On the West Coast of the United States, atmospheric rivers deliver winter rain and snow, such as the Pineapple Express, which carries tropical moisture from Hawaii to California. But atmospheric rivers can form around the world, even in Antarctica.
Heat, drought and dissolving ice
The findings help explain why it's so rare for the entire Greenland ice sheet to melt in toasty summer weather.
Atmospheric rivers reach Greenland only when atmospheric pressure patterns, such as ridges and lows, free a path for North America's heat and moisture to travel northward.
"Distortions in the jet stream must happen in just the right place to direct atmospheric rivers toward Greenland," Neff said. "That may be one reason extreme melt events there have been relatively rare."
A sophisticated computer model of historic weather patterns suggests the same factors underlie Greenland's 1889 surface melt. In the summer of that year, heat waves in the Rocky Mountains and eastward drove temperatures as much as 15 to 20 degrees Fahrenheit (8 to 11 degrees Celsius) higher than average, and a severe drought gripped the northwestern and Upper Midwest states, according to weather records.
In the summer of 2012, temperatures east of the Rocky Mountains were also about 15 F (8 C) higher than normal.
Neff did note one significant difference between the two heat waves — Rocky Mountain forest fires. In 1889, forest fires burned freely, and soot from the extensive flames can be found in Greenland ice cores. Studies show snow and ice coated in soot will melt faster than clean snow and ice.
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