Recent changes in Icelandic climate

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1 Recent changes in Icelandic climate Edward Hanna 1 Trausti Jónsson 2 Jason E. Box 3 1 Department of Geography, University of Sheffield, UK 2 Icelandic Meteorological Office, Reykjavík, Iceland 3 Byrd Polar Research Center, Ohio State University, USA Iceland, located in a climatically critical part of the North Atlantic (Fig. 1), is an island where earth, air, fire and ice literally meet, while its environs are a key region of ocean atmosphere interaction, atmospheric dynamics and mid-latitude cyclone formation. As such, Icelandic climate is sensitive to changes in storm tracks and positions. Moreover, the north-west coast of Iceland is situated only about 400 km from Greenland, so Iceland can be climatically influenced by this huge land/ice mass nearby, although it is more often affected by relatively mild Atlantic Ocean currents. Seaice in the Greenland Sea sometimes extends south to the north Icelandic coast in winter, lowering regional temperatures (Ogilvie et al. 2000). Iceland has large temperate glaciers and ice caps, which are potentially sensitive to changes in climate. It is somewhat surprising that Icelandic climate records have been little presented in the English science literature in recent years, given the prominent geographical position and relevance of Iceland to present-day climatic change issues. Here we comment on some aspects of long-running (at least a century) temperature and precipitation records from Iceland. We also analyse and discuss the data in the context of recent (sub) Arctic and global climatic change. Pressure The south-west Iceland pressure series dates back to 1820 and combines data from Reykjavik and nearby Stykkishólmur (Jones et al. 1997). It is used in part to reconstruct the North Atlantic Oscillation (NAO) Fig. 1 Satellite picture of Iceland and its surroundings, showing meteorological stations used in the analysis. The day in question (16 May 2002) featured high pressures centred over southern Greenland (1028 mbar) and just north of Iceland (1025 mbar), a mature depression off the south-west approaches (975 mbar) and a weaker low (1005 mbar) in the Norwegian Sea. The main band of cloud is associated with occluded frontal systems stretching from around the Faeroes south-west across the Atlantic. Iceland itself is comparatively cloud-free (except in the far south) and the main ice caps and glaciers are well shown. (Azores Iceland)* pressure index series (Hurrell 1995). Figure 2 shows a winter (December, January, February (DJF)) version of the south-west Iceland pressure series, here updated to As might be expected with the vigorous North Atlantic circulation, there is a distinct seasonal cycle in atmospheric pressure in Iceland, with winter minima the month with the lowest mean pressure was January at mbar; the *The NAO index used here was calculated by Dr Jim Hurrell ( stat.html) and is based on the difference of normalised sea-level pressure data between Ponta Delgada, Azores, and Stykkishólmur/ Reykjavík, Iceland. Station data were originally obtained from the World Monthly Surface Climatology. highest was May at mbar. Perhaps surprisingly, despite the long record, there were no significant overall trends for either annual averages or any of the months. However, in recent decades, the winter low pressure season lasted longer and ended later, and this was linked with a positive tendency in the NAO from the 1960s to 2000 (Jónsson and Miles 2001) (Fig. 2). Temperature The Reykjavík continuous temperature record begins in 1871, although there are also readings between 1822 and 1854 (Jónsson and Gardarsson 2001). Stykkishólmur has the longest running and most uniform temperature record in Iceland, 3

2 Recent changes in Icelandic climate Fig. 2South-west Iceland winter (DJF) pressure series, , with 10-year running average Table 1 Summary of Icelandic meteorological data used in this study (T temperature, P precipitation) Site name Lat. N Long. W Elevation Earliest Latest (deg) (min) (deg) (min) (m) year year Stykkishólmur T P Reykjavík T 2003 Vestmannaeyjar T P Grímsey T 2003 Teigarhorn T 2003 and P and continuous observations start in The early Reykjavík data have been used to reconstruct the Stykkishólmur series back to 1823 by comparing the two stations during the early common period ( ). Other long-term records are presented for Vestmannaeyjar, on the south coast, Grímsey a small island off the north coast and Teigarhorn in the eastern fjords (Table 1, Fig. 1). Jónsson and Gardarsson (2001) include detailed discussions of early Icelandic temperature records. Annual averages of these series were compared over the post-1870 period to check their consistency. Temperature variations in the pre-1900 part of the Stykkishólmur record were much greater than the twentieth-century period, consistent with a more irregular sea-ice influence noted by Ogilvie and Jónsson (2001) (Fig. 3). Table 2 lists long-term means and standard deviations (SDs) for for selected stations. Mean January temperatures are around or slightly below freezing and mean July temperatures around 8 10 C at most stations. Unsurprisingly, the mildest station is Vestmannaeyjar and the coldest is Grímsey. However, Reykjavík has the highest temperatures in summer. Year-to-year variations in temperature (indicated by SDs in brackets) are at least twice as great in winter as in summer given the more variable and vigorous winter atmospheric circulation around Iceland (Serreze et al. 1997). All the Icelandic station records indicate significant warming, concentrated in winter (Table 3).