Pack Ice Extent And Its Spatiotemporal Variation In Summer Arctic

Variation of pack ice extent (PIE) in summer Arctic is important for understanding thesea-ice change in Arctic Ocean. As the ice concentration smoothly decreases frompack ice to marginal ice zone(MIZ), there is no definite boundary between them.Therefore a threshold of ice concentration, C0, is required to define a margin of thepack ice in order to study the spatiotemporal variation of pack ice coverage. Thispaper made a comparison of AMSR-E6.25km and12.5km resolution sea iceconcentration data，and found that the latter decreased excessively fast withinice-coverage area with ice melting. Thus, two characteristics of ice concentrationvariation were obtained using the former. The pack ice areas decrease with higherthresholds of ice concentration, whereas the decreasing rate with ice concentration isnot uniform. A value of ice concentration could be chosen as a threshold at which thedecreasing rate shifts from faster to slower. Meanwhile, the differences of iceconcentration with neighboring grids will have a biggest statistical change nearby themargin. The suggested thresholds by the two characteristics mentioned above are all89%, indicating a well defined threshold with clear physical significance. Based onthe threshold of ice concentration, an erosion algorithm is used to eliminate the MIZand a connected domain algorithm is used to remove the small ice pieces out of thepack ice. The larger PIE appeared during2002-2006with little annual variation. Since2007, PIE decreased obviously. The minimum PIE was renewed in2010and2011,especially the PIE of2011is only64%of2006. The variation of PIE is quite differentfrom the ice-coverage extent (PIE plus MIZ), being an relative independent parameterin reflecting the sea ice variation in the area with high ice concentration. Sea-icemelting and variability of MIZ are two main factors influence the PIE. MIZ couldextend or shrink by dynamic factors, which will induce some ice blocks moving from or into pack ice, leading to an associated transition between PIE and MIZ. It foundthat melt ponds in higher latitudes had begun to freeze during a period from lateAugust to early September, which resulted in decrease of melt ponds. During this time,melt pond fraction was so low that it had little influence on minimum PIE.This paperprovides a new insight about sea ice variation in the Arctic, and the decrease of PIEindicates the ongoing reduce of pack ice in central Arctic Ocean.