The Southern Boundary of the Northern Hemisphere Periglacial Zone at the Faroe Islands
Total Page：16
File Type：pdf, Size：1020Kb
Permafrost, Phillips, Springman & Arenson (eds) © 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7 The southern boundary of the Northern Hemisphere periglacial zone at the Faroe Islands H.H. Christiansen Institute of Geography, University of Oslo, Blindern, Norway O. Humlum The University Courses on Svalbard, Norway ABSTRACT: The Faroe Islands are used as the observational basis for determining meteorological and periglacial conditions at the southern boundary of the Northern Hemisphere periglacial zone in the North Atlantic. MAAT for the period spanning the Little Ice Age to present are reconstructed for altitudes 0–850 m a.s.l., using the long term (AD 1867–2002) Tórshavn meteorological station record and recent (1995–2002) records from four stations located from sea level up to 850 m a.s.l. Late 20th century temperatures and the alti- tudinal FDD, TDD and GDD distributions do not suggest modern permafrost at the Faroe Islands, but the highest mountains presumably are close to permafrost conditions. Since about 1930, a cooling trend has prevailed, cul- minating around 1980, followed by a slight warming trend. Widespread shallow sorting producing small-scale sorted circles and stripes occur in the highlands were shallow seasonal freezing reach 10–20 cm. The mountains have a continuous winter snow cover from December to April. 1 INTRODUCTION of the global thermohaline circulation, and is con- sidered of global importance (Broecker, 1991). In com- Northern Hemisphere periglacial terrain near the paratively warm North Atlantic periods, when generally southern (warm) limit is at risk of losing its periglacial strong, or northward-displaced, circulation occurs in character over the next 50–100 years, as indicated by the atmosphere and ocean, the Faroe Islands lie con- projections of climate change from Global Circulation tinually in the main arm of the North Atlantic Drift Models (GCMs) (Houghton et al. 2001). The Faroe (the Gulf Stream). In colder periods, when the North Islands (62°N) in the North Atlantic lie within this Atlantic Drift weakens or its main branch takes a more potentially zone of change. Motivated by the impor- southerly position, a tongue of polar water from the tance of estimating the impact of projected climate East Iceland branch of the East Greenland Current change on periglacial environments, we have recon- approaches the Faroe Islands from the north. As a structed Faroese air temperatures from the years consequence, the Faroe Islands are well placed to regis- 1867–2002, over elevations of 0–850 m a.s.l., to ter periglacial imprints of any large amplitude shifts examine climate change in the Faroes within the in North Atlantic oceanic variations, both past and observational period. We also present short-term present. (1995–2002) measurements of freezing degree days The Faroe Islands are characterised by alpine (FDD), thawing degree days (TDD), growing degree topography due to Quaternary glaciations. There is no days (GDD), snow cover, and ground temperatures tree vegetation, except where planted in few sheltered to characterize Faroese periglacial conditions and to locations below 100 m a.s.l. At sea level the warmest provide baseline measurements for comparison with month is near 10°C and sorted ground phenomena are projected future climate change. widespread above 200–300 m a.s.l. (Humlum and Christiansen, 1998a). Most of the Faroese landscape is therefore within the periglacial zone and is exposed 2 STUDY AREA to Arctic conditions. By representing a series of modern periglacial envi- Warm and saline Atlantic surface water presently flows ronments ranging from marginal close to sea level to around the Faroe Islands into the Norwegian and full periglacial in the highlands, the modern Faroese Greenland Seas, where evaporation and cooling during landscape may provide useful information regarding winter produces a gradually higher water density. This the climatic constraints on the North Atlantic periglacial dense water then overturns, resulting in deep convec- environment. tion (Bigg, 1996). The sinking cold water represents a Meteorological observations were initiated in major constituent of North Atlantic Deep Water, part Tórshavn in AD 1867 (Brandt, 1994). The data series 139 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 are further used to reconstruct MAAT for different TORSHAVN,FAROE ISLANDS altitudes back to the final parts of the LIA. 2000 annual values and 11-yr running mean 2000 1600 1600 1200 Precipitation (mm w.e) 1200 3 AIR TEMPERATURES 800 JJA temp (deg.C) 800 11 11 In the highest mountain massif, Slættaratindur (50 km 10 10 NNE of Tórshavn), air temperatures at different