Climate Processes in CMIP5: Extreme summer weather and atmospheric blocking over North America

Atmospheric blocking patterns in the mid-upper troposphere are believed to be responsible for a variety of extreme weather and climate events. For example, extended droughts over the western and central U.S. are associated with persistent (omega) blocking. This study examines the skill of the newly available CMIP5 simulations in reproducing the characteristics of atmospheric blocking events. We computed frequency, location, strength, and duration of blocking in the CMIP5 historical experiment based on the daily 500 hPa geopotential height from 1950-2005. Globally, most models we evaluated realistically captured the Euro-Asian and North America blocking ridges, including their seasonality. An annual peak frequency that occurred from 13 to 15% of the time was simulated. Over North America, the models reproduced blocking frequency over the West Cost, with a maximum around the date line and a minimum near 90oW. Eleven of the 12 models that were chosen simulated a similar blocking frequency. The majority of them slightly under-predicted the frequency by 20-40%. On average, model-simulated centers are slightly north (~2o) of the observed center, around 45oN, with larger error in the fall. However, the model simulated blocking intensity, measured by the height difference between the blocking ridge and flanking troughs, is overwhelmingly stronger than the observed. The blocking characteristics are highly associated with record temperature statistics in the U.S. as is observed.