Northern Hemisphere atmospheric blocking as simulated by a 60-km-mesh MRI AGCM in the period 1872-2099

In this study, we conducted 228-year integrations using a 60-km-mesh MRI AGCM (TL319L64). Model integration was conducted for the period 1872-2099 using observed and prescribed, interannually varying SSTs as lower boundary conditions. The prescribed SST was estimated by the CMIP3 multi-model ensemble mean to which the detrended interannual variations in HadISST have been added. The IPCC SRES A1B scenario was assumed for future emissions of greenhouse gases. We focused on Euro-Atlantic (EA) and Pacific (PA) atmospheric blockings in winter (December-February) and summer (June-August).

The TL319L64 AGCM has a superior ability in simulating the blocking frequency and duration throughout the year, compared with the NCEP/NCAR reanalysis data for the period 1950-2005. Pacific blocking was observed more frequently during La Nina (El Nino) conditions than during El Nino (La Nina) conditions for the winter (summer) periods 1950-2005. The relationships between the PA blocking and the oceanic condition, El Nino and La Nina, are well simulated for the period 1950-2005. No apparent relationships between EA blocking and the El Nino/La Nina phase were observed and simulated for the period 1950-2005.

In terms of the timeseries of simulated areal mean blocking frequency for the period 1872-2099, the wintertime EA blocking frequencies show the most remarkable decreasing trend, whereas the summertime EA blocking frequencies show a decrease trend only in the 21st Century. Given that El Nino condition is predicted to be preferable in the future climate and that there are no possible relationships between the EA blocking and the El Nino/La Nina phase, the reduction in the EA blocking frequency might be related to other possible reasons. On the other hand, the wintertime PA blocking frequencies show a decreasing trend for the period 1872-2099. The reduction in the PA blocking frequency might be related to preferable El Nino conditions in the future climate, unlike that in the EA blocking frequency. The summertime PA blocking frequencies show a decreasing trend in the 20th and 21st Centuries with a notable jump in the last quarter of the 20th Century.