Relationship between zonal position of the North Atlantic Oscillation and Euro-Atlantic blocking events and its possible effect on the weather over Europe

The North Atlantic Oscillation (NAO) exhibited a marked eastward shift in the mid-1970s. Observations show that the extreme weather events in Europe have emerged frequently in the past decades. In this paper, based upon the daily NAO index, we have calculated the frequency of in-situ NAO events in winter during 1950-2011 by defining the Eastern-type NAO (ENAO) and Western-type NAO (WNAO) events according to its position at the east (west) of 10A degrees W. Then, the composites of the blocking frequency, temperature and precipitation anomalies for different types of NAO events are performed. Results show that the frequency of Euro-Atlantic blocking events is distributed along the northwest-southeast (southwest-northeast) direction for the negative (positive) phase. Two blocking action centers in Greenland and European continent are observed during the negative phase while one blocking action center over south Europe is seen for the positive phase. The action center of blocking events tends to shift eastward as the NAO is shifted toward the European continent. Moreover, the eastern-type negative phase (ENAO(-)) events are followed by a sharp decline of surface air temperature over Europe (especially in central, east, and south Europe), which have a wider and stronger impact on the weather over European continent than the western-type negative phase (WNAO(-)) events do. A doublebranched structure of positive precipitation anomalies is seen for the negative phase event, besides strong positive precipitation anomalies over south Europe for ENAO(-) event. The eastern-type and western-type positive phase (ENAO(+) and WNAO(+)) can lead to warming over Europe. A single-branched positive precipitation anomaly dominant in central and north Europe is seen for positive phase events.