In this study, we focus on the Northern Hemisphere wintertime (DJF) blocking frequency simulated by a 20-km mesh atmospheric global circulation model (AGCM; TL959L69) for the present-day (1979-2003) and the future (2075-2099) climates.For the present-day simulation, the model was integrated with observed historical SST (HadISST). For the future simulation, SST was made by adding change in SST projected by the CMIP3 multi-model ensemble (MME) mean to annual variation of the HadISST. IPCC SRES A1B scenario was assumed for future emission of green house gases.

In the present-day climate, the 20-km model shows good agreement with the Japanese reanalysis (JRA25) for the Atlantic blocking frequency, whereas it greatly overestimates the Pacific blocking frequency. In the future climate, the 20-km model shows the decreases in the Atlantic and Pacific blocking frequencies. Especially, the frequencies greatly decrease on the west side of 20E and 180E.

In order to estimate uncertainty in the future projection of blocking frequency, SST and initial value ensemble simulations were conducted using a 60-km mesh AGCM (TL319L60). Future SSTs were made by adding changes in SST projected by the CMIP3 MME mean, CSIRO_Mk3.0, MIROC3.2 (hires), and MRI-CGCM2.3.2 to annual variation of the HadISST. Each future projection was conducted from 3 arbitrary initial values.

In the present-day climate, the 60-km model shows better agreement with JRA25 than the 20-km model for the Pacific blocking. On the other hand, the 60-km model somewhat underestimates the Atlantic blocking frequency.

In the future climate, 60-km mesh AGCM ensemble simulations support the decreases in the Atlantic and Pacific blocking frequencies. Especially, the Pacific blocking frequency decreases much more than the Atlantic blocking frequency does. Also, a 60-km ensemble simulation with the MIROC3.2 (hires) SST tends to indicate large decreases in the Atlantic and Pacific blocking frequencies.