Melting momentum, it's what I call the absorption of heat and solar radiation during May and June that does not directly lead to melt and a reduction in ice cover, but rather comes into play during July and August (I had a more wordy explanation last year).
As important as it seems to be for the final outcome of the melting season, it is one of those aspects of Arctic sea ice that has proven to be really difficult to measure. Observations based on satellite images have been developed in the recent past, but for some strange reason, this research didn't receive further funding.
I will discuss some indirect and experimental measurements further below, but start off with the best tool we have by far to get a feel for how much melting momentum is being built up during this crucial period in the melting season: the melt pond fraction simulation model developed by CPOM researchers at the University of Reading (more info).
Again this year, Dr David Schröder has been so kind as to send me the May melt pond fraction maps he uses for his SIPN 2016 Sea Ice Outlook prediction, showing melt pond anomalies for the years 2012, 2014, 2015 and 2016 compared to the average for the last 10 years:
This past May has been similar to last year's, but nowhere near May 2012. Here are two more maps that show the difference in melt pond fraction with May 2012 and 2015 (red = more melt ponds now, blue = less melt ponds now):
Here's the text accompanying Dr Schröder's entry to the 2016 Sea Ice Outlook:
We predict the September ice extent 2016 to be slightly lower than last year and slightly below the trend line from 1979 to 2015: 4.5 +/- 0.5 million km2. Given the exceptional low May sea ice extent, one might expect a stronger decline this summer. In our prediction the Arctic mean May melt pond fraction is not strongly affected by the low May sea ice extent, but it will be an intriguing question whether and to what extent the current Arctic sea ice and SST conditions might result in a lower sea ice minimum.
Looking at the May 2016 melt pond fraction in our sea ice simulation, the pond fraction is higher in the Kara Sea, north of Svalbard and in the Fram Strait compared to May 2015 and with a lesser extent to May 2012, but lower in the East Siberian Sea and the Arctic Basin (see attached figures). The weighted Arctic wide mean May pond fraction 2016 is higher than in 2015, but much lower than in 2012. While the ice thickness is generally thinner in May 2016 compared to previous years, the air temperature has been several degrees above the last 10 year mean in the northern North Atlantic and the Beaufort Sea, but slightly colder in the Eastern Siberian Sea causing the described melt pond pattern.
I want to mention that the observed positive SST anomalies are not accounted for in our sea ice simulation in which the mixed layer ocean temperature is restored to climatology. This shortcoming had not been important in the past, but might have a stronger impact this year due the magnitude of the positive SST anomalies.
Nevertheless, we do not expect a new record low for September 2016.
So, that's May. How about June so far? As we've seen in the latest ASI update, clouded skies have been blocking the Sun's rays from reaching large parts of the Arctic. Here too, the difference with 2012 weather conditions for the first half of June has been quite large. The image below shows surface air temperature anomalies on the left, and average sea level pressure on the right (from the ESRL daily composites page):
In 2012, temperatures were much higher over all of the Arctic during the first two weeks of June, and pressure was much higher too, which means solar radiation wasn't blocked by clouds. A very large amount of melting momentum came into being, the foundation on which the 2012 record minimum was built.
What is the effect of all this on melting momentum, you may ask. We'll have to wait a month for Dr Schröder to share his melt pond fraction maps for June. In the meantime there's a melt pond extent map on the Arctic Data archive System website that has taken over the role of IJIS in providing AMSR2 data from the JAXA GCOM-W1 satellite. This map showing melt pond concentration on the ice pack is "an essentially experimental and research product" (hat-tip to Wipneus), so I can't vouch for its accuracy. It is an interesting piece of information nevertheless.
Below is an animation showing the progress of melt pond formation since the start of the month:
The ever prolific Wipneus is even producing a melt pond extent graph based on these images, so we can compare this year to 2013, 2014 and 2015 (AMSR2 data started coming on-line back in 2012, so that year isn't complete):
This year started out slow, melt pond formation then speeded up at the start of June, but has been slowing down again in the past few days (as can be seen on the animation as well), and is now below the 2013 and 2014 trend lines. Mind you, given the fact that 2016 sea ice extent is currently more than 250K, 500K and 800K lower than 2015, 2014 and 2013 respectively, there is also less ice cover for melt ponds to form on, in the peripheral regions where temps and solar radiation should be higher than in the ice pack's core.
Although 2016 has been leading in extent, volume and snow cover almost all year so far, the build-up of melting momentum isn't following suit. It doesn't look like June will be very different from May in this respect. Based on the evidence presented in this blog post, it already looks highly unlikely that the 2012 record low minimum will be broken in September. This also depends on what happens during July and August, of course. Last year a similar unspectacular preconditioning phase was followed by the warmest July on record, eventually leading to one of the lowest September extents on record.
And there's one other of those aspects that are largely invisible to us: ocean heat flux. We simply don't know how much heat is coming into the Arctic via the Atlantic and Pacific Oceans, and both seem to have been quite warm all winter. Despite a cloudier and colder June so far, sea surface temperature anomalies are considerably higher than they were in 2012 around this time of year, on both sides of the Arctic:
With the ice pack being dispersed as it is right now and all the open water in the Beaufort and Kara Seas, one or two periods of predominantly clear skies over the Arctic, combined with high SSTs, can still cause this melting season to go low. Provided these periods occur, of course. As things currently stand, and with the forecast in mind, a more apt question is: Can the 2016 September minimum end up among the 5 lowest on record?
I will be providing one more line of evidence in part 2 of this series, as enough has been said already.