Recent climate of southern Greenland

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1 Recent climate of southern Greenland Edward Hanna 1 and John Cappelen 2 1 Institute of Marine Studies, University of Plymouth 2 Danish Meteorological Institute, Copenhagen, Denmark The polar regions are among the most interesting regarding the ongoing debate on global climate change because, due to several key climatic feedbacks, they are potentially extremely climatically sensitive. The best known is the ice± albedo feedback by which an initial perturbation (slight warming) melts some ice; this new, more extensive, darker, melt-water area absorbs more incoming sunlight, which accelerates the warming and melting of surrounding ice. Therefore, it is crucial to improve our understanding of current conditions and past history of the major ice sheets and sea-ice, and to model how they are likely to behave in future (e.g. in response to man-made global warming). This requires not only glaciological observations but also meteorological ones. Unfortunately, polar areas are noted for their dearth of observations. Greenland, the world s largest island, hosts the world s second-largest ice sheet (after Antarctica) and is important in both meteorological and glaciological terms. Its predominantly ice-covered surface reflects some 60± 90% of incoming solar radiation. The Greenland ice sheet is a huge ice dome, 3 km thick in the middle, which locally rises to more than 3 km above sea-level. Cold, dense air streams off the surface and fierce katabatic winds roar down ice fjords near the outer edges. If the ice 320

2 * Growth minus loss processes, i.e. snow accumulation minus surface melt runoff and iceberg calving, to a good approximation. sheet were to melt in its entirety, which would take some thousands of years at projected rates of global warming, it could contribute around 6± 7 m to global sea-level (Hvidberg 2000). Recent studies, especially airborne laser measurements of surface elevation of the ice, suggest that most of the interior of the ice sheet is relatively stable or growing slightly, whereas the margins have thinned substantially over the past few years (Krabill et al. 2000). Most melting of the ice, and resulting loss by runoff of surface melt water, occurs within a relatively thin zone (about 100± 200 km) around the edges, and depends strongly on summer temperatures, so the mass balance* of the ice sheet is highly sensitive to climatic change. Likely future warming could also affect the way the ice flows outwards near the edges, and the rate of iceberg calving (formation) ± the latter another important mass-loss process. Meanwhile, changes in precipitation and snowfall are paramount for snow accumulating on the ice sheet. Southern Greenland is potentially especially climatically sensitive, and ice-core data and meteorological models have shown the adjacent ice sheet margin to have large interannual variations in snow accumulation and surface melt runoff (McConnell et al. 2001; Hanna et al. 2002). Changes in the North Atlantic Oscillation (NAO) and cyclone formation off Iceland are associated with variations in precipitation and accumulation (Appenzeller et al. 1998; Bromwich et al. 1999) and also in ablation (which depends mainly on summer temperatures but also on accumulated snow). Greenland climate is sensitive to changes that affect UK weather ± a high NAO index which gives mild conditions over north-west Europe tends to give relatively cold conditions over Greenland. Climatic change in Greenland has been important historically. The period AD 800± 1100 was relatively warm and favoured Viking expansion to Greenland (Ogilvie et al. 2000). A climatic deterioration then set in from c.1250 until 1350 (Lamb 1995). The fourteenth century is considered the coldest in the last 1000 years in Greenland. The subsequent chill of the Little Ice Age (LIA) (over Europe) may have been muted over western Greenland due to the antiphase with northern European climate arising from the NAO (Barlow 2001), although there is evidence for an LIA cooling over central Greenland (Hoffman et al. 2001). A glacial advance that began in Greenland about 400 years ago continued until the eighteenth, nineteenth or even early twentieth centuries (Humlum 1999). Humlum (1999) examined the instrumental record in coastal western Greenland from 1873 to 1997, albeit for just two stations, and found a marked (3± 5 degc) temperature rise during the 1920s, followed by a recession back towards the LIA level. He noted that, in contrast to general global warming, Greenland (also Iceland and the Faeroes) cooled significantly during the latter half of the twentieth century. Greenland climate stations The Danish Meteorological Institute recently published a report on the observed climate of Greenland (Cappelen et al. 2001) and made monthly data from its meteorological stations publicly available. These data include temperature, pressure, accumulated precipitation and falling/lying snow. Although records are in many cases broken, several stations have complete (and several others nearly complete) records covering the period 1958± 99. Six stations were selected for the present study because their records are among the most comprehensive and reliable available for Greenland (Table 1). They are all situated on or near the coast of southern Greenland, well south of 708N (Fig. 1). All available information concerning the stations in question must lead to the conclusion that it is very hard to believe that any relocations or recent local developments could be an issue in relation to the homogeneity of the climatic (especially temperature) time-series. Large-scale maps show that all sensors have been in nearly the same positions for the period in question. Regarding the station sites, is on the north-west side of a small island at the southern end of Disko Bay. Figure 2 shows various 321

N 468 3 E 32 04360 Tasiilaq 658 36 N 378 38 E 50 raingauges at 04220.

