An ENSO impact on Europe? - A review

The possible influence of the El Ni±o/Southern Oscillation (ENSO) warm and cold extremes on mid-latitude circulation regimes in the North Atlantic/European sector is described in terms of a phenomenological, statistical and physical analysis of observational data. (1) The European circulation patterns (after Hess-Brezowsky, 1977) are combined to a binary set of cyclonic and anticyclonic (low and high pressure) centres of action. They reveal a regional ENSO response with predominantly cyclonic (anticyclonic) Grosswetter for warm (cold) ENSO events in winters at the peak of the episode. (2) Standard climate statistics of the same winter seasons (surface pressure, temperature and precipitation anomalies) supplement the Grosswetter phenomenology. They suggest a shift of the tail end of the cross Atlantic storm track and its rainbearing frontal systems from a more northern route (during warm events) to a more zonal orientation (in cold events). (3) Finally, the transient and stationary eddy—mean flow interaction is diagnosed from daily hemispheric 500 mb geopotential height fields. They are composited about most extreme anomalies in Europe (independent of ENSO) defined by the amplitude of the first simultaneous EOF of normalized monthly mean pressure, temperature and precipitation at 40 stations. Thus (upstream location and intensity of) dynamic sources of wave activity flux in the western North-Atlantic cyclogenesis area can be identified. They are associated with European climate anomalies and may represent the connection linking ENSO and Europe under favourable conditions.