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- Title
Mean Southern Hemisphere Extratropical Cyclone Behavior in the 40-Year NCEP--NCAR Reanalysis.
- Authors
Simmonds, Ian; Keay, Kevin
- 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 Environ-mental 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 3 10 23 cyclones (deg lat)22 ] found south of 608S 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 508S, and a net destruction to the south of this latitude. Having said this, the most active cyclogenesis takes place south of 458S. 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, 608S. 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 608S, 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 508 and 708S (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...
- Subjects
SOUTHERN Hemisphere; CYCLONES; CLIMATOLOGY
- Publication
Journal of Climate, 2000, Vol 13, Issue 5, p873
- ISSN
0894-8755
- Publication type
Article
- DOI
10.1175/1520-0442(2000)013<0873:MSHECB>2.0.CO;2