THE INTERDECADAL VARIABILITY OF NORTHERN HEMISPHERE BLOCKING

A 55-year (1948-2002) Northern Hemipshere (NH) blocking climatology is presented here. Traditional blocking indexes and methodologies are revised in order to detect better blocking events objectively. The index also provides other quantities automatically such as the central point, duration, intensity, and extension. Additionally, this algorithm allows for the tracking of blocking events. This results in the longest known and most comprehensive blocking climatology, and the results are compared to those of previous studies. A new regional classification scheme is developed here, and blocking is examined on the time-scale of seasonal to interdecadal. The long term trend shows a significant trend toward weaker and less persistent events, as well as regional increases in blocking frequency over the western Pacific Ocean and Europe. The influence of teleconnection patterns on blocking are also explored, and these are primarily confined to the winter season. There are interannual variations in NH block intensity due to El Nino and Southern Oscillation (ENSO), but no significant variations in occurrence. Also, significant variability in the occurrence and duration of blocking have been found and are highly correlated with variability in the North Atlantic Oscillation (NAO).