In this exciting battle between Greenland and the Arctic sea ice (who gets the most media coverage?), it's now time for one of those boring comparisons that give the current melting season some more context. For that I have used the excellent maps on the daily mean composites page from NOAA's Earth Science Research Laboratory.
Let me start off with sea level pressure (SLP) from June 1st to July 15th, divided into three two-week periods. Since 2012 is still so low on SIE and SIA graphs, I figured it's best to compare it with other record years such as 2007, 2010 and 2011 (2008 became competitive towards the end of the melting season).
These maps show SLP averaged over two weeks, which makes them a bit crude, but they still tell the story of each respective melting season during June and the first half of July. 2010 was going down very fast on the area/extent graphs during June, with a lot of high pressure in the right spot. But in July this decrease came to a grinding halt, with low-pressure systems taking over (see orange arrow on the IJIS SIE graph on the right). 2011 took over the lead, because it kept going strong all the way to mid-July. However, a switch in weather patterns would cut that lead short as well (purple arrow), with 2007 taking over for the remainder of the melting season with its ideal decrease conditions from start to late finish. High pressure on the American side of the Arctic, low pressure on the Siberian side, all the way.
And now 2012. We can clearly see on the SLP map in the top right what was causing that nosedive in June. The rapid decrease slowed down when that big high-pressure system over the Beaufort Sea was replaced by an equally big and powerful low-pressure system. Interestingly though, we didn't see the trend line on the IJIS SIE graph bend to the right as sharp as the 2010 trend line, when the weather patterns switched back then. Despite relatively unfavourable conditions for area and extent decrease trend lines on all graphs kept plodding downwards steadily. It's only in the past week that we see the trend line veer off slightly to the right (red arrow), and that probably has to do with this:
That's one big low-pressure area right there, and though area/extent decrease has slowed down a bit, we are certainly not witnessing the grinding halt we saw at the beginning of July in 2010 (orange arrow) or mid-July in 2011 (purple arrow). This leaves us with only one possible conclusion - if we exclude the nefarious influence of icebreakers, undersea volcanoes and aliens - and that's that a large part of the ice pack is so thin it doesn't really care what the weather does. I concluded the same at the end of last year's melting season, but this is at least 6 weeks earlier. If weather patterns switch to that ideal mix of high-pressure and low-pressure areas, there's no telling what can happen...
Hold on a minute.
Although I really believe my conclusion is broadly correct, I must admit that I am jumping to it. There are two other factors besides SLP and ice thickness that influence sea ice area and extent decrease, namely air and sea surface temperatures. So here's a comparison for July 8th to July 22nd:
2012 looks slightly warmer than the other years, especially over the Canadian Archipelago and Greenland (no kidding!). The same goes for sea surface temperatures, although the maps don't look nearly as exciting as the winter maps. Except for the Bering Strait, the sea water is warmer everywhere, with Baffin Bay and the Beaufort Sea standing out with a tad of orange and red (4-5 °C warmer than the long-term average). And of course Barentsz and Kara, our WACC-y Weather regions.
So temps play a part as well, but it definitely looks like PIOMAS is on the right track. And the Arctic sea ice isn't. This weekend's ASI update will have more detailed info on what we can expect in the coming 1-2 weeks.