Multidecadal global and regional trends in 1000 mb and 500 mb cyclone frequencies

Trends in seasonal and annual frequencies of 1000 mb and 500 mb closed low pressure centers, or "cyclones", over the 40 year period 1958-1997 are determined for latitudinal zones and regions of cyclogenesis. Statistically significant trends are found throughout the northern hemisphere and in the southern hemisphere tropics, though the trends vary by season. Opposite trends were observed at the two levels in some regions. No significant differences in cyclone frequencies were found when El Nino and La Nina years were compared. Cyclone frequencies in North America and Europe are poorly correlated with the North Atlantic Oscillation.