Northern Hemisphere extratropical cyclone activity for four mid‐season months

Abstract Cyclone frequency, cyclogenesis and the primary and secondary tracks followed by extratropical cyclones within the Northern Hemisphere are examined for the four mid‐season months: January, April, July and October. The data for these four months were compiled from several sources for the period 1958–1977. Cyclogenesis was defined as the point of origin of a cyclone provided it maintained a closed isobar for 24 hours. Cyclone frequency was determined from the number of occurrences of an extratropical cyclone passing through a 5° latitude‐longitude grid box. Analyses of the grid values of cyclone frequency reveal relative maxima which define the cyclone tracks. The tracks are partitioned into primary and secondary tracks depending on the size of the relative maxima of cyclone frequency. Cyclogenesis is concentrated in the lee of major mountain ranges and along the east coasts of Asia and North America, with cyclogenesis in coastal North America found nearer the coastline than in the Asian area. A major cyclogenetic region also exists in the Gulf of Genoa‐Northern Italy region of the Mediterranean Sea, but here cyclogenesis is confined to the cooler seasons. As expected the areas of most frequent cyclogenesis shift poleward during the warmer months. Similarly, cyclone frequency exhibits a poleward displacement during the warmer months. It is also noted that whereas winter (January) is characterized by a significantly greater cyclone frequency over the ocean than over land, summer (July) finds the frequency over the continent about as large as over the oceans. Similarly there are significant poleward displacements of cyclone tracks in summer, but the displacement is in no way uniform over the hemisphere. These results can be compared with earlier hemispheric studies of cyclone activity.