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).
This is part 3 in the series.
As we saw last month, the melt pond fraction during May wasn't particularly high compared to previous years, and quite below that of 2012, effectively cancelling out the advantage that was built up on the extent charts. Back then it already looked highly unlikely that the 2012 record low minimum will be broken in September. This trend continued during June.
Dr David Schröder has been so kind as to send me the June melt pond fraction maps he uses for his SIPN 2016 Sea Ice Outlook prediction (blog post on June Report here). These results are based on the melt pond fraction simulation model developed by CPOM researchers at the University of Reading (more info). The comparison below shows melt pond anomalies for the years 2010, 2012, 2015 and 2016 compared to the average for the last 10 years:
Caveat: This is a model result, and so the distribution of melt ponds doesn't necessarily reflect reality.
The difference with 2010 and 2012 is clearly quite large. Just like last year May and June saw very little melting momentum being built up. Last year then caught up a bit because of one of the hottest Julys on record, eventually ending in third place with a September average of 4.63 million km2 (just like 2011, with 2007 and 2012 coming in at 4.30 and 3.63 million km2 respectively).
Dr. Schröder writes:
Based on May/June melt pond fraction, we predict the September ice extent 2016 to be larger than last year: 5.2 +/- 0.44 million km2.
In spite of the mild Arctic winter resulting in thinner ice and an exceptional low May ice extent, the atmospheric conditions during June did not allow much melting. Therefore, our simulated June melt pond fraction is clearly below average, lower than in any other year since 2001. Taking into account our prediction from last month and our current prediction, we expect the September ice extent 2016 to be between 4.5 and 5.2 million km2.
Given that I voted 'between 4.5 and 4.75 million km2' on this month's NSIDC September minimum poll over on the Forum, I agree with the lower range of this prediction. Weather conditions in the past few weeks have been such (see last week's ASI update), and volume not nearly low enough (see latest PIOMAS update), that an exceptional July and/or August are needed for this year to make it into the top 3 lowest minimums on record.
This year hasn't exactly been cold, but air temperatures haven't been extremely high either. Here's the ranking for June, taken from Dr. Andrew Slater's website, with this year coming in 22nd:
One aspect this melting still has going for it, is sea surface temperatures, which shouldn't be underestimated. Below is a comparison of SST anomalies around this date:
This year is clearly running hotter than last year, and similar to 2012 (with 4 days left to go). I'm not sure yet what role this is going to play with regards to the September minimum or the longer term.
Another intriguing aspect is that this year is currently very low on various compactness graphs. Compactness is obtained by dividing area numbers by extent numbers (see the Melting momentum part 2 blog post for a more extensive explanation). I'm currently updating two such graphs, one based on Cryosphere Today sea ice area numbers divided by JAXA sea ice extent numbers (CAJAX), the other on the same area numbers divided by MASIE (CAMAS):
The 2016 trend line is lowest on the CAMAS graph, and almost tied with 2008 on the CAJAX graph. Of course, these 'bastard' graphs aren't showing pure compactness. For that you need a graph with SIE and SIA data that come from the exact same provider. Luckily, Wipneus has one for us showing NSIDC, JAXA, and Uni Hamburg compactness (see ASIG Regional Graphs page):
Now, we know that this low compactness isn't due to melt ponds. In fact, it must be due to dispersal given the weather conditions of the past weeks, cyclones blowing the ice floes away from each other, causing regions of low-concentration ice within the ice pack, such as this one not too far from the Pole (image courtesy of commenter A-Team):
This isn't out of the ordinary, we've seen it too during 2010 and 2013, but it means that there is some compaction potential that could come into play if winds start blowing the ice inwards again. Or perhaps the combination of more dispersal and high SSTs causes a lot of in-situ melting, preventing a top 3 ranking perhaps, but at the same time keeping the ice pack in the dire state it has been for the past decade. Who knows, a big cyclone might cause another substantial piece of the ice pack near Siberia to break off, like we saw in 2012.
Again, we see that you can have a killer start to the melting season, go 1 million km2 lower extent-wise than any other year on record for weeks on end, but if you don't build up melting momentum during May and June you are not going to break records. Unless you start with really low volume, which may take some time if Chris Reynolds and his Slow Transition theory are to be believed.
I have a feeling this melting season will teach us some more valuable lessons.