Upper-level cut-off lows in southern South America

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 degrees Co80 degrees CeW and 30 degrees Co45 degrees CeS. 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.