CUT OFF LOWS AND ITS RELATION WITH EXTREME PRECIPITATION EVENTS OVER SOUTHERN SOUTH AMERICA

GODOY, ALEJANDRO; ROBLEDO, FEDERICO ARIEL; CAMPETELLA, CLAUDIA

Foz do Iguazu

Conferencia; The Meeting of the Americas; 2010

American Geophysical Union

The aim of this paper is to explore the relationship between upper level cut-off lows (COLs) occurrence and extreme precipitation over southern South America for the 2001-2006 cold season (March to August). The definition of an extreme event will largely depend on the activity and region affected. In the particular case of extreme precipitation events, their definition depends, moreover, on the nature of the rainfall in the region under study. Daily rainfall data used in this study were provided by different national institutions such as the National Weather Service, the National Institute of Agricultural Technology for Argentina, and the National Agricultural Research Institute for Uruguay. We define a rain day as one on which the rainfall is greater than 0.1 mm and extreme rainfall is considered when the daily rainfall is greater than a given threshold. These thresholds are based on statistical values such as the 75th, 90th and 95th daily percentiles which were determined empirically. The different daily percentiles are calculated on all the daily rainfalls for each meteorological station. COLs events affecting South America continental area (between 15 and 45 S) during certain stages of its life cycle for at least 24 hours were selected. For the 5 cold seasons studied, 52 events satisfies the criterion. The contribution of COLs to the total seasonal precipitation is greater in autumn (MAM), more than 50%, over central western Argentina and central Chile (25 - 35 S and 70 - 85 W), while in winter the major contribution is over central eastern and NE Argentina and Uruguay. Synoptic stations were selected over these particular regions in order to analyze the occurrence of extreme precipitation events (above percentile 75th). The results show almost the 40% of the COLs events affecting Mendoza (central western Argentina) during autumn produce extreme precipitation indicating the relevance of these phenomena in this region. In winter, there is an increased percentage of extremes events for the stations over central and NE Argentina, Junin (43%) , Parana (21%) and Corrientes (37%). Analyzing the total precipitation extremes during autumn for Mendoza, the 43% of these extremes corresponds to COLs affecting the area.