Southern Hemisphere Winter Extratropical Cyclone Characteristics and Vertical Organization Observed with the ERA-40 Data in 1979–2001

Abstract

The mean characteristics and trends of Southern Hemisphere (SH) winter extratropical cyclones occurring at six levels of the troposphere over the period 1979?2001 have been investigated using the 40-yr ECMWF Re-Analysis (ERA-40) data. Cyclonic systems were identified with the Melbourne University cyclone finding and tracking scheme. This study shows that mean sea level pressure (MSLP) cyclones are more numerous, more intense, smaller, deeper, and slower moving than higher-level cyclones. The novel vertical tracing scheme devised for this research revealed that about 52% of SH winter MSLP cyclones have a vertically well organized structure, extending through to the 500-hPa level. About 80% of these vertically coherent SH cyclones keep their westward tilt until the surface cyclones reach their maximum depths, and the mean distance is 300 km between the surface and the 500-hPa level cyclone centers when the surface cyclones obtain their maturity. According to the authors? definition of vertical organization, explosively developing cyclones are vertically very well organized systems, whose surface development is antecedent to their 500-hPa level counterpart. Over 1979?2001 cyclones have increased in their system density, intensity, and translational velocity but decreased in their scale at almost all levels. However, some of the trends are not statistically significant. The proportion of vertically well organized systems in the entire population of SH winter extratropical cyclones has considerably increased over the last 23 yr, and the mean distance between the surface and the 500-hPa- level cyclone centers has decreased. Such changes in vertical organization of extratropical cyclones are statistically significant at the 95% confidence level.