alti- 9 9 8 8 tudes have been recorded since 1995. Temperatures MAAT (deg.C) 7 7 were measured using miniature single channel 6 6 Tinytalk (1995–1997) and TinyTag (1997–2002) data 5 5 loggers with external sensors located 10 cm above the 4 4 3 3 terrain surface in small stone cairns, protecting the 2 DJF temp (deg.C) 2 sensors from sheep, people and direct solar radiation 1 1 and exposing them to efficient ventilation. Presumably 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 the recorded temperatures are slightly higher than if Figure 1. Variations of air temperatures (°C) and precipi- recorded at standard height, 2 m above ground. Data tation (mm w.e.) at Tórshavn since 1867. Solid lines show recording intervals were 5 hours May 1995–1996, running 11 yr mean values. 2 hours May 1996–1999 and 1 hour since May 1999. The accuracy of the sensors is 0.1°C. To describe the altitudinal variation data from 303 m, 634 m and 850 m a.s.l. are used. All these three document the final part of the Little Ice Age (LIA) stations were located in wind-exposed parts of the and its termination shortly after 1920, as indicated by landscape, in passes or on mountain summits, without a temperature increase (Fig. 1). The transition was any significant snow coverage. Data were down- mainly signalled by higher winter temperatures and loaded annually. They showed no signs of snow cover only to a lesser degree by other seasonal temperatures. preventing quick temperature variations. The data After 1940 the mean annual air temperature (MAAT) series are almost continuous, as only very short periods gradually decreased to typical LIA values until of maximum 34 hours of failure occurred. However, around 1980, again mainly caused by changes in win- for the 850 m station, the sensor was damaged after a ter temperatures. Since then, a slight warming has few months of operation in 1995 and was not replaced occurred. From a temperature point of view, the LIA before May 1996. thus more or less still continues on the Faroe Islands, The mean monthly air temperature (MMAT) (Fig. 2 and was only shortly interrupted by a relatively warm and table 1) shows the same general annual variation period 1925–1940. There is no clear evidence for an at the different altitudes, but with the largest ampli- association between MAAT and annual precipitation tude (13°C) registered for the higher parts of the land- in the Faroe Islands during the observational period scape, while the amplitude is somewhat smaller (r-squared: 0.12), and precipitation is presumably also (10°C) close to sea level. controlled by local factors such as wind direction and The coldest month varies from November to March. orographic effects. In contrast, the warmest month is always either July or Until recently all Faroese meteorological stations August, but it varies at each station for each indi- were located close to sea level with only a few stations vidual year. The MMAT difference between stations is operating at higher altitudes (up to 282 m a.s.l.). The largest during winter when the lapse rate generally is establishment of a mountain meteorological station in at maximum (Ϫ0.7 to Ϫ0.9°C/100 m), while in sum- the year 2000, however, suggested a position of the mer it is generally smaller (Ϫ0.4 to Ϫ0.5°C/100 m) low arctic boundary at only 200 m a.s.l. (Christiansen presumably due to increased insolation and lack of and Mortensen, 2002). This altitude corresponds to a topographic induced shading on the high ground. Dur- mean annual air temperature (MAAT) of 5.0 to 3.5°C, ing summer, the highest ground may penetrate above depending upon exposure to the prevailing wind the cloud cover, and therefore receive more direct (Humlum and Christiansen, 1998). radiation than the valleys below. In the present paper new 1995–2002 data series of The fact that the monthly variations show an identi- air- and ground temperatures, supplemented by auto- cal pattern for all stations, including the official station matic digital snow cover observations are used to in Tórshavn, suggests that the distribution of stations improve the description of the modern Faroese used in the present analysis ensures well-ventilated periglacial environment and to analyse the magnitude temperature sensors due to a combination of an open and significance of interannual variations. The data landscape without trees and frequent high wind speeds. 140 MMAT 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 Torshavn 303 m asl 14 634 m asl 14 8 8 13 850 m asl 13 12 12 7 7 11 11 10 10 6 Torshavn 6 9 9 8 8 7 7 5 5 6 6 303 m 5 5 4 4 4 4 3 3 3 3 degrees C 2 2 634 m 1 1 2 2 0 0 -1 -1 850 m -2 -2 1 1 -3 -3 -4 -4 0 0 -5 -5 Jun-1995 Jun-1996 Jun-1997 Jun-1998 Jun-1999 Jun-2000 Jun-2001 Jun-2002 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 Figure 2.