The tropics breathe with an annual rhythm: air temperature rises and falls, and within this breath lie two main pulses. The first pulse peaks in February and weakens in August; the second is shifted by half a year. These two pulses are not just background noise for the famous El Niño; they are its conductors.

Scientists decomposed daily temperature variations into two main modes using eigenanalysis and introduced a 'micro-correlation' measure to track how the El Niño region is connected to the Northern and Southern Hemispheres. It turns out that before El Niño onset, the region becomes strongly coupled to the Northern Hemisphere, and at the event's end it switches to the Southern Hemisphere. This resembles a tug-of-war: first one side pulls harder, then the other.

The tug-of-war explains why El Niño decays rapidly after its peak: the connection to the Southern Hemisphere dampens the warm phase. During neutral phases, the El Niño region lies at the boundary between hemispheres and is weakly linked to the first mode, so the rope is barely taut.

Returning to the tropical breath: the two pulses, once seen as mere seasonal noise, actually drive the planet's most famous climate seesaw. They can be used to improve seasonal forecasts of El Niño, especially in predicting rapid event decay.