Variability of Southern Hemisphere Extratropical Cyclone Behavior, 1958–97

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Abstract
An analysis of the variability and trends exhibited by many aspects of Southern Hemisphere (SH) mean sea level extratropical cyclones during the period 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 undertaken to date. Across the 40-yr period the annual and seasonal mean cyclone densities have undergone reductions at most locations south of about 40°S (with the greatest reductions near 60°S), and increases to the north. This pattern of change resembles the “high-latitude mode” identified in many studies of SH circulation features. It is shown that the mean radius of SH extratropical cyclones displays almost everywhere a significant positive trend, and there are also increases in annual mean cyclone “depth” (i.e., the pressure difference between the cen... Abstract An analysis of the variability and trends exhibited by many aspects of Southern Hemisphere (SH) mean sea level extratropical cyclones during the period 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 undertaken to date. Across the 40-yr period the annual and seasonal mean cyclone densities have undergone reductions at most locations south of about 40°S (with the greatest reductions near 60°S), and increases to the north. This pattern of change resembles the “high-latitude mode” identified in many studies of SH circulation features. It is shown that the mean radius of SH extratropical cyclones displays almost everywhere a significant positive trend, and there are also increases in annual mean cyclone “depth” (i.e., the pressure difference between the cen...

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