In summer, the Arabian Sea breathes like a pair of bellows: one moment it pumps heat, the next it exhales moisture over a billion people. But sometimes this breathing stumbles, and the water off the coasts of India and Pakistan runs a few degrees hotter than usual. Such episodes are called marine heat waves, and they act like a conductor who suddenly quickens the tempo of the entire monsoon orchestra.

Scientists tracked these waves through forty years of satellite data and saw two different stories. Short waves, which last about nine days, are born when the atmosphere above the sea goes quiet: clouds dissipate, winds slacken, the sun beats down at full strength, and evaporation nearly stops. The water heats up like a pot with the lid taken off. Long waves, which stretch beyond three weeks, come from afar: they are warmed by El Niño in the Pacific and by temperature swings in the Indian Ocean itself.

But the most interesting part comes next. The warmed water starts to command the sky. Over a warm patch, air rises, and a vortex spins up around it, pulling moisture from the sea onto the land. In the eastern Arabian Sea, such a vortex intensifies downpours on India's southwestern coast, and the risk of extreme rainfall climbs by a factor of two to three. In the north, it aims at Pakistan and northwestern India.

Previously, it was thought that the ocean simply obeyed the atmosphere: calm warmed the water, and that was that. Now it is clear that this is a two-way street. A heat wave born of calm returns to the sky as a strengthened vortex and squeezes more rain out of the monsoon. This explains why rainfall forecasts in the region often miss the mark: they track the large-scale currents but overlook the local warm patches right under their noses.

If the ocean is the conductor, we now know when it picks up the baton. Forecasts of atmospheric calms can already look several weeks ahead, so we can anticipate where the water will start to overheat. That will not stop the downpours, but it buys time to warn the people who live in the path of the monsoon orchestra.