CLIMATOLOGICAL FEATURES OF BLOCKING ANTICYCLONES IN THE NORTHERN-HEMISPHERE

Blocking anticyclones have long been of interest to the atmospheric sc ience community because of their profound effect on local and regional climates. Previous climatologies of blocking anticyclones have been p erformed using subjective or objective techniques to locate individual blocking events using observational data sets typically of greater th an 10 years. In this study, a 3-year climatology of Northern Hemispher e 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 eas tern Atlantic Ocean, the Pacific Ocean, and the Ukraine/western Russia and that most blocks occurred in winter, as seen in other climatologi cal studies. Block half-wavelengths, which averaged about 3000 km, wer e positively correlated with block intensity at the 99% confidence lev el. 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 b lock onset. However, only 34 of these cyclones could be characterized as ''explosively'' developing, with half of these preceding winter sea son blocks and none preceding summer season blocks. A positive correla tion 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 occurrin g over the oceanic regions. Finally, the intensity of the precursor cy clone development was correlated with other blocking characteristics a nd no significant relationships were found.