Annual Cycle of the Main Northern Extratropical Modes of Circulation Variability in Different Reanalyses

Modes of circulation variability detected in 500 or 700 hPa geopotential height fields on monthly or seasonal scale can be considered as main building blocks of a low-frequency variability in free atmosphere. Thus the mean state of atmosphere can be composed just from a few modes typical for a given season (Trenberth and Paolino, 1981; Clinet and Martin, 1992; Moore et al., 2013). Nine to ten modes have been identified in the Northern Hemisphere (NH) Extratropics; the most notable ones include the well known North Atlantic Oscillation (NAO) and Pacific/North American pattern (PNA), as well as the East-Atlanic and Eurasian patterns, which have been focused on only recently. The circulation modes have been used in several ways: (i) to describe multidecadal changes in tropospheric circulation (Hurrell and van Loon, 1997; Beranová and Huth, 2008; Moore et al., 2013), (ii) to explain relationships between atmospheric circulation and surface climatic elements (Castro-Diez et al., 2002; Bueh and Nakamura, 2007; Chronis, 2011; Ionita, 2014 and many others), (iii) to detect the effect of the climate change on atmospheric circulation (Cohen and Barlow, 2005) and recently (iv) for the validation of climate models and interpretation of future climate scenarios (Casado and Pastor, 2012).

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.

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