In 1972, anchovies vanished off the coast of Peru. Fishermen were left empty-handed, and the world learned the word El Niño. Since then, scientists have been searching for ways to predict when that warm current will once again upend the planet's weather. It turns out the phenomenon has a hidden conductor, one that resides in the western Pacific and breathes on a timescale of several decades.

The climate of the Indo-Pacific region resembles a symphony orchestra without a conductor. The violins play their melody, the cellos theirs, and together they produce the complex music of weather. Until recently, scientists could only hear the loud parts: El Niño, monsoons, the Indian Ocean Dipole. But this music also contains quiet, slow themes that last for decades. They are almost inaudible beneath the noise of short-term fluctuations, yet they set the tempo for the entire orchestra.

To pick out that slow theme, the researchers took a 1,300-year record of climate generated by the CCSM4 computer model. It is as if we had a recording of the orchestra spanning a thousand years. They then applied nonlinear spectral analysis, a method that requires no prior filtering and can separate the voices of instruments even when their frequencies are close. Previously, principal component analysis was used, but it could mix signals like a poor microphone that records all instruments into one jumble.

The results revealed a slow mode of temperature oscillation in the western Pacific, dubbed the western Pacific multidecadal mode (WPMM). When it is in its cold phase, meaning temperatures in the western Pacific are below normal, the amplitude of El Niño and monsoons increases. In other words, a cool western Pacific amplifies warm events in the east. This connection is not coincidental: the correlation between WPMM and decadal rainfall over Australia reaches −0.62, far stronger than that of the classic Pacific Decadal Oscillation (0.44).

Such a result implies that multi-year droughts and floods in Australia could be predicted by monitoring slow temperature swings in the western Pacific. It is as if we could predict the orchestra's volume by watching the conductor's baton, even if the conductor stands behind the scenes. In the 1,300-year model record, the cold phase of WPMM can produce anomalously wet periods over Australia lasting nearly a century.

The study shows that internal multidecadal climate oscillations exist and can be extracted from data without prior filtering. This opens the way to more accurate indices of climate oscillations and improved decadal forecasts. The conductorless orchestra turns out not to be so chaotic after all: it has its own slow rhythms, audible to those who listen carefully.