Regime Transition of the North Atlantic Oscillation and the Extreme Cold Event over Europe in January–February 2012

In this paper, large-scale aspects for the onset of the extreme cold European weather event in January–February 2012 are investigated. It is shown that the outbreak of this extreme cold weather event may be attributed to the transition from a positive North Atlantic Oscillation (NAO+) event to a long-lasting blocking event over the eastern Atlantic and western Europe (hereafter ENAO−). A persistent decline of the surface air temperature (SAT) is seen over all of Europe during the long-lived ENAO− event, while the main region of enhanced precipitation is located over southern Europe and part of central Europe, in association with the presence of a persistent double storm track: one over the Norwegian and Barents Seas and the other over southern Europe. The NAO+ to NAO− transition events are divided into NAO+ to ENAO− and NAO+ to WNAO− transition events [ENAO− (WNAO−) events correspond to eastward- (westward-) displaced NAO− events whose positive center is defined to be located to the east (west) of 10°W], and a statistical analysis of the NAO+ to ENAO− transition events during 1978–2012 is performed. It is found that there has been a marked increase in the frequency of the NAO+ to ENAO− transition events during the period 2005–12. Composites of SAT anomalies indicate that the marked decline of the SAT observed over much of Europe is primarily associated with NAO+ to ENAO− transition events. Thus, NAO+ to ENAO− transition events may be more favorable for the extreme cold events over Europe observed in recent winters than other types of NAO− events.