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 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 degrees 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.