Cyclone frequency and 700 mb. circulation : trends, periodicities, and high versus low cyclone distributions

Bibliography: pages [90]-94.The seasonal and annual cyclone frequencies, derived from an equal-area sampling grid over North America from 1950-1980, are used to determine linear and periodic tendencies. Also, the composites of the 7 highest and 7 lowest cyclone distributions are compared to each other and to the associated 700 mb circulation patterns. The annual cyclone number has declined by 30% during the study period. This declining trend indicates that the frequency of low cyclone distributions has increased. There is little difference between the high and low cyclone distribution patterns except for the level of cyclone activity. The cyclone activity centers most affected are the lee slopes of the Rocky Mountains followed by the west Canadian coast and then the east coast. High cyclone distributions are associated with relatively stronger westerlies over the Pacific Ocean and the west coast and zonal flow across the continent while low cyclone distributions are associated with relatively weaker westerlies and meridional flow. The declining trend in cyclone number is primarily due to lower cyclone frequencies north of 40° N and west of 90° W. Essentially, fewer cyclones are tracking across the western mountain ranges due to the blocking effects of an enhanced west coast ridge. The cyclone data also exhibit some periodic tendencies which cause some of the fluctuations in cyclone number; an annual cycle and a 10.3-year cycle are found. The zonal index and sunspot data also exhibit a 10.3-year cycle during the study period, but the relationships of the latter two parameters to the cyclone cycle are uncertain.M.S. (Master of Science