A new research paper by scientists of the Polar Science Center of the Applied Physics Laboratory of the University of Washington, has been published this week in the online version of Geophysical Research Letters. It's called The impact of an intense summer cyclone on 2012 Arctic sea ice retreat and discusses the role the Great Arctic Cyclone of 2012 - or GAC-2012 - played in the incredible, unequivocal and unspinnable record streak
we witnessed towards the end of the melting season.
From the UW press release:
Cyclone did not cause 2012 record low for Arctic sea ice
It came out of Siberia, swirling winds over an area that covered almost the entire Arctic basin in the normally calm late summer. It came to be known as “The Great Arctic Cyclone of August 2012,” and for some observers it suggested that the historic sea ice minimum may have been caused by a freak summer storm, rather than warming temperatures.
But new results from the University of Washington show that the August cyclone was not responsible for last year’s record low for Arctic sea ice. The study was published online this week in Geophysical Research Letters.
“The effect is huge in the immediate aftermath of the cyclone, but after about two weeks the effect gets smaller,” said lead author Jinlun Zhang, an oceanographer in the UW’s Applied Physics Laboratory. “By September, most of the ice that melted would have melted with or without the cyclone.”
Recent research [covered on the ASI Blog here, N.] showed that the Arctic cyclone was the most powerful ever seen during the month of August, and the 13th most powerful of all Arctic storms in more than three decades of satellite records.
“The storm was enormous,” said co-author Axel Schweiger, a polar scientist in the Applied Physics Laboratory. “The impact on the ice was immediately obvious, but the question was whether the ice that went away during the storm would have melted anyway because it was thin to begin with.”
The UW team performed the climate scientist’s equivalent of a forensic exam: They ran a computer simulation of last summer’s weather and compared it against a second scenario that was identical except that there was no cyclone.
Results showed the storm caused the sea ice to pass the previous record 10 days earlier in August than it would have otherwise, but only reduced the final September ice extent by 150,000 square kilometers (almost 60,000 square miles), less than a 5 percent difference. By comparison, the actual minimum ice extent was 18 percent less than the previous record set in 2007.
The storm apparently caused 150K km2 of extra melted ice and so the record(s) would have been broken anyway, as the difference with the 2007 minimum was much larger. The main reason the storm had such an effect, was that the ice was exceptionally thin. Just 10 years ago a storm, even of this size, wouldn't have had such an impact.
Read for instance this Skeptical Science article from October, with the figure below showing September Arctic sea ice extent data from NSIDC (blue) and years with Arctic summer storms similar to the 2012 storm depicted in red.
Remember that we started noticing that the ice seemed to be exceptionally thin weeks before the storm hit. It was the only way to explain the steadily rapid drop in extent and area levels, even though the weather wasn't conducive to that rate of melt.
Weather patterns haven't been conducive to sea ice decrease, trend lines on graphs should be stalling, but they don't. As I've shown in yesterday's blog post comparing this year's weather patterns in June and July with previous record years, the decrease should have slowed down significantly like it did in 2010 and 2011, but it didn't. The 2012 SIE trend line shouldn't follow 2007 so closely, but it does. The 2012 SIA trend line shouldn't lead, but it does.
Now air and sea surface temperatures are higher than in previous years, but does that explain all of it? Personally, I don't think so. I think ice thickness is playing a big part in all of this.
And from the conclusion of that ASI 2012 update:
With ice that doesn't seem to care what the weather does and warm temperatures in many places above and below the ice, it's no wonder that SIE and SIA keep decreasing steadily. This melting season doesn't resemble 2007 in any way really, but 2012 is keeping up with the tempo easily. The longer the weather stays like this, the stronger the evidence becomes that the ice is thinner than it has been for a long, long time.
And two weeks earlier:
Things look a bit like they did in 2010 and 2011 when the rapid decline came to a grinding halt, but so far trend lines continue to go down steadily. This could have something to do with the position of low-pressure areas that refuse to replace the high-pressure areas on the American side of the Arctic. Or it could have to do with the fact that there is enough weak ice left for weather patterns not to matter as much.
So even without the storm, mighty as it was, records would have most probably been broken, but with almost opposite conditions to those that made 2007 such an impressive recordbreaker at the time. The massive,
but short-lived effect the storm had, might have been caused by something that was the subject of much discussion and speculation even as the storm was going on (see for instance, this comment by Rob Dekker I highlighted on August 10th): Upwelling of warmer waters due to Ekman pumping caused by strong
anti-clockwise blowing winds.
The authors of the paper seem to have come to the same conclusion:
The study also revealed a surprising mechanism for the cyclone-related melting. Earlier discussions about the cyclone’s effect had focused on winds breaking up the ice or driving ice floes into areas of warmer water. The results suggest that neither process led to much increase in melting.
Relatively recent research shows that in the summertime, thin ice and areas of open water allow sunlight to filter down to the water below. As a result, while a layer of ice-cold fresh water sits just beneath the sea ice, about 20 meters (65 feet) down there is a layer of denser, saltier water that has been gradually warmed by the sun’s rays.
Blowing on polar water is like blowing on a layered cocktail. When the cyclone swept over the drifting ice floes, underside ridges churned up the water to bring sun-warmed seawater to the ice’s bottom edge. The model suggests that during the cyclone there was a quadrupling of melting from below, and that this was the biggest cause for doubling ice loss during the three-day storm.
Read the last part of the press release here.
Now, this is all very interesting and it never hurts to look back and learn. But what I find even more interesting,
is the question whether this amazing storm was just a fluke, a freak event, a coincidence. Or did the large expanse of exceptionally warm waters have to do with the storm's unusual intensity and duration? If it did,
we might see more of these storms in the near future.
I wouldn't be surprised if we saw something similar in the coming 3-5 years.