Published: April 24, 2014

In 2012, temperatures at the summit of Greenland rose above freezing for the first time since 1889, raising questions about what led to the unusual melt episode. Now, at the Â鶹ĘÓƵ shows that some of the same weather and climate factors were at play in both 1889 and 2012: heat waves thousands of miles upwind in North America, higher-than-average ocean surface temperatures south of Greenland and atmospheric rivers of warm, moist air that streamed toward Greenland’s west coast.

“These rare melt events on the highest elevations of Greenland require an unusual coincidence of factors. Understanding how they come together may help us better forecast the future of Greenland’s ice and snow,” said William Neff, a fellow at CIRES, a joint institute of CU-Boulder and the National Oceanic and Atmospheric Administration. Neff is lead author of the new analysis, accepted for publication in the Journal of Geophysical Research.

Neff and colleagues at CIRES, NOAA’s and at the University of California San Diego began digging into the underlying reasons for Greenland’s extreme melt year after project showed that warm air and thin clouds were key to the 2012 warmth and melt. To figure out where the warm air and clouds came from, the scientists started with satellite observations of moisture in the air over the Atlantic Ocean, looking for atmospheric rivers. Atmospheric rivers are narrow filaments of water vapor in the atmosphere that can stream significant amounts of moisture northward in the midlatitudes.

The researchers also studied sea-surface temperatures, which might have influenced the temperature and moisture content of air moving toward Greenland. And to better understand atmospheric and oceanic conditions back in 1889, the research team drew on data in the , a sophisticated computer reconstruction of the weather going back to 1871.

Neff and his colleagues found that several key factors conspired to melt Greenland’s surface in both 1889 and 2012:

First, heat waves and drought gripped North American regions upwind of Greenland. In the summer of 2012, temperatures over the mid-to-eastern United States were about 15 degrees Fahrenheit hotter than normal, and a persistent drought plagued the Midwest. In the summer of 1889, temperatures south of Hudson Bay, in the Upper Midwest and over the Rocky Mountains rose in heat waves as much as 15-20 degrees Fahrenheit higher than average, and a severe drought stretched across the northwestern and Upper Midwest states. During periods of melt in both 2012 and 1889, back-trajectory analyses from Greenland showed that incoming air had originated in those unseasonably warm areas upwind—so that air was already warm.

Second, in both years, the ocean surface temperatures south of Greenland were higher than average: by about 2 degrees Fahrenheit in 1889 and nearly 4 degrees in 2012. In both years, that extra warmth came from a natural “oscillation” that periodically seesaws temperatures in the northwest Atlantic Ocean. Air flowing toward Greenland over warmer oceans would have picked up extra warmth and moisture.

Finally, wind and pressure patterns in North America in both years were ideal for steering atmospheric rivers of relatively warm, moist air up along the west coast of Greenland and then over the ice sheet. “These distortions of 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.”

Neff and his colleagues found intriguing evidence that a fourth factor—soot from intense U.S. wildfires swept up toward Greenland and deposited on the snow—may have played a role in 1889. Other researchers have found significant deposits of soot in ice core records from the summer of 1889; when darkened by soot or “black carbon,” snow and ice can melt faster. In 1889, Major John Wesley Powell, then director of the U.S. Geological Survey, traveled by train through the northern Rockies during the fire season and later reported to Congress, “The fires in the mountains created such a smoke that the whole country was enveloped by it and hidden from view.”

“Better understanding how factors that can occur naturally, such as long-term droughts or short-term atmospheric rivers, combine to produce an extreme event, such as Greenland’s melt, can help researchers better explain and forecast these events,” said co-author Gilbert Compo, a CIRES scientist working at ESRL. “This is especially important because we expect climate changeĚý to continue to warm the oceans and warm and moisten the atmosphere, raising the possibility of more frequent melt episodes.”

Authors of “Continental heat anomalies and the extreme melting of the Greenland ice surface in 2012 and 1889,” published in the Journal of Geophysical Research: Atmospheres, are: William Neff and Gilbert Compo (CIRES and NOAA ESRL); F. Martin Ralph (UC San Diego, Scripps Institution of Oceanography ); and Matthew D. Shupe (CIRES and NOAA ESRL).

CIRES is a partnership of and .

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Graphics:

  • Animations: Atmospheric river events on and swept warm, moist air up toward Greenland’s west coast, contributing to extreme melt that year swept warm, moist air up toward Greenland’s west coast, contributing to extreme melt events both years. Credit: Don Murray, CIRES/NOAA.
  • Still images depicting and atmospheric river events are on our . Credit: Don Murray, CIRES/NOAA.

An atmospheric river event from July 9, 2012, swept warm, moist air to Greenland's west coast and likely contributed to the major melt event. More intense colors (green, yellow, red) show higher concentrations of water vapor in the air. Photo courtesy of Don Murray, CIRES/NOAA Earth System Research Laboratory

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