2000 years of North Atlantic-Arctic climate (original) (raw)
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Variability of the North Atlantic Oscillation over the past 5,200 years
Nature Geoscience, 2012
Climate in the Arctic region and northwestern Europe is strongly affected by the North Atlantic Oscillation 1,2 (NAO), the dominant mode of atmospheric variability at mid-latitudes in the North Atlantic region. The NAO index is an indicator of atmospheric circulation and weather patterns: when the index is positive, Europe and the eastern US are mild and wet, whereas Greenland and northern Canada are cold and dry. A negative index is associated with the reverse pattern. Reconstructions of the NAO have so far been limited to the past 900 years 3. Here we analyse a 5,200-year-long, highresolution lake sediment record from southwestern Greenland to reconstruct lake hypolimnic anoxia, and link the results to an existing reconstruction of the NAO index from tree rings and speleothems 3. Using the relationship between the two records, we find that around 4,500 and 650 years ago-around the end of the Holocene Thermal Maximum and the beginning of the Little Ice Age, respectively-the NAO changed from generally positive to variable, intermittently negative conditions. We suggest that variability in the dominant state of the NAO tend to coincide with large-scale changes in Northern Hemisphere climate. However, the onset of the Medieval Climate Anomaly was not associated with any notable changes in the NAO. Future climate change is predicted to warm Arctic regions more than elsewhere, accelerating melting of the Greenland ice sheet with a resultant increase in global sea level of ∼0.5-1.0 m (ref. 4). Injection of freshwater into the North Atlantic Ocean may disrupt the thermohaline circulation and weaken meridional energy exchange. A better delimitation of natural Arctic climate variability is therefore essential to obtain a more complete understanding of the mechanisms driving climate change in this area 5,6. At mid-latitudes in the North Atlantic region, the NAO represents the dominant mode of atmospheric circulation variability. The NAO, defined as the difference in atmospheric pressure at sea level between the Icelandic low and the Azores high, controls the strength and direction of westerly winds and storm tracks across the North Atlantic. This phenomenon exerts a major influence on temperature and precipitation patterns in the region bounding the northern North Atlantic, and affects the oceanic meridional overturning circulation and Arctic sea-ice distribution 7-9. Moreover, it has been demonstrated that the NAO influences ecosystem functioning and, in particular, the length of the growing season, which has a significant impact on biological productivity in the Arctic 1. The NAO is strongly linked to the Arctic Oscillation 10-12 , which represents the leading mode of variability for the whole Northern Hemisphere circulation. At Kangerlussuaq in southwestern Greenland, the NAO is highly correlated to regional
Past changes in the North Atlantic storm track driven by insolation and sea-ice forcing
Geology, 2017
Changes in the strength and location of winter storms may cause significant societal and economic impacts under future climate change, but projections of future changes in Northern Hemisphere storm tracks are highly uncertain and drivers of long term changes are poorly understood. Here we develop a Late Holocene storminess reconstruction from northwest Spain and combine this with an equivalent record from the Outer Hebrides, Scotland, to measure changes in the dominant latitudinal position of the storm track over the past 4000 years. The north-south index shows storm tracks moved from a southerly position to higher latitudes over the past 4000 years likely driven by a change from meridional to zonal atmospheric circulation, associated with a negative to positive North Atlantic Oscillation (NAO) shift. We suggest that gradual polar cooling caused by decreasing solar insolation receipt in summer and amplified by sea-ice feedbacks, and mid-latitude warming caused by increasing winter insolation, drove a steepening of the winter latitudinal temperature gradient through the Late Holocene, resulting in the observed change to a more northerly storm track. Our findings provide palaeoclimate support for short-term observational and modelling studies linking changes in the latitudinal temperature gradient and sea-ice extent to the strength and shape of the circumpolar vortex. Together, the evidence now suggests that North Atlantic storm tracks will shift southward under future warming as sea ice extent decreases, increasingly affecting southern Europe.
An analysis of Icelandic climate since the nineteenth century
International Journal of Climatology, 2004
New, long monthly series of Icelandic air pressure, temperature, precipitation and sunshine data are presented and analysed to determine possible evidence of recent climatic changes in Iceland. Climatic series are compared with the North Atlantic oscillation (NAO) indices; Icelandic temperature and precipitation are moderately but significantly correlated with the NAO. An updated south-north Iceland temperature index is discussed in relation to 20th century reductions in seaice coverage. Net warming over Iceland occurred over all long-term records from the mid19th century to the present, consistent with observed global warming trends, but superimposed on this was a marked cooling between the 1940s and early 1980s; Icelandic warming resumed around 1985. The mid-late 20th century cooling is in agreement with observed cooling in southern Greenland, suggesting that large-scale changes in atmospheric circulation were probably responsible. The 1930s was the warmest decade of the 20th century in Iceland, in contrast to the Northern Hemisphere land average. There was a distinct 20th century dipole in temperatures between Iceland and northwestern Europe, with 1941 serving as an extreme year, i.e. cold Europe and warm Iceland and Greenland. There are also signs of a precipitation increase since the late 19th century, although this is significant for only one out of three stations. Moreover, precipitation rates exhibit a positive correlation with temperature. There were no statistically significant overall long-term changes in pressure or sunshine duration. However, there are statistically significant negative correlations of precipitation with the sunshine data. There is evidence of possible solar forcing of Icelandic temperature and pressure. Results from the analysis aid our understanding of recent and ongoing changes in Icelandic and North Atlantic climate. The results should help us interpret these changes in the context of larger scale atmospheric/subpolar variability and future climate-change predictions.
Tellus Series A-dynamic Meteorology and Oceanography, 2007
The atmosphere-ocean model ECHO-G, run with solar, volcanic and greenhouse gas forcing for the past millennium, is used to analyse winter and summer temperature variability in Scandinavia. Relationships with atmospheric circulation, North Atlantic SSTs and Northern Hemisphere (NH) temperatures are investigated at timescales longer and shorter than 10 yr. The simulated response to volcanic forcing is also analysed. Realistic relationships with the atmospheric circulation, with some deficiencies in summer, are found. High-frequency co-variations with SSTs and NH temperatures are too weak, but low-frequency co-variations with NH temperatures in winter are apparently too strong. The summer cooling response to volcanic forcing is realistic, but the expected winter warming is absent. The simulated long-term temperature evolution agrees broadly with proxy data. Combinations of several forcing factors can lead to decadal and multidecadal anomalies from the centennial trends. Decreased solar forcing can account for cold intervals in both summer and winter. A systematic negative North Atlantic Oscillation (NAO) phase can explain the coldest winter temperatures during 1590-1650. Several strong volcanic forcing events can have contributed to a simultaneous summer cooling. Proxy data also indicate cold summers and winters, and a negative NAO in winter, in the same period.