Unusual North Atlantic temperature dipole during the winter of 2006/2007

Transcription

1 Unusual North Atlantic temperature dipole during the winter of 2006/ J. J.-M. Hirschi National Oceanography Centre, Southampton, United Kingdom Over most of western Europe and generally over the Northern Hemisphere the winter season 2006/2007 was particularly mild. Large parts of countries such as Germany, the Netherlands, Austria, Switzerland and also the south of England experienced their warmest winter since measurements began (MetOffice, 2007; NOAA, 2007; SpiegelOnline, 2007; WMO, 2007; ZAMG, 2007). In the Netherlands regular measurements started in 1706 and for the other regions temperature records date back to the early nineteenth century (Austria), 1864 (Switzerland), 1901 (Germany) and to the mid-nineteenth century (south of England). Mild winters over western and northern Europe usually coincide with a positive phase of the North Atlantic Oscillation (NAO) related to a higher-than-normal pressure difference between the Icelandic Low and the Azores High (Hurrell, 1995). The increased meridional pressure gradient favours the advection of warm and moist air masses into the continent leading to warmer and often wetter-than-normal conditions. At the same time, the western flank of the vigorous Icelandic Low favours outbreaks of cold Arctic air masses into the Labrador region. Positive NAO winters are normally linked to a temperature anomaly pattern that increases the zonal temperature difference across the North Atlantic and the warming over north-western Europe coincides with colder-than-normal winter conditions over the north-eastern regions of North America (Rogers and van Loon, 1978; van Loon and Rogers, 1978; Meehl and van Loon, 1979; Wallace and Gutzler, 1981). The zonal temperature dipole between the eastern and western sides of the North Atlantic is a consequence of the predominantly westerly winds over the North Atlantic mid- to subpolar latitudes. Regardless of the sign of the NAO, western Europe feels the maritime influence of the North Atlantic Ocean whereas in the eastern regions of North America westerly winds in winter time advect cold continental air masses to the coastal areas. A similar albeit less pronounced temperature anomaly pattern is also found for the North Pacific region with the west coast of the northern USA and of Canada enjoying milder winters than locations at the same latitudes in coastal Asia. In the North Atlantic the zonal temperature dipole between north-western Europe and the Labrador/Greenland region is amplified by the oceanic northward heat transport related to the meridional overturning circulation (Dickson and Brown, 1994; Trenberth and Caron, 2001). The westerly winds over the North Atlantic carry much of that heat westward into Europe thus further increasing the North Atlantic winter temperature dipole. Positive and negative phases of the NAO modulate the amplitude of this dipole: a negative NAO reduces the temperature difference across the Atlantic whereas this difference tends to increase during positive NAO winters. Even though the winter of 2006/2007 coincided with a positive NAO the mild temperatures found over western and northern Europe coincided with warmer than normal conditions over north-eastern America. The aim of this study is to compare the last winter with previous warm European winters and to highlight the atmospheric circulation patterns responsible for the simultaneous occurrence of warm anomalies on both sides of the Atlantic. North Atlantic temperature dipole As in Hirschi and Sinha (2007) the 1948 to 2007 reanalysis data of the National Centers for Environmental Prediction and the National Center for Atmospheric Research (NCEP/NCAR) are the basis for this investigation (Kalnay et al., 1996; Kalnay, 2003). Since these data result from the assimilation of observations into a numerical model some differences are likely if the NCEP/NCAR data are compared with station data at corresponding locations and times. This can especially be the case during the first years of the reanalysis dataset over areas where only few observations can be assimilated. Since 1980, the availability of satellite data means that the risk of looking at anomaly patterns that are model artefacts rather than real features is small. Additionally, the anomalies associated with warm (or cold) winter seasons that will be discussed in this study are generally large in amplitude and spatial extent thus further reducing the risk of looking at model-induced features. The increase of the temperature dipole across