Abstract

The Polar MM5 regional climate model was run over the North Atlantic region for 1991-2000. We analyze 24 km output over the Greenland ice sheet to evaluate spatial and temporal surface mass balance variability. The model output is compared with three years of automatic weather station (AWS) data from 15 ice sheet sites to identify biases in simulated melt energy. Using in situ data, we derive simple corrections for biases in melt energy and sublimation from the surface and from blowing snow. Simulated accumulation rate is in general agreement with AWS and snow pit observations. Estimates for runoff and the surface mass balance distribution over the ice sheet are produced using modeled melt volume and a meltwater retention scheme. From the decade investigated, the magnitude of interannual variability in surface mass balance components is tentatively established. Variability in simulated equilibrium line altitude is suggestive of a dominance of thermal variability in the south with increasing importance of accumulation variability with increasing latitude. Empirical functions for the sensitivity of surface mass balance to temperature and precipitation anomalies are presented. The precise locations and regions of maximum and minimum surface energy and mass fluxes are identified. Using an estimate for iceberg discharge and bottom melting, the total ice sheet mass balance is estimated be -94 Gt y-1, approximately 40 Gt y-1 more negative than previous estimates, and would produce 2.7 mm of eustatic sea level rise and contribute roughly one fifth to the observed (1.5 mm y-1) global sea level rise. The increasingly negative mass balance is attributed to incorporating blowing snow sublimation and a recent increase in temperatures. The simulated interannual fluctuations in implied ice sheet mass balance suggest a very large natural variability in surface mass balance.