Preferential pathways for Southern Hemisphere cold surges

Cold surges are high-impact weather events that pose a distinct threat to the agriculture industry in subtropical regions. Previous literature on cold surges in the Southern Hemisphere (SH) has focused on singular events, particularly those along the Brazilian Highlands affecting eastern Brazil. Such events typically occur in response to surface anticyclogenesis initiated by ridge amplification and subsequent anticyclonic wave breaking associated with downstream development. In this presentation, SH cold surges are examined from broader climatological and synoptic perspectives. Specifically, preferred SH cold surge pathways are identified from a 25-yr (1977¬–2001) SH climatology of oceanic and continental cold surges, and a comparison of regional cold surge thermodynamic and dynamic characteristics by SH region is conducted via a composite analysis. The dataset used to construct both the climatology and the composite analysis is the 2.5° ECMWF 40-year Reanalysis (ERA-40).

Results of the SH cold surge climatology indicate that preferred continental SH cold surge pathways are found near high terrain east of the Andes and the Brazilian Highlands, along the eastern shores of southern Africa and Madagascar, and, to a lesser extent, along the eastern shore of Australia, whereas oceanic SH cold surge pathways are present on the eastern sides of ocean basins. Preliminary results of the composite analysis suggest that thermodynamic and dynamic characteristics of cold surges may vary significantly by SH region, but that enhanced low-level southerly flow due to anticyclonic wave breaking is a common characteristic of both continental and oceanic cold surges.