland is the biggest land mass and the most important source of terrestrial records of environmental and climatic changes in the predominantly marine environment of the northern North Atlantic. Important atmospheric and oceanic boundaries such as the polar front and the southern limit of the North Atlantic sea-ice oscillate around Iceland. Thus Iceland lies in a key position for the understanding of the North Atlantic circulation system, which controls climatic conditions over wide parts of Europe.
The ca. 250 small corrie and valley glaciers of the Tröllaskagi and Flateyjarskagi peninsulas in Northern Iceland react very rapidly to changes in temperature and precipitation, thus they provide high-resolution climate proxy data. The climatic parameters governing glacier behaviour in Northern Iceland are highly correlated to the occurrence of sea-ice around Iceland. Therefore, the latter is an important element for the calibration of climate - glacier relationships in this area, and provides the link to the larger-scale North Atlantic atmospheric and oceanic circulation.
A detailed Holocene glacier history has been established in Northern Iceland in recent years. It shows that glacier extents in the second half of the 19th century are comparable to the maximum advances since the end of the Preboreal. While a simple pattern of sea-ice - climate - glacier relationships in Northern Iceland has been determined (Stötter et al., 1999), quantitative palaeoclimatic reconstructions on the basis of the glacier record are hampered by the lack of more sophisticated models of climate - glacier relationships.
Against this background, the aim of the research project is the development and calibration of a model describing the relationships between sea-ice, temperature and precipitation conditions and glacier extents in Northern Iceland for the period since the mid-19th century, when continuous and homogeneous instrumental recording of temperature and precipitation in Iceland starts.
The established sea-ice - climate - glacier model allows quantitative reconstructions of climatic parameters on the basis of glacier history in Northern Iceland for the period before the beginning of meteorological measurements. This improves the quality of Holocene palaeoclimatic reconstruction in this area. The adopted approach of relating glacier behaviour to temperature and precipitation guarantees that the climatic parameters reconstructed by means of this model can be compared to palaeoclimatic data based on other proxy records in Northern Iceland.
As an additional result of the model calibration, a GIS-based glacier inventory following the standard of the UNESCO World Glacier Inventory is established for Northern Iceland.
Methods
The requirements for the climate - glacier model are determined by its projected application for reconstructions of palaeoclimatic conditions on the basis of glacial proxy data for the period before the beginning of meteorological measurements. The model type chosen is a degree-day glacier mass balance model establishing a relationship between summer ablation and winter accumulation at the equilibrium line altitude (ELA) as a function of summer temperature and winter precipitation.
Data-bases
Meteorological data-base
The meteorological data-base comprises (i) the official meteorological temperature and precipitation records (continuous records from Stykkishólmur since 1845, Grímsey since 1873, Akureyri since 1881 and Siglunes since 1943 plus additional data from a number of sites around the research area), the meteorological measurements by Björnsson (1971, 1972) on Bægisárjökull for two glaciological years (1967/1968 and 1968/1969), as well as the temperature measurements carried out during this project and (ii) the official sea-ice records.
The official meteorological and sea-ice data is provided by the Icelandic Meteorological Office. A regional sea-ice index is established representing the area of the sea north of Iceland which is covered by ice.
Glacier data-base
The glacier data-base includes (i) the glaciological measurements by Björnsson (1971, 1972) on Bægisárjökull as well as simple mass balance measurements on selected glaciers carried out during this project and (ii) a geographical information system (GIS)-based inventory of all glaciers in Northern Iceland (Tröllaskagi and Flateyjarskagi peninsulas) as well as the reconstruction of the glacier history since the Little Ice Age maximum based on dated moraine sequences in selected glacier forefields.
The glacier inventory for Northern Iceland documents the present extent of the glaciation. The glaciological parameters following the standard of the World Glacier Inventory (see IAHS(ICSI)-UNEP-UNESCO, 1989) are acquired from mapping the glaciers on the basis of the most recent aerial photographs covering the two peninsulas (summer 1994 for Tröllaskagi, summers 1986 and 1992 for Flateyjarskagi) and by extensive ground truthing of the aerial photograph interpretation.
The reconstruction of the glacier history covers both the Little Ice Age maximum extent of the glaciation, which was reached in the second half of the 19th century, and the advances marked by moraines in the glacier forefields. For the corresponding glacier extents, the glaciological parameters required for the sea-ice - climate - glacier model (surface area of the glacier, position of the ELA) are calculated in the GIS. The reconstruction of the glacier history is based on the geomorphological mapping of the glaciers and glacier forefields (aerial photograph interpretation, ground truthing). As there are virtually no historical sources on former glacier extents in Northern Iceland, the moraines are dated by means of lichenometry.
