Large-Scale Circulation Features Associated with Decadal Variations of Tropical Cyclone Activity over the Central North Pacific

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Abstract
Tropical cyclone frequency in the central North Pacific (CNP) from 1966 to 2000 has exhibited decadal-scale variability. A statistical changepoint analysis reveals objectively that the shifts occur in 1982 and 1995, with fewer cyclones during the 1966–81 and 1995–2000 epochs and more during the 1982–94 epoch. A bootstrap resampling method is then applied to determine the frequency distribution of the mean annual cyclones for the 1966–81 and 1982–94 epochs, as well as to infer the confidence intervals of the observed mean and variance of cyclones for each epoch. Large-scale environmental conditions conducive to cyclone incidences during the peak hurricane season (July–September) for the inactive (1966–81) and active (1982–94) epochs are investigated. A nonparametric Mann–Whitney test is used to investigate whether the differences in location between the two epochs are significant. In contrast to the first epoch, warmer sea surface temperatures, lower sea level pressure, stronger low-level anomalous cyclonic vorticity, reduced vertical wind shear, and increased total precipitable water covered a large domain of the tropical North Pacific in the second epoch. These changes in environmental conditions favor more cyclone incidences for the second epoch. Many of the aforementioned changes were already established prior to the peak season. In addition, atmospheric steering flows have changed remarkably in October and November so that tropical cyclones in the eastern North Pacific have a better chance to enter the CNP, and cyclones formed in the CNP are more likely to be steered through the western Hawaiian Islands in the second epoch.