3 Table 1 Details of Danish Meteorological Institute stations used in the study WMO station No. Name Latitude Longitude Elevation (m) Aasiaat N E Nuuk N E Paamiut N E Narsarsuaq N E Qaqortoq N E Tasiilaq N E 50 raingauges at This is the most northerly of the stations examined, and there is usually a seasonal sea-ice cover in this area (National Snow and Ice Data Center, Boulder, Colorado, ``State of the cryosphere Web page*). Station is near the mouth of a south-west-facing fjord, and is the station furthest from the edge of the ice sheet. Figure 3 shows the new automatic weather station at Station is more open to the coast, whilst lies inland, quite near the ice-sheet edge. Station lies near the coast in a south-west-facing fjord in extreme southern Greenland; it is the Fig. 1 Location of meteorological stations in Greenland used in the study * URL: html Fig. 2 Raingauges at Aasiaat, west Greenland 322

Fig. 3 New automatic weather station at 04250 Nuuk, south-west Greenland most southerly station considered here.

4 Fig. 3 New automatic weather station at Nuuk, south-west Greenland most southerly station considered here. Station is on an island at the southern end of the Denmark Strait, sheltered by land to the west and north-east, and is the only station on our list in eastern Greenland; the other five are in the west (or extreme south). The site photographs show the rather barren, forbidding and inhospitable conditions. Climate of southern Greenland Standard normal values (climatic means), mainly for the period 1961± 90 (a standard climatological reference period), were used to construct graphs of mean monthly temperatures and accumulated precipitation (rainfall plus snowfall) (Fig. 4 shows examples for 04270). These standard normal values are from Cappelen et al. (2001). Precipitation was recorded using gauges set at 3 m height to reduce (mainly positive) errors from windblown snow. However, this almost certainly led to an undercatch of precipitation due to wind turbulence around the gauge orifice, especially with frequent drifting snow in winter, although attempts were made to minimise this using a specially constructed windshield ± a standing vertical shelter placed around each gauge. There are, nevertheless, probably substantial underestimates in the precipitation data, and correction factors of +25± 90% may need to be applied (Yang et al. 1999). Overall, the temperature data are considered more reliable. Mean annual temperature (MAT) values, for `normal reference periods (usually 1961± 90), are listed in Table 2. The coldest station was 04220, with MAT of ± 4.9 8C; only the two southernmost stations have MATs (just) above freezing, with 0.9 8C for and 0.6 8C for Winter temperatures sank below ± 29 8C at all six stations, and as low as ± 39.88C at On the other hand, absolute maximum temperatures exceeded 21 8C at all the stations, and C was recorded at 04360, so (at least in the south, a little inland and off the ice sheet) Greenland summers can be warm! Comparison of temperatures at stations 323

5 Fig. 4 (a) Mean and absolute temperatures for `normal periods, and (b) mean accumulated precipitation for 1961± 90 at Narsarsuaq, south Greenland and reveals the effect of continentality ± colder winters and warmer summers. Although these two stations are quite close to each other, (inland) has mean monthly temperatures ranging from ± 6.8 8C (January) to C (July), whereas (coastal) temperatures have a much smaller seasonal variation from ± 5.5 8C (January) to 7.2 8C (July and August). Concerning precipitation, with 304 mm per year is by far the driest station (with less than half the rainfall of any of the others) (Table 3). The stations become progressively wetter moving south down the south-west coast, but the south-east coast is even wetter (04360 is the wettest station with mm per year). Since these are also `normal values, some years may be consider- 324

