Map of the minimum (august) and maximum (april) expanse covered by ice at greater than 15% concentration, in 2013. Data obtained from http://nsidc.org/data/seaice_index/ ( ftp://sidads.colorado.edu/DATASETS/NOAA/G02135/shapefiles/Aug/shp_extent/extent_S_201308_polygon.zip , ftp://sidads.colorado.edu/DATASETS/NOAA/G02135/shapefiles/Apr/shp_extent/extent_S_201304_polygon.zip ) from research by: 
Fetterer, F., K. Knowles, W. Meier, and M. Savoie. 2002, updated daily. Sea Ice Index. [indicate subset used]. Boulder, Colorado USA: National Snow and Ice Data Center. https://dx.doi.org/10.7265/N5QJ7F7W.

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Atomicdryad