the North Atlantic during mild northern European winters is illustrated in Figure 1(a). The values show the average 2 m temperature anomalies for winters (December to February) coinciding with warmer-than-normal conditions over northwestern Europe. The European temperature anomaly is centred over southern Finland with values of +3 degc. The positive temperature anomaly extends well into Russia and values larger than +1 degc are found over most of Siberia. On the other side of the Atlantic the temperature anomaly is most pronounced over the Labrador Sea with values of 3 degc and negative values are also found in the adjacent regions. Note that in terms of absolute temperatures even a cold European winter is generally milder than its north-east American counterpart. The antiphase of the temperature anomalies found in the European and Labrador regions is illustrated for the areas delimited by the green rectangles. Between 1949 and 1997 all winters with positive temperature anomalies in excess of +1degC over the European region coincide with colder-than-normal conditions over the Labrador region (e.g. 1982/1983 or at the end of the 1980s). During cold European winter seasons on the other hand the temperature in the Labrador region is mostly above average (e.g. 1962/1963, 1968/1969, 1984/1985). Before 1996 only few winters (e.g. 1954/1955) show positive temperature anomalies in both the European and the Labrador regions and the temperature anomalies are small (0 to 1 degc). An increasing number of years with positive temperature anomalies in both regions is found after Between 1996/1997 and 2006/2007 only two winters (2002/2003,

2 (a) (b) Figure 1. (a) Change of the temperature dipole between Europe and the Labrador Sea region during warm European winters. The anomalies are calculated with respect to the mean for the winter (Dec Jan Feb) seasons 1948/1949 to 2006/2007 covered by the NCEP/NCAR reanalysis dataset. The anomalies are based on the 2m temperature field of all winters with a positive temperature anomaly exceeding one standard deviation in the European region since 1948 (see Figure 1(b)). (b) temporal evolution of temperature anomalies in the European (red) and Labrador (blue) regions indicated by the green boxes in Figure 1(a). Horizonal black lines indicate one standard deviation for the European region. Units are degc. Figure 2. Temperature anomaly averaged over the European and Labrador regions indicated in Figure1. Horizontal green lines indicate the standard deviation of the combined European-Labrador temperature anomaly. Units are degc. 2005/2006) have different temperature anomaly signs over the European and Labrador regions. However, the temperature anomalies for the pre-2006/2007 winters of the last decade, which coincided with warmer-than-normal conditions on both sides of the Atlantic, are rather small with typical values of around +1 degc. For the winter of 2006/2007 this is no longer true and large anomalies are found in both regions: +2.3 degc in the European region and +3.0 degc in the Labrador region. For the entire 1948 to 2006 period analyzed here this appears to be a unique event. The shift that occurred after 1996 becomes even more obvious when the temperature anomaly averaged over both the European and Labrador domains is considered (Figure 2). Since the anomalies in both regions tend to cancel each other out, the variability is reduced compared to the separate timeseries of the two regions shown in Figure 1. During the last decade the combined anomaly has always been in the order of +1 degc and the value of +2.6 degc in 2006/2007 is by far the highest since at least 1948 (more than three times the standard deviation of 0.8 degc). Regardless of whether there is an anti-phase between the European and Labrador regions, the combined temperature anomaly has always been positive since In the pre-1997 years, positive and negative signs of the combined temperature anomaly alternate and there is no period of comparable length with either positive or negative temperature anomalies. Spatial temperature anomaly patterns For a better understanding of the processes that might have affected the winter of 2006/2007 it is useful to compare the spatial temperature anomaly pattern of that winter with those of previous mild European winters. Based on Figure 1 the six warmest winters in the European region since 1948 are identified as 1948/1949, 1956/1957, 1974/1975, 1988/1989, 1989/1990 and 2006/2007. The temperature anomaly patterns reveal large differences between the different years (Figure 3). During the winter of 