Naturwissenschaften 83, 127-129 (1996) © Springer-Verlag 1996 ENSO-Controlled Flooding in the Paranfi River (1904-1991) P.J. Depetris Universidad Nacional de Cdrdoba, EC.E.F. y N., 5000 Cdrdoba, Argentina S. Kempe Technische Hochschule Darmstadt, Geologisch-Palfiontologisches Institut, D-64287 Darmstadt, Germany M. Latif Max-Planck-Institut ft~r Meteorologie, D-20146 Hamburg, Germany W.G. Mook Netherlands Institute for Sea Research (NIOZ), NL-1790 AB Den Burg, Texel, The Netherlands Since the poineering work of Walker [1, 2], evidence has been mounting that the E1Nifio-Southern Oscillation (ENSO) phenomenon causes rainfall anomalies in various parts of the globe [3, 4]. These anomalies are particularly well defined over the region of South Amer- ica [4- 6]. Rivers exhibit the advantage of being rainfall integrators over large areas. This approach was used by Richey et al. [7], who have shown a significant (5% level) coherent relationship in the cross- spectral analysis of deseasonalized Am- azon discharge data and atmospheric pressure anomalies at Port Darwin, Australia. Precipitation over the Ama- zon basin appears to be stimulated by the ascending motions associated with the Southern Oscillation (SO) positive phase and, hence, its positive level departures occur mainly during cold events, also known as La Nifia [31. Early in the study of the SO, a keen in- terest arose on the connection which might exist between the SO and the flow regime of the Paranfi River. Origi- nally Mossman [8], and, a few years la- ter, Bliss [9], pointed to the relationship of the SO with rainfall over southern Brazil, Paraguay, and northern Argenti- na, as presented by the level of the Paranfi River 420 km above the mouth, at Rosario, Argentina. In opposition to the Amazon, the Paranfi positive level departures are significantly correlated with ENSO warm extremes. Moreover, the "season" of ENSO-related precipi- tation in the Paranfi River drainage ba- sin is in phase with the normal annual precipitation cycle [10], thus causing flooding events - often disastrous - during E1 Nifio years. To the best of our knowledge, the Paranfi is the largest riv- er system in the world which exhibits such a behavior. A relatively long record of the Paranfi discharge oscillations is available for the Corrientes section (Fig. l). In analyzing this daily record (Fig. 2A), we seek to establish the frequency of the variability and the significance of the teleconnection with the ENSO phe- nomenon. This record reflects the hydrologic conditions prevailing over 1.95 x 106 km 2 (ca. 70o7o of the total ba- sin area), including the runoff supplied by the Andean headwaters of the Bermejo and Pilcomayo rivers, the dis- charge provided by the Mato Grosso through the Paraguay River, and the water output delivered by the Upper Paranfi River basin with headwaters in Brazil's Serra dos Preneos. When the discharge time series of Fig. 2A is considered, the most out- standing features which arise are the pronounced seasonality - with high waters in February/March and low wa- ters in August/September - the sub- stantial variability, and the magnitude of the 1982/83 flood. This flood, obvi- ously related to the strong 1982 E1 Nifio event, caused profound modifications in the biogeochemical functioning of the river [ll, 12]. Other large floods, such as those of 1904, 1911, 1932, and 1965, also occurred in accordance with ENSO warm events [4]. Worthy of at- tention are the negative deviations in the record (Fig. 2B), which persisted longer than positive ones, at least until I \ M. ;J Fig. 1. Map of the Paranfi River drainage basin showing main tributaries. The cities of Corrientes and Rosario are referred to in the text. Other cities shown for reference Naturwissenschaffen 83 (1996) © Springer-Verlag 1996 127