Arctic marginal ice zone trending wider in summer and narrower in winter

Declines in Arctic sea ice extent and thickness suggest scientifically important changes in the spatial pattern of the marginal ice zone (MIZ)—a dynamic and biologically active band of sea ice cover close to open ocean. Arctic MIZ widths were measured in satellite era sea ice concentrations over the years 1979–2011 using a recently published MIZ width analysis method. Over the record, the warm season (July–September) MIZ width increased by 13 km decade−1, amounting to a 39% widening. This widening trend resulted from a marked poleward advancement of the MIZ poleward edge into regions where sea ice was increasingly younger and thinner at the beginning of annual melt, while the MIZ's equatorward edge moved comparatively little. The warm season MIZ widening contrasted in sign and strength with a cold season (February–April) MIZ narrowing trend (4 km decade−1, amounting to a 15% narrowing). Key PointsThe warm‐season Arctic MIZ widened by 39% while trending poleward.The MIZ widening is linked to decline of thick, multiyear sea ice.The MIZ widening contrasts with a modest cold-season narrowing (15%).