Contribution du bilan de masse de surface antarctique à l’évolution du niveau des mers avec le modèle atmosphérique régional MAR

peer reviewe

Evolution of the Antarctic surface mass balance by physical downscaling and impact on the change in sea-level

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. 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

Added value of the regional climate model MAR for simulating the surface mass balance of the Antarctic ice sheet compared to a global climate model (ACCESS1.3)

Due to their ability to produce climate projections, General circulation models (GCM) are often used to provide estimates of the surface mass balance (SMB) of the Antarctic ice sheet that can be used to constrain ice sheet models. However, GCM still benefit from a poor representation of polar climate specificities such as stable boundary layers, polar clouds or interactions between snow-covered surfaces and the atmosphere. In this study, we highlight the importance of downscaling GCM outputs from the Fifth Climate Model Intercomparison Project (CMIP5) with a regional climate model to provide accurate estimates of the Antarctic SMB. For that purpose, the regional climate model MAR is forced by 6-hourly outputs from ACCESS1.3 that is currently considered as one of the best GCM from CMIP5 over the Antarctic ice sheet. Estimates of the SMB computed by MAR and ACCESS1.3 are evaluated against SMB observations. Even if the temporal variability of the SMB is forced by the driving GCM, the comparison shows that MAR improves the spatial variability of the Antarctic SMB, emphasizing the added value of using a polar RCM for downscaling GCM outputs at high latitudes

Evolution of Antarctic Surface Mass Balance by high-resolution downscaling of LMDZ4 AGCM and contribution to sea-level change

Most of the IPCC-AR4 Atmospheric Global Circulation Models (AGCM) predict an increase of the Antarctic Surface Mass Balance (SMB) during the 21st century that would mitigate global sea level rise. Present accumulation and predicted change are largest at the ice sheet margins because they are driven by snowfall, which mostly comes from warm, moist air arising over the land slopes. The coastal belt is also where complex processes of sublimation, melt and redistribution by the wind occur. Thus, high-resolution modelling (5 to 15 km) is necessary to adequately capture the effects of small-scale variations in topography on the atmospheric variables in this area, but limitations in computing resources prevent such resolution at the scale of Antarctica in full climate models. We present here a downscaling method leading to 15-km SMB resolution for century time-scales over Antarctica. We compute precipitation fields by considering orographic processes induced by the broad-scale and the fine-scale topography, and we estimate sublimation, melting and refreezing with a surface scheme validated for snow and ice-covered land surface. We display the SMB downscaled from LMDZ4 AGCM outputs (~60-km resolution), and compare the agreement of the broad-scale SMB and the downscaled SMB with 20th century observations. Then, we present hi-resolution features of the Antarctic SMB evolution during the 21st century downscaled from LMDZ4 and discuss the effect of the resolution on the Antarctic SMB contribution to sea level change. The downscaling model is a powerful tool that will be applied to others climate models for a better assessment of future sea level rise

Acceleration of dynamic ice loss in Antarctica from satellite gravimetry

The dynamic stability of the Antarctic Ice Sheet is one of the largest uncertainties in projections of future global sea-level rise. Essential for improving projections of the ice sheet evolution is the understanding of the ongoing trends and accelerations of mass loss in the context of ice dynamics. Here, we examine accelerations of mass change of the Antarctic Ice Sheet from 2002 to 2020 using data from the GRACE (Gravity Recovery and Climate Experiment; 2002–2017) and its follow-on GRACE-FO (2018-present) satellite missions. By subtracting estimates of net snow accumulation provided by re-analysis data and regional climate models from GRACE/GRACE-FO mass changes, we isolate variations in ice-dynamic discharge and compare them to direct measurements based on the remote sensing of the surface-ice velocity (2002–2017). We show that variations in the GRACE/GRACE-FO time series are modulated by variations in regional snow accumulation caused by large-scale atmospheric circulation. We show for the first time that, after removal of these surface effects, accelerations of ice-dynamic discharge from GRACE/GRACE-FO agree well with those independently derived from surface-ice velocities. For 2002–2020, we recover a discharge acceleration of -5.3 ± 2.2 Gt yr−2 for the entire ice sheet; these increasing losses originate mainly in the Amundsen and Bellingshausen Sea Embayment regions (68%), with additional significant contributions from Dronning Maud Land (18%) and the Filchner-Ronne Ice Shelf region (13%). Under the assumption that the recovered rates and accelerations of mass loss persisted independent of any external forcing, Antarctica would contribute 7.6 ± 2.9 cm to global mean sea-level rise by the year 2100, more than two times the amount of 2.9 ± 0.6 cm obtained by linear extrapolation of current GRACE/GRACE-FO mass loss trends

