The largest floods in the Uruguay River and their climate

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: inesc@at.fcen.uba.ar
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.