CLIMATOLOGICAL FEATURES OF BLOCKING ANTICYCLONES - A STUDY OF NORTHERN-HEMISPHERE CCM1 MODEL BLOCKING EVENTS IN PRESENT-DAY AND DOUBLE CO2 CONCENTRATION ATMOSPHERES

Using output made with the National Center for Atmospheric Research (N CAR) Community Climate Model Version 1 (CCM1), the characteristics of blocking events over the Northern Hemisphere in a ten-year present day control simulation with a CO2 concentration of 330 ppm were compared to those in a previously analyzed observational three-year climatology . The characteristics of blocking events in a double present-day CO2 c oncentration simulation were then compared to those in the control sim ulation in order to evaluate how these characteristics might change in an increased CO2 atmosphere. The results demonstrated that in the Nor thern Hemisphere the CCM1 correctly simulated many characteristics of blocking events such as average annual number of occurrences, annual v ariations is size and intensity, and preferred formation regions. A mo re detailed analysis (i.e., by region and season) revealed some differ ences between the CCM1 and observed blocking events for characteristic s such as mean frequency of occurrence, intensity, size and duration. In addition, the model failed to capture adequately the occurrence of blocking events over the western Asian continent. A comparison of the double CO2 concentration run to the control showed that, in general, b locking events were more persistent and weaker, but of similar size in the increased CO2 atmosphere. Also, some statistically significant re gional and seasonally dependent changes were found in the frequency of occurrence, duration, and intensity. Finally, a correlation between b lock size and intensity, significant at the 99% confidence level, was found in each climatology. This result is similar to a correlation fou nd in the analysis of observations.