Climatological Features of blocking anticyclones in the Northern Hemisphere

Abstract

Blocking anticyclones have long been of interest to the atmospheric science community because of their profound effect on local and regional climates. Previous climatologies of blocking anticyclones have been performed using subjective or objective techniques to locate individual blocking events using observational data sets typically of greater than lO years. In this study, a 3-year climatology of Northern Hemisphere blocking anticyclones was developed using ECMWF analyses to derive a comprehensive set of blocking anticyclone characteristics, including location, frequency, duration, intensity, size, seasonal and regional distribution, and relationship to precursor cyclones and jet streaks. Results show that preferred blocking regions were located over the eastern Atlantic Ocean, the Pacific Ocean, and the Ukraine/western Russia and that most blocks occurred in winter, as seen in other climatological studies. Block half- wavelengths, which averaged about 3000 km, were positively cQrrelated with block intensity at the 99% confidence level. However, block duration, which averaged 8.6 days, was only weakly correlated with both size and intensity. Also, this study reveals that all 63 blocking anticyclones were preceded by an identifiable surface cyclone, which began its most rapid deepening 36 h or more prior to block onset. However, only 34 of these cyclones could be characterized as "explosively" developing, with half of these preceding winter season blocks and none preceding summer season blocks. A positive correlation was found between the intensity of blocking anticyclones and the intensity of the precursor cyclone development, significant at the 95 % confidence level. This correlation was also found for events occurring over the oceanic regions. Finally, the intensity of the precursor cyclone development was correlated with other blocking characteristics and no significant relationships were found.