By Will Dunham
WASHINGTON, Feb 6 (Reuters) - It's no surprise to any sailor who has braved the southern seas to hear that the region is one of the world's most stormy.
What might come as a surprise is that the storm cycles in a huge part of the Southern Hemisphere are not haphazard. Instead, they pulsate year-round to a 20- to 30-day beat.
That's the finding of U.S. researchers who studied three decades of atmospheric data and found this pattern across a wide expanse of the Southern Ocean, spanning from the southern reaches of South America, Africa and Australia down to the coast of Antarctica.
The scientists examined the large-scale atmospheric circulation in the mid-latitudes of the Southern Hemisphere. They found regularity in the pulsating flow of heat carried by storms from the tropics down toward cooler latitudes closer to the South Pole.
Here's how they think the cycle works. The sun heats up the atmosphere in tropical latitudes to the north, creating a contrast with the cooler temperatures nearer the polar region.
This temperature disparity causes atmospheric instability and unleashes storms. The storms, in turn, alleviate the north-south temperature differences, letting the cycle begin again.
"The energy of the storms - averaged over the middle latitude of the Southern Hemisphere - has a 20- to 30-day rhythm to it," Colorado State University meteorologist David Thompson, who led the study published in the journal Science, said in a telephone interview.
The discovery adds to the understanding of the workings of the Southern Hemisphere atmosphere and could have implications for weather forecasting in the region, the researchers said.
Thompson emphasized, however, that the finding does not mean that specific locations - for example, Buenos Aires - can expect to be lashed by storms every 20 to 30 days.
"It doesn't necessarily mean that storms spontaneously break out everywhere at the same time," Thompson added.
Thompson and colleague Elizabeth Barnes examined data derived from satellite observations, atmospheric balloon measurements and surface temperature readings spanning more than 30 years. They then turned to computer models to confirm their findings about the repeating cycle.
The region affected by the cycle is primarily covered by seas. Parts of the region are notorious for tempestuous weather, and the new research helps explain some of the atmospheric factors at play.
"There are a lot of mid-latitude cyclones spinning around the Southern Ocean. It's a very stormy part of the world," Thompson said.
If a similar cyclical phenomenon is at work in the Northern Hemisphere, it has proven difficult to detect - "but we're looking," Thompson said. A few widely known rhythmic changes in the atmosphere are known to occur in the tropics.
(Reporting by Will Dunham; editing by Andrew Hay)
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