INTERANNUAL VARIABILITY OF FLOODING IN THE SOUTHERN ISTHMUS

In the past two decades, the most studied example of the meteorological forcing of flows in large basins has been the El Niño/Southern Oscillation (ENSO) phenomenon. Variability in flow regime, which clearly manifests changes in climatic inputs, impinges directly upon human activity through floods and droughts, and associated loss of life, infrastructure and economic activity. The study of rivers and lakes has also aided in detecting the influence of ENSO, both in the short and long run. River basins integrate noisy at-a-site meteorological signals, combining information from large, frequently unmonitored, areas. They may act to amplify the ENSO signal and become part of a series of complex environmental feedbacks.

Flood regimes of two basins on opposing flanks of the Central American Cordillera, show the regional controls on the annual hydrologic regime at varying temporal and spatial scales. This area is influenced directly by the atmosphere-ocean regime of the eastern equatorial Pacific and also by the tropical North Atlantic and mid-latitude North America. The interaction of these "centers of action" with local low level jets and regional topography explains very different flood responses at both the regional and local scales.

Standard frequency analyses of annual flood and partial duration series are applied to records from both rivers, paying particular attention to the distributions of the number (Poisson), timing (Gaussian) and magnitude (Pareto) of events arising from each generating process. These records and their derived statistical properties are sub-divided into three categories via an a priori classification of phases of ENSO. Observed differences in the stochastic properties are interpreted in terms of the proposed regional forcing mechanisms.

The results also illustrate that consideration of ENSO alone is insufficient to explain all interanual variability. Initial conditions, non-linearities and feedback mechanisms, both local and regional, serve to modify the effects of ENSO. Further, consideration of low frequency changes in the atmosphere-ocean system of the tropical North Atlantic demonstrates that these modify the response of the regionally significant "mid-summer drought" to ENSO.