1948/1949 most of the northern half of Eurasia was much warmer than average with temperature anomalies of up to +10 degc. Much of the Arctic as well as the eastern half of North America were colder than normal with the largest anomalies of 7 degc centred over the northern part of the Labrador Sea. In the winter of 1956/1957, the positive temperature anomaly was less pronounced over Eurasia but the area of positive temperature deviations includes most of the Arctic Ocean and Greenland. Colder-than-normal conditions prevailed over the northern half of North America and the Labrador Sea as well as in an area reaching from central Asia to China. Most areas of the USA were much warmer than average during that winter. In 1974/1975 the warm anomaly was mostly confined to north-western Europe and northern Russia. South-east of a line from the Balkans to north-eastern Siberia the temperature over Eurasia was mostly below average. Colder-than-normal conditions are found over Greenland, the Labrador Sea and in large parts of the Arctic. For the winter of 1988/1989 the temperature anomaly pattern is similar to that of 1948/1949 over Eurasia with values of up to +7 degc over eastern Siberia. The regions surrounding the Labrador Sea as well as the Atlantic sector of the Arctic were colder than normal. In the following winter of 1989/1990 the anomaly values were smaller than in the other years, the main pattern being the positive and negative anomalies over the European and Labrador regions, respectively. One feature common to the global temperature anomaly patterns of the pre-2006/2007 winters coinciding with mild European temperatures is the simultaneous occurrence of positive and negative anomalies of similar spatial extent compared to the long-term mean. The picture is different during the winter 5

3 1948/ / / / / /2007 Figure 3. Winter (Dec-Jan-Feb) temperature anomalies for mild European winters. Units are degc. 6 of 2006/2007. Almost all land areas found north of N experienced warmer than normal winter conditions. Over Eurasia, the largest anomalies occurred over an almost zonal band between 40 N and 60 N extending from western Europe to the Far East. Over North America mild conditions occurred over most of Canada and Greenland. Apart from the Atlantic sector with deviations of up to +10 degc relatively small temperature anomalies were found in the Arctic ( 1 to +2 degc). Only few areas experienced colder-than-normal conditions during the last winter. Over large parts of the western and central USA the temperature anomaly was between +1deg and 2degC and a cold winter occurred over the Middle East ( 2 to 3 degc). The largest negative deviation of 5 degc occurred over a small region north of Japan. Both the amplitude and the spatial extent of the cold anomalies are modest compared to the values found for the warm anomalies. Even with the recent upward trend of temperatures since the 1980s, the last winter stands out as exceptionally mild. The relatively even distribution of the warm anomaly means that record warm winter temperatures have only been recorded over few areas (mostly parts of western and central Europe and around Lake Baikal in Siberia). Over most of the Northern Hemisphere warmer conditions have occurred in the past. What made the winter of 2006/2007 unusual was the absence of pronounced cold anomalies over larger areas and the zonal character of the positive anomalies. North of about 45 N all latitudes experienced warmer-than-normal winter conditions (Figure 4(a)). The temporal evolution of the zonally averaged winter temperature anomalies suggests that this has not

4 occurred since at least There is also little meridional variability between 45 N and 85 N where the temperature anomaly is between +2 degc and +3 degc. Previous years are characterized by alternating meridional bands of small and large anomalies. For example, during the winter of 2005/2006 which was exceptionally warm in the Arctic, a band of much smaller or even slightly negative anomalies was found further south. It is worth noting that with +1degC in 2006/2007 the latitudes north of 85 N are much colder than during the winters of 2004/2005 and 2005/2006 when the Arctic Ocean experienced temperature anomalies of +5 degc or more. Over the Arctic Ocean the past winter was the coldest since 1998/1999 but still warmer than the average for the 1948 to 2006 period. Another feature apparent from Figure 4 is the recent warming trend of the winter season. There has been a shift towards higher temperatures after the winter of 1997/1998, especially north of 60 N. Since