Reconstructions of the 1900–2015 Greenland ice sheet surface mass balance using the regional climate MAR model

With the aim of studying the recent Greenland ice sheet (GrIS) surface mass balance (SMB) decrease relative to the last century, we have forced the regional climate MAR (Modèle Atmosphérique Régional; version 3.5.2) model with the ERA-Interim (ECMWF Interim Re-Analysis; 1979–2015), ERA-40 (1958–2001), NCEP–NCARv1 (National Centers for Environmental Prediction–National Center for Atmospheric Research Reanalysis version 1; 1948–2015), NCEP–NCARv2 (1979–2015), JRA-55 (Japanese 55-year Reanalysis; 1958–2014), 20CRv2(c) (Twentieth Century Reanalysis version 2; 1900–2014) and ERA-20C (1900–2010) reanalyses. While all these forcing products are reanalyses that are assumed to represent the same climate, they produce significant differences in the MAR-simulated SMB over their common period. A temperature adjustment of +1 °C (respectively −1 °C) was, for example, needed at the MAR boundaries with ERA-20C (20CRv2) reanalysis, given that ERA-20C (20CRv2) is ∼ 1 °C colder (warmer) than ERA-Interim over Greenland during the period 1980–2010. Comparisons with daily PROMICE (Programme for Monitoring of the Greenland Ice Sheet) near-surface observations support these adjustments. Comparisons with SMB measurements, ice cores and satellite-derived melt extent reveal the most accurate forcing datasets for the simulation of the GrIS SMB to be ERA-Interim and NCEP– NCARv1. However, some biases remain in MAR, suggesting that some improvements are still needed in its cloudiness and radiative schemes as well as in the representation of the bare ice albedo.Results from all MAR simulations indicate that (i) the period 1961–1990, commonly chosen as a stable reference period for Greenland SMB and ice dynamics, is actually a period of anomalously positive SMB (∼ +40 Gt yr−1) compared to 1900–2010; (ii) SMB has decreased significantly after this reference period due to increasing and unprecedented melt reaching the highest rates in the 120- year common period; (iii) before 1960, both ERA-20C and 20CRv2-forced MAR simulations suggest a significant precipitation increase over 1900–1950, but this increase could be the result of an artefact in the reanalyses that are not well-enough constrained by observations during this period and (iv) since the 1980s, snowfall is quite stable after having reached a maximum in the 1970s. These MAR-based SMB and accumulation reconstructions are, however, quite similar to those from Box (2013) after 1930 and confirm that SMB was quite stable from the 1940s to the 1990s. Finally, only the ERA-20C-forced simulation suggests that SMB during the 1920–1930 warm period over Greenland was comparable to the SMB of the 2000s, due to both higher melt and lower precipitation than normal

Experimental protocol for sea level projections from ISMIP6 stand-alone ice sheet models

Projection of the contribution of ice sheets to sea level change as part of the Coupled Model Intercomparison Project Phase 6 (CMIP6) takes the form of simulations from coupled ice sheet–climate models and stand-alone ice sheet models, overseen by the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6). This paper describes the experimental setup for process-based sea level change projections to be performed with stand-alone Greenland and Antarctic ice sheet models in the context of ISMIP6. The ISMIP6 protocol relies on a suite of polar atmospheric and oceanic CMIP-based forcing for ice sheet models, in order to explore the uncertainty in projected sea level change due to future emissions scenarios, CMIP models, ice sheet models, and parameterizations for ice–ocean interactions. We describe here the approach taken for defining the suite of ISMIP6 stand-alone ice sheet simulations, document the experimental framework and implementation, and present an overview of the ISMIP6 forcing to be used by participating ice sheet modeling groups