Role of terrestrial hydrologic memory in modulating ENSO impacts in North America

Relationships among the terrestrial hydrologic processes over the North American continent associated with the El Niño-Southern Oscillation (ENSO) are investigated using a large-area basin-scale land surface model driven by the European Centre for Medium-Range Weather Forecasts Re-Analyses 15-yr (1979-93) dataset. The modeling approach allows for the study of the relationships of ENSO with several hydrologic variables simultaneously, such as soil water storage, basin runoff, snow-water equivalent, and precipitation. The cross-correlation coefficients between terrestrial variables and the ENSO index are computed. The runoff from the northern part of North America was found to be most often negatively correlated with ENSO, and there are four distinct coherent regions over the continent where the runoff anomalies are positively correlated. The terrestrial systems have a delayed response to the ENSO signal, as compared to the precipitation, and the delay may range from a month to a season or longer. The shorter and longer delays are typically associated with rainfall runoff, and snow accumulation and melt processes, respectively. The soil moisture storage plays a very vital role in delaying the effects of the climate variability on the terrestrial hydrologic processes and in extending the influences of the El Niño or La Niña events on the terrestrial climate.