Key Dynamical Features of the 2005/06 European Winter

A three-part study of the anomalously cold European winter of 2005/06 is undertaken. Climatological analysis indicates that the dominant pattern of climate variability in the Euro-Atlantic sector during this winter was not a negative phase of the North Atlantic Oscillation (NAO), but a pattern with a blocklike'' center located immediately upstream of the continent. Synoptic-dynamical diagnosis of the winter indicates the frequent occurrence of long-lasting blocks in this region, and a Lagrangian trajectory analysis points to the significant role of cloud-diabatic effects in the dynamics of block inception. A series of heuristic numerical simulations lend credence to the hypothesis that the occurrence of the blocks was sensitive to, and significantly influenced by, the warm surface temperature anomalies upstream over the western Atlantic Ocean and North America. Brief comments are made on the significance of the foregoing results for seasonal numerical weather prediction and also their relevance to the consideration of interannual climate variability.