The Variability Of The North Atlantic Oscillation In Winter And Its Impact On European Weather

The North Atlantic Oscillation (NAO), one of the most prominent low frequencydipole mode in the Northern Hemisphere, has a significant adjustment on the weatherin surrounding areas especially over Europe. Although a great deal of studies haveindicated the relationship between NAO and European weather, most of them are onlyfocus on monthly or even seasonal mean scale. The quasi two weeks life cycle ofNAO is not well discussed, especially the variability of associated blocking,temperature and precipitation within the life cycle of NAO. Moreover, traditionally,NAO events are divided into positive and negative NAO phases (NAO+and NAO-).However, the traditional NAO classification is unable to describe the characteristicsof variability of NAO.In this paper, the life cycle of NAO is further divided into four phases: beginningphase (B1), growing phase (B2), decaying phase (A1) and end phase (A2). Then theassociated blocking, temperature and precipitation variability within the sub-stagesare examined. Results show that the blocking distribution is controlled by the phasesof NAO. The blocking is distributed from eastern Atlantic to northeast Europe alongthe southwest-northeast (SW-NE) direction during the positive phase (NAO+).However, for the negative NAO (NAO-) phase the enhanced blocking frequency isdistributed along the southeast-northwest (SE-NW) direction from the central Europeto North Atlantic and Greenland (SE-NW pattern, hereafter). Especially, it is foundthat the blocking frequency can vary from the SW-NE (SE-NW) to SE-NW (SW-NE)pattern as NAO+(NAO-) events transition into NAO-(NAO+) events. Finally, thecausal relationship between European blocking and the phase of NAO is alsodiscussed. Moreover, it is shown that for the NAO-(NAO+) phase the most markeddecrease (increase) in the surface air temperature in winter over northern Europe is in its decaying phase of the NAO event. For the NAO+phase the dominant positivetemperature and precipitation anomalies exhibit the SW-NE oriented distribution fromwestern Europe to northeast Europe, which are parallel to the NAO+-related blockingfrequency distribution. For the NAO-phase the dominant negative temperatureanomaly is in the North and Central Europe, whereas the dominant positiveprecipitation anomaly is distributed over the South Europe along the SW-NE direction.Thus, the NAO’s phase can act as an asymmetric impact on the European climatethrough producing the different spatial pattern of the Euro-Atlantic blockingfrequency distribution.A case study of the physical processes that drove the extreme cold Europeanweather event in January-February2012is presented. It is shown that the outbreak ofthis extreme cold weather event can be attributed to the transition from a positiveNorth Atlantic Oscillation (NAO+) event to a long-lasting blocking event over theeasternmost North Atlantic and western Europe (Eastern NAO, ENAO-hereafter). Apersistent decline of the surface air temperature (SAT) is seen over all of Europeduring the long-lived ENAO-event, while the dominant positive precipitationanomaly is located over southern Europe and a small part of central Europe due topersistent double storm tracks: one along the Norwegian sea and Barents sea andanother along the southern Europe.By dividing the NAO+to NAO-transition events into NAO+to ENAO-andNAO+to WNAO-events (the ENAO-and WNAO-event corresponds to an eastward(westward) displaced NAO-event whose positive center is defined to be located tothe east (west) of10W), a statistical analysis of the NAO+to ENAO-transitionevents during1978-2012is performed. It is found that there has been a markedincrease in the frequency of the NAO+to ENAO-transition events during the periodafter2005. The composites of SAT anomalies indicate that the marked decline of theSAT observed over much of Europe is primarily associated with the NAO+to ENAO-transition events. Thus, NAO+to ENAO-transition events may be more important forthe extreme cold events over Europe observed in recent winters than other NAO-events. Furthermore, based upon the daily NAO index, we have calculated the frequencyof in-situ NAO events in winter during1950-2011by defining the Eastern-type NAO(ENAO) and Western-type NAO (WNAO) events according to its position at the east(west) of10W. Then, the composites of the blocking frequency, temperature andprecipitation anomalies for different types of NAO events are performed. Resultsshow that the frequency of Euro-Atlantic blocking events is distributed along thenorthwest-southeast (southwest-northeast) direction for the negative (positive) phase.Two blocking action centers in Greenland and European continent are observedduring negative phase while one blocking action center over south Europe is seen forthe positive phase. The action center of blocking events tends to shift eastward as theNAO is shifted toward the European continent.Moreover, the ENAO+and WNAO+events can lead to warming over Europe.A single-branched positive precipitation anomaly dominant in central and northEurope is seen for positive phase events. In addition, it is found that the zonal positionof NAO dipole modes may be associated with the strength of the zonal wind. Thedouble-branched structure of the positive precipitation anomaly for NAO-events islikely to be controlled by the double-branched storm tracks. The southward transportof cold airs during the ENAO-life cycle may create a suitable condition for the coldweather occurrence in Europe.