19th Conference on Climate Variability and Change


Atmospheric blockings - Their influence on the NAO and PNA

Mischa Croci-Maspoli, ETH, Zurich, Switzerland; and C. B. Schwierz and H. C. Davies

Atmospheric blocking can generally be defined as relatively long-living barotropic structures that considerably disrupt the prevailing westerly circumpolar flow in the extra-tropics. Typical blocking life-times are on the order of days to weeks, and hence influence both synoptic and short-term climate conditions. The relatively long life-time and the predilection for the Northern Hemispheric Atlantic-European and Pacific sector prompts considerations of a possible linkage between blocking and large scale climate modes (e.g.\,North Atlantic Oscillation --- NAO, Pacific North American pattern --- PNA).

A dynamical explanation of the interrelation of these phenomena is still open and not well understood. Here a recently developed blocking indicator is applied to the ERA-40 ECMWF-reanalysis data set (1958 - 2001) in the Northern Hemisphere. The focus is set on the Atlantic and Pacific basin for the NAO and PNA respectively.

A statistical composite analysis indicates a significant correlation between blocking occurrence in the North Atlantic/Pacific and the negative NAO/PNA phases. An event-based approach is adopted that identifies every single blocking track during the opposed pattern phases. Distinctively different blocking tracks and hence different genesis and lysis regions are found for opposed phases of the climate variability patterns in each basin.

Furthermore, to get some insight whether blocking events can modify/determine the pattern index values, these index values are investigated during every individual blocking track. Evidence is given, suggesting that in particular long-lasting blocking events in the North Atlantic can be of importance in the determination of the negative NAO phase. Together the results provide further insight into the concomitant dynamics of blocking and the large scale modes of atmospheric variability.

extended abstract  Extended Abstract (212K)

Session 2B, General Session on Climate Variability
Monday, 15 January 2007, 11:45 AM-5:30 PM, 214C

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