Blocking signatures in the northern hemisphere: Frequency distribution and interpretation

• Published in: Journal of Climatology
• Main Authors: Knox, John L., Hay, John E.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1985
• Subjects: Baffin Island; Baffin; Canadian Archipelago
• Online Access: http://dx.doi.org/10.1002/joc.3370050102; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1002%2Fjoc.3370050102; https://rmets.onlinelibrary.wiley.com/doi/pdf/10.1002/joc.3370050102

Abstract In a companion paper (Knox and Hay, 1983) the authors developed objective identification criteria, adaptable to machine processing methods, by relating the blocking anticyclone to its associated positive anomaly of 5‐day mean 500 mb height. Anomalies meeting the criteria were called ‘blocking signatures’. Moreover, by using an empirically derived pentad displacement threshold, it was possible to identify ‘blocking signature sequences’ corresponding to the life history of blocking episodes. In this paper we present and interpret the spatial and temporal distribution of blocking signatures during the 33 year period 1946 to 1978 inclusive. The frequency of occurrence is presented by area, and by longitude for each season and for the year. The results are, in general, in agreement with published studies for the oceans, but they also reveal a high frequency of blocking signatures over the North‐eastern Canadian Archipelago. This particular result, ‘the Baffin Island paradox’, is further investigated and rationalized. The interannual variation of blocking signature frequency over the Northern Hemisphere showed substantial fluctuations, particularly with regard to the more persistent episodes. However, because of the small size of the sample (33 years) relative to the time scale of the fluctuations (order of one decade) it was not possible to draw statistical conclusions.