Geopotential height reanalysis data (NCEP-NCAR) at 1000 hPa and 500 hPa and infrared longwave radiation data from 18 years (1979-1996) are used for the analysis. They are compared with daily precipitation data in Chile (26°S to 36°S). Cutoff cyclones events are identified by means of the meridian gradient of geopotential height at 500 hPa between the regions 30°-35°S, 80°-75°W and 35°-40°S, 80°-75°W. A negative gradient is considered as an indication of the presence of a cutoff cyclone (discarding the cases in which the index is negative due to a frontal system).
The typical atmospheric configuration for the development of a cutoff cyclone in the mid-troposphere is a profound trough followed by a ridge with NW-SE orientation (between 25°S and 40°S). At the surface an anticyclonic configuration is observed. Southward of this region the flow is zonal. An important factor for the formation of a cyclonic nucleus seems to be the intensification of a downstream ridge which, associated with a westerly flow, tends to close the cyclonic nucleus in its southern part.
On average, seven events per year were observed (130 during the period considered) with a significant interannual variability (between 2 and 16). Summer presents the lowest frequencies of cutoff cyclones (20 events in total), followed by fall (32 events), winter (37 events) and spring (41 events). Of the 130 cases only 17 did not produce precipitation in the target region between day-1 and day+1. In the rest of the cases, there was significant variability in the spatial distribution of rainfall. In most of the cases the cutoff cyclones produce precipitation between 32°S and 34°S. The region from 32°S northward is affected by less cases.
The structures of cutoff cyclones with rainfall in the whole region (39 cases) and without precipitation (17 cases) are similar, however, those events with precipitation are more intense, i.e. the cyclonic nucleus is more profound, colder, and presents a stronger cyclonic vorticity advection (about three times stronger). There are also differences in the atmospheric configuration before the formation of cutoff cyclones with and without rainfall. Before a cutoff cyclone that produces precipitation a stronger positive tendency of geopotential height towards SW of the trough can be observed. The maximum positive tendency in this case is situated farther northward (at about 47.5°S) than in the case of cutoff cyclones without precipitation (at a
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