Topographical data-base
The topographical data-base consists of the digital terrain model (DTM) of the research area, the digitized aerial photos and digital orthophotos of the relevant areas.
A DTM of the research area is important for several aspects of the project. (i) It is necessary for the rectification of the aerial photographs and thus the production of orthophotos and orthophoto maps, which serve both as documents for the World Glacier Inventory and as the basis for ground truthing the mapped glacier surface areas and moraines of the glacier forefields. (ii) Using a DTM and a GIS the parameters for the glacier inventory can be calculated in a homogeneous quality. (iii) Based on the DTM and the geometry of the glacier surface areas, the ELAs both for the present situation and for glacier extents documented by moraines in the glacier forefields can be determined, avoiding the subjective errors of manual reconstructions.
Fieldwork
Meteorological measurements
automatic air temperature logger in Hamarsdalur As the official meteorological stations are all close to sea level, automatic air temperature loggers are installed at different altitudes in different maritime versus continental positions over Tröllaskagi as a basis for calculating the lapse rate. In addition to this, two automatic weather stations measure temperature, relative humidity, radiation, wind speed and wind direction at the valley bottom and the ELA level respectively.
Glaciological and glacial-geomorphological investigations
ablation stakes on Gljúfurárjökull Measurements of winter accumulation and summer ablation at the ELA are carried out on three selected glaciers representing N - S maritime - continental gradients on Tröllaskagi (Hálsjökull ca. 65°52'N, 18°27'W, Gljúfurárjökull ca. 65°43'N, 18°39'W and Bægisárjökull ca. 65°36'N, 18°23'W). The winter accumulation is measured in late April or early May by means of snow pits. The summer ablation is measured in September using ablation stakes.
The glacier limits and moraine positions determined from the aerial photograph interpretation are checked and complemented in the field before they are integrated into the glacier data-base. The limits of glaciers and positions of moraines which cannot accurately be determined from the aerial photographs are measured in the field and directly incorporated into the glacier data-base by means of a differential GPS (D-GPS). The moraines in selected glacier forefields are dated by means of lichenometry.
Bægisárjökull 09/2000 In addition to this, almost 2000 photographs taken during a flight over Tröllaskagi and Flateyjarskagi in early September 2000 document the extent of the glaciation at the end of the ablation season. As the summer 2000 had extremely little snow in Northern Iceland, these pictures provide an excellent data-base for mapping the glaciers and forefield areas.
Topographical survey
GPS measurement Prominent points in the aerial photographs are measured in the field using a D-GPS to provide additional ground control points for the production of the orthophotos.
Model development
Approach
The sea-ice - climate - glacier relationships are modelled by means of a temperature - precipitation approach establishing a relationship between summer ablation and winter accumulation at the ELA as a function of summer temperature and winter precipitation. Such a model allows to express glacier mass balance in terms of temperature and precipitation, which are the desired output parameters for palaeoclimatic reconstructions. The sea-ice - climate - glacier model is developed by progressive integration of the results of the meteorological and glaciological measurements during this project.
The initial model (climate - glacier model, step I) correlates the meteorological and glaciological data measured at the ELA of Bægisárjökull for the glaciological years 1966/1967 and 1967/1968 (Björnsson, 1971) with the temperature and precipitation records from Akureyri.
The climate - glacier relationship thus established is further developed by integrating the meteorological and glaciological data collected for the mass balance years from 1999/2000 during the course of the project (climate - glacier model, step II). This goes along with a spatial extension of the meteorological and glaciological data-bases over the whole research area. Thus spatial trends of temperature and precipitation gradients in Northern Iceland can be established.
The developed climate - glacier model can then be applied to reconstructed glacier extents since the Little Ice Age maximum (climate - glacier model, step III). The model allows to calculate the winter accumulation for reconstructed ELAs from the official temperature records and the lapse rates determined in step II. The obtained magnitude, spatial and temporal pattern of the variations in winter accumulation for the corresponding periods are compared to the recorded winter precipitation at the official meteorological stations.
In a final step, the established climate (temperature/precipitation) - glacier relationships are related to the sea-ice conditions off Northern Iceland given by the regional sea-ice index developed in this project (sea-ice - climate - glacier model, step IV).
