The largest floods in the Uruguay River and their climate forcing Inés A. Camilloni 1 Departamento de Ciencias de la Atmósfera y los Océanos. Universidad de Buenos Aires Centro de Investigaciones del Mar y la Atmósfera (CIMA/CONICETUBA) Buenos Aires, Argentina and Ruben M. Caffera Unidad Docente Meteorología. Facultad de Ciencias. Universidad de la República Montevideo, Uruguay 1 Corresponding author address: Inés A. Camilloni Departamento de Ciencias de la Atmósfera y los Océanos Ciudad Universitaria. Pabellón 2. Piso 2. (1428) Buenos Aires. Argentina. Tel. (5411) 4576-3398 Fax (5411) 4576-3356/3364 ext. 12 e-mail: [email protected] Abstract The Uruguay river is the second tributary in importance of the La Plata river with a basin that includes a vast region of Southeastern South America. The objective of this article is to examine the largest discharge events in the Uruguay River and to explore their atmospheric forcing. Extreme daily Uruguay River levels registered during the austral warm season (October to March) are related to intense rainfall in the upper basin particularly in the period -12 to -9 days before the date of maximum level at the Salto Grande gauging station while those occurred during the austral cold semester (April to September) are mostly due to large rainfall amounts registered over and upstream Salto Grande in two separate periods before the flooding dates (-12 to -9 and -4 to -1 days). The composites of the low-level wind fields for the warm season events show a northwestern flow over Bolivia and Paraguay that converges with a weak northern component over the Uruguay basin during the 12 days before the flooding date. The composites for the cold season events show an intense northwesterly flow over the same region that continues over southeastern South America and penetrates in the Atlantic ocean for the days -12 to -9 and -4 to -1 in accordance with the two heavy rainfall periods identified over the Uruguay basin. The circulation pattern during the -8 to -5 period shows northern flow and weak convergence over the basin. The enhanced precipitation due to moisture flux convergence in the South American low-level jet (SALLJ) region could be responsible for almost half of the largest floods of the Uruguay River at Salto Grande. For both the warm and cold seasons, the SALLJ occurrence largest frequencies are observed during the periods -12 to -9 and -4 to -1 before the flooding dates with a major decrease in the frequency during the intermediate period of the cold season events. In most cases, the extreme daily Uruguay River levels induced extraordinary discharges at the monthly scale. The largest monthly discharge anomalies at the Paso de los Libres gauging station occurred mostly during warm ENSO phases explained by the large positive rainfall anomalies observed in the region during these events.