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We identify Dry and Wet months for the Canadian Prairies during the 40-year period 1946-1985

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.KEY WORDS Blocking Blocking signature Blocking signature sequences Positive anomaly (of 5-day mean 500 mb height) Secular variation (of blocking signatures)

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