Intensity of extreme rainfall in south-eastern South America

FEDERICO ARIEL ROBLEDO; OLGA C. PENALBA; VERA, CAROLINA

París

Workshop; Workshop on metrics and methodologies of estimation of extreme climate events; 2010

WCRP-UNESCO (GEWEX/CLIVAR/IHP

Extreme events are infrequent meteorological phenomena and their severity depends on the natural environment affected. This means that the definition of an extreme event will largely depend on the activity envolved 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. In the south-eastern South America (SESA), as a direct consequence of the annual cycle, we can observed different atmospheric circulations on the synoptic scale and mesoscale. They will be the cause of events which result in the different rainfall values observed. Annual rainfalls in SESA have been increasing during the last decades of the twentieth century. This temporal variability is usually quantified in terms of total rainfall, for example, annual, seasonal, or monthly. In some cases number of rain days appeared to be responsible for the fluctuations in rainfall amounts. In some opportunbities this variation depended on rainfall intensity; and in others, both variables played a part. The pattern of the trends of extremes and of annual rainfall in South America during the last decades was generally the same. At it was mentioned, changes observed in monthly rainfall may be due to changes in the number of rain days, in rainfall intensity, or both. This work focuses on the first issue, especially extreme rainfall, and tries to show that the extreme rainfall variability presents seasonal and regional behavior and becomes more pronounced according to the season. The region of analysis is in the south-east of South America and stretches over central and eastern Argentina, south of Paraguay, south of Brazil and Uruguay. In this region, it was possible to identify and collect only 45 series of daily precipitation that were completed in the period 1961-2000. 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 as follows. The different daily percentiles are calculated on all the daily rainfalls for each meteorological station. The indices selected in this study are the number of days with rainfall equal to or greater than the different percentile thresholds mentioned above, calculating the percentage of events (PE). We also calculated he monthly mean of daily intensity of extreme rainfall (DIER) is the quotient between monthly accumulated extreme rainfall (AE) and the number of days with extreme precipitation in one month (PE).The different indices were calculated per austral season: summer (December, January and February, DJF), autumn (March, April and May, MAM); winter (June, July and August, JJA), spring (September, October and November, SON), and the year as a whole (December to November of the following year). The trend test applied in this study for the common period 1961-2000 is the non-parametric Kendall-Tau test. We observed increases in the annual frequencies in spatially coherent areas. This coherence was more marked in austral summer, autumn, and spring during which the greatest increases occurred in southern Brazil, especially during extreme events. In winter, central east Argentina showed negative trends, some of which were significant.