Ocean Currents Responsible for Wetter Northern Hemsiphere

Ocean currents originating from the poles cause it to rain more above the equator than below it, according to a new study published in the journal Nature Geoscience.

The Palmyra Atoll, located at 6 degrees north, receives an average of 174 inches of rain annually. Six degrees south, in contrast, receives just 45 inches.

For years, scientists believed this was the result of a diagonal tilt of the ocean basins combined with the planet's spin, together pushing tropical rain bands north.

"But at the same time, a lot of people didn't really believe that explanation because it's kind of a complicated argument," co-author Dargan Frierson, a University of Washington associate professor of atmospheric sciences, said in a statement. "For such a major feature there's usually a simpler explanation."

Using detailed measurements from NASA's Clouds and Earth's Radiant Energy System (CERES), Frierson and his colleagues clearly demonstrated that sunlight provides more heat to the Southern Hemisphere. This means that, in terms of atmospheric radiation alone, the Southern Hemisphere should receive more rain since hotter places are wetter given the fact that hot air rises and moisture precipitates out.

Strangely enough, the Northern Hemisphere is hotter, and thus receives more rain. Says Frierson,"The question is: What makes the Northern Hemisphere warmer?"

The answer, the researchers determined, was ocean circulation.

The team calculated the ocean heat transport and then used computer models to determine the effect of the conveyor-belt current that sinks near Greenland, travels to Antarctica along the bottom of the ocean and then rises, flowing north along the surface. When they eliminated this current using the models, the tropical rain bands were flipped to the south.

The reason for this, the researchers explain, is that during the many decades it takes for the water to move north, it heats up, carrying roughly 400 trillion watts of power across the equator.

"This is really just another part of a big, growing body of evidence that's come out in the last 10 or 15 years showing how important high latitudes are for other parts of the world," Frierson concluded.