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 Oce an over the past 50 years is found. The frequency and intensity of extreme cyclones has increased markedly, with associated upward trends in extreme s urface winds between 25 degrees and 40 degreesN and major changes in cyclon e-related circulation patterns in the Gulf of Alaska. Related increases in extreme wave heights are inferred from wave measurements and wave-model hin dcast results. The more vigorous cyclone activity has apparently resulted f rom increasing upper-tropospheric winds and vertical wind shear over the ce ntral North Pacific. Such changes, which create an environment more favorab le for cyclone formation and intensification, may be related to the observe d modulation of El Nino-related teleconnections at decadal and longer times cales. It is intriguing that this trend has been relatively steady rather t han the sudden or stepwise shifts documented for other aspects of North Pac ific climate change. Increasing sea surface temperatures in the western tro pical Pacific are suggested as a plausible cause of the observed changes, t hough other underlying mechanisms may also contribute.