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A midlatitude climatology of 200- and 500-hPa cut-off lows and its usefulness in categorizing cut-off lows in central Chile

Munoz Castillo, Cristian

[Thesis]. Manchester, UK: The University of Manchester; 2020.

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Abstract

In this thesis, a midlatitude climatology of cut-off lows was done and then used for studying the synoptic-scale patterns associated with extreme rainfall rate events due to the passage of cut-off lows in central Chile. The climatology was developed using the NCEP-NCAR reanalysis data from 1960 to 2017 to detect 200- and 500-hPa cut-off lows. It was found that the seasonality of cut-off lows is level-dependent in both the Northern and Southern Hemispheres. Additionally, the climatology detected a positive trend in the yearly number of events especially in the Southern Hemisphere that is not associated with any of the natural climate variability modes. The results of these analyses are summarized in paper 1. For paper 2 the applicability of the climatology was tested by examining 500-hPa cut-off lows impinging upon central Chile between 1979 and 2017. From these cut-off lows, only those that were associated with extreme rainfall rates were selected for further analyses. Those cut-off lows associated with high precipitation rate (WET events) had a different moisture distribution than cut-off lows associated with low precipitation rate (DRY events). Whereas in WET events the moisture plume is mostly located equatorward of the cut-off low centre, in DRY events the moisture plume is mostly located westward and poleward. For WET events, its associated configuration of the flow facilitates the input of moisture to the upper-level low's leading edge. In contrast, for DRY events the moisture input is mostly directed towards the polar edge of the cyclonic circulation associated with the cut-off low, thus preventing moisture reaching the leading edge. Additionally, WET cut-off lows were less persistent than DRY cut-off lows even though both groups tended to occur more frequently during the rainy season, with no statistically significant trend at the interannual timescale.