A climatology of cutoff lows in the Southern Hemisphere

Abstract

The occurrence of cutoff lows (COLs) in the Southern Hemisphere at 500 hPa is studied for a 31-year period using atmospheric NCEP-NCAR reanalysis. The methodology combines objective detection and tracking for cyclonic systems with visual inspection in order to select those systems that segregate equatorward from the main westerlies. Hemispheric charts and frequency distributions are presented for COL distribution, initial and final locations, duration, intensity, and motion. COLs tend to cluster around the three main continental areas and to have a low frequency of occurrence over the oceans. Because particular features are shown by the COLs associated with each continent, three longitudinal sectors centered in Africa, Australia, and South America are defined. From the total of 1253 COLs detected, 10% were found in the African sector, 48% were found in the Australian sector, and 42% were found in the South American sector. Marked seasonal cycles with summer minima are found around South America and Africa but not over Australia. Over South America, net dissipation of COLs prevails while over Australia, generation is the dominant process. Active storm tracks and moist conditions seem to be responsible for fewer COLs, while low winds in the upper troposphere and dry conditions in the lower troposphere are associated with a higher frequency of occurrence. A trend in the number of COLs is significant only near South America with higher COLs after 1990. No relation was found between COL frequencies and the ENSO phenomena.