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Blocking patterns: How global warming might have worsened US drought

Two teams of researchers find that subtle changes brought about by global warming might be amplifying atmospheric blocking patterns, which keep weather conditions in place for a long time.

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As the summer of 2012 winds down, with drought and searing temperatures its hallmark for much of the continental United States, researchers are trying to get a better handle on the factors that contribute to the persistence of weather patterns responsible for the extremes.

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The immediate culprit: patterns of atmospheric flow that steer storms along a given path for weeks, heating and depriving some areas of needed rain while drenching others. Such blocking patterns are a global phenomena, a normal component of Earth's weather systems.

But some researchers suggest that global warming's influence on the Arctic and on the tropics can change circulation patterns in ways that keep blocking patterns in place longer than they otherwise might.

For the continental US, blocking has been a byword for much of the year. The first eight months of 2012 have gone into the books as the warmest January-August period on record for the continental US, according to the National Oceanic and Atmospheric Administration's National Climatic Data Center in Asheville, N.C. The 12-month span ending in August 2012 was the warmest 12 months on record. The summer itself ranks third among the warmest summers on record.

At the end of August, 62.9 percent of the continental US was experiencing moderate to exceptional drought.

"I can tell you that 2011 and 2012 are shaping up to be ripe for study. It's really been a sustained period of climate extremes up to this point," says John Fasullo, a researcher at the National Center for Atmospheric Research (NCAR) in Boulder, Colo.

As researchers try to untangle the atmospheric influences behind persistent weather patterns, the Arctic and the tropics have come under increasing scrutiny.

The decline in summer sea ice, which this summer has reached a record low, deprived the Arctic Ocean of much of its reflective coat of white. This leaves more open ocean from which heat escapes back into the atmosphere in fall and winter. This tends to slow the jet stream's wind speeds and stretch its meanders north and south, according to researchers from Rutgers University and the University of Wisconsin at Madison.

Both effects tend to slow the movement of these meanders – the boundary between warm air from the south and cold Arctic air, and the track storms follow – as they work their way from west to east across the continent in the fall and winter.

In addition, the researchers show how earlier snow melt in the spring and a later onset of snow in the fall dries out the region, contributing to warmer land temperatures in spring and summer – and with a similar, if somewhat less-pronounced, effect on the jet stream then.

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