Évolution du bilan de masse de surface Antarctique par régionalisation physique et conséquences sur les variations du niveau des mers

The Antarctic surface mass balance (SMB, i.e. the snow accumulation from which we sub- tract ablation by sublimation, run-off or erosion) is a major yet poorly known contribution to changes in the present-day sea level. Water storage by snow accumulation at the top of the ice- sheet is expected to increase during the 21st century, which would moderate the rise in sea level. Three-quarters of the Antarctic SMB are concentrated below 2000 m above sea level whereas this area represents only 40% of the grounded ice sheet area. Orographic precipitation is a major contributor to snow accumulation in this region, which is why a better estimation of this term is important. The representation of this process by models depends to a great extent on the resolu- tion of the model, since precipitation amounts depend on the ice sheet slopes. Sublimation and snowmelt also depend on elevation. Global and regional atmospheric climate models are unable to achieve a 40-km resolution over Antarctica at a century time scale, due to their computing cost. At this resolution, ice-sheet margins are still badly resolved.
<br />That is why we developed the downscaling model SMHiL (surface mass balance high-resolution downscaling), which estimates the Antarctic SMB components at a high resolution (∼15 km) from large-scale atmospheric forcings. We compute the impact of the high-resolution topography on orographic precipitation amounts and on the boundary-layer processes that lead to sublima- tion, melting and refreezing. To validate SMHiL, we compare our results with more than 2700 field data recently updated and quality-controlled. However, we exhibit that field data below 2000 m above sea level are too scarce to settle SMHiL efficiency. In light of this, we show that the GLACIOCLIM-SAMBA stake lines located on the ice sheet coast-to-plateau area is an ap- propriate reference to evaluate model performance. Finally, we downscale the atmospheric global climate model LMDZ4 to estimate the SMB changes during the 21st and 22nd centuries. The high-resolution SMB is significantly different from the SMB given by LMDZ4. Our results sug- gest that running LMDZ4 at a finer resolution may give a future increase in SMB in Antarctica between 15% to 30% higher than at its standard resolution. Future changes in the Antarctic SMB at low elevations will result from the conflict between higher snow accumulation and ru- noff. The downscaling model is a powerful tool that can be applied to climate models for a better assessment of a future rise in sea level.