CR 31:3-18 (2006)  -  doi:10.3354/cr031003

ABSTRACT: The occurrence of warm (cold) events in winter has increased (decreased) over large areas of Europe in a recent-past period (1961–1990). These trends follow asymmetrical patterns and are detected in both maximum and minimum temperatures. It is shown that these trends can be partially explained by changes in the strength of wintertime large-scale forcing. The North Atlantic Oscillation (NAO) forcing is by far the leading coupling between the large-scale atmospheric circulation and frequencies of occurrence of temperature extremes. In fact, their occurrence is strongly connected to NAO-driven anomalies in geopotential, vorticity and temperature advections. As the third generation of the Hadley Centre global coupled models (HadCM3) is skilful in reproducing currently observed major couplings, it was used as a tool for assessing how, and to what extent, these couplings are projected to change under human-induced climate change. A strong wintertime warming is projected to occur under the B2 and A2 scenarios from the Special Report on Emission Scenarios (SRES), particularly over northeastern Europe and in the A2 scenario. An overall increase (decrease) in the occurrence of warm (cold) events is also projected to occur in future scenarios, mainly due to the increase in mean temperatures (changes in variance are generally irrelevant). An eastward extension of the southern centre-of-action of the NAO towards the Mediterranean Basin is also projected to occur by the end of this century (2070–2099), yielding significant changes in the local/regional relationships between the occurrence of temperature extremes and this ‘modified NAO’. Furthermore, projected changes in the surface large-scale forcing are associated with vertically coherent changes in the Northern Annular Mode in response to human-induced changes in radiative forcing.

KEY WORDS: Temperature extremes · Large-scale circulation · Europe · Scenarios · HadCM3