Implementation
The climate - glacier relationship was modelled by means of a degree-day approach. In a first step, daily temperature and precipitation values from Akureyri, as well as temperature measurements on Öxnadalsheiði (540m a.s.l.) and near the investigated glaciers (576m - 879m a.s.l.), were used to calibrate the climate - glacier model using the mass balance data of Björnsson (1971) for the years 1966/1967 and 1967/1968 and the glaciological measurements in the course of the project (1999/2000 - 2004/2005). In a second step, the model was recalibrated using monthly temperature and precipitation values.
Integrating the sea-ice record in the model failed, however, as the two main climatic effects of the presence of sea-ice, i.e. lower temperatures and less (winter) precipitation, offset each other in the glacier mass balance (colder summers = less ablation, drier winters = less accumulation).
Results
It was possible to establish a climate - glacier model which well describes the mass balance at the measured sites (Lippert et al., 2006). This model allows quantitative reconstructions of glacier behaviour since the start of meteorological measurements in Akureyri. Glacier tongues respond within a few years to positive and negative mass balance changes. Climate conditions in the 1960s turned out to be not as glaciologically favourable as had been assumed, as they were, especially in the beginning, too dry.
It was however not found that the presence of sea-ice had a major effect on glacier mass balance in Northern Iceland. On the average colder and drier years with sea-ice off the coast showed more positive mass balance conditions than ice free years, but there are many exceptions with negative mass balances due to the dryness, which prevents a general correlation (Lippert, in prep.).
References
Björnsson, H. (1971). Bægisárjökull, North-Iceland. Results of glaciological investigatons 1967-68. Part I. Mass balance and general meteorology. Jökull, 21, 1-23.
Björnsson, H. (1972). Bægisárjökull, North-Iceland. Results of glaciological investigations 1967-68. Part II. The energy balance. Jökull, 22, 44-61.
IAHS(ICSI)-UNEP-UNESCO (1989). World Glacier Inventory - Status 1988. Haeberli, W., Bösch, H., Scherler, K., Østrem, G., Wallén, C.C. (eds.).
Lippert, J. (in prep.). Connections between climate, glaciers and sea ice in Northern Iceland.
Lippert, J., Wastl, M., Stötter, J., Moran, A.P., Geist, Th., Geitner, C. (2006). Measuring and modelling ablation and accumulation on glaciers in Northern Iceland. Zeitschrift für Gletscherkunde und Glazialgeologie, 39, 87-98. Abstract
Stötter, J., Wastl, M., Caseldine, C. and Häberle, T. (1999). Holocene palaeoclimatic reconstruction in Northern Iceland: approaches and results. Quaternary Science Reviews, 18, 457-474. Abstract
Publications
Geist, T., Stötter, J. and Wastl, M. (2000). Development and calibration of a quantitative model describing sea-ice - climate - glacier relationships in Northern Iceland. In: Russell, A.J. and Marren, P.M. (eds.), Iceland 2000: Modern Processes and Past Environments, Keele University, Department of Geography Occasional Papers Series, 21, 37-38. Abstract
Wastl, M., Geist, T., Geitner, C., Meißl, G. and Stötter, J. (2001). Entwicklung und Eichung eines Meereis - Klima - Gletscher Modells für Nordisland. In: Innsbrucker Geographische Gesellschaft (ed.), Innsbrucker Jahresbericht 1999/2000, pp. 158-164. Innsbruck.
Geitner, C., Grießer, M., Moran, A., Stötter, J., Waltle, E. and Wastl, M. (2003). Lichenometrische Erhebungen an aufgelassenen Höfen sowie erste Anwendung der Ergebnisse auf gletschergeschichtliche Fragestellungen in Nordisland. Norden, 15, 151-162. Abstract
Wastl, M., Stötter, J. and Venzke, J.-F. (2003). Neue Beiträge zur spätglazialen und holozänen Gletschergeschichte in Nordisland. Norden, 15, 137-150. Abstract
Wastl, M. and Stötter, J. (2005). Holocene glacier history. In: Caseldine, C., Russel, A., Harðardóttir, J. and Knudsen, Ó. (eds.), Iceland - Modern Processes and Past Environments (Developments in Quaternary Science, 5), pp. 221-240. Elsevier, Amsterdam.
Lippert, J., Wastl, M., Stötter, J., Moran, A.P., Geist, Th., Geitner, C. (2006). Measuring and modelling ablation and accumulation on glaciers in Northern Iceland. Zeitschrift für Gletscherkunde und Glazialgeologie, 39, 87-98. Abstract
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