July 2012 melt event far surpassed previous record
By Summit Voice
SUMMIT COUNTY — Along with an unusual constellation of low clouds, changes the jet stream were also a factor in last summer’s exceptional surface melt of the Greenland Ice Sheet, according to a research team led by the University of Sheffield’s geography professor Edward Hanna.
A NOAA study published in March indicated that a lingering layer of thin, low clouds helped intensify atmospheric conditions leading to the meltdown.
Hanna and his colleagues used a computer model simulation (called SnowModel) and satellite data to confirm that last summer’s meltdown was unprecedented in the past 50 years. About 90 percent of the ice-sheet surface melted July 11, far surpassing the previous known surface melt extent record of 52 percent in 2010.
The team also analyzed weather station data from on top of and around the GrIS, largely collected by the Danish Meteorological Institute but also by US programmes, which showed that several new high Greenland temperature records were set in summer 2012.
The study published this week in the International Journal of Climatology, shows that the record surface melting ice sheet was mainly caused by highly unusual atmospheric circulation and jet stream changes, which were also responsible for last summer’s unusually wet weather in England.
The analysis shows that ocean temperatures and Arctic sea-ice cover were relatively unimportant factors in causing the extra Greenland melt.
“The Greenland ice sheet is a highly sensitive indicator of regional and global climate change, and has been undergoing rapid warming and mass loss during the last five to 20 years,” Hanna said. “Much attention has been given to the NASA announcement of record surface melting of the GrIS in mid-July 2012. This event was unprecedented in the satellite record of observations dating back to the 1970s and probably unlikely to have occurred previously for well over a century.
“Our research found that a ‘heat dome’ of warm southerly winds over the ice sheet led to widespread surface melting. These jet stream changes over Greenland do not seem to be well captured in the latest Intergovernmental Panel on Climate Change computer model predictions of climate change, and this may indicate a deficiency in these models.
“According to our current understanding, the unusual atmospheric circulation and consequent warm conditions of summer 2012 do not appear to be climatically representative of future ‘average’ summers predicted later this century,” Hanna said.
“Taken together, our present results strongly suggest that the main forcing of the extreme GrIS surface melt in July 2012 was atmospheric, linked with changes in the summer North Atlantic Oscillation (NAO), Greenland Blocking Index (a high pressure system centered over Greenland) and polar jet stream which favored southerly warm air advection along the western coast,” he explained.
“The next five to 10 years will reveal whether or not 2012 was a rare event resulting from the natural variability of the NAO or part of an emerging pattern of new extreme high melt years.
“Because such atmospheric, and resulting GrIS surface climate, changes are not well projected by the current generation of global climate models, it is currently very hard to predict future changes in Greenland climate. Yet it is crucial to understand such changes much better if we are to have any hope of reliably predicting future changes in GrIS mass balance, which is likely to be a dominant contributor to global sea-level change over the next 100-1000 years,” he concluded.