Mean Southern Hemisphere Extratropical Cyclone Behavior in the 40-Year NCEP–NCAR Reanalysis

Abstract

This paper presents a new climatology of Southern Hemisphere (SH) extratropical cyclones. This has been compiled by applying a state-of-the-art cyclone tracking scheme to the 6-hourly National Centers for Environmental Prediction?National Center for Atmospheric Research (NCEP?NCAR) global reanalyses spanning the period 1958?97. The results show there to be, on average, between 35 and 38 cyclonic systems per analysis (depending on season), with the greatest density [exceeding 6 ? 10?3 cyclones (deg lat)?2] found south of 60°S in all seasons and in the Indian and west Pacific Oceans in autumn and winter. For the most part, there is a net creation of cyclones (i.e., cyclogenesis exceeds cyclolysis) north of about 50°S, and a net destruction to the south of this latitude. Having said this, the most active cyclogenesis takes place south of 45°S. The NCEP?NCAR reanalyses indicate that most SH cyclogenesis occurs at very high latitudes, and the axis of the maximum lies on, or to the south of, 60°S. This is in agreement with the deductions of many modern studies of SH cyclone behavior. The region is also host to even greater levels of cyclolytic activity. The authors consider measures of the importance and influence (e.g., for eddy fluxes) of cyclonic systems. It is suggested that the ?depth? of a system (the pressure difference between the center and the ?edge? of a cyclone) is a relatively bias-free and useful measure of a cyclone?s status and effect on the circulation. The greatest climatological depths are seen to lie at about 60°S, well to the north of the circumpolar trough and of the region of greatest cyclone density. The mean lifetime of cyclones that last at least 1 day is just over 3 days. Those that are located between 50° and 70°S (at their half-lifetime) endure, on average, almost one day longer than all other systems. The mean track length of winter systems is 2315 km, which reduces to 1946 km in summer. The significance of the work presented here lies in a number of factors. First, the climatology has been derived from 40 yr of analysis, a period longer than any considered heretofore. Further, the (re)analyses used can be regarded as one of the best representations of the global atmosphere. The availability of these analyses at 6-hourly intervals means that the uncertainties with tracking of cyclones are greatly diminished. Finally, it has been compiled using one of the most sophisticated and reliable automatic cyclone finding and tracking schemes. This climatology of SH extratropical cyclones is arguably the most accurate and representative set yet assembled.