Streamflow responses to the El Niño/Southern Oscillation (ENSO) phenomenon in the tropical Pacific are detectable in many regions. During warm-tropical El Niño and cool-tropical La Niña episodes, streamflows are affected throughout the Americas and Australia, in northern Europe, and in parts of Africa and Asia. In North and South America, correlations between peak-flow season streamflows and seasonal Southern Oscillation Indices (SOIs) show considerable persistence. In South America, correlations between flows in other seasons with December–February SOIs also are notably persistent, whereas, in North America, correlations are smaller when other, non-peak season time periods are considered.
At least two modes of streamflow response to ENSO are present in the Western Hemisphere. When interannual North and South American streamflow variations are analyzed together in a single principal components analysis, two of the leading components are found to be associated with ENSO climate variability. The more powerful of these modes corresponds mostly to ENSO responses by the rivers of tropical South America east of the Andes, along with rivers in southern South America and the southwestern United States, with Brazil experiencing less runoff during El Niños and the other regions experiencing more runoff. This streamflow mode is correlated globally with ENSO-like sea surface temperature (SST) patterns on both interannual and interdecadal timescales; indeed, the tropical South American rivers east of the Andes are coherent with SOI on virtually all historical timescales.