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UW Today

It seems counterintuitive that clouds over the Southern Ocean, which circles Antarctica, would cause rain in Zambia or the tropical island of Java. But new research finds that one of the most persistent biases in global climate models – a phantom band of rainfall just south of the equator that does not occur in reality – is caused by poor simulation of the cloud cover thousands of miles farther to the south.

University of Washington atmospheric scientists hope their results help explain why global climate models mistakenly duplicate the inter-tropical convergence zone, a band of heavy rainfall in the northern tropics, on the other side of the equator. The study appears this week in the Proceedings of the National Academy of Sciences.

“There have been tons of efforts to get the tropical precipitation right, but they have looked in the tropics only,” said lead author Yen-Ting Hwang, a UW doctoral student in atmospheric sciences. She found the culprit in one of the most remote areas of the planet.

“What we found, and that was surprising to us, is the models tend to be not cloudy enough in the Southern Ocean so too much sunlight reaches the ocean surface and it gets too hot there,” Hwang said. “People think of clouds locally, but we found that these changes spread into the lower latitudes.”

Previous studies looking at the problem investigated tropical sea-surface temperatures, or ways to better represent tropical winds and clouds. But none managed to correctly simulate rainfall in the tropics – an important region for global climate predictions, since small shifts in rainfall patterns can have huge effects on climate and agriculture.

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