6 Table 2 Mean annual temperature values (8C), mainly for the reference period 1961± 90, at Greenland stations 04220, 04250, 04260, 04270, and ± ± ± ± ± Mean temp Average daily max Absolute max Average daily min Absolute min Table 3 Mean annual accumulated precipitation (mm), mainly for the reference period 1961± 90, at Greenland stations 04220, 04250, 04260, 04270, and Precipitation ably wetter (or drier) than this, and 2± 3 m per year precipitation is recorded in some years at `favoured locations on the south-east coastal margin (Cappelen et al. 2001). `Real amounts of precipitation (allowing for loss due to wind) may be even greater. For the south-west Greenland stations, precipitation tended to peak in mid± late summer (July± September), with a secondary peak around November. But for (south-east Greenland), precipitation peaked in January and was at a minimum in July. This reflects the cyclonic influence of low pressures forming off the south-east coast of Greenland (cyclogenesis) as part of the activity associated with the Icelandic low. This process, and the orographic enhancement of resulting cyclones, is more vigorous in winter. Meanwhile, an increased intensity of low pressure systems may explain the greater summer/autumn precipitation recorded in south-west Greenland. Climatic trends When searching for climatic trends in any timeseries, a big question is: how reliable are the data? The Greenland station climatic timeseries were closely examined and compared with time-series for related climatic elements from the same stations (Cappelen et al. 2001). Although there is a big problem finding neighbouring stations in Greenland, in southern Greenland (where stations are relatively closer together and with which this study is concerned) station data were also inter-compared to search for systematic offsets which (where they could not be accounted for by local conditions) were also corrected. Trends were found by fitting a least-squares linear regression (best-fit) line through the data series. Trends were regarded as statistically noteworthy if the trend-line change exceeded the standard deviation (a measure of dispersion or variation about the mean) in the data; this gives results very similar to the more rigorous F-test outlined in Shaw and Wheeler (1994). Means, standard deviations and trend results for the six stations and various climatic parameters over the four decades are summarised in Table 4. The most apparent result is a general cooling. Mean temperature decreased by about 1.5 degc for 04220, 04250, and over the 40 or so years. This cooling was largely a daytime feature, as revealed by significant declines in the daily maximum temperature for five out of six stations; so too was an increase in the number of `ice days (days with maximum temperature below 0 8C). (Table 4 defines ice, cold and frost days.) Further analysis (not shown) revealed that the cooling occurred mainly in winter. Only had a significant decline in the daily minimum temperature, whereas nights actually became significantly warmer and there was a significant decrease in the number of `cold days at The most notable cooling occurred at 04260, followed by (Fig. 5). These sta- 325

7 Table 4 Means, standard deviations (SDs) and trends of climatic elements for six stations in southern Greenland. Significant trend values are emboldened. Values are for period 1958± 99, but 1961± 99 for stations and 04272, and *1958± 98 and {1958± 91 Climatic element Statistic ± ± ± ± ± ± ± ± ± Mean temp. mean (8C) SD (degc) trend (degc) ± Mean daily max. temp. mean (8C) SD (degc) trend (degc) No. ice days per year mean * (max. temp. 40 8C) SD * trend * ± ± ± ± ± Mean daily min. temp. mean (8C) SD (degc) trend (degc) No. cold days per year mean (min. temp. 4± 10 8C) SD trend ± 28.0 No. frost days per year mean (min. temp. 40 8C) SD trend ± ± 6.8 ± 8.5 ± Mean air pressure mean (mbar) SD trend Mean precipitation mean * 840.5{ (mm) SD * trend ± 31.1* { ± No. snowfall days mean * 73.9{ per year SD * 27.0{ trend * +24.7{ tions are right on the south-west coast of Greenland. The cooling at (Nuuk, the most heavily populated settlement in Greenland) is the more remarkable considering possible urbanisation at this site and its effect on the temperature record. The only station without any significant cooling was (on the south-east coast). Although at most of the stations there was a short warming and reduction in ice/cold/frost days during the mid± late 1990s, this was insignificant in the context of the overall record (e.g. Fig. 5). There were no significant trend-line changes in the last 20 years of the station records (1980± 99) except for a warming at There were no obvious trends in precipitation; barely significant increases were seen at stations and but, due to uncertainties in the data, these can be ignored. Four stations (04220, 04250, and 04360) 326 showed strongly significant increases in the number of snowfall days: these were between 29 and 68 days, so quite substantial. A snowfall day was defined according to three-hourly visually observed present- and past-weather conditions (rain, sleet, snow of any kind/ different showers of any kind), temperature (40 8C or, for drizzle, 4± 0.5 8C) and precipitation (50.1 mm). Conclusions Despite global warming over the past few decades (Mann 2001), the south-west marginal areas of southern Greenland seem to have actually cooled, especially during daytime in winter. This may be related to a stronger NAO, which yields warmer conditions over northwest Europe while strengthening cold northerly winds over Greenland. It will be intriguing to

