Determining Cyclone Frequencies Using Equal-Area Circles

This paper examines recent cyclone frequencies with a methodology that incorporates the use of 5° latitude equal-area circles located in a grid for a region centered from 25° to 70°N and 60° to 140°W. Cyclones were counted in 82 equal-area circles for the period 1950–93 over North America. The grid of equal-area circles eliminated two related problems associated with conventional grid systems, area-inequality, and area-normalization, and allows for comparison of frequency counts among circles. An analysis of winter cyclones among four north-to-south circles on a meridian indicated latitudinal variability in year-to-year raw cyclone counts. Correlation coefficients developed from relating winter cyclone counts from one circle to another were less than ±0.29, implying that the location of the axis of maximum winter cyclone frequency varies annually. Results revealed that winter and spring had the greatest number of cyclones and the southernmost position of the axis of maximum frequency. Summer and autumn had the smallest cyclone number and northernmost displacement of the axis of maximum frequency. The maximum frequency of cyclones extended eastward from the lee of the Rockies to the Great Lakes basin and off the Northeast coast. Temporal analyses identified that annual cyclone counts decreased from the early 1950s to the mid-1980s before reversing and increasing into the mid-1990s. Similar trends occurred in all four seasons and suggest that major shifts in the atmosphere's general circulation occur over multiple decades in all seasons.