Evidence for Intensification of North Pacific Winter Cyclones since 1948

Using NCEP–NCAR reanalysis and in situ data, evidence of important changes in the winter (December–March) cyclone climatology of the North Pacific Ocean over the past 50 years is found. The frequency and intensity of extreme cyclones has increased markedly, with associated upward trends in extreme surface winds between 25° and 40°N and major changes in cyclone-related circulation patterns in the Gulf of Alaska. Related increases in extreme wave heights are inferred from wave measurements and wave-model hindcast results. The more vigorous cyclone activity has apparently resulted from increasing upper-tropospheric winds and vertical wind shear over the central North Pacific. Such changes, which create an environment more favorable for cyclone formation and intensification, may be related to the observed modulation of El Niño–related teleconnections at decadal and longer timescales. It is intriguing that this trend has been relatively steady rather than the sudden or stepwise shifts documented for other aspects of North Pacific climate change. Increasing sea surface temperatures in the western tropical Pacific are suggested as a plausible cause of the observed changes, though other underlying mechanisms may also contribute.