抖阴旅行射

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 University of Colorado Boulder 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鈥檚 west coast.

鈥淭hese 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鈥檚 ice and snow,鈥� said William Neff, a fellow at CIRES, a joint institute of 抖阴旅行射-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鈥檚 and at the University of California San Diego began digging into the underlying reasons for Greenland鈥檚 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鈥檚 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鈥攕o 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 鈥渙scillation鈥� 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. 鈥淭hese distortions of the jet stream must happen in just the right place to direct atmospheric rivers toward Greenland,鈥� Neff said. 鈥淭hat may be one reason extreme melt events there have been relatively rare.鈥�

Neff and his colleagues found intriguing evidence that a fourth factor鈥攕oot from intense U.S. wildfires swept up toward Greenland and deposited on the snow鈥攎ay 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 鈥渂lack carbon,鈥� 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, 鈥淭he fires in the mountains created such a smoke that the whole country was enveloped by it and hidden from view.鈥�

鈥淏etter 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鈥檚 melt, can help researchers better explain and forecast these events,鈥� said co-author Gilbert Compo, a CIRES scientist working at ESRL. 鈥淭his is especially important because we expect climate change听 to continue to warm the oceans and warm and moisten the atmosphere, raising the possibility of more frequent melt episodes.鈥�

Authors of 鈥淐ontinental heat anomalies and the extreme melting of the Greenland ice surface in 2012 and 1889,鈥� 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 .

Contacts:

• William Neff, CIRES Fellow and lead author, 303-497-6265, william.neff@noaa.gov

• Katy Human, CIRES communications director, 303-735-0196, kathleen.human@colorado.edu

• Gil Compo, CIRES scientist in NOAA鈥檚 Earth System Research Laboratory, 303-497-6115, gilbert.p.compo@noaa.gov

• Robert Monroe, Scripps Institution of Oceanography communications office, 858-822-4487, scrippsnews@ucsd.edu

Graphics:

• Animations: Atmospheric river events on and swept warm, moist air up toward Greenland鈥檚 west coast, contributing to extreme melt that year swept warm, moist air up toward Greenland鈥檚 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.