El Niño and water

El Niño shifts where the rain falls. The same event brings flooding to some regions and drought to others, and the pattern is broadly repeatable.

How it is measured: By the Oceanic Niño Index — a three-month running mean sea surface temperature anomaly in the central Pacific. Sustained values above +0.5 °C define El Niño.
What it does and does not tell you: ENSO shifts the odds of a wet or dry season in particular regions. It does not determine any individual season, and its influence is much stronger in some regions than others.
Water planning: Where the teleconnection is strong and well established, ENSO forecasts give several months of useful warning for reservoir and irrigation planning.

Regional effects are statistical tendencies, not certainties. Presenting an ENSO overlay alongside a rainfall series shows co-occurrence and must not be read as proof of causation in any single year.

Text description

Under normal conditions, trade winds push warm surface water westward across the Pacific, producing rainfall over Indonesia and Australia and cold upwelling off South America. During El Niño the trade winds weaken, warm water shifts east, and the rainfall follows it. This brings drought to Australia, Indonesia and southern Africa, and heavy rain and flooding to Peru, Ecuador and the southern United States. La Niña is the opposite phase.