* By significant, we mean that the Fig. 3 Annual mean temperature at Reykjavík, Stykkishólmur and Teigarhorn, , with 10-year running averages *The length of the seasons (the conventional three months each season here) is different from the Icelandic custom, so the ranking of warm and cold seasons does not conform to the publications of the Icelandic Meteorological Office. The main reason is that it is considered preposterous to include March as a spring month in Iceland as it is quite often the coldest month of the year. It is therefore always a part of the winter that consists of four months (December to March). The Icelandic spring is only two months long (April and May) and the autumn also (October and November). The long summer is defined as June to September and thus has four months like the winter. Table 2 4 Icelandic annual and monthly temperature means ( C) (standard deviations (degc) in brackets) for Site Ann. Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Stykkishólmur (0.8) (2.2) (2.2) (2.3) (1.7) (1.5) (0.9) (0.9) (1.0) (1.3) (1.5) (1.5) (1.8) Reykjavík (0.7) (2.0) (1.9) (2.1) (1.5) (1.3) (0.8) (0.9) (0.9) (1.4) (1.6) (1.6) (1.9) Vestmannaeyjar (0.6) (1.6) (1.6) (1.8) (1.3) (1.0) (0.7) (0.7) (0.8) (1.1) (1.3) (1.4) (1.6) Grímsey (1.0) (2.4) (2.2) (2.6) (1.8) (1.6) (1.4) (1.5) (1.4) (1.3) (1.5) (1.6) (1.9) Teigarhorn (0.8) (2.0) (1.8) (2.3) (1.6) (1.4) (1.1) (1.0) (1.0) (1.1) (1.5) (1.6) (1.8)

3 underlying least-squares linear regression trend-line change exceeds the SD in the data. The warming was non-uniform in time, occurring in three distinct phases, approximately: , , and Warming was most rapid in , reaching some of the highest temperatures over the entire record in 1939 and Early twentieth-century warming may be due at least partly to a reduced sea-ice cover around Iceland and consequent sea-ice forcing, as sea-ice is responsible for many key climatic feedbacks many of them positive (see e.g. Hanna 1996). Moreover, the most significant Icelandic trends during are for winter and spring, when sea-ice incidence in the area decreased markedly. Grímsey, the station most sensitive to Icelandic sea-ice, warmed the most by 1.64 degc over The Icelandic warming for is consistent with warming trends at northwest European sites (Hanna et al. 2004). The strongest period of Icelandic warming, roughly from 1919 to 1933, is characterised by the largest increases of temperatures during spring, e.g. 3.2 degc at Reykjavík. The 1930s warming was followed by cooling between about 1940 and the early 1980s, concentrated in summer over the period (Table 3). After 1980, trends are consistently positive. Thus, in some sense, Iceland did share in this recent period of warming which is also observed for the Northern Hemisphere average, while the cooling between the 1930s and c is not so consistent with the global average (Intergovernmental Panel on Climate Change (IPCC) 2001; Parker et al. 2004). Some of the coldest years and seasons of the twentieth century in Iceland were in the late 1970s and early 1980s (especially 1979, 1983 and 1981) (Table 4). There is a difference of about 3.5 degc between the coldest and warmest years in Reykjavík, and 6 7 degc between the coldest and warmest winters and springs. The 1990s was definitely not the warmest decade of the twentieth century in Iceland, in contrast to the Northern Hemisphere land average (IPCC 2001). It was cooler than the 1930s by 0.45 degc for Reykjavík, 0.41 degc for Stykkishólmur and 0.16 degc for Teigarhorn. The overall coldest years occurred in the 1880s. Strong autumn and summer warming occurred between 1991 and 2003 unlike the 1920s/1930s warming (Table 3); 2003 was an unusually warm year in Iceland, one of the three or four warmest years ever recorded and in some parts of the country even the warmest (Table 4). The average temperature in Reykjavík was 6.1 C, which is 1.8 degc above the mean. After relocations of the stations have been accounted for, this is the warmest year in Reykjavík since the commencement of continuous observations in However, the Table 3 Icelandic temperature trends (degc) over the periods stated* Time period and site Winter Spring Summer Autumn Annual Stykkishólmur Stykkishólmur Reykjavík Vestmannaeyjar Grímsey Teigarhorn Stykkishólmur Reykjavík Vestmannaeyjar Grímsey Teigarhorn Stykkishólmur Reykjavík