Thursday, June 9, 2016

Wobbly Jet Stream Is Sending the Melting Arctic into 'Uncharted Territory'

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.

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.
Increasing loss of Arctic sea ice is leading to more blocking patterns, which are contributing to the increasing surface melt on Greenland. Accounting for these shifts is crucial to being able to model how much sea level will rise and how fast Greenland's melting is contributing to rising seas.

Changes in the jet stream can explain intense warming in Alaska, the "polar vortex" weather that has frozen parts of the United States, as well as stronger storms in some regions and variations in tropical monsoons. Overall in the summer of 2015, the melt season was about 30 to 40 days longer than average in the western, northwestern and northeastern regions of Greenland. For the first time since 2012, the melt area exceeded more than half the ice sheet.

An extreme melting event in Greenland's ice sheets have scientists worried.

Melt water pours through a channel in Greenland’s ice.

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