Most tropical rain falls in the Northern Hemisphere is because of ocean currents originating from the poles, thousands of miles away, shows a new study.
The Palmyra Atoll, at 6 degrees north, gets 175 inches of rain a year, while an equal distance on the opposite side of the equator gets only 45 inches.
Scientists long believed that this was a quirk of the Earth's geometry - that the ocean basins tilting diagonally while the planet spins pushed tropical rain bands north of the equator.
However, the findings explain a fundamental feature of the planet's climate, and show that icy waters affect seasonal rains that are crucial for growing crops in places like Africa's Sahel region and southern India.
In general, hotter places are wetter because hot air rises and moisture precipitates out.
Corresponding author Dargan Frierson, a University of Washington associate professor of atmospheric sciences, said that it rains more in the Northern Hemisphere because it's warmer.
Frierson and his co-authors first used detailed measurements from NASA's Clouds and Earth's Radiant Energy System, or CERES, satellites to show that sunlight actually provides more heat to the Southern Hemisphere - and so, by atmospheric radiation alone, the Southern Hemisphere should be the soggier one.
After using other observations to calculate the ocean heat transport, the authors next used computer models to show the key role of the huge conveyor-belt current that sinks near Greenland, travels along the ocean bottom to Antarctica, and then rises and flows north along the surface. Eliminating this current flips the tropical rain bands to the south.
The reason is that as the water moves north over many decades it gradually heats up, carrying some 400 trillion watts of power across the equator.
The study has been published in journal Nature Geoscience.