In the summer of 2022, rains poured over western India while the east stayed dry. Usually, the monsoon brings moisture evenly, but that year things went wrong. Scientists traced the cause to the deep ocean, where water moves in waves that are invisible from the surface.

When the Pacific Ocean is cool (La Niña) and the eastern Indian Ocean is warm (negative dipole), westerly winds blow over the equator. They push water eastward, and a Kelvin wave runs along the equator. Off the coast of Sumatra, it reflects and returns westward as a Rossby wave. This wave reaches the Arabian Sea and pushes the thermocline 20–30 meters deeper than usual. Warm water accumulates like in a bathtub when you leave the plug in.

The warm water heats the air, which rises. Over the Arabian Sea, the air begins to converge like water swirling down a drain, and moisture comes along. A second such center forms over the Indonesian islands. The two centers act like twin pumps, lifting moist air skyward, and rains fall over western India. In years when La Niña and the negative dipole did not align, this mechanism was absent, and rains were distributed differently.

Now, by looking at water temperatures in the depths of the Arabian Sea, meteorologists can better predict what the monsoon will bring. This will help farmers and cities prepare for rains or drought. As it turns out, the ocean steers rain not from the surface but from the depths.