I vaguely remembered a short discussion about discrepancies between various area and extent datasets in the comment section of SIE update 21 (perhaps somewhere else too). One of the possible causes mentioned was what I have just started referring to as the North Hole, also called the 'pole hole' by NSIDC:
Note that unlike ice extent, the Arctic values for ice area do not
include the area near the pole not imaged by the sensor, called the pole
hole. This area is (...) 0.31 million square
kilometers for SSM/I (from July 1987 to present).
IJIS also makes mention of this on their extent page:
The black dot seen at the North Pole is an area lacking data where AMSR-E cannot observe the Earth’s surface due to the limit of its observational coverage (i.e., orbit inclination of 98deg. and swath width of 1600km). Please note that this area is also counted as sea-ice cover in our estimation of sea-ice extent. We may change the policy (i.e., filling the gap with full coverage of sea ice) in the near future due to the recent drastic reduction of Arctic sea ice. We will announce this if it is implemented.
The black dot is also visible on the sea ice concentration map from Cryosphere Today at the top of the post. In previous years the ice there was practically 100% throughout the summer melt, but this year is different. Large swathes of low concentration ice have recently started to appear under the North Hole, like I noticed when commenting on the latest Race to Fram Strait animation. This could be important, because it means that for instance NSIDC area and IJIS extent are actually lower than reported (I haven't found any info yet on how other agencies, such as DMI, Arctic Roos or Uni Bremen handle the North Hole). But how much lower? That's what the 'in progress' is all about (check out the update below). Let me explain what I did so far.
One of the lessons from Patrick Lockerby's MODIS Rapidfire for Citizen Scientists-series I remembered, is that the North Pole can easily be found on the MODIS Arctic Mosaic because it's in the middle of the four quadrants on row 3 and 4, column 3 and 4.
So I decided to download these 4 quadrants from day 247 (September 4th) and combine them in one large image. So far, so good. But how to determine how big the North Hole is in that image? If I really wanted to get to the bottom of this, it was inevitable I did some calculating. Oh my, not my speciality. Bear with me, please.
As we've seen, the North Hole covers 310,000 square km according to NSIDC. If I've understood correctly each pixel in the MODIS images I downloaded is 1 square km. In other words, the total area of the North Hole circle is 310,000 pixels. The formula for the area enclosed by a circle is A = π r², so 310,000 = π r² (r being the radius of the circle). Moving π to the other side makes 310,000/π = 100,000 = r² (that's not entirely correct, but I'm not a scientist, so I can round off any way I like). The square root of 100,000 is 316, so the radius is 316. Multiplied by 2 I now have the diameter of my circle, which enables me to set the Fixed Size of my Elliptical Marquee Tool in Photoshop to a width and height of 632 pixels.
Here's what the result looks like:
That's from this combined image of the 4 quadrants:
Conclusion: I'm seeing quite a bit of dark water under the North Hole. Assuming I did everything right (maybe the circle is smaller or bigger). This means that IJIS extent is probably lower than reported. To determine exactly how much I'd probably need to do something with the colours or use a filter or something, and then count pixels. These are things I currently know nothing about.
To be continued...
I've gridded the image and I don't think there are any grid cells that have ice concentrations below 15%. This means that NSIDC extent isn't overestimated. Area is another story, but whether it's relevant remains to be seen.
Here's the gridded image (25x25 pixel/square km grid cells):
And here's a grid cell with relatively a lot of blue in it:
UPDATE 2: Here's the combined image for day 250 (Sept. 7th):