then, the temperature anomalies have remained positive for all winters. This increase is reflected in the average winter temperature of the Northern Hemisphere. During the last nine winters the values have constantly been high and the largest warm anomaly occurred in 2006/2007. Sea-level pressure anomalies As mentioned earlier, warm winters in the European region normally coincide with a positive NAO index. Figure 5(a) illustrates the average sea-level pressure anomaly pattern for all winter seasons with a temperature anomaly exceeding one standard deviation in the European domain. As expected, a dipole structure characterizes the North Atlantic area with lower-thannormal sea-level pressure values over the subpolar North Atlantic and higher-thannormal values over the southern half of Europe. The pressure difference between the two areas indicated by red rectangles shows the evolution of the Dec-Jan-Feb NAO from 1948/1949 to 2006/2007. All winters discussed in the previous section coincide with positive NAO values. Despite being positive for the winter of 2006/2007 the NAO index was lower than during other warm European winters (e.g. 1988/1989, 1989/1990). Sea-level pressure anomalies are considered for four recent warm European winters: 1974/1975, 1988/1989, 1989/1990 and 2006/2007 (Figure 6). Over Europe and the North Atlantic all these winters show the pressure anomaly pattern expected during a positive phase of the NAO: over the southern half of Europe the atmospheric pressure is higher than normal while lower-thannormal values are found over the subpolar North Atlantic and Scandinavia. However, (a) (b) Figure 4. (a) Temporal evolution of zonally averaged winter (Dec Jan Feb) temperature anomalies since (b) Time series of Northern Hemisphere temperature anomalies. Units are degc. (a) (b) Figure 5. (a) Mean sea level pressure anomalies for all winters with positive temperature anomalies in excess of one standard deviation over the European domain. (b) Sea-level pressure difference between south-west Iceland and Gibraltar (indicated by the red squares in Figure 1(a)). Units are hpa. there are differences between the pressure anomaly patterns over North America, Siberia and the Arctic Ocean. Except for the winter of 2006/2007 the pressure gradient along the western flank of the North Atlantic pressure anomaly favours the advection of air masses from the central Canadian Arctic into the Labrador region thus leading to a cold temperature anomaly. A different picture is seen in 2006/2007. Despite being negative, the pressure anomaly is not very pronounced over the North Atlantic around 7

5 1974/ / / /2007 Figure 6. Illustration of winter (Dec Jan Feb) pressure anomalies for years with large positive temperature anomalies in the European domain. Units are hpa. 8 Iceland. Instead, two centres of action with lower-than-normal pressure are found: one south of Greenland and another over western Russia and Scandinavia. The pressure difference between the higher-thannormal pressure over southern Europe and the lower-than-normal pressure over Russia is consistent with strong westerly winds and warm conditions over most of Europe. At the same time, the second centre of action south of Greenland makes the advection of Arctic air masses into the Labrador region more difficult, thus preventing a cold anomaly from developing. There are similarities between the pressure anomaly patterns of the winters 1988/1989 and 2006/2007. In both years the annular nature of the anomaly pattern can clearly be recognized. In 1988/1989, the negative pressure anomaly is centred over the Arctic Ocean and encompasses most of northern Siberia and the subpolar North Atlantic. The Arctic pressure anomaly is almost enclosed by the positive anomaly except for Asia where the anomalies are close to zero. A similar albeit weaker positive pressure anomaly characterizes the winter of 2006/2007 and, as before, the only gap in the positive anomaly ring is found over Asia. In contrast to 1988/1989, the pattern of the negative pressure anomaly over the Arctic region is ring shaped as well; small pressure anomalies over the Arctic Ocean are enclosed by a band of more pronounced negative anomalies. Monthly anomaly patterns A better insight into the atmospheric dynamics of the winter of 2006/2007 is obtained by looking at monthly anomaly patterns for sea-level pressure and surface temperature (Figure 7). The monthly maps reveal that the winter pressure anomaly pattern was mainly set in December and January. During both months, a negative pressure anomaly was found over the Arctic whereas pressure was higher than normal further south. Larger-than-normal meridional pressure gradients over much of Eurasia and North America favoured the advection of maritime air masses well into the continents. This prevented the build-up of the cold continental air masses which nor-

