NORTHERN-HEMISPHERE ATMOSPHERIC BLOCKING AS SIMULATED BY 15 ATMOSPHERIC GENERAL-CIRCULATION MODELS IN THE PERIOD 1979-1988

As a part of the Atmospheric Model Intercomparison Project (AMIP), the behaviour of 15 general circulation models has been analysed in order to diagnose and compare the ability of the different models in simula ting Northern Hemisphere midlatitude atmospheric blocking. In accordan ce with the established AMIP procedure, the 10-year model integrations were performed using prescribed, time-evolving monthly mean observed SSTs spanning the period January 1979-December 1988. Atmospheric obser vational data (ECMWF analyses) over the same period have been also use d to verify the models results. The models involved in this comparison represent a wide spectrum of model complexity, with different horizon tal and vertical resolution, numerical techniques and physical paramet rizations, and exhibit large differences in blocking behaviour. Nevert heless, a few common features can be found, such as the general tenden cy to underestimate both blocking frequency and the average duration o f blocks. The problem of the possible relationship between model block ing and model systematic errors has also been assessed, although witho ut resorting to ad-hoc numerical experimentation it is impossible to r elate with certainty particular model deficiencies in representing blo cking to precise parts of the model formulation.