The vast northern ice sheet of Greenland melts every summer, pooling lakes of meltwater on its surface and losing fleets of icebergs from its finger-like glaciers. That’s not surprising — it’s summer — although in a warming climate, there are reasons to think these melt seasons are getting more intense, pouring more and more freshwater into the ocean.
But when a group of scientists looked back at the last summer melt season — 2015 — they found something odd and troubling.
Specifically, they found that Greenland had shown much more unusual melting in its colder northern stretches than in the warmer south, and that this had occurred because of very strange behavior in the atmosphere above it. During the month of July, an atmospheric phenomenon called a “cutoff high” — a region of high pressure that stayed relatively immobile over the ice sheet, bringing with it sustained sunny conditions — lingered for many days and produced unusual warmth at the surface and record melting for northwest Greenland.
A cutoff high “describes this atmospheric high pressure system that detaches from the jet stream, in this case, and then basically sits there, it’s almost like living by itself,” said Marco Tedesco, the lead author of the study just published in Nature Communications, and a researcher with the Lamont-Doherty Earth Observatory at Columbia University. “You can imagine something with enough energy to sustain itself, and there’s nothing bothering it. And it’s sitting right there, and that is driving the clear sky conditions over northwest, and also blowing the cold air over southwest.”
The high was accompanied, in this case, by a northward departure of the mid-latitude jet stream — a stream of air in the northern hemisphere that can travel in a more or less wavy route as it progresses from west to east — that set a record for its northward extent, the study found.
Here’s a video that Tedesco made of what happened:
The idea that the jet stream has been becoming more loopy and wavy as the Arctic melts, and that this is producing more stuck-in-place weather conditions, is one of the more contentious ones in recent atmospheric and climate science. Yet the new research seems not only to affirm it, but to suggest that it could be an as-yet unacknowledged factor driving Greenland’s melting.
“It is really the first study that is linking what is happening in Greenland with the mid-latitudes, potentially through the jet stream factor,” Tedesco said.
And while the topic remains highly debated, several other recent studies have also highlighted a potential connection between melting Arctic sea ice, changes in the jet stream, and atmospheric “blocking events” over Greenland that can trigger melting events.
“I think we can start to connect these dots and say that increasing loss of Arctic sea ice is leading to more blocking patterns, which are contributing to the increasing surface melt on Greenland,” said Jennifer Francis, the Rutgers University Arctic expert whose ideas about Arctic melting distorting the jet stream have ignited one of the biggest ongoing debates in climate science, and who is familiar with the new study by Tedesco and his colleagues. “Of course, this is bad news for sea-level rise and maybe also for the ocean circulation as the extra meltwater appears to be partially responsible for the ‘Cool Blob’ south of Iceland.”
But many experts still challenge Francis’s theory. One of them, atmospheric scientist Elizabeth Barnes of Colorado State University, told me last year that “natural variability is very large, and the Arctic is not acting in isolation in terms of its influence on mid latitude weather. To date, I can’t think of a study that shows that the Arctic is more important on influencing mid-latitude weather than these other factors, like tropical warming.”
It was already known that the 2015 Greenland melt season featured the oddity of more northern melting than usual, combined with pretty standard southern melting. “The number of melt days in the northeastern, western and northwestern regions, was up to 30-40 days above the 1981-2010 average and setting new records for meltwater production and runoff in the northwestern region,” noted a report on the melt season from the National Oceanic and Atmospheric Administration, which Tedesco also worked on.
The result was a large amount of water atop the northern parts of the ice sheet that, in turn, reduced its “albedo,” or ability to reflect sunlight away from the surface. Thus, melting fed upon melting.
The key issue, then, is whether 2015 is a harbinger of a future in which the jet stream keeps sending Greenland atmospheric systems that drive major melt — and in turn, whether the Arctic amplification of climate change is driving this. If so, that could be a factor, not currently included in many climate change simulations, that would worsen the ice sheet’s melt, drive additional sea level rise and perhaps upend ocean currents due to large influxes of fresh water.
“The atmospheric changes that are occurring, there is a trend in these quantities, and these quantities are the ones that are also affecting the intensity, and the duration, of the melting over Greenland,” Tedesco said.
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