In 1998, the Pacific Ocean off the coast of Peru suddenly cooled, and tropical rains seemed to take a deep breath. Wet zones, where more than three meters of water falls in a year, crept westward, while dry patches, where less than two hundred millimeters accumulate, expanded off the coasts of Australia and the Americas. This was not a one-year fluke but a phase shift in El Niño, a natural cycle that alternately warms and cools the equatorial ocean.
To see how this seesaw reshapes the rain map, scientists compared annual precipitation totals in the tropics over several decades. They took GPCP satellite data from 1979 and GPCC ground data from 1950, split them into epochs with warm and cold El Niño phases, and built histograms: how often each grid point received less than 200 mm or more than 3000 mm per year. The Kolmogorov–Smirnov test showed a significant difference between epochs, meaning this is not random fluctuation.
The picture turned out to be almost mirror-like. In the transition from the warm phase of 1979–1997 to the cold phase of 1998–2013, wet zones appeared near the western part of the Maritime Continent, over the Indian Ocean, and off the west coast of South America, while dry zones expanded off the western shores of Australia, North America, and South America. When comparing the cold epoch of 1960–1978 with the warm epoch of 1979–1997, everything reversed: wet zones shrank, dry zones appeared in southern Africa and the Sahel, and aridity retreated in Pakistan, the western United States, and Australia.
Such a reversal is hard to explain by global warming, since average temperature rose in the second comparison while precipitation extremes declined. Instead, it aligns precisely with the rhythm of El Niño: the warm phase compresses wet zones, the cold phase expands them. This suggests that natural variability can dominate the anthropogenic signal, at least for annual tropical precipitation totals.
The breathing of tropical rains continues today. Understanding that it follows El Niño phases helps improve seasonal precipitation forecasts and interpret climate trends more cautiously. After all, if the ocean's inhale and exhale can redraw the map of wet and dry zones, then not every drought or flood is a consequence of human activity.
The threshold for a 'dry' year in this work is 200 mm. For comparison, Moscow receives on average about 700 mm of precipitation per year, meaning a tropical point with 200 mm is three times drier than the Russian capital.