Because of a very interesting discussion in the comment section of the last SIE update, I've decided to report some more on the area and extent figures. Although it deserves a blog post of its own, I unfortunately lack the skills and time to produce the necessary graphs. But some of the commenters are busy with this, so I will probably be adjusting and expanding this post, making it more comprehensive, this coming week.
As we have seen ever since atmospheric patterns changed radically since the beginning of the month and caused extent melt rates to stall in a big way, the ice pack, instead of being compacted and transported out of Fram Strait, has been diverging and spreading out over the Arctic Ocean. So much in fact that halfway through the month we started to notice 'holes' in the ice pack where relatively large leads and patches of blue ocean water were increasingly becoming visible.
Because of this phenomenon of cyclones dominating over the Central Arctic (preventing for instance the setting up of a stable and strong positive Arctic Dipole Anomaly, a key factor in 2007's record minimum extent) and causing the ice pack to be spread out, an interesting thing might be happening with regards to sea ice area and sea ice extent, both different methods of calculating the total surface of ocean water that is covered with sea ice.
I keep forgetting how this works, and other people probably do too, so I think we can't have enough explanations and analogies to stress the differences between both methods. The Arctic is divided up into grid cells, measuring 25 km x 25 km. For every grid cell that contains an ice concentration (percentage of the seawater that is covered by sea ice) of at least 15%, the whole surface area of the grid cell, 625 square km is added up to the total extent number. For sea ice area the surface area of every grid cell that is covered with sea ice is added to the total, except grid cells that have an ice concentration lower than 15%.
Say, for instance, we have three grid cells. One is covered with 10% ice, the second with 50% ice and the third with 100% ice. The first grid cell has less than 15% ice concentration and so isn't counted, but the other two are. Total extent is thus 1,250 square km (two grid cells of 625 km). For total area we take 10%, 50% and 100% of the surface areas of the grid cells, adding up to 937.5 square km (0 + 312.5 + 625). Total area will practically always be lower than extent.
Here is a great graph made by Larry Hamilton that shows us the monthly decline in area and extent from 1979 till now:
Of course, we know that IJIS reports its extent numbers on a daily basis, forming the basis of my SIE updates. Unfortunately, although they also have a sea ice area graph that looks just like the extent graph, they do not provide a spreadsheet or document with sea ice area data for us to download and play around with. Cryosphere Today has a regularly updated sea ice area document, but they only do area, not extent. NSIDC does have numbers for both methods. Larry Hamilton's graph is based on them and professional statistician Tamino has used the same numbers in his recent blog post, Sea Ice Curiosity, where he subtracted area numbers from extent numbers:
2007 is blue, 2008 is green, 2009 is brown, 2010 is red.
But unfortunately these are monthly numbers. And we can't wait for that. We Arctic junkies like to be fed data on a daily, near real-time basis, thank you very much. That's why some of the commenters of the Arctic Sea Ice blog have downloaded IJIS extent numbers and CT area numbers to play around with. Even though it isn't a perfect combination of data, it's still interesting to subtract the numbers from each other or divide them.
Why is this interesting? Mostly, it tells us something about how much the ice pack is spread out. When the pack gets compacted area and extent will come closer together. For instance in Tamino's graph we can see that during winter the difference between extent and area - when area is subtracted from extent - fluctuates between 1 and 2 million square km. But in the melting season the gap gets greater. This is because ice is melting, gets spread out and melt ponds start to form.
The last aspect is important as I believe it is the key factor in the early peaks of 2007 and 2010 to a lesser extent. When air temperatures get high enough and the sun reaches its maximum elevation around summer solstice melt ponds start to form on the ice, fooling satellite sensors into believing this is open water. For extent numbers this isn't so much a problem because a melt pond would have to get really big to cover a grid cell of 625 square km in such a way (more than 85%) for it to not be counted. With area however every percent of the surface that doesn't look like ice gets added to the total, making sea ice area numbers go much lower than extent numbers. But at this point in the melting season melt ponds aren't that big any longer, having been drained or starting to freeze over again, so we are mostly looking at the spreading of ice when fiddling around with area and extent numbers.
We are now entering the zone of speculation. I will probably alter and adjust this final part of the blog post as numbers and graphs are being crunched by commenters and there is still a lack of clarity on several points.
Like I said some of the commenters on this blog have started combining area and extent numbers from Cryosphere Today and IJIS respectively. To be more precise, they are dividing area by extent to see if the resulting percentages have been dipping in the last few weeks.
For instance here's the percentage (area/extent) for the 30th of July:
FrankD came up with different numbers because he managed to divide IJIS area numbers (probably by inferring them from the IJIS area graph) with IJIS extent numbers, which of course would be much better than using numbers from different organisations who use different methods and algorithms to calculate their numbers. Nevertheless I've decided to post his preliminary graph, because it is the thing we are looking for:
Unfortunately the 2010 trend line stops around July 8th, the point at which we'd expect the percentage to drop because of sea ice area dropping faster than extent, or in other words: because the ice pack kept a relative same shape at the exterior, but started to become less concentrated in parts of the interior, as shown by the 'holes' we have seen on MODIS satellite images.
In all years the percentages drop, but this year stands out. The number for July 31st 2010 is 63.8%. According to my data the minimum was reached on the following dates in previous years:
- September 25th 2006: 67.2%
- August 20th 2007: 60.6%
- August 14th 2008: 60.2%
- September 10th 2009: 64.7%
Larry Hamilton produced another graph with the NSIDC monthly area and extent data that shows how over the last 30 years the yearly minimum of ice area as a percentage of extent has dropped:
Okay, that's it for now. My poor little alpha brains can't take anymore. Stay tuned for more (when I have recovered).