When low-pressure areas take over the Arctic, they do two things: make the ice pack diverge and increase cloudiness. The diverging of the pack is hidden by the clouds, but we can sometimes see the holes in the ice pack through holes in the cloud cover on the LANCE-MODIS satellite images. We saw some early examples of that this year, ever since lows started to invade the central Arctic (for instance here and here), but it was and remained difficult to get an idea of the extent of holes in the ice pack. Until commenter Dr Tskoul came up with this tip:
MODIS Composites by Environment Canada.
Every Wednesday EC releases a 7 day MODIS satellite composite image, compiled to create a mosaic image of the Arctic Basin. They make a True-Colour Composite image as well as a False-Colour Composite image. I find the latter particularly interesting because it shows the holes best, as it highlights colour differences between sea ice and cloud. As a result, sea ice and snow appear light blue and clouds appear white (and purple over water).
Update July 25th:
Commenter dabize sent me this cleaned up version of the latest composite image:
I took the liberty of zooming in on the area with the most holes, notably in the East Siberian Sea and part of the Chukchi Sea:
The effects aren't just showing in the MODIS composite above, but also on various sea ice concentration maps:
Interesting as it may look, and in a way spectacular too, it isn't unprecedented (check for instance the concentration maps for July 25th on the ASI Graphs website). In fact, around this time in 2010 the ice pack had also started to show holes here and there, as can be seen on the Arctic Mosaic from July 22nd.
Amazingly, the holes back then were showing up in the centre of the ice pack and even close to the Canadian Archipelago, where ice is thickest. This only got worse as the melting season progressed, with 2010 being besieged by low-pressure areas from the start of July to mid-August. At the start of September large holes appeared around the North Pole, which led me to speculate about the accuracy of extent and area calculations in a blog post aptly called North Hole (followed by a second part). Environment Canada unfortunately started making these composite images in September 2010, but they have one composite image from 2010 that shows some of that patchiness: September 7-13.
This composite image for the period July 19-25, 2011, shows there weren't as many holes last year (full-resolution image here):
The reasons for year-to-year discrepancies are first and foremost due to atmospheric patterns during the melting season. Whereas 2011 was spared the worst, 2010 had a lot of lows for a period of six weeks. This year so far the Arctic has been dominated by low-pressure areas for a two-week period in June and now for the last 10 days.
Another reason, I presume, is what happened during the freezing season. The winter of 2009/2010 was characterized by an extremely negative Arctic Oscillation (see this graph from the Climate Prediction Center). This means there were a lot of high-pressure systems with winds pushing away the ice from Greenland and the Canadian Archipelago towards Siberia. These winds caused leads to open on the American side of the Arctic, leads that would re-freeze and thus have thinner ice when the melting season started. Under the right circumstances this thin ice melts first, showing up as holes in the ice pack.
The opposite occured for a large part of the winter preceding the current melting season. Winds blew the ice from Siberia towards America, which could partially explain why we see holes on the Siberian side of the Arctic, whereas things look pretty cohesive on the American side of the Arctic.
Speaking of Siberia, are we seeing another hole emerging in the Laptev Sea, just like last year's melting season (hat-tip to Lars K)? In the image below on the left we see a low-concentration spot in about the same place where last year a hole developed into a full-fledged polynya towards the end of August (image on the right from August 27th 2011):
Not too much later this hole merged with the rest of the open water in the Laptev Sea, which I dubbed the Laptev Bite, fond as I am of silly names.
It remains to be seen if the hole emerges again this year, and if so what could be causing it: Methane seeps (I don't think so), volcanoes (nah), Atlantic ocean inflow coming up due to seafloor relief(perhaps). I'll get back to it, if there are further developments.
In the meantime we keep peeking through the clouds to see how patchy the ice pack can become. It all depends on the weather, of course. The forecasts are all over the place right now and keep changing radically from day to day, so there's no telling what could happen. But if low-pressure areas keep dominating, we'll see more diverging, more and bigger holes. That much is sure.