6 Jan Dec Feb Figure 7. Pressure (left) and temperature (right) anomalies for December 2006 (top), January 2007 (middle) and February 2007 (bottom). Units are hpa and degc for pressure and temperature anomalies, respectively. 9

7 mally occurs due to radiative cooling over the surface of Siberia and Canada at that time of the year. This is clearly visible in the spatial distribution of warm temperature anomalies which affected the northern halves of Eurasia and North America. At the same time weaker-than-normal zonal pressure gradients reduced the inflow of mild Atlantic or Pacific air masses into the Arctic. This explains the relatively small temperature deviations (locally even negative) found in 2006/2007 over the Arctic Ocean compared to the long-term mean. South of the Eurasian and North American warm temperature anomalies the values are mainly negative but the amplitudes are comparatively small. The most notable feature is the cold anomaly over the Middle East, a pattern expected during positive phases of the NAO (Wallace and Gutzler, 1981). The positive NAO during the last winter results from the pressure anomalies found in December 2006 and January February was characterized by a different picture for the Northern Hemisphere. The temperature anomaly pattern is almost the inverse of that seen in December and January: the Arctic Ocean region is up to 5 degc warmer than normal, northern Eurasia and North America are characterized by cold anomalies whereas warm anomalies occur further south (especially over Eurasia). However, the sign of the anomaly remained unchanged over western Europe and the Labrador/Greenland region. With anomalies of up to +7 degc, February was the warmest winter month over the Labrador Sea. The change in February is reflected in the sea-level pressure anomalies; higher-thannormal pressures occurred over the Arctic Ocean whereas negative deviations are found over the subpolar North Atlantic and Siberia. More pronounced zonal pressure differences also favoured the meridional exchange of air masses. The pressure gradient between the high-pressure anomaly ridge from the Arctic Ocean into the Pacific and the lower-than-normal pressures over Siberia allowed the inflow of milder maritime air masses from the Pacific Ocean into the Polar region. Over North America, the western flank of the negative pressure anomaly over the North Atlantic favoured outbreaks of Arctic air masses into Canada and the north-eastern USA bringing an end to the mild conditions prevailing in December and January. Western Europe finds itself on the eastern flank of a low-pressure anomaly over the North Atlantic which continued the inflow of mild air masses. However, in contrast to December and January, the Atlantic air masses no longer influenced Scandinavia, which experienced much colder conditions in February. The warm anomaly over the European domain used to define the zonal temperature dipole (Figure 1) is mainly due to the deviations in the first two winter months. In the Labrador Sea region, the pressure gradient between the North Atlantic and the Arctic regions advects mild Atlantic air masses into Greenland and the Labrador Sea thus keeping the temperatures well above average in the last winter month as well. Conclusions The winter of 2006/2007 was not only very mild but it was also linked to an unusually zonal character of the warm anomaly. Whereas previous warm European/Eurasian winters coincided with cold anomalies at other mid- to high-latitude locations on the Northern Hemisphere, almost all areas north of 45 N experienced mild winter conditions in 2006/2007. In particular, the usual antiphase relationship between the northern half of Europe and the north-eastern part of North America during positive NAO phases didn t apply: in both areas the winter season was 2 to 3 degc warmer than average. In the time period considered here (1948 to 2006) it is the first time that such large, positive temperature deviations occurred simultaneously on both sides of the North Atlantic. Due to the even distribution of the temperature anomalies, only few areas actually experienced their warmest winter in 2006/2007. Nevertheless, a new record was set in parts of several western European countries such as Germany, Austria, Switzerland and the south of England. In