Two pendulums hang on the same wall, swinging out of sync, but suddenly they start moving as one, only with a delay of eighteen months. One pendulum is the Indian monsoon, the seasonal wind that brings rain to the fields of South Asia. The second is the deep waters of the tropical oceans, which slowly accumulate heat and remember it for years.
Previously, scientists looked at the surface temperature of the Pacific Ocean to predict rains in India. But the connection would strengthen and weaken, and it seemed that forecasting was impossible. Then they thought to look deeper, at the water layer where the temperature reaches 20°C. This layer changes slowly and holds the ocean's memory for years, unlike the surface, which forgets everything within a few months.
Now it is possible to make forecasts of the Indian monsoon 18 months ahead. This is important for farmers planning their sowing, and for cities stocking up on water. Two pendulums on the same wall do not just swing side by side; they hear each other through the ocean's depths.
The depth of the 20°C isotherm in the tropics changes by tens of meters, and these slow waves remember the ocean state for years, while the surface forgets everything within a few months.