For years we've talked about one of the most fascinating phenomena of Arctic sea ice loss as it progresses from year to year: The moment when the ice disappears regardless of the weather. We've seen some of that, back in July 2012, when extent and area numbers kept going down steadily, even though weather conditions weren't conducive to melt and should have caused a marked slowdown on the graphs (like they did in the preceding years). Some time later I concluded that this had to do with what I now call melting momentum.
The idea is that if during May and July the ice gets hit by lot of sunshine, lots of melt ponds form and they soak up a lot of heat. This doesn't immediately cause the ice to melt, but starts to make itself felt towards the later stage of the melting season. The built-up momentum keeps the melting going. For this reason I've been focusing on things like compactness, that tells us something about the amount of water within the ice pack, whether open water or melt ponds (satellite sensors can't see the difference).
But does this still hold true? Will there be a moment when the ice disappears regardless of melting momentum as well? We saw in 2016 that there wasn't much melting momentum built up during May and June, but despite the cloudiness, a record warm winter and persistent high air temperatures were enough to make the ice vulnerable enough for a big cyclone and massive dipole to have 2016 come in second on almost all graphs (see this overview). Yes, melting momentum is important, but there are other things at play as well, of course.
Now, this year has been extremely interesting so far. For the third time in a row, the winter preceding the melting season was relatively mild, the maximum was low (second lowest on record), and the trend line on many a graph went low from the get-go. Not as low as 2016, but low. However, as in previous years, clouds moved in and things slowed down to a crawl, taking the trend line as high up as 11th lowest on record on the JAXA sea ice extent chart last week. But then this happened in the last couple of days, suggesting spectacular melting on the Pacific side of the Arctic:
So, what's going on?
The first thing you need to do if you want to say something about Arctic sea ice is compare, compare, compare. Even if we're in uncharted waters here, and the New Normal (2005-2011) quickly evolved into the New Abnormal (2012-now), we still need to compare current data as much as we can to that of the past, especially if that data was gathered in a consistent way from year to year.
Before looking in more detail at the weather conditions during this melting season so far, I'd like to dig a bit deeper into the melting momentum aspect of things. Above, the NSIDC compactness graph shows how the 2018 trend line swooped down at the end of June, but picked up again after that, where years like 2012 and 2016 continued to go down. And before it went lowest on record, during those crucial months of May and June, it was actually relatively high. This would suggest very little melting momentum was built up during that time.
But NSIDC compactness is far from perfect as a melt pond measurement, because it is also heavily influenced by open water between floes, patches of open water inside the ice pack. Luckily, David Schröder, a scientist from the University of Reading, developed a model together with his colleagues that calculates melt pond fraction. And even more luckily, Dr Schröder is so kind as to share some of his model output with me. Here's how May and June look according to the model:
Edit 31-7: The legend is correct for May, but should be ten times higher for June, running from -20 to +20.
Caveat: This is a model result, and so the distribution of melt ponds doesn't necessarily reflect reality.
2012 really stands out and 2018 doesn't come close to it, but it does seem that during June more extensive melt ponding took place than in 2016 and 2017. It moved Dr Schröder to lower the CPOM June prediction for the SIPN Sea Ice Outlook by half a million km2 compared to the June prediction. The submitted prediction also had this:
While melt pond fraction has been generally low in May, the area covered by ponds in June is above the mean (the last 10 years) in the Central Arctic and below in the Siberian part.
So, let's have a closer look at weather conditions during May, June and July. For that, I have downloaded weather maps, both for sea level pressure (SLP) and surface air temperature (SAT), from the Daily Mean Composites website, based on NCEP/NCAR Reanalysis data and other datasets. Mind you, the two maps on the right run to July 25th, not 31st:
First, SLP. It's always good to remember that green, yellow and red represent high pressure, while blue and purple represent low pressure. Basically, this tells us something about how cloudy the Arctic has been, because in high pressure areas clouds are less prevalent than in low pressure areas (or cyclones). As we can see, high pressure dominated over the Central Arctic during the second half of May, but after that clouds took over, for a large part explaining the slowdown in sea ice extent/area decrease.
Of course, temperatures play a role as well. In July they were anomalously high over the Siberian side of the Arctic, but on this front things slowed down too during July. It's interesting to note that CPOM's melt pond fraction model had below average melt ponding in the Siberian part in June (which is why I underlined it in the quote), whereas skies were reasonably open during the first half of June, and anomalously warm throughout the whole month. I'll get back to why this is important at the end of the blog post.
But first we compare, compare, compare some more. It's interesting to see how things have evolved, pressure- and temperature-wise, this melting season so far, but without context it doesn't mean much. And so I've downloaded the SLP and SAT maps for every year since 2007, or actually, re-downloaded them because the format had changed, cropped them, and combined them into the largest images I've probably posted on the blog since starting it (click for full size, SLP left, SAT right):
Don't try to take it all in. Just compare the different periods of this year with those of previous years. For instance, on the SLP side (left), 2018 can't keep up with 2010 and 2012, the big volume droppers. And as for SAT (right), 2011 and 2012 stand out much more. It's all about those periods of intense sunshine and cyclones. The longer they last, the more impact they make. They usually don't last longer than a week, although there are exceptions, like 2007. This year did see two feisty cyclones which stirred things up a bit, and then quickly fizzled out. But no real sunny spells to speak of.
There's one more measure we can look at and that's sea surface temperature. For that I use DMI's SST anomaly maps:
And here too it seems 2018 can't keep up with the Joneses. But is that it? You can never know with the Arctic. As we saw in that animation of UB SIC maps huge swaths of lower concentration suddenly appeared. Now, this ice won't melt out in the coming week, but it's a sign that the ice in these regions may not be making it to September. In fact, we might be seeing some of the flash melting and detachment that made 2012 legendary.
As to how low 2018 may still go: It has caught up quite a bit in the past few days on the JAXA sea ice extent chart, almost as the past two years now:
This probably has to do with the warm winter, the intense cyclones, and if the CPOM melt pond fraction model has it right, more melting momentum than in 2016 and 2017 as well. All of this is based on the tools I have used for years, tools that allow one to compare to previous periods. Over on the ASIF, commenters use many more sources, like different thickness models, but it's difficult to ascertain their reliability, and they often don't allow one-on-one comparisons with the past.
Also, we can't know what may happen in the last 5-8 weeks that this melting season has left, but we can look at what the next week has in store for the coming week, according to the ECMWF weather forecast model (source: Tropical Tidbits, click for a larger version):
Now, that's some serious high pressure, and though not positioned over the Beaufort Sea, it is covering a large part of the Central Arctic and western Siberia. This is going to diverge the ice pack some more, bombard it with solar radiation, and push the vulnerable parts towards the Pacific.
And this brings us to why what's happening on the Siberian side of the Arctic is so important. According to PIOMAS the ice is thicker there than it was compared to the 2011-2017 period (red means thicker, blue means thinner):
If those red zones near the Siberian coast have disappeared on the next PIOMAS update, this 'average' melting season without any real ice melting weather to speak of so far and no Fram Strait export whatsoever, could still end up among the lowest on record. Does that mean we may be witnessing another moment where ice extent keeps going down steadily, regardless of the weather, regardless of melting momentum even? Or will August weather present a couple of surprises that cause this year to crash down to a top 3 position, making it even more difficult to make heads or tails of dynamics?
Lots of questions, and I guess we're about to find out. Either way, this has been the longest prelude to a PIOMAS update I've ever written.