A Global Climatology of Surface and
A 40-year climatology of cyclonic systems has been developed based on the 1000 mb and 500 mb geopotential height data from the National Center for Environmental Prediction (NCEP) and NCAR. This is the NCEP/NCAR "reanalysis" data. Cyclones at these two levels are identified automatically and tracked from one 12-hour grid to the next.
Statistically significant trends in cyclone frequencies have been observed at 1000 mb and 500 mb in certain seasons for all regions and latitude zones. In some cases the trends at the two levels are of opposite sign. While differences in annual average frequencies for El Nino and La Nina years are apparent, the majority of the differences are not statistically significant. The North Atlantic Oscillation index is correlated with Arctic cyclone frequencies, but not with frequencies in northeastern North America or northwestern Europe.
The figures below illustrate the trends in cyclone frequencies at the two levels for the mid-latitudes Opposite trends in the frequency of cyclones at the two levels can be seen, particularly in summer. Trend magnitude and sign vary from region to region.
The data set from which these trends were derived is available for download.
This research is being (or was) performed by the following people: Jeff Key and Alan Chan.