In the summer of 2022, the sky over western India seemed to have forgotten about drought: rains fell more often than usual, while eastern states languished in dryness. Such an imbalance puzzled forecasters, because ocean currents in the Pacific had promised a steady, abundant monsoon across the country. The answer was hidden not on the water's surface but in the depths, where invisible waves carry heat like underground rivers carry water.
When La Niña (the cold phase) sets in over the Pacific, and at the same time a negative dipole (warm east, cold west) develops in the Indian Ocean, westerly winds begin to blow over the equator. These winds push water eastward, giving birth to a Kelvin wave: a wave trapped at the equator that travels east. Off the coast of Sumatra, the wave reflects and returns westward as a Rossby wave, a slow wave moving west. Upon reaching the Arabian Sea, it deepens the thermocline, lowering the boundary between warm and cold water by 20–30 meters below normal.
The warm water accumulated in the upper 700 meters of the Arabian Sea acts like a heating pad under a pot: it warms the air, causing it to rise. A center of wind convergence forms over the sea, drawing moist air from the ocean. A second such center arises over the Maritime Continent due to the warm surface in the eastern equatorial zone. The two centers, like two pumps, lift moisture into the sky, and rains pour over western India.
In years when La Niña and the negative dipole coincided (1974, 1998, 2016), this mechanism was absent: convergence over the northern Arabian Sea was missing, and heat content grew more weakly. Only in 2010 and 2022 did subsurface waves act like a conductor, gathering moisture in the right place. The model from the Indian Institute of Tropical Meteorology reproduced this chain, confirming its reality.
Today, this mechanism can be woven into seasonal monsoon forecasts, especially for western India and Pakistan. A farmer looking at a map of temperatures in the depths of the Arabian Sea could understand whether to expect rains or prepare for drought. The ocean, it turns out, conducts the rain not from the surface but from the dark depths.
A Kelvin wave born at the equator takes several weeks to reach Sumatra, and the reflected Rossby wave returns to the Arabian Sea even more slowly. The full cycle takes about two months, comparable to the duration of the active phase of the monsoon.