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Saturday, April 26, 2014

Neven: More on Melt Ponds [on the Arctic sea ice]

Melt_Ponds_Connected 2by Neven, Arctic Sea Ice Blog, April 25, 2014
A really good paper has been published online a couple of days ago in Nature, called September Arctic sea-ice minimum predicted by spring melt-pond fraction. It's really good because it's interesting, short, and it confirms what I've been suspecting for a while now. And when a paper confirms what one is suspecting, it must be really good, right?
All joking aside, the paper by Schröder et al. presents evidence that melt ponds play a very important role at the start of the melting season, to the point that it can heavily influence the September minimum. The last two melting seasons actually proved to be a great lesson in this respect. 2012 had a really good* start to the melting season, so good that when bad weather showed up, it didn't really slow down sea ice loss, the trend lines just kept dropping (low sea ice volume also played a role, of course). The reverse was true in 2013: cold and cloudy weather during the first half of the melting season caused a lagged response during the short periods when the Sun and higher temps finally got to the ice.
What Schröder et al. did was develop a melt pond model and incorporate it into the larger Los Alamos sea-ice model called CICE. Here's what they came up with:

Our simulations show that melt ponds start to form in May, a maximum extent of 18% is reached in the climatological mean at mid-July, and there are hardly any exposed ponds left by mid-August. The strong interannual variability and the positive trend are striking. Whereas in 1996, the year with the highest September ice extent since 1979, the maximum pond fraction reaches only 11%, in 2012, the year with the lowest September ice extent, up to 34% of the sea ice is covered by ponds.
This is accompanied by the following figure:
 Schröder et al fig 2
 Based on their results, Schröder et al. conclude:
[T]he melt-pond fraction in May seems to have the strongest impact on the sea-ice state in the subsequent September. Our results confirm that the early melt season is decisive for the strength of the summer ice retreat.
We conclude that the inclusion of a realistic melt-pond model will transform future forecast and climate models in the Arctic regions and beyond.
 Although they didn't participate in last year's Sea Ice Outlook, they did make a prediction:
For September 2013 we forecast a mean ice extent of 5.55 0 ± 44 million km2, which is closer to the observed mean value of 5.35 million km2 than any of the 23 statistical,model and heuristic predictions presented at the Arctic Sea Ice Outlook webpage in July (median value of 4.0 million km2).
As we all know, last year's ASIB community prediction for the SEARCH Sea Ice Outlook was overly pessimistic, and off the mark by almost 2 million km2, much more than the other predictions. Because of low volume and a record amount of first-year ice, I personally thought the September minimum couldn't but end up somewhere between 2007 and 2012, at the very least. Most of us would probably have guessed differently, knowing just how low the melt pond cover fraction was in May 2013, and how influential this can be.
Which goes to show how incredibly handy it would be to have near real-time melt pond cover fraction data at our disposal, which we could then compare to data from the last 5-6 years. It wouldn't tell us anything conclusive about the melting season's final outcome (weather determines this), but combined with ice age distribution, and data on thickness and volume, it would give us a good idea of where things could or couldn't head.
image from
The image above comes from a blog post from 2 years ago, wherein I mentioned melt pond data having been developed by the University of Hamburg's Klima Campus for the 2000-2011 period (also referenced in the Schröder et al. paper), using MODIS satellite data. Hopefully something similar comes along next year, as May 2014 is just around the corner.
Still, we now know that the start of the melting season can make it or break it.
* When I say 'good,' I mean 'good for melting,' not that Arctic sea ice loss is a good thing in itself.
The image at the top of the blog post was found here, with a nice quote by Don Perovich:
When Arctic melt ponds are sufficiently connected, as pictured here, they exhibit a property called universality that researchers believe is common to all complex, correlated systems. 

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