Upper-level cut-off lows in southern South America |
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| Authors: | C M Campetella N E Possia |
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| Institution: | (1) Departamento de Ciencias de la Atmósfera y los Océanos, Universidad de Buenos Aires and Centro de Investigaciones del Mar y la Atmósfera – CONICET/UBA, Buenos Aires, Argentina |
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| Abstract: | Summary This paper presents a statistical study of the spatial and seasonal distribution and duration of cut-off low systems over
the southern South American region based on the NCEP- NCAR reanalysis data for the period 1979–1988. Cut-off lows were first
objectively determined as minimum geopotential values at the 250 hPa level and then subjectively imposing a cut-off circulation
and a cold core. A total of 171 cut-off low events were detected, being more frequent in austral autumn followed by winter,
spring and summer. There is a preferential region of occurrence in spring and autumn located between 68°–80° W and 30°–45° S.
The Pacific area showed the greatest frequency of occurrence followed by the Atlantic and the continental areas. Most of the
cut-off lows last 2 or 3 days (around 90% of the cases) though there is a tendency of the continental events to be longer.
The cut-off low event developed upwind the Andes on 22–28 September 1986 was selected as a case study. Low-level cold air
advection was the main forcing of the deepening of the upper level low system. |
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