Ocean Currents Responsible For Wetter Northern Hemisphere
October 21, 2013

Asymmetry In Northern Hemisphere Rainfall Explained In New Study

redOrbit Staff & Wire Reports - Your Universe Online

Ocean circulation is responsible for causing more tropical rainfall in the Northern Hemisphere than in the southern part of the world, according to new research published Sunday in the journal Nature Geoscience.

Scientists have long known that the Northern Hemisphere receives the majority of this type of precipitation. For example, the researchers note that Palmyra Atoll, located at 6 degrees north, receives 175 inches of rain annually while the same location on the other side of the equator gets just 45 inches of precipitation.

Previously, experts believed that this phenomenon was the result of the planet’s ocean basins tilting diagonally while the Earth rotates forcing bands of tropical rain to the northern half of the planet. However, a team led by University of Washington (UW) scientists shows that the pattern is actually the result of polar ocean currents.

Their findings, the university said in a statement, help “explain a fundamental feature of the planet's climate, and show that icy waters affect seasonal rains that are crucial for growing crops in such places as Africa's Sahel region and southern India. In general, hotter places are wetter because hot air rises and moisture precipitates out.”

“It rains more in the Northern Hemisphere because it's warmer," explained corresponding author Dargan Frierson, an associate professor in the Department of Atmospheric Sciences. “The question is: What makes the Northern Hemisphere warmer? And we've found that it's the ocean circulation.”

He and colleagues from UW, the University of Hawaii, Columbia University, the Ulsan National Institute of Science and Technology in South Korea and the Massachusetts Institute of Technology (MIT) conducted their analysis using data obtained from NASA's Clouds and Earth's Radiant Energy System (CERES).

They found that sunlight actually provides more heat to the Southern Hemisphere, and that based solely on atmospheric radiation, it – not the Northern Hemisphere – should actually receive more annual rainfall.

Frierson’s team then used other observations to calculate ocean heat transport, and created computer simulations to show the role that 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” plays in the process.

“Eliminating this current flips the tropical rain bands to the south,” the university explained. “The reason is that as the water moves north over many decades it gradually heats up, carrying some 400 trillion (that's four with 14 zeroes after it) watts of power across the equator.”

While slanting ocean basins had been the officially accepted explanation for the discrepancy in rainfall between the two hemispheres, Frierson said that many experts did not truly believe that theory because of its overly-complicated nature. He said that these types of phenomenon typically have a simple explanation.

“The ocean current they found to be responsible was made famous in the 2004 movie ‘The Day After Tomorrow,’ in which the premise was that the overturning circulation shut down and New York froze over,” the university said. “While a sudden shutdown like in the movie won't happen, a gradual slowing – which the recent United Nations report said was ‘very likely’ by 2100 – could shift tropical rains south… as it probably has in the past.”

“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 added, “A lot of the changes in the recent past have been due to air pollution. The future will depend on air pollution and global warming, as well as ocean circulation changes. That makes tropical rainfall particularly hard to predict.”