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^-). Apersistent decline of the surface air temperature (SAT) is seen over all of Europe during the long-lived ENAO² 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 withNAO⁺ 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 NAO2 events.