Present-Day Climate at Zackenberg

At Zackenberg (74°30′N, 20°30′W), the polar night lasts 89 days and the polar day 106 days. During the polar night, when solar energy is low or equal to zero, radiative cooling of the snow-covered surface leads to monthly mean air temperatures below -20 °C and daily minimum temperatures well below -30 °C often occur. Calm and weak winds (<2 m/s) combined with strong low-level temperature inversions are present 55-79% of the time during the winter months, when northerly winds dominate. Cyclone activity over the Greenland Sea or over the Greenland Ice Sheet often takes place during the winter season and can generate pronounced changes in the weather, which is the main factor producing the large temporal variability during the cold period. Within a few hours, the air pressure can drop drastically and the wind can increase well above 20 m/s. The temperature rises rapidly because of destruction of the cold bottom layer. Once or twice during the snow-covered period, strong foehn events can make the temperature reach the melting point and can stay well above it for several hours even in the core of the winter. In the snow-free summer period with continuous daylight all around the clock, an almost constant weak sea breeze from south to southeast dominates the daytime weather at Zackenberg. The mean monthly air temperatures (MMATs) vary between 3 °C and 7 °C in July and August, and the air temperature rarely gets below zero for 4-6 weeks during the warmest part of the summer season. Foehn events can occur and the wind speed can suddenly increase up to 20 m/s, when the air temperature remains above 20 °C for several hours, and the relative humidity drops well below 40%. Precipitation at Zackenberg is strongly influenced by the cyclone activity. For the hydrological year (October 1 to September 30), the average annual accumulated precipitation was 261 mm w.eq. (water equivalent) for the years 1996-2005, of which mixed precipitation or sleet accounted for 17 mm w.eq. (7%) primarily from May until November, while liquid precipitation was 27 mm w.eq. (10% of total) and occurred only from June to September. The observational time series have been used to downscale the datasets from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis in order to obtain longer time series and to analyse these time series for both spatial and temporal variability at various scales. For the period 1958-2005, the NCEP/NCAR datasets show a significant increase of 1.9 mm/year in the annual accumulated precipitation. The average for the whole period 1958-2005 was 178, 15 and 22 mm w.eq. for solid, mixed and liquid precipitation, respectively. The spatial variability reveals a strong overall precipitation gradient along the east coast with nearly 2000 mm w.eq./year in the southern part (61°N) decreasing to less than 100 mm w.eq. in the northern part (83°N) (Cappelen et al., 2001). A downscaled time series of air temperatures from the Climate Research Unit (CRU) at Tyndall Centre for Climate Change Research shows a strong annual variability in the seasonal temperatures for the period 1901-2005 and a significant annual warming of 2.25 °C during the latest 15 years period (1991-2005), with the five warmest years (2005, 2004, 2002, 1996 and 2003) during the last century, all occurring within the last 10 years. The air temperature also reveals a strong latitudinal gradient along the east coast of Greenland with -1.41 °C/latitude in the coldest month (February) and a very weak gradient of only -0.12 °C/latitude in the warmest month (July). Two external factors have a great influence on the spatiotemporal variability of the climate at Zackenberg. The annual variability in the export of Arctic Ocean sea ice through the Greenland Sea and the annual variability in the North Atlantic Oscillation (NAO) both have a great influence on the frequency of cyclones passing Zackenberg, especially during the snow-covered wintertime. Cold, dry winters (<100 mm w.eq.) seem to be caused by an above normal sea-ice export along the coast and by tree or more negative NAO months, while warm/wet winters (>200 mm w.eq.) seem to be caused by below normal sea-ice export in the Greenland Sea and tree or more positive NAO months.