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- Folland, Chris K., et al.
(författare)
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The Summer North Atlantic Oscillation: past, present and future
- 2009
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Ingår i: Journal of Climate. - 0894-8755. ; 22:5, s. 1082-1103
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Tidskriftsartikel (refereegranskat)abstract
- Summer climate in the North Atlantic-European sector possesses a principal pattern of year-to-year variability that is the parallel of the well-known North Atlantic Oscillation in winter. This ‘Summer North Atlantic Oscillation’ (SNAO) is defined here as the first empirical orthogonal function (EOF) of observed summertime extratropical North Atlantic pressure at mean sea level. It is shown to be characterised by a more northerly location and smaller spatial scale than its winter counterpart. The SNAO is also detected by cluster analysis and has a near equivalent barotropic structure on daily and monthly time scales. Although of lesser amplitude than its wintertime counterpart, the SNAO exerts a strong influence on Northern European rainfall, temperature and cloudiness through changes in the position of the North Atlantic storm track. It is, therefore, of key importance in generating summer climate extremes, including flooding, drought and heat stress in North Western Europe. The El Niño/Southern Oscillation (ENSO) phenomenon is known to influence summertime European climate; however, interannual variations of the SNAO are only weakly influenced by ENSO. On interdecadal time scales, both modelling and observational results indicate that SNAO variations are partly related to the Atlantic Multidecadal Oscillation. It is shown that SNAO variations extend far back in time, as evidenced by reconstructions of SNAO variations back to 1706 using tree-ring records. Very long instrumental records, such as Central England Temperature, are used to validate the reconstruction. Finally, two climate models are shown to simulate the present-day SNAO and predict a trend towards a more positive index phase in the future under increasing greenhouse gas concentrations. This implies the long-term likelihood of increased summer drought for North Western Europe
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| 4. |
- Scaife, Adam, et al.
(författare)
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European climate extremes and the North Atlantic Oscillation
- 2008
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Ingår i: Journal of Climate. - 1520-0442. ; 21:1, s. 72-83
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Tidskriftsartikel (refereegranskat)abstract
- The authors estimate the change in extreme winter weather events over Europe that is due to a long-term change in the North Atlantic Oscillation (NAO) such as that observed between the 1960s and 1990s. Using ensembles of simulations from a general circulation model, large changes in the frequency of 10th percentile temperature and 90th percentile precipitation events over Europe are found from changes in the NAO. In some cases, these changes are comparable to the expected change in the frequency of events due to anthropogenic forcing over the twenty-first century. Although the results presented here do not affect anthropogenic interpretation of global and annual mean changes in observed extremes, they do show that great care is needed to assess changes due to modes of climate variability when interpreting extreme events on regional and seasonal scales. How changes in natural modes of variability, such as the NAO, could radically alter current climate model predictions of changes in extreme weather events on multidecadal time scales is also discussed.
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