Dynamical aspects of Euro-Atlantic Blockings during the winter 2005/06

Long-lasting weather phenomena can exert a sustained and significant impact upon a region's weather conditions. Among the most prominent of these phenomena is that of extra-tropical blocking. A block can disrupt the prevailing westerly airflow for periods in excess of several days and concomitantly modify the weather conditions over an extended area. Indeed individual blocking events can account for several historical examples of extreme weather conditions and short-term climate anomalies. A recent topical example is the distinctively anomalous cold winter of 2005/06 over Europe. The accompanying dynamics that led to the establishment, maintenance and breakdown of the associated blocks is both of intrinsic theoretical interest and of practical forecasting concern since accurate block prediction remains a major challenge for medium-range and seasonal forecasting. In this presentation a systematic analysis is conducted of the winter's 2005/06 prevalent weather characteristics that contributed to the anomalous cold conditions over Europe. It is shown that these anomalies were linked to a sequence of atmospheric blocking events rather than to the dominant in-situ pattern of large-scale climate variability (the North Atlantic Oscillation) that normally influences European winter climate. In turn, novel insights are presented of the mechanisms for the establishment of the blocks by applying a regional climate model (RCM) and a diagnostic approach based upon a Lagrangian framework. A striking result of the analysis is the deduction that cloud-diabatic processes contribute seminally to the block formation. In light of these results comments are also made on the extant monthly to seasonal time scale predictions of the winter.