Interannual and Decadal Variability of Tropical Cyclone Activity over the Central North Pacific

Time series of the annual number of tropical cyclone counts over the Central North Pacific (CNP) during 1966-97 exhibit both distinct interannual and decadal-scale variability. On the interannual time scales, tropical cyclone activity appears to be modulated by the El Nino-Southern Oscillation (ENSO) phenomenon and, to some extent, influenced by the quasi-biennial zonal wind oscillation in the stratosphere.

During ENSO years, when the monsoon trough extends farther eastward and often into the CNP, tropical cyclone genesis is more frequent in the region between the dateline and 160 degrees W. The corresponding low level relative vorticity within this region doubles its value from a cold composite (i.e., La Nina) to an ENSO composite. An ENSO composite of tropospheric vertical wind shear also shows a two to three times reduction equatorward of 16 degrees N where most hurricanes traverse when compared to a cold composite. Consequently, cyclones entering from the far east into the CNP would have a greater chance to maintain their strength. More tropical cyclones are also observed over the CNP when the stratospheric winds are easterlies than westerlies, a feature in contrast to the Atlantic basin.

On the decadal time scale, tropical cyclone incidences over the CNP appear to have undergone a step-like change around the early 1980s, with fewer cyclones during the early epoch and more cyclones since 1982. This increase in cyclone incidences is probably related to the more frequent and stronger ENSO events as seen during 1982-97, which broadly coincides with a warm phase of the decadal oscillation of the sea surface temperature and atmospheric circulation in the Pacific Ocean. An investigation is currently undertaken to understand the reason why cyclone occurences become more favorable during the recent epoch.