Jeff Id did an interesting post a few days ago on Antarctic sea ice in which he provided the following interesting graphic of Antarctic seaice area anomalies, which, from the texture, is daily, rather than monthly:
Figure 1. Antarctic seaice area anomaly (Jeff Id version)
Jeff’s data reference was to the following webpage at NSIDC, but, to my knowledge, this only provides gridded data in binary form and does not provide information on a daily basis. Last year, I scraped binary gridded data for the Arctic and it’s not a small job. The data has to be scraped on a day by day basis and it took me about half a day on high-speed cable. The scraping program requires a little ingenuity. Then the form of the binary data has to be figured out – which “endian” is it, the location and area of the cells has to be obtained (online in an information area), then the definitions of “area” and “extent” have to be applied. Unless Jeff found some daily data that I couldn’t find, there’s more work in this graphic than most readers realize.
Last year, I’d done this for the Arctic and felt a little out-flanked :) by another blogger managing to scrape data that I hadn’t. So I modified by scraping script, downloaded and organized the Antarctic data and produced my own version of Jeff’s graphic, which is substantially the same, but with a few interesting differences.
In producing one of these graphics, there are 5 satellites, with very short overlaps between the early satellites but a longer overlap between the two most recent – f13 and f15. I checked the overlaps for each pairing – there were noticeable differences, but in the tens of thousands of sq km, not hundreds of thousands of sq km. So I used the data as archived without attempting to make intersatellite adjustments (though that would be desirable in a lengthier exposition) and averaged the two satellites where there was an overlap.
While the two graphics are reassuringly similar, as noted above, there are interesting differences. In my version, there’s a remarkable downspike in the latter part of 2008. Re-examining the underlying data, this seems to result from an error in satellite f15 in October 2008 – on October 18, 2008, f15 (improbably) showed Antarctic seaice area of over 4 million sq km less than f13. There were only a few days of huge discrepancies, but they don’t seem to have picked up this error yet. f15 was the same satellite that had problems in the Arctic in February 2009 (discussed at Watt’s Up) and results from this satellite are no longer reported.
I’ll post my scraping scripts up as CA/scripts/seaice/collation.antarctica.txt, but warn that I’ve not tried to make them fully turnkey. I’ve posted daily area and extent in tab-separated ASCII form up to May 3, 2009 at CA/data/seaice/antarctica.csv .