Vestmannaeyjar Grímsey Teigarhorn Stykkishólmur Reykjavík Vestmannaeyjar Grímsey Teigarhorn Stykkishólmur Reykjavík Vestmannaeyjar Grímsey Teigarhorn *Correlation coefficients with statistical significance at or above 90% are in bold type. Italic bold values indicate significance at or above 99%. Table 4 Ranking of coldest and warmest years at Reykjavík during Season Coldest 5 years (coldest on left) Warmest 5 years (warmest on right) Annual Winter Spring Summer Autumn difference between 2003 and the previous two warmest years (1939 and 1941) is not significant in the light of the relocations. At Stykkishólmur 2003 is also the warmest year recorded; the average temperature being 5.4 C or 0.3 degc higher than the second warmest year, which at this station is At Vestmannaeyjar the mean 2003 temperature equalled that of 1941, 1939 being slightly colder. In Reykjavík 2003 was also the warmest summer on record (12.1 C), the summers of 1880 and 1939 being slightly colder (Table 4). Preliminary results suggest that summer 2004 was unusually warm in Iceland, but not as warm as At the time of writing (end of September 2004), the monthly mean temperature in Reykjavík has now been above or equal to the average for 30 consecutive months, which is unparalleled in the historical record. In an analysis of Greenland temperature records, Box (2002) lists 1941 and 1939 among the five warmest years and 1983 and 1907 among the five coldest years for the nearest site to Iceland, Tasiilaq, south-east Greenland. This is largely consistent with the results from Reykjavík (Table 4) (1907 was 8th coldest and th coldest in Reykjavík in the 126-year record). Furthermore, this is partly consistent with the often-cited temperature dipole (opposites) between Greenland and north-west- Recent changes in Icelandic climate 5

4 Recent changes in Icelandic climate 6 ern Europe (e.g. Van Loon and Rogers 1978). Thus 1941 was one of the coldest years of the twentieth century, whereas 1983 was relatively warm, in north-west Europe (Hanna et al. 2004). The apparent dipole in temperature and the NAO is illustrated statistically in Table 5. However, other years do not exhibit the dipole; e.g and 1907 were cold in both Iceland and north-west Europe (Hanna et al. 2004). The Icelandic cooling from the 1940s to the 1980s is in broad agreement with a general cooling between the late 1950s and the 1990s observed in western and southern Greenland (Hanna and Cappelen 2003; Box 2002) and also agrees with the P. D. Jones/Hadley Centre data shown in Serreze et al. (2000) which indicate a widespread cooling (or at least muted warming) over southern Greenland Iceland the northwestern North Atlantic (Fig. 4). These regions experienced a prolonged and deeper midtwentieth-century cooling when compared with the global warming trend (e.g. IPCC 2001). The contrast is probably attributable to changes in atmospheric circulation linked with the NAO (e.g. Hanna and Cappelen 2003). There is a weak, yet statistically significant, negative correlation of Icelandic temperature anomalies and the NAO index for Reykjavík in all seasons (Table 5). The Reykjavík correlations are very similar in magnitude to those for Tasiilaq, Greenland. There is apparently a strong gradient in the effect of the NAO across Iceland, so that Teigarhorn in the south-east has a weak but significant positive correlation of its temperature with the NAO in summer. The overall coldest years occurred in 1859 and 1866 in the longer ( ) Stykkishólmur record. These temperature minima coincide with the latter of two temperature minima (c. AD 1850) that define the Little Ice Age, as measured by Greenland ice sheet borehole temperatures (Dahl-Jensen et al. 1998). The earlier minimum near AD 1500 was roughly 0.2 degc higher than the c minimum. The 1850s and 1860s are also in a period of relatively few and declining sunspots and presumed low solar activity, which might perhaps partly explain this cold period (e.g. Crowley 2000). Elevenyear running averages of Stykkishólmur temperature and sunspot numbers*, , are significantly correlated (r = 0.47). Precipitation The Icelandic Meteorological Office has three precipitation records extending back to the *Sunspot numbers were obtained from the National Geophysical Data Center, Boulder, CO, USA, website: ftp://ftp.ngdc.noaa.gov/stp/ SOLAR_DATA/ SUNSPOT_NUMBERS/MONTHLY. Table 5 NAO index correlation with temperature anomalies over the standard century, * Azores Iceland (Rogers 1997) NAO index Ann. DJF MAM JJA SON Nuuk/Godthåb Tasiilaq/Ammassalik Reykjavík Grímsey Teigarhorn Tórshavn Oslo Copenhagen Stockholm *Correlation coefficients with statistical significance at or above 90% are in bold type. Italic bold values indicate significance at or above 99%. Fig. 4 Arctic regional temperature trends, (courtesy of William Chapman, University of Illinois) late 1800s. Reykjavík is not one of these. Although precipitation measurements started there in 1884, a lapse of observations occurred from 