Handout (1.8 MB)
It is widely assumed that the circulation modes occur in all seasons. Commonly it is supposed that the modes dominating in winter remain the leading modes also in other seasons with only slight changes of their character. However, it turned out that their shape, the intensity of their action centres and the portion of circulation variability explained by individual modes vary considerably during year (Bueh and Nakamura, 2007; Clinet and Martin, 1992; Folland et al., 2009; Pokorná and Huth, 2015).
In this contribution we aim to clarify the character of two dominant modes of NH atmospheric circulation, NAO and PNA, within the year. Subsesonal changes of the modes are detected using the method of sliding seasons of the 30-day length shifting with a one-day step. Rotated principal component analysis (PCA) is applied for each sliding season separately; this allows the annual variation in the variability modes, both in terms of their spatial structure and intensity, to be captured in full detail.
The procedure was applied to three reanalyses: NCEP/NCAR, ERA-40 and JRA-55 (Japanese 55-year Reanalysis), which cover common period 1957-2002. The presence of NAO and PNA was confirmed throughout the whole year in all reanalyses. The seasonal and subseasonal changes in their character are compared among the reanalyses.
References
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