Future change in the frequency and intensity of wintertime North Pacific blocking in CMIP5 models

The current status and future changes in the frequency and intensity of climatological blocking activity over the North Pacific region are investigated using historical and two Representative Concentration Pathway (RCP4.5 and 8.5) simulations in the coupled climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) for boreal winters (December-February) over a 30-year period. The future change in the Pacific blocking frequency and intensity are examined in terms of the projected meridional thickness gradient, Hadley circulation changes, and changes in the probability distribution of categorized blocking strength. The five CMIP5 models that show better performance in reproducing climatological blocking events in the historical simulations for the Pacific region are selected for the analyses of the projected blocking activities. The climatological winter Pacific blocking frequencies of most of the individual models and model mean values show a tendency to decrease under global warming conditions. The trend is closely linked with the strong upper level westerly wind, resulting in less meandering air flow, consistent with the enhanced meridional temperature gradients at mid-latitude in the future climate. The decreased frequency in climatological atmospheric blocking over the Pacific under warming may also be influenced by the strengthening of the north-south temperature gradients due to the poleward extension of Hadley circulation in the subtropics. The climatology of the Pacific blocking intensity in boreal winter also tends to decrease slightly due to a future reduction in the number of strong blocking events.