contrast to other positive NAO years, when high European winter temperatures coincided with the inflow of cold Arctic air masses into north-eastern North America along the western flank of the Icelandic Low, last winter s pressure anomaly pattern constantly steered mild air into the Labrador region thus maintaining the warm anomaly from December 2006 to February The pressure anomaly maps (Figures 6 and 7) show the annular nature of the pressure fluctuations with anomalies of opposite sign over the Polar regions and in a band of latitude centred at about 45 N. This illustrates that a positive (or negative) NAO is a regional (North Atlantic) manifestation of the Arctic Oscillation which describes an anti-phase between atmospheric pressure anomalies over the Polar region and the mid- to subpolar latitudes (Thompson and Wallace, 1998; Thompson and Wallace, 2000a; Thompson and Wallace, 2000b). Compared to other mild European winters (e.g., 1988/1989) the pressure anomaly between the Polar region and the mid-latitudes was not very high in 2006/2007. What distinguishes the last winter from other warm winters is that (in December and January) there is only little zonal structure in the pressure anomalies. As a consequence, the exchange between the Arctic and lower latitudes was reduced. This favoured the development of the relatively cold conditions over the Arctic and the higher-thannormal temperatures in mid- to high latitudes during the first two winter months. This pattern was reversed in February, but in most areas this was not enough to compensate for the temperature excess of the previous two months and the overall winter temperatures were well above average over a large fraction of the Northern Hemisphere north of 45 N. Acknowledgements The author wishes to thank NCAR/NCEP for making their reanalysis datasets available. He would also like to thank two reviewers for their helpful comments. This work was supported by NERC as part of the RAPID programme. References Dickson RR, Brown J The production of North Atlantic Deep Water: sources, rates, and pathways. J. Geophys. Res. 99: Hirschi JJ-M, Sinha B Negative NAO and cold Eurasian winters: How exceptional was the winter of 1962/1963? Weather 62: Hurrell J W Decadal trends in the North Atlantic oscillation: regional temperatures and precipitation. Science 269: Kalnay E Atmospheric Modeling, Data Assimilation and Predictability. Cambridge University Press, 341pp. Kalnay E, Kanamitsu R, Kistler R, Collins W, Deaven D, Gandin L, Iredell M, Saha S, White G, Wollen J, Zhu Y, Leetmaa A, Reynolds B, Chelliah M, Ebisuzaki W, Higgins W, Janowiak J, Ropelewski CR, Jenne R, Joseph D The NCEP/ NCAR reanalysis project. Bull. Amer. Meteor. Soc. 77: Meehl GA, van Loon H The seesaw in winter temperatures between Greenland and Northern Europe. Part III: Teleconnections with lower latitudes. Mon. Weather Rev. 79: Met Office Winter 2006/2007 figures. Available at: metoffice.gov.uk/corporate/ pressoffice/2007/pr html NOAA US winter temperature near average, Global December-February temperature warmest on record. NOAA Magazine. Available at: noaanews.noaa.gov/stories2007/ s2819.htm Rogers JC, van Loon H The seesaw in winter temperatures between Greenland and Northern Europe. Part II: Some atmospheric and oceanic effects in middle and high latitudes. Mon. Weather Rev. 107:

8 SpiegelOnline Ein winter wie am Mittelmeer. Available at: spiegel.de/wissenschaft/natur/ 0,1518,468933,00.html Thompson DWJ, Wallace JM The Arctic Oscillation signature in the wintertime geopotential height and temperature fields. Geophys. Res. Let. 25: Thompson DWJ, Wallace JM. 2000a. Annular modes in the extratropical circulation. Part I: Month-to-month variability. J. Clim. 13: Thompson DWJ, Wallace JM. 2000b. Annular modes in the extratropical circulation. Part II: Trends. J. Clim. 13: Trenberth, KE, Caron JM Estimates of meridional atmosphere and ocean heat transports. J. Clim. 14: van Loon H, Rogers JC The seesaw in winter temperatures between Greenland and Northern Europe. Part I: General description}. Mon. Weather Rev. 106: Wallace JM, Gutzler DS Teleconnections in the geopotential height field during the northern hemisphere winter. Mon. Weather Rev. 109: WMO World Meteorological Organisation. Available at: newsmarch07.html ZAMG Klima Saisonrückblick Winter 2006/2007. Zentralanstalt für Meteorologie und Geodynamik. Available at: Correspondence to: Joël J.-M. Hirschi, National Oceanography Centre Southampton, University of Southampton, European Way, Southampton, SO14 3ZH, United Kingdom Royal Meteorological Society, DOI: /wea