Blocking Action in the Middle Troposphere and its Effect upon Regional Climate: II. The Climatology of Blocking Action

The results of a semi-statistical study of 112 cases of blocking action in the upper westerlies are presented; areas of most frequent occurrence, characteristic movement and persistence, and seasonal and yearly trends of blocking activity are determined. The regional precipitation and mean surface-temperature anomalies associated with blocking over Europe are illustrated and compared with those produced by strong zonal flow aloft. The effects produced by variations in blocking activity upon European climatic trends are discussed. The influence of blocking action upon Scandinavian glaciation and certain indications as to the rôle played by blocking in an ice-age climatic cycle are discussed.