yThe aim of this study is to document some of the key systematic errors of the atmospheric circulation as simulated by the ECMWF model during the winter season (December-March). Two major topics are addressed. First, key systematic circulation errors are described and it is shown how they grow from the short range all the way to the extended range. Second, it is shown how systematic circulation errors changed during the last two decades. Results are presented for 500 hPa geopotential height fields, eddy kinetic energy, synoptic variability, and atmospheric blocking. A concise summary is somewhat hampered by the fact that systematic error characteristics significantly depend on the parameter, region, and the forecast range being considered. In general, though, it has been found that systematic circulation errors have been reduced considerably in recent years, particularly from the 1980s to the early 1990s. Model improvements are primarily manifest through a reduction of the rate at which systematic errors grow; their spatial structure, however, remains more or less unchanged for most of the parameters. On the other hand, and somewhat surprisingly, it has been found that for some parameters and regions systematic circulation errors have slightly deteriorated since the mid-1990s. Moreover, it turns out that during the first few days of the integration the spatial structure of systematic circulation errors undergoes considerable changes; in the medium range and beyond their spatial structure remains virtually unchanged. The most prominent systematic circulation errors identified in this study are: the development of a large anticyclonic bias in the central North Pacific in the medium-range and extended-range, the underestimation of kinetic energy of the transient eddies, a pronounced underestimation of synoptic activity in high latitudes, and the underestimation of atmospheric blocking in the extended range. In the medium range over Europe, however, it is shown that recent cycles of the ECMWF model are able to produce realistic blocking frequencies.
