Climatological features of blocking anticyclones: a study of Northern Hemisphere CCM1 model blocking events in present-day and double CO2 concentration atmospheres

Abstract.

 Using output made with the National Center for Atmospheric Research (NCAR) 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 concentration simulation were then compared to those in the control simulation in order to evaluate how these characteristics might change in an increased CO₂ atmosphere. The results demonstrated that in the Northern Hemisphere the CCM1 correctly simulated many characteristics of blocking events such as average annual number of occurrences, annual variations is size and intensity, and preferred formation regions. A more detailed analysis (i.e., by region and season) revealed some differences between the CCM1 and observed blocking events for characteristics 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, blocking events were more persistent and weaker, but of similar size in the increased CO₂ atmosphere. Also, some statistically significant regional and seasonally dependent changes were found in the frequency of occurrence, duration, and intensity. Finally, a correlation between block size and intensity, significant at the 99% confidence level, was found in each climatology. This result is similar to a correlation found in the analysis of observations.