1907 to 1920, which can only be partly filled with information from a station nearby. The Reykjavík station has been moved repeatedly around the town, making successive measurements difficult to compare. A particular problem was the 1931 site relocation from a very sheltered backyard to a relatively exposed rooftop. Vestmannaeyjar, off the southern coast (Fig. 5), has the most reliable and complete long-term precipitation record ( ). Therefore, the Vestmannaeyjar record is used as the main dataset here. The two other long-running precipitation series are from Stykkishólmur ( ) and Teigarhorn ( ) (Fig. 6, Tables 1 and 6). Mean annual precipitation for was 1446 mm at Vestmannaeyjar, 1263 mm at Teigarhorn and 701 mm at Stykkishólmur. The heaviest precipitation usually occurs in south-easterly and southerly winds so that Stykkishólmur, in the centre of the west of the country and north of a mountain barrier, is partly protected. However, there are very large local effects due to complex topography, which complicates generalisations. Easterly winds are dry in the west but very wet in the east of Iceland. Winds directly from the west are usually dry, probably owing to the influence of Greenland, but south-westerly winds are wet in the west, south, and also the south-east, including Teigarhorn. So Teigarhorn has a more favourable exposure to the wettest wind directions than Stykkishólmur but is not completely dry when there is a wet southwesterly wind. Vestmannaeyjar is exposed

5 to many directions, so is the wettest of the three stations. Precipitation has a seasonal minimum in May or June and maximum in October, and this is reflected by the seasonal totals, which are lower in summer and higher in autumn/winter (Table 6). There is reasonable agreement between 10-year running averages of the Stykkishólmur and Teigarhorn precipitation series, and less good agreement of either of these series with Vestmannaeyjar precipitation (Fig. 6). This is not too surprising given the large distances between the stations and possible uncertainties with measurement. There were overall positive trends at all three stations but the increase is statistically significant only at Vestmannaeyjar, with a 22% increase from 1881 to However, at least part of the precipitation increase that we observe may be due to the introduction of wind shields in the 1950s and 1960s and/or more of the precipitation falling as rain rather than snow (Førland and Hanssen-Bauer 2000). There are relatively weak correlation coefficients of 0.45, 0.36 and 0.29 of annual precipitation at Teigarhorn, Vestmannaeyjar. and Stykkishólmur, respectively, with the annual (Hurrell) NAO index; however, given the long records, these are statistically significant. Correlating monthly precipitation against Hurrell's monthly values of the NAO index gives the strongest results in February at Vestmannaeyjar and Teigarhorn (r = 0.51 and 0.51 respectively), and in March at Stykkishólmur (0.31). These results suggest that the amount of precipitation is influenced by the state of the NAO, although correlations are relatively low and other factors are likely to be involved. Climatic change might be expected to change the position, intensity and frequency of storms, and hence the storm tracks, over Iceland (Serreze et al. 1997), but this may only partly be reflected in the precipitation data. Unfortunately, there is no similar long-term wind speed record available from Iceland, which might shed further light on this question. Vestmannaeyjar Grímsey temperature difference Figure 7 shows the temperature difference between the extreme south (Vestmannaeyjar) and the extreme north (Grímsey) of Iceland. This difference is used as an indicator of the continentality of the island. In its form, presented for March , this temperature index shows some qualitative agreement with the Koch index of sea-ice around Iceland (Ogilvie and Jónsson 2001). However, these authors did not present a direct statistical comparison. The Koch index measures residence time of sea-ice near the Icelandic coast and the length of coast affected each year (Wallevik and Fig. 5 Table 6 The Stórhöfði lighthouse and weather station in Vestmannaeyjar, 1974 ( Guðmundur Ingólfsson) Fig. 6 Annual precipitation at Vestmannaeyjar, Stykkishólmur and Teigarhorn, , with 10-year running averages Icelandic station precipitation means (M), standard deviations (SDs) and least-squares linear regression trend-line changes (T) (mm), Trends more than 1 SD are shown in bold. Site Ann. DJF MAM JJA SON Vestmannaeyjar M SD T Stykkishólmur M SD T Teigarhorn M SD T Recent changes in Icelandic climate 7

