Impacts of Water Variability: Benefits and Challenges


Low-frequency precipitation and river flow variability in Cuenca del Plata


J.-P. Boulanger, LODYC, CNRS, Paris, France; and F. Lafon, A. Lazar, C. Menkes, O. Penalba, M. Rusticucci, and W. Vargas

Cuenca del Plata is the second largest hydrological basin of South America covering parts of the five MERCOSUR countries (Argentina, Bolivia, Brazil, Paraguay and Uruguay). Understanding and forecasting the hydrological cycle over that region is a great challenge for preventing dramatic socio-economical impacts of climate variability as those observed in the past. As a contribution toward that objective, we studied the low-frequency variability of precipitation over Cuenca del Plata focusing on two periods: first, the 1979-2001 period using satellite data (Xie and Arkin, 1997); second, the 1950-1998 period using continental stations. Over the most recent period, we identified three major modes of variability which all together explain about 60% of the total variance. The first mode (28%) is well correlated to the Niņo3 index as already described in the litterature. However, it is worth noting that the relationship with Niņo3, while quasi-linear during extreme events (1982-1983, 1997-1998), is not as clear during smaller ENSO events. The second mode (17%) is more complex as it displays some variability on top of a low-frequency trend. The short period under study does not allow to clarify whether that trend is real or part of a much lower frequency variability. That second mode is mainly characterized by a weakening of the precipitations over the Bolivian Altiplano. Studying as well sea surface temperature (SST) and sea level pressure (SLP) data over the same period, it is suggested that that variability is related to a strengthening of the subtropical South Atlantic anticyclone which induces a cooling of sea surface temperature in the central part of the subtropical South Atlantic Ocean (certainly due to stronger latent heat release), a warming along the Argentinian coasts in relation with a southward extension of the Brazil Current and an eastward warming along 40°S-50°S as the warm waters are entrained by the ocean circulation. Finally, the third mode (13%) is mainly characterized by a 6-8 year variability with a spatial pattern displaying a dipole structure in the vincinity of the South Atlantic Convergence Zone (SACZ). We infer from SST and SLP data that that mode reflects changes in the intensity of the SACZ activity (characterized by a meridional displacement of the SACZ together with either stronger or weaker subsidence on the northern and southern flanks of the SACZ region). While the third mode is well correlated to local SST variability along the southern coasts of Brazil, this SST pattern may actually result from a larger scale variability. We suggest that both changes of the meridional interhemispheric SST gradient (i.e. the Atlantic Dipole) and the equatorial Atlantic mode may influence the SACZ activity and thus precipitations over Cuenca del Plata. The previous investigation is extended to a much longer period using continental stations and confirms most of our results. Finally, we investigate the low-frequency variability of precipitations in the subbasins of Cuenca del Plata and relate the local principal modes to the major river flows.

extended abstract  Extended Abstract (292K)

Session 5, Global Perspectives on Impacts
Tuesday, 11 February 2003, 1:30 PM-4:45 PM

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