1. Renata Gonçalves Tedeschi Alice Marlene Grimm Universidade Federal do Paraná, Curitiba, Paraná 1. OBJECTIVES 1)To asses the influence of ENSO on the frequency of extreme precipitation events; 2)To examine the influence of ENSO on the daily precipitation distribution. THE INFLUENCE OF ENSO ON THE FREQUENCY OF EXTREME PRECIPITATION IN SOUTH AMERICA 2. METHODS AND DATA Rain gauge data from about 10,000 stations are gridded to 1º latitude by 1º longitude, in the period 1956- 2002. Three-day running means of precipitation are computed and the values attributed to the central days. Gamma distributions are fitted to these means (above 0.1 mm), one distribution for each day of the year. Extreme events are those with a three-day mean percentile above 90. The years are classified as El Niño (EN), La Niña (LN) and Neutral (NN). El Niño (0): 1957, 1963, 1965, 1969, 1972, 1976, 1982, 1986, 1987, 1991, 1997 La Niña (0): 1964, 1967, 1970, 1971, 1973, 1975, 1985, 1988, 1999 The mean frequency of extreme events for each month, within each category of year, and the differences between these mean frequencies for EN and neutral years, and for LN and NN years are computed, as well as their statistical consistency, using the hypergeometric distribution. The daily rainfall frequency distributions are calculated separately for EN, LN, and NN years and the frequency ratio for each rainfall category is also computed for some grid points. 3. RESULTS Figure 1 shows the difference between the frequency of extreme events between EN and neutral years and LN and neutral years for some months: November (0), January (+), and April (+). In Nov (0) of EN years the frequency increases significantly over most of the La Plata Basin (LPB), with exception of its upper part. In January (+) the EN impact on extreme events reverses signal in Central-East Brazil and part of southern Brazil. This is probably due to a surface-atmosphere interaction triggered by the anomalies in spring (Grimm et al., 2007, Grimm and Zilli, 2008). The enhancement of extreme events is, however, maintained in the lower part of the LPB. In April (+) of EN episodes, there is increase in extreme precipitation events over most of the LPB, which is consistent with floods observed in autumn of these episodes. In LN episodes the influence on the frequency of extreme events is nearly opposite, but weaker in April (+) over the LPB. Figure 2 shows the location of the grid boxes for which the average number of extreme rainfall events for each category of year is displayed in Table 1, and for which the frequency distribution (and ratio) of daily rainfall is shown in Figure 3. The hystograms of daily rainfall and the logarithm of the ratio between frequencies for EN and NN years, as well as for LN and NN years show results consistent with Figure 1, and indicate that the ENSO impact tends to be relatively stronger on the categories of higher rainfall. Fig. 1: (Upper panels) Differences between numbers of extreme events in El Niño years and neutral years, and (lower panels) between numbers of extreme events in La Niña years and neutral years. Contour interval is 1 event. Positive (negative) differences significant over the 90% confidence level are represented in each grid box by red (lblue) shade. Fig. 2: Location of the grid boxes for which the average number of extreme rainfall events for each category of year is displayed in Table 1, and for which the frequency distribution (and ratio) of daily rainfall is shown in Figure 3. 4. CONCLUSIONS EN and LN episodes influence significantly the frequency of extreme precipitation events in several regions of South America during certain periods of the ENSO cycle, and most of the impact occurs during the rainy season and thus has strong practical consequences. All regions with consistent changes in total monthly rainfall exhibit also consistent changes in the frequency of extreme events but the reverse is not true. The impact of ENSO episodes on extreme events is more significant and extensive than on monthly or seasonal precipitation totals. There is more sensitivity to ENSO in the extreme ranges of daily rainfall. The ENSO-related changes in the intensity of extreme events do not show as much consistency and spatial coherence as the changes in the frequency of extreme events. The frequency of extreme events increases (decreases) when the large-scale perturbations favor (hamper) the circulation anomalies associated with extreme events in the affected regions. As the most dramatic social and economic impacts of climate variability are due to the associated variability in the extreme events, it is worth detailing the association between ENSO and extreme rainfall events. Fig. 3: (Upper panels) “Scaled” histograms of daily rainfall in the indicated grid boxes for El Niño (EN) episodes, La Niña (LN) episodes, and neutral (NN) years; (lower panels) logarithm of the frequency ratio EN/NN (black bars) and LN/NN (white bars). When ratio is infinite (no occurrences in one category), the value plotted is ± 0.75. (a) (b) (c) (d) (e) (f) Grid boxEN (11years) LN (9years) NN (27years) A ( November) 4,91,03,6 B (January)5,90,83,8 C (April)1,43,21,8 Table 1: Average number of extreme rainfall events for each category of year in each grid box marked in the map. (a) Region A – November (b) Region B – January (c) Region C- April A B C