6 Recent changes in Icelandic climate Fig. 7 Vestmannaeyjar Grímsey (Icelandic south north) annual mean temperature difference and Koch sea-ice index, with 10-year running averages Sigurjónsson 1998). When ice is present along the northern coast, Iceland temporarily assumes a more continental type of climate. Whereas southern coastal temperature anomalies are quite small (due to the proximity to open ocean), temperature anomalies on the northern coast are much greater than normal, giving a large south north temperature difference (Ogilvie and Jónsson 2001). Such periods occurred in the late nineteenth century and late 1960s (Ogilvie and Jónsson 2001). In Table 7 we present an updated form of the temperature difference data for ; former gaps in the March series are filled and series are now given for other months of the year. Temperature differences are also correlated with the existing Koch ice index (to 1990) for the first time (Fig. 7). The mean annual Vestmannaeyjar Grimsey temperature difference (Table 7) was 2.6 degc, with mean monthly values ranging from 1.9 degc in September to 3.6 degc in April; March at 3.5 degc was only marginally lower. Seasonal variation in this continentality index reflects the late winter early spring peak of sea-ice near or around Iceland. There was a notable downward trend of 0.9 degc for the year as a whole, with particularly strong downward trends of 1.3 to 1.7 degc for the key spring months of March May. This reflects an overall reduction of sea-ice in Icelandic waters during the twentieth century. Indeed, the Icelandic south north annual mean temperature difference and sea-ice index for are significantly correlated with r = Icelandic sea-ice since 1990 (not yet indexed) has been conspicuous by its absence. Greenland Sea ice cover (which affects Iceland) experiences large interannual fluctuations due to frequent transient storms and their variations. However, widespread significant reductions of Arctic, including Greenland Sea, sea-ice cover have been reported for the past few decades and are linked with regional anomalies (warming) of surface temperatures (Parkinson and Cavalieri 2002; Comiso 2002). Conclusions Our analysis suggests that Iceland experienced a warming of between ~0.8 and 1.6 degc during , consistent with twentieth-century global warming trends (the latter taken from IPCC (2001)). The warming was not gradual through time but concentrated in the 1920s and 1930s and more recently from 1987 to There was a marked cooling from the 1940s to the 1980s which, as with southern Greenland (Hanna and Cappelen 2003), is much more pronounced and prolonged than in the global temperature series. This result is perhaps no surprise since regional amplitudes of change tend to be greater than global, but it is in line with other recent studies of climate change in this part of the (sub) Arctic, and suggests that distinct changes in atmospheric circulation probably gave cooler conditions over Iceland. Warming resumed in the late 1980s and 1990s; however, as for Greenland, this was not the warmest period in Iceland. The warmest year in the twentieth-century Reykjavík record was 1941 and the coldest 1979, although most recently 2003 was an exceptionally warm year (but not significantly warmer than 1939 or 1941). The full records indicate that the coldest years occurred before the twentieth century, and 1859 and 1866 were very cold years in the long-running ( ) Stykkishólmur record. These coldest years effectively occurred during the latter part of the Little Ice Age (Lamb 1995) and can be attributed to a lull in solar activity and advance of the Arctic sea-ice. Sea-ice around the north Icelandic coast has become rarer over the past 120 years, and this is reflected by a decrease in the south north Icelandic temperature difference. Precipitation appears to have increased somewhat in line with the overall warming. However, precipitation trends are less significant and uncertainties greater than with the temperature records, to some extent owing to precipitation sampling error. An inverse pattern of extreme temperatures is sometimes, but not always, observed for the twentieth century when Icelandic temperatures are compared with north-western European temperatures. These patterns are linked with variations in the intensity of the North Atlantic atmospheric circulation, as defined by the NAO index. The fact that Iceland lies near one node of the NAO is probably a major factor weakening correlation of its climate with the NAO. Also, the NAO index is a purely statistical measure, imperfectly representing the underlying physical mechanisms and causes. Nevertheless, the temperature and precipitation records discussed here form a coherent picture of the regional atmospheric circulation, which is dominated by the Iceland low. Acknowledgements We acknowledge the Icelandic Meteorological Office for provision of data. EH would like to thank Helen Nance for help with Fig. 1. Table 7 Vestmannaeyjar Grímsey (Icelandic south north) temperature difference: means, standard deviations (SDs) and least-squares linear regression trend-line changes (degc) for * Ann. Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Mean SD Trend * Correlation coefficients with statistical significance at or above 90% are in bold type.

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