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Blocking patterns: How global warming might have worsened US drought

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"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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