A Long-Term Arctic Snow Depth Record from Abisko, Northern Sweden, 1913–2004
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A long-term Arctic snow depth record from Abisko, This article was modifi ed in northern Sweden, 1913–2004 February 2008: Fig. 8 has been corrected. Jack Kohler, Ola Brandt, Margareta Johansson & Terry Callaghan A newly digitized record of snow depth from the Abisko Scientifi c Research Station in northern Sweden covers the period 1913–present. Mean snow depths were taken from paper records of measurements made on a profi le comprising 10 permanent stakes. This long-term record yields snow depths consistent with two other shorter term Abisko records: measurements made at another 10-stake profi le (1974–present) and at a single stake (1956–present). The measurement interval is variable, rang- ing from daily to monthly, and there are no data for about half of the winter months in the period 1930–1956. To fi ll the gaps, we use a simple snowpack model driven by concurrent temperature and precipitation measurements at Abisko. Model snow depths are similar to observed; dif- ferences between the two records are comparable to those between profi le and single stake measurements. For both model and observed snow depth records, the most statistically signifi cant trend is in winter mean snow depths, amounting to an increase of about 2 cm or 5 % of the mean per decade over the whole measurement period, and 10 % per decade since the 1930–40s, but all seasonal means of snow depth show positive trends on the longest timescales. However, the start, end, and length of the snow season do not show any statistically signifi cant long-term trends. Finally, the relation between the Arctic Oscillation index and Abisko temperature, precipitation and snow depth is positive and highly signifi cant, with the best correlations for winter. J. Kohler & O. Brandt, Norwegian Polar Institute, Polar Environmental Centre, NO-9296 Tromsø, Norway, [email protected]; M. Johansson, Abisko Scientifi c Research Station, SE-981 07 Abisko, Sweden and Dept. of Physical Geography and Ecosystem Analyses, Lund University, SE-223 62 Lund, Sweden; T. Callaghan, Dept. of Physical Geography and Ecosystem Analyses, Lund University, SE-223 62 Lund, Sweden and Shef- fi eld Centre for Arctic Ecology, University of Sheffi eld, S10 5BR Western Bank, Sheffi eld, UK. The importance of snow cover to hydrology, dom- have been increasing both on the century time- estic water supply, hydropower, albedo and Arctic scale and, more rapidly, over the past few dec- ecology is overarching (e.g. Jones et al. 2001). In ades. The pattern for precipitation also shows an ecological context, snow provides insulation an overall increase over the 20th century, but a for plants and soils (Sokratov & Barry 2002), a more varied spatial response in recent decades. source of soil moisture in the growing season, The ACIA report concludes that the climate of the shelter for animals and protection from preda- Arctic is already changing, that the rate of change tors (Callaghan et al. 2004). A recent assessment is faster than at other latitudes and that the chang- (ACIA 2005) fi nds that pan-Arctic temperatures es are very likely to continue. Kohler et al. 2006: Polar Research 25(2), 91–113 91 Tromsø We then present and discuss data and trends of Øverbygd Narvik Abisko such parameters as the length of snow season, Abisko dates of fi rst and last snow, and various means of winter snow depths. We also compare our data to rway n o N ede long-term snow records from Tromsø and Øver- w S Kiruna bygd, Norway, to see if trends observed at Abisko are part of a regional pattern. Finally, we examine the connection between N 50 km (a) large-scale climate indices and the Abisko snow record. The Northern Hemisphere annular mode (NAM) is the most prominent pattern of atmos- 100 m Western snow profile (WP) pheric circulation variability in the Northern Hem- isphere, infl uencing climate variability through- N out the Arctic, particularly in winter (Thompson & Wallace 1998). The NAM is often referred to as the North Atlantic Oscillation (NAO) or the Arctic Oscillation (AO), but the basic phenome- Eastern snow non is essentially the same (e.g. Wallace 2000). profile (EP) Temporal variations in the NAM are embodied Single snow stake (SS) in this study by the AO index, the fi rst principal component of the Northern Hemisphere sea level pressure fi eld (e.g. Thompson & Wallace 1998). (b) Fig. 1. (a) Location map of Abisko, Tromsø and Øverbygd. (b) Site description Close-up view of station with buildings (black rectangles) and stakes used for snow measurements. Abisko is in northernmost Sweden (68° 21' N, 18° 49' E), near the Norwegian border (Fig. 1), and There is also increasing recognition that the lies