Assimilating satellite concentration data into an Arctic sea ice mass balance model, 1979–1985

Abstract

Using satellite ice concentration data, observed buoy velocities, and a thermodynamic ice growth model, we compute the time histories of the thickness distributions of four ice types: level and ridged first‐year ice, and level and ridged multiyear ice. These histories are computed for seven regions covering the Arctic Ocean with a 10‐day time step for 1979 through 1985. A blend of observed buoy motions and wind‐driven, free‐drift velocities drives the advective and ridging parts of the model, while climatological growth and melt rates determine the ice thickness. The model adjusts the thickness distributions so that the modeled concentrations of open water, first‐year ice, and multiyear ice match the concentrations derived by Kalman filtering of satellite passive microwave observations. The major adjustment is to reduce modeled ice concentrations in summer and particularly in the peripheral seas to account for unmodeled melt. The model includes an ice salinity parameterization which, along with changes in ice volume, gives the salt or freshwater flux from the ice. Our results show that the Arctic ice cover consists primarily of multiyear ice, 60% by area and 82% by volume, and that most of this ice is ridged. The average ice thickness is 2.7 m with a seasonal variation of 30% and an interannual variation of 10%. The study also shows that for each cell and the Arctic as a whole, the changes in ice volume caused by growth minus melt approximately balance the changes caused by import minus export. The annual ice export through Fram Strait varies from 1100 to 3000 km3, primarily because of variations in ice velocity. All regions except the Chukchi Sea have a net ice growth, a net export, and a net salt input to the ocean surface. The annual mean salt flux to the ocean surface from net ice production is 7.3 kg m−2 downward. Fluxes along the transpolar drift stream are larger than elsewhere; the largest downward flux of 33 kg m−2 yr−1 in the Nansen Basin can be attributed to the strong divergence near Fram Strait. A companion paper [Steele et al., this issue] uses these fluxes and ice velocities to derive the Arctic Ocean freshwater balance.