DETERMINING CYCLONE FREQUENCIES USING EQUAL-AREA CIRCLES

This paper examines recent cyclone frequencies with a methodology that incorporates the use of 5 degrees latitude equal-area circles located in a grid for a region centered from 25 degrees to 70 degrees N and 6 0 degrees to 140 degrees W. Cyclones were counted in 82 equal-area cir cles 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 cy clones among four north-to-south circles on a meridian indicated latit udinal variability in year-to-year raw cyclone counts. Correlation coe fficients developed from relating winter cyclone counts from one circl e to another were less than +/-0.29, implying that the location of the axis of maximum winter cyclone frequency varies annually. Results rev ealed that winter and spring had the greatest number of cyclones and t he southernmost position of the axis of maximum frequency. Summer and autumn had the smallest cyclone number and northernmost displacement o f the axis of maximum frequency. The maximum frequency of cyclones ext ended eastward from the lee of the Rockies to the Great Lakes basin an d off the Northeast coast. Temporal analyses identified that annual cy clone counts decreased from the early 1950s to the mid-1980s before re versing and increasing into the mid-1990s. Similar trends occurred in all four seasons and suggest that major shifts in the atmosphere's gen eral circulation occur over multiple decades in all seasons.