at an altitude of 385 m a.s.l. The Abisko Sci- most profound changes will be in winter. Although entifi c Research Station was established as a year- less is known about this period than summer, it round station in 1912. In 1913, climate monitoring is clear from satellite measurements that precipi- began at the station. The mean annual tempera- tation and snow cover in northern latitudes have ture at Abisko is 0.7 °C for the period 1913–2000. decreased during recent decades (Brown 2000; Due to its location in a rain shadow, Abisko is rel- Dye 2002; Bamzai 2003). However, long-term atively dry, with a mean annual precipitation of observational data records are rare and are fur- 310 mm for the period 1913–2000. July is both the thermore prone to inhomogeneities since precip- warmest and rainiest month (Fig. 2); over 40 % of itation and snow measurements are easily infl u- the annual precipitation occurs in summer (JJA), enced by local factors. Snow depth and snow while only 15 % occurs in spring (MAM). properties such as density can vary over relative- ly short timescales, such that minor changes in the surrounding terrain, like the construction of Data buildings or the growth or removal of a stand of trees near the measurement site, can infl uence the Abisko snow measurements deposition of snow. Here we present a newly digitized long (1913– Snow data for the period 1913–1929 are taken 2004) record of snow depth measurements from from meteorological yearbooks (Rolf 1930). the Abisko Scientifi c Research Station in north- Data from 1929 until 1988 were digitized direct- ern Sweden. We describe and present the record, ly from paper records held at the Abisko Station. check it for consistency against other shorter term Some snow data have been previously presented snow measurements at the Station, and compare as graphs (Eriksson 1989). Since 1988, snow data it to a model snow record generated from concur- have been entered digitally at the time the meas- rent temperature and precipitation measurements. urements were made. All snow measurements are 92 A long-term Arctic snow depth record, northern Sweden, 1913–2004 Fig. 2. Average climatology for 15 60 Abisko: mean for the period Monthly mean T 1913–2000 of the monthly mean temperatures and 10 Monthly sum P 50 monthly summed precipitation. 5 40 C) 0 30 (º (mm) T T P -5 20 -10 10 -15 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec made just after the daily morning meteorological The monthly mean EP snow depths are pre- observations at 07:00 local time. sented in Table 1. Months for which there were no Snow depth measurements were started on an measurements are indicated with a dash. Prior to eastern snow profi le (EP) in 1913 (Fig. 1). From 1929, monthly means are based on 20 - 31 meas- January until May 1913, snow depth was meas- urements. For the period January 1930–Septem- ured at fi ve unmarked points equally divid- ber 1956, there are substantial gaps, and a lower ed between the main station building and the measuring frequency, usually about 4 - 6 meas- temperature screen at the meteorological sta- urements per month. For several months there is tion. In the autumn of 1913, a line of 10 stakes just one measurement; means for these months was established 50 m east of the original main are not included in Table 1. In other cases, 2 - 3 building. These stakes were relocated in January measurements are reported for only part of the 1914, since which time the stake locations have month; these are not included either when it was remained unchanged. Until 1929, measurements not obvious that the unreported days in the month of the EP stakes were carried out daily, with only were days with zero snow depth. After 1956 the a few gaps (Fig. 3). From 1930 to 1956 the meas- measurements are made regularly enough (4 - 7 urements were made at more variable intervals, times per month) to be considered complete and typically 3 - 5 measurements per month, but occa- suffi ciently representative for averaging. sionally there are signifi cant gaps; several years are completely missing (Fig. 3). After 1956 the Snow density measurements become regularized and the EP series is then essentially uninterrupted from 1956 Between December 1914 and 1929, snowpack until the present, with measurements made usual- density was measured at Abisko Station through- ly every 5th day, year round, when appropriate. out the winter at irregular intervals (Rolf 1930). A single stake (SS) snow measurement was No information is provided as to how or where started in 1956, at the meteorological station (Fig. exactly the density data were obtained. There are 1), and has been carried out daily since then. This no density data after 1929. is a nearly complete series; there are no more than a few days without measurements.