8 Fig. 5 (a) Mean annual temperature, and (b) mean annual maximum temperature at Nuuk see if this trend continues, as it could substantially influence the mass balance (through changes in snow accumulation, surface melt runoff and iceberg calving) of the southern parts, at least, of the ice sheet. The overall cooling may also have caused a sharp increase in snowfall days at some of the coastal stations, where snow as a fraction of precipitation is critically dependent on temperature. These results demonstrate the regional vagaries of the global weather machine; climatic change is not a simple uniform process. Yet, as one of these regions, Greenland greatly influences the surface heat budget, atmospheric circulation and (through the waxing and waning of its ice sheet) global sea-level.* Therefore, it is very important to preserve and extend observational/ instrumental records and accompanying metadata (details on sites and instruments, including any changes) from long-running Greenland stations. * Another such anomaly, which merits further attention, is the slightly increasing Antarctic sea-ice area observed from satellites over the past two or three decades (Hanna 2001). 327

9 References Appenzeller, C., Schwander, J., Sommer, S. and Stocker, T. F. (1998) The North Atlantic Oscillation and its imprint on precipitation and ice accumulation in Greenland. Geophys. Res. Lett., 11, pp. 1939± 1942 Barlow, L. K. (2001) The time period A.D. 1400± 1980 in central Greenland ice cores in relation to the North Atlantic sector. Clim. Change, 48, pp.101± 119 Bromwich, D. H., Chen, Q.-S., Li, Y. and Cullather, R. I. (1999) Precipitation over Greenland and its relation to the North Atlantic Oscillation. J. Geophys. Res., 104, pp ± Cappelen, J., Jù rgensen, B. V., Laursen, E. V., Stannius, L. S. and Thomsen, R. S. (2001) The observed climate of Greenland, 1958± 99 ± with climatological standard normals, 1961± 90. Danish Meteorological Institute Ministry of Transport Technical Report 00-18, Copenhagen Hanna, E. (2001) Anomalous peak in Antarctic seaice area, winter 1998, coincident with ENSO. Geophys. Res. Lett., 28, pp. 1595± 1958 Hanna, E., Huybrechts, P. and Mote, T. (2002) Surface mass balance of the Greenland Ice Sheet from climate analysis data and accumulation/runoff models. Ann. Glaciol., 35 (In Press) Hoffmann, G., Jouzel, J. and Johnsen, S. (2001) Deuterium excess record from central Greenland over the last millennium: hints of a North Atlantic signal during the Little Ice Age. J. Geophys. Res., 106, pp ± Humlum, O. (1999) Late-Holocene climate in central west Greenland: meteorological data and rock-glacier isotope evidence. The Holocene, 9, pp.581± 594 Hvidberg, C. S. (2000) When Greenland ice melts. Nature, 404, pp. 551± 552 Krabill, W., Abdalati, W., Frederick, E., Manizade, S., Martin, C., Sonntag, J., Swift, R., Thomas, R., Wright, W. and Yungel, J. (2000) Greenland ice sheet: high-elevation balance and peripheral thinning. Science, 289, pp. 428± 430 Lamb, H. H. (1995) Climate, history and the modern world, second edition. Routledge, London McConnell, J. R., Lamorey, G., Hanna, E., Mosley- Thompson, E., Bales, R., Belle-Oudry, D. and Kyne, J. D. (2001) Annual net snow accumulation over southern Greenland from 1975 to J. Geophys. Res., 106, pp ± Mann, M. E. (2001) Climate during the past millennium. Weather, 56, pp. 91± 102 Ogilvie, A. E. J., Barlow, L. K. and Jennings, A. E. (2000) North Atlantic climate c. AD 1000: Millennial reflections on the Viking discoveries of Iceland, Greenland and North America. Weather, 55, pp. 34± 45 Shaw, G. and Wheeler, D. (1994) Statistical techniques in geographical analysis, second edition. David Fulton Publishers, London Yang, D., Ishida, S., Goodison, B. E. and Gunther, E. (1999) Bias correction of daily precipitation measurements for Greenland. J. Geophys. Res., 104, pp. 6171± 6181 Correspondence to: Dr E. Hanna, Institute of Marine Studies, University of Plymouth, Drake Circus, Plymouth, Devon PL4 8AA. e.hanna@plymouth.ac.uk # Royal Meteorological Society,

Polar Portal Season Report 2013

Polar Portal Season Report 2013 All in all, 2013 has been a year with large melting from both the Greenland Ice Sheet and the Arctic sea ice but not nearly as large as the record-setting year of 2012.