Tropical and Extratropical Cyclones 



Tropical cyclones develop in the vicinity of 15° to 20° N and near 20° S latitude. In 

 the northern hemisphere, tropical storms and hurricanes evolve from tropical cyclones 

 which have grown from low level disturbances originating in the intertropical convergence 

 zone (ICZ), an area where northeast and southwest trade winds meet (5° to 10°N). Figure 

 26 gives the average number of cyclones per 5-degree square per year, and the principal 

 season for storm activity is also given for each region. Large areas off the east coast of the 

 United States and the east coast of Asia, Austraha, India, and Madagascar are characterized 

 by relatively frequent tropical cyclones. Off the east coast of the U.S. there is at least one 

 tropical cyclone per year per 5-degree square as far north as 42° (Fig. 26), due in part to the 

 influence of the warm, northerly moving Gulf Stream. In the area of study (eastern Pacific), 

 the region of cyclone frequency greater than or equal to one per year per 5-degree square 

 only extends to 27°N because of the cooler California Current water moving south. The 

 greatest frequency of cyclones, however, is in the eastern tropical Pacific, where it reaches a 

 mean of six per year per 5-degree square in the vicinity of 16°N, 1 10°W (Fig. 26, Ref. 34). 

 In the eastern Pacific, tropical cyclones generally occur in the period June through October, 

 with the greatest frequency occurring during September. 



The various stages of tropical cyclones, as well as other meteorological data, are 

 given in Appendix D. Figure D.l shows the areal distribution plus frequency and speed 

 statistics for cyclonic winds of 18 m/sec (34 knot) or greater (gale-force winds) for 5-degree 

 squares in the northeastern Pacific. During the 22-year period of record, there was only a 

 single tropical cyclone in the vicinity of San Diego (4 percent probability of occurrence) 

 whereas in the square 15° to 20°N and 105° to 1 10°W there were 106 such cyclones. The 

 frequency and distribution of tropical stonns (with winds of 25-32 m/sec or 48-63 knots) 

 and hurricanes (winds > 33 m/sec) are given in Figs. D.2 and D.3, respectively. 



Figure 27 shows the maximum extent and mean tracks of both tropical and extra- 

 tropical cyclones for the northeastern Pacific. Most tropical cyclones originating southwest 

 of the area of maximal cyclone frequency (indicated by crosshatched line) move west- 

 northwestward and reach as far as 180°. The maximum northerly extent of tropical 

 storms with winds of 18 m/sec or greater is approximately 35°N. Hurricane-force stonns 

 (winds > 33 m/sec, in the period of record) have not extended beyond 30°N (322 kilome- 

 ters south of San Diego) and have reached the southern portion of the Hawahan Islands 

 (Fig. 27). 



During autumn and early winter the cooling of the Asiatic continent and adjacent 

 seas strengthens the surface temperature gradients and the polar front. This cooling trend 

 and an intensification of the jet stream aloft leads to more frequent fonnation and greater 

 development of extratropical cyclones. 



The usual extratropical cyclone track is east to southeast (Fig. 27). The Gulf of 

 Alaska is frequented by more extratropical cyclones than any other region of the north 

 Pacific. Their maximum frequency and intensity is attained during early and middle winter. 

 Off the west coast there are fewer than one extratropical cyclone per 5-degree square (dur- 

 ing February) south of approximately 44°N. The intensity of extratropical cyclones 



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