Revisiting the climatology of atmospheric blocking in the Northern Hemisphere

In addition to the occurrence of atmospheric blocking, the climatology of the characteristics of blocking events, including duration, intensity, and extension, in four seasons over the Northern Hemisphere was analyzed for the period 1950-2009. The seasonality and spatial variations of these characteristics were studied according to their longitudinal distributions. In general, there were sharp discrepancies in the blocking characteristics between winter and summer, and these differences were more prominent over the Atlantic and Pacific Oceans. The blocking not only occurred more frequently but also underwent stronger amplification in winter; likewise, the blocking occurred less frequently and underwent weaker amplification in summer. There are very strong interrelationships among different blocking characteristics, suggesting that they are supported by similar physical factors. In addition, the relationship between blocking over different regions and East Asian circulation was examined. Ural-Siberia is a major blocking formation region in all seasons that may exert a downstream impact on East Asia. The impact is generally weak in summer, which is due to its lower intensity and smaller duration. However, the extratropical circulation over East Asia in summer can be disturbed persistently by the frequent occurrence of blocking over the Asian continent or the Western Pacific. In particular, the blocking frequency over the Western Pacific significantly increased during the study period. This climatological information provides a background for studying the impact of blocking on East Asian circulation under both present and future climate conditions.