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, a new analysis that included researchers from Scripps Institution of Oceanography at UC San Diego 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 (NOAA’s Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder). Neff is lead author of the new analysis, published in the American Geophysical Union’s Journal of Geophysical Research.
Neff and colleagues at CIRES, NOAA’s Earth System Research Laboratory, and Scripps began digging into the underlying reasons for Greenland’s extreme melt year after another research 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.
“Research in the last 10 years has revealed the major role that atmospheric rivers play in mid-latitude weather and precipitation, and this study is an important extension of those findings to high latitudes,” said study co-author Marty Ralph, director of the new Center for Western Weather and Water Extremes at Scripps.
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 20th Century Reanalysis, 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.”
“Because melting can be a very episodic phenomenon that is favored by conditions that are both warm and moist, as demonstrated in this study, it is possible that atmospheric river events could play a significant role in warming of the Arctic more generally,” said Ralph.
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