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Cyclone Frequencies in the Southern Hemisphere
Reference
Simmonds, I. and Keay, K.  2000.  Variability of Southern Hemisphere extratropical cyclone behavior, 1958-97.  Journal of Climate 13: 550-561.

What was done
The authors employed a new cyclone finding and tracking scheme to conduct what they say "is arguably the most reliable analysis of Southern Hemisphere cyclone variability undertaken to date."

What was learned
It was found that the annual average number of cyclones in the Southern Hemisphere experienced a steady increase from the start of the assessment period.  After peaking in 1972, however, there was an overall decline; and the authors state that "the counts in the 1990s have been particularly low."  Simultaneously, they detected a small increase in mean cyclone radius; but they note that this effect has only "served to partially offset the effect of the remarkable decrease in cyclone numbers."

What it means
The authors note that the time series of Southern Hemisphere cyclone numbers shows an out-of-phase relationship with the Southern Hemisphere mean annual temperature record, which suggests, in their words, "that the downward trends in cyclone numbers are associated with a warming Southern Hemisphere."


Reviewed 1 March 2000