Climatology of Northern Hemisphere blocking in the ECHAM model

An analysis of blocking events is conducted on the output of four integrations of the ECHAM3 atmospheric general circulation model with different configurations of horizontal spectral truncation and oceanic surface temperatures. The definition of local blocking and sector blocking is obtained by using an objective index based on a measure of the local zonality of the flow. Model variability and systematic errors are assessed and frequency diagrams of different quantities relative to blocking statistical and synoptic properties are produced. Effects of resolution and of physical parametrizations on model performance are considered. All versions of the model show a tendency to underestimate blocking occurrence. Evidence of a different nature of the blocking phenomenon in the Euro-Atlantic and in the Pacific sectors is found. The two sectors are characterized by a different sensitivity of blocking frequency to SST prescription, by a different annual cycle and by a different sensitivity of the distribution of blocking lifetime to model resolution. Euro-Atlantic blocking is found to be more the result of internal dynamics of the flow, while Pacific blocking appears more dependent on the oceanic boundary conditions.