Blocking signatures in the Northern Hemisphere: Rationale and identification

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Abstract
It is shown that blocking anticyclones are closely related to positive anomalies of 5‐day mean 500‐mb geopotential height satisfying criteria involving amplitude, latitude and time of year. Anomalies meeting the criteria are called “blocking signatures”. The criteria are used objectively to construct a Catalogue that identifies the pentad, location and magnitude of each blocking signature that occurred from 1 January 1946 to 31 December 1978. Through the use of an empirically derived displacement threshold, the Catalogue is designed to identify sequences of signatures corresponding to the life history of blocking episodes. The probability that a sequence which is 2 pentads or longer in duration will be related to a blocking episode is greater than 90%. In a subsequent paper, the authors will present and interpret the spatial and temporal distributions of blocking signatures during the 33‐year period.