Decadal variations of tropical cyclone activity over the central North Pacific

Tropical cyclone activity (tropical depressions, tropical storms, and hurri canes combined) in the central North Pacific has been found to be on the ri se and this increase amounts to about 3.2 cyclones over the last 32 years ( 1966-97). An examination of time series of tropical storms and hurricanes a nd hurricane records alone also reveals an increasing trend in both series since 1966. The upward trend is characterized by decadal-scale variability as manifeste d by fewer cyclones during the first half of the record (1966-81) and more during the second half of the record (1982-97). The maximum hurricane inten sity has also increased in the central North Pacific, as well as the number of intense hurricanes from the first to the second half of the record. Rel ative to 1966-81, sea surface temperatures in the tropical Pacific and rela tive vorticity near the surface to the south of Hawaii have increased drama tically during 1982-97. The changes in the frequency and intensity of tropi cal cyclones in the central North Pacific appear to be modulated by decadal -scale variability of the basic state of SST, which transitioned from a col d to a warm phase in the late 1970s; this warm phase may have resulted in s tronger and more frequent El Nino events as seen during the second half of the record, leading to more cyclones in the central North Pacific.