In 1960, a meteorologist at a station in Peru records water temperature, while a farmer in India watches the sky for the monsoon. There is no direct link between these two events, but months later one will echo in the other. Earth's climate is like a conversation where the speakers reply not immediately but after a pause: the ocean warms, and only months later does the rainfall pattern change on another continent. Old methods could only hear simultaneous remarks, so half the conversation remained unintelligible.

The scientists took data on sea surface temperature and land precipitation from 1960 to 2019, smoothed them, and turned them into complex signals. Then they found common patterns and rotated them so that each pattern became clear and interpretable. The result was six main modes: the seasonal cycle (which explains 62.7% of the total relationship), El Niño, global warming, the Pacific Decadal Oscillation, El Niño Modoki, and the Atlantic Meridional Mode. For example, the seasonal temperature maximum in the northern Pacific occurs 196 days later than in the southern Pacific, and in the Indian Ocean, rains lag the heat by 81 days.

Now the ocean–land conversation can be heard in full, with all its pauses. Previously, each such connection was sought separately, with a prior assumption about which delay mattered. The new method does everything in one procedure and does not require detrending, which is important in a changing climate. The farmer in India and the meteorologist in Peru are linked by an invisible thread of delayed signal, and we are now learning to discern it amid the noise.