Mean Southern Hemisphere extratropical cyclone behavior in the 40-year NCEP-NCAR reanalysis

This paper presents a new climatology of Southern Hemisphere (SH) extratrop ical cyclones. This has been compiled by applying a state-of-the-art cyclon e tracking scheme to the 6-hourly National Centers for Environmental Predic tion-National Center for Atmospheric Research (NCEP-NCAR) global reanalyses spanning the period 1958-97. The results show there to be, on average, bet ween 35 and 38 cyclonic systems per analysis (depending on season), with th e greatest density [exceeding 6 X 10(-3) cyclones (deg lat)(-2)] found sout h of 60 degrees 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 cyclone s (i.e., cyclogenesis exceeds cyclolysis) north of about 50 degrees S, and a net destruction to the south of this latitude. Having said this, the most active cyclogenesis takes place south of 45 degrees S. The NCEP-NCAR reana lyses indicate that most SH cyclogenesis occurs at very high latitudes, and the axis of the maximum lies on, or to the south of, 60 degrees S. This is in agreement with the deductions of many modern studies of SH cyclone beha vior. The region is also host to even greater levels of cyclolytic activity . The authors consider measures of the importance and influence (e.g., for ed dy fluxes) of cyclonic systems. It is suggested that the "depth" of a syste m (the pressure difference between the center and the "edge" of a cyclone) is 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 degrees 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 degr ees and 70 degrees S (at their half-lifetime) endure, on average, almost on e day longer than all other systems. The mean track length of winter system s is 2315 km, which reduces to 1946 km in summer. The significance of the work presented here lies in a number of factors. Fi rst, the climatology has been derived from 40 yr of analysis, a period long er than any considered heretofore. Further, the (re)analyses used can be re garded as one of the best representations of the global atmosphere. The ava ilability of these analyses at 6-hourly intervals means that the uncertaint ies 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 cyclone s is arguably the most accurate and representative set yet assembled.