56.85% vs 57.39% on August 12th 2011.
CAPIE stands for Cryosphere-today Area Per IJIS Extent, and it tells us something about the compactness (official scientific term) of the ice. It all revolves around the different definitions for sea ice area and sea ice extent, which are two ways of calculating the total ice cover. The NSIDC explains it well:
Area and extent are different measures and give scientists slightly different information. Some organizations, including Cryosphere Today, report ice area; NSIDC primarily reports ice extent. Extent is always a larger number than area, and there are pros and cons associated with each method.
A simplified way to think of extent versus area is to imagine a slice of swiss cheese. Extent would be a measure of the edges of the slice of cheese and all of the space inside it. Area would be the measure of where there is cheese only, not including the holes. That is why if you compare extent and area in the same time period, extent is always bigger. A more precise explanation of extent versus area gets more complicated.
Extent defines a region as “ice-covered” or “not ice-covered.” For each satellite data cell, the cell is said to either have ice or to have no ice, based on a threshold. The most common threshold (and the one NSIDC uses) is 15 percent, meaning that if the data cell has greater than 15 percent ice concentration, the cell is considered ice covered; less than that and it is said to be ice free. Example: Let’s say you have three 25 kilometer (km) x 25 km (16 miles x 16 miles) grid cells covered by 16% ice, 2% ice, and 90% ice. Two of the three cells would be considered “ice covered,” or 100% ice. Multiply the grid cell area by 100% sea ice and you would get a total extent of 1,250 square km (482 square miles).
Area takes the percentages of sea ice within data cells and adds them up to report how much of the Arctic is covered by ice; area typically uses a threshold of 15%. So in the same example, with three 25 km x 25 km (16 miles x 16 miles) grid cells of 16% ice, 2% ice, and 90% ice, multiply the grid cell areas that are over the 15% threshold by the percent of sea ice in those grid cells, and add it up. You would have a total area of 662 square km (255.8 square miles).
If we divide area by extent, we get an idea about how spread out the ice pack is or not (which is determined by wind patterns). When the ice pack is compact, for instance during winter, all the 'holes in the cheese' freeze over, and so the area and extent numbers come very close, and CAPIE oscillates around the 95% mark.
At the start of the melting season melt ponds fool satellite sensors into thinking that there is open water where there is none (which get calculated for area, but not for extent). This makes CAPIE go down. As the melting season progresses, melt ponds become less and less of an influence because they drain, or the ice floes they are on break up.
We are now at a point in the melting season where the CAPIE percentage tells us something about how much the ice pack is spread out by 1) winds and 2) thinner ice that melts out in situ and leaves open water behind, when compared to other years. As of today CAPIE is the lowest it has ever been in the 2005-2012 period (and probably in the entire satellite record), and very early in the season as well. This is hardly a surprise as low pressure systems have been dominating the Arctic for quite a while now, and we have seen plenty of evidence of the effects.
I have to add one more possible influence on the CAPIE numbers besides divergence and melt ponds. Up till November last year IJIS used data from the AMSR-E sensor. Due to failing of the sensor IJIS has now switched to the lower-resolution WindSat radiometer. In contrast with previous years bigger jumps have been observed this year in the daily IJIS sea ice extent numbers (up as well as down). For instance, whereas Cryosphere Today sea ice area has decreased very fast in the past couple of days, IJIS sea ice extent numbers slowed down. This of course means that the CAPIE percentage will go down fast as well.
I wouldn't say it's an apples to oranges comparison, but rather a comparison between Golden Delicious and Jonagold. There's plenty of evidence the ice pack is being spread out a lot by diverging winds from low-pressure areas, and I wouldn't be surprised if it went a tad lower in days to come.
Read more about how CAPIE came into being on the ASI Blog, back in 2010 (followed by this blog post). But remember, just like the PICT thickness chart that is used for monthly PIOMAS updates, CAPIE is a crude method that gives us extra context for current developments. Nothing more, nothing less.
To cut a long story short: there's a lot of potential for extent numbers to drop. If a high-pressure system takes over again, its winds will start to compact the ice pack again. And there's a lot to compact. More than ever in fact.