Toward a marginal Arctic sea ice cover: changes to freezing, melting and dynamics
Abstract. As summer Arctic sea ice extent has retreated, the marginal ice zone (MIZ) has been widening and making up an increasing percentage of the summer sea ice. The MIZ is defined as the region of the ice cover that is influenced by waves, and for convenience here is defined as the region of the ice cover between ice concentrations (area fractions) A of 15 to 80 %. The MIZ is projected to become a larger percentage of the summer ice cover, as the Arctic transitions to ice free summers. We compare individual processes of ice volume gain and loss in the ice pack (A>80 %) to those in the MIZ to establish and contrast their relative importance and examine how these processes change as the summer MIZ fraction and amplitude of the seasonal sea ice growth/melt cycle increases over decadal timescales. We use an atmosphere-forced, physics-rich sea ice-mixed layer model that includes a prognostic floe size distribution (FSD) model including brittle fracture and form drag. The model has been compared to FSD observations, satellite observation of sea ice extent and PIOMAS.
The MIZ fraction of the July sea ice cover, when the MIZ is at its maximum extent, increases by a factor of 2 to 3, from 14 % (20 %) in the 1980s to 46 % (50 %) in the 2010s in NCEP (HadGEM2-ES) atmosphere-forced simulations. In a HadGEM2-ES forced projection the July sea ice cover is almost entirely MIZ (93 %) in the 2040s. Basal melting accounted for the largest proportion of melt in regions of pack ice and MIZ for all time periods. During the historical period, top melt was the next largest melt term in pack ice, but in the MIZ top melt and lateral melt were comparable. This is due to a relative increase of lateral melting and a relative reduction of top melting by a factor of 2 in the MIZ compared to the pack ice. The volume fluxes due to dynamic processes decreases due to the reduction in ice volume in both the MIZ and pack ice. As the ice cover becomes marginal (MIZ), it melts earlier: in the region that was pack ice in the 1980s and became marginal in the 2010s, peak melting starts 20/12 days earlier (NCEP/HadGEM2-ES). This continues in the projection where melting in the region that becomes MIZ in the 2040s shifts 14 days earlier.
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