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152.7.6.2 Weather Types and Blocking

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In AR4, weather types were not assessed as such, but an increase in blocking frequency in the Western Pacific and a decrease in North Atlantic were noted.

Changes in the frequency of weather types are of interest since weather extremes are often associated with specific weather types. For instance, persistent blocking of the westerly flow was essential in the development of the 2010 heat wave in Russia (Dole et al., 2011) (Section 9.5.2.2 and Box 14.2). Synoptic classifications or statistical clustering (Philipp et al., 2007) are commonly used to classify the weather on a given day. Feature-based methods are also used (Croci-Maspoli et al., 2007a). All these methods require daily SLP or upper-level fields.

Trends in synoptic weather types have been best analysed for central Europe since the mid-20th century, where several studies describe an increase in westerly or cyclonic weather types in winter but an increase of anticyclonic, dry weather types in summer (Philipp et al., 2007; Werner et al., 2008; Trnka et al., 2009). An eastward shift of blocking events over the North Atlantic (fewer cases of blocking over Greenland and more frequent blocking over the eartern North Atlantic) and the North Pacific was found by Davini et al. (2012) using NCEP/NCAR reanalysis since 1951 and by Croci-Maspoli et al. (2007a) in ERA-40 reanalysis during the period 1957–2001. Mokhov et al. (2013) find an increase in blocking duration over the NH year-round since about 1990 in a study based on NCEP/NCAR reanalysis data from 1969–2011. For the SH, Dong et al. (2008) found a decrease in number of blocking days but increase in intensity of blocking over the period 1948–1999. Differences in blocking index definitions, the sensitivity of some indices to changes in the mean field, and strong interannual variability in all seasons (Kreienkamp et al., 2010), partly related to circulation variability modes (Croci-Maspoli et al., 2007b), complicate a global assessment of blocking trends.

In summary, there is evidence for a poleward shift of storm tracks and jet streams since the 1970s. Based on the consistency of these trends with the widening of the tropical belt (Section 2.7.5), trends that are based on many different data sets, variables, and approaches, it is likely that circulation features have moved poleward since the 1970s. Methodological differences between studies mean there is low confidence in characterizing the global nature of any change in blocking.

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