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Forty Years of Cyclone Activity
Key, J.R. and Chan, A.C.K.  1999.  Multidecadal global and regional trends in 1000 mb and 500 mb cyclone frequencies.  Geophysical Research Letters 26: 2053-2056.

What was done
Trends in seasonal and annual frequencies of low-pressure centers (cyclones) at 1000 mb (near-surface) and 500 mb heights were analyzed for six latitude regions (0-30°N, 0-30°S, 30-60°N, 30-60°S, 60-90°N and 60-90°S) over the four-decade period 1958-1997.  Trends in cyclone frequencies were also determined for El Niņo vs. La Niņa years.

What was learned
Trends in cyclone frequency at both atmospheric levels turned out to be pretty much a wash when considering results for all latitudes, with both positive and negative trends (some significant and some not) observed over the 40-year period.  Cyclone frequencies at both atmospheric levels during El Niņo years were found to be lower at all latitude regions except two (30-60°S and 60-90°S) as opposed to La Niņa years.

What it means
Results of this analysis suggest that, although there have been regional changes in cyclone frequencies, there is no indication of any global trend, positive or negative.  The authors' finding that fewer cyclones occur during warmer El Niņo years, as opposed to cooler La Niņa years, refutes the oft-made claim that global warming will result in more storms.

Reviewed 1 October 1999