Decadal Variations of Tropical Cyclone Activity over the Central North Pacific

Abstract

Tropical cyclone activity (tropical depressions, tropical storms, and hurricanes combined) in the central North Pacific has been found to be on the rise 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 and hurricane records alone also reveals an increasing trend in both series since 1966. The upward trend is characterized by decadal-scale variability as manifested 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 intensity 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. Relative to 1966-81, sea surface temperatures in the tropical Pacific and relative vorticity near the surface to the south of Hawaii have increased dramatically during 1982-97. The changes in the frequency and intensity of tropical cyclones in the central North Pacific appear to be modulated by decadal-scale variability of the basic state of SST, which transitioned from a cold to a warm phase in the late 1970s; this warm phase may have resulted in stronger and more frequent El Niño events as seen during the second half of the record, leading to more cyclones in the central North Pacific.