The impact of El Nino-southern oscillation upon weather regimes over Europe and the North Atlantic during boreal winter

The influence of the warm and cold sea-surface temperatures in the eastern and central equatorial Pacific associated with El Nino-southern oscillation (ENSO) on the probability of occurrence of weather regimes (WRs) over the North Atlantic sector is investigated for the period November-March. Five WRs are identified from daily sea-level pressure anomalies (SLPAs) during 119 winters (1882-2000) over this sector by applying cluster analysis: the positive North Atlantic oscillation (NAO; called ZO for zonal) and negative NAO (called WBL for west blocking) patterns; GA (for Greenland anticyclone), with a positive SLPA shifted north of 60degreesN; EA (for European anticyclone) with a positive SLPA over Europe but enhanced north-south SLPA gradient over the western and central North Atlantic; and AR (for Atlantic Ridge) with a positive (negative) SLPA over the central North Atlantic (northern and central Europe). El Nino winters are associated with a significant increase (decrease) in the prevalence of ZO (WBL) in November-December and a significant increase (decrease) in the prevalence of GA and WBL (EA and ZO) in January-March. During La Nina winters, ZO (WBL and AR) occurs significantly less (more) frequently in November-December, and GA and WBL (EA and AR) are less (more) frequent in January-March. So, the anomalies of the WR frequencies are almost inverted between November-December and January-March. The response of the WR frequencies to ENSO extremes is most pronounced in February. On the inter- and multi-decadal time scales, the typical ENSO signals tend to be stronger during preferred phases of the basinwide westerlies, especially in January-March. The typical El Nino signal in January-March (e.g. more GA and WBL and less ZO and EA than normal) is strong when westerlies are slower than normal, around 1900, 1915 and mainly from 1930 to 1970. The generally reversed association during La Nina winters (e.g. more EA and AR and less GA and WBL than normal) in January-March is strong mainly when westerlies are faster than normal. Anomalies are weaker and quite different during 'slow westerlies-La Nina' and 'fast westerlies-El Nino' January-March winters. Such a modulation also appears in November-December with reversed association (i.e. stronger ENSO signal during 'slow westerfies-La Nina' and 'fast westerlies-El Nino' November- December winters), but the difference between the slow. and fast westerlies phases is weaker than in January-March. Copyright (C) 2003 Royal Meteorological Society.