Trends in Northern Hemisphere surface cyclone frequency and intensity

One of the hypothesized effects of global warming from increasing concentra tions of greenhouse gases is a change in the frequency and/or intensity of extratropical cyclones. In this study, winter frequencies and intensities o f extratropical cyclones in the Northern Hemisphere for the period 1959-97 are examined to determine if identifiable trends are occurring. Results ind icate a statistically significant decrease in midlatitude cyclone frequency and a significant increase in high-latitude cyclone frequency. In addition , storm intensity has increased in both the high and midlatitudes. The chan ges in storm frequency correlate with changes in winter Northern Hemisphere temperature and support hypotheses that global warming may result in a nor thward shift of storm tracks in the Northern Hemisphere.