A hammock woven from thousands of cords is stretched between continents. Each cord represents a link between two points on the planet: when it warms in one place, it cools in another. As long as the hammock is intact, the climate behaves as usual. But every few years El Niño arrives, a warm current off the coast of Peru, and the cords begin to snap.
Scientists took daily temperature measurements from 1979 to 2006 in four regions: the tropical Pacific, North America, Europe, and Southeast Asia. For each pair of points, they calculated how synchronously the temperatures changed. If the link was strong and lasted long, they considered it stable. During eight El Niño events, the number of such stable links dropped sharply, even though the average temperature barely changed. The hammock lost cords but remained taut.
This view of climate helps reveal how El Niño affects regions where the average temperature does not change. Previously this was unclear, but now it is evident: it rewires the connections between points, and this is visible in the number of surviving cords. The method can be used to monitor climate anomalies and possibly provide early warnings of El Niño onset.
The correlation significance threshold of 2 was not chosen arbitrarily: in calm years, correlations below this value are almost nonexistent, so any drop below 2 immediately signals an anomaly. It is like a sensor that only triggers on a real alarm.