Variability of Southern Hemisphere extratropical cyclone behavior, 1958-97

An analysis of the variability and trends exhibited by many aspects of Sout hern Hemisphere (SH) mean sea level extratropical cyclones during the perio d 1958-97 is presented. The investigation is undertaken by applying a state -of-the-art cyclone finding and tracking scheme to the 6-hourly reanalyses produced by the National Centers for Environmental Prediction. The outcome of this is arguably the most reliable analysis of SH cyclone variability un dertaken to date. Across the 40-yr period the annual and seasonal mean cyclone densities have undergone reductions at most locations south of about 40 degrees S (with t he greatest reductions near 60 degrees S), and increases to the north. This pattern of change resembles the "high-latitude mode" identified in many st udies of SH circulation features. It is shown that the mean radius of SH ex tratropical cyclones displays almost everywhere a significant positive tren d, and there are also increases in annual mean cyclone "depth" (i.e.. the p ressure difference between the center and the "edge" of a cyclone). The annual average number of cyclones per SH analysis rose from the start o f the period to a maximum of about 39 in 1972. Since then, the numbers have shown an overall decline, the counts in the 1990s being particularly low. Similar behavior was evident when the count was confined to the 30 degrees- 50 degrees S and 50 degrees-70 degrees S latitude bands. Least squares best lit to the three lime series exhibit significant slopes of -0.58, -0.26, a nd -0.58 cyclones per analysis per decade, respectively. Between 30 degrees and 70 degrees S the annual mean number of cyclones found per analysis ass umed a maximum about 1970. bur that number has dramatically decreased by ab out 10% since then. (This analysis suggests that the downward trends in cyc lone numbers are associated with a warming Southern Hemisphere.) The overal l structure of the time series of annual cyclone per analysis over 30 degre es-50 degrees S and 50 degrees-70 degrees S are similar, but their year-to- year changes are shown to be negatively correlated; hence, there tends to b e an interannual compensation of cyclone density between the middle and hig her latitudes. The extent to which changes in the semiannual oscillation over the last few decades could be said to have influenced how cyclones are distributed acro ss seasons is briefly examined. The results show. in particular. that the i nterannual relationship between spring and winter cyclone density cannot be explained in terms of a response to a change in the amplitude of the semia nnual oscillation.