Comment on tc-2023-15

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> The Greenland Ice Sheet is a key contributor to sea level rise. By melting, the ice sheet thins, inducing higher surface melt due to lower surface elevations, accelerating the melt coming from global warming. This process is called the melt-elevation feedback that can be considered by using two types of models: atmospheric models, which can represent the surface mass balance, usually using a fixed surface elevation, and the ice sheet models, which represent the surface elevation evolution but do not represent the surface mass balance as well as atmospheric models. A new coupling between the regional climate model MAR (Mod&egrave;le Atmosph&eacute;rique R&eacute;gional) and the ice sheet model PISM (Parallel Ice Sheet Model) is presented here following the CESM2 (SSP5-8.5) scenario until 2100 at the MAR lateral boundaries. The coupling is extended to 2200 with a stabilised climate (+ 7 &deg;C compared to 1961&ndash;1990) by randomly sampling the last 10 years of CESM2 to force MAR and reaches a sea level rise contribution of 64 cm. The fully coupled simulation is compared to a 1-way experiment where surface topography remains fixed in MAR. However, the surface mass balance is corrected to the melt-elevation feedback when extrapolated on the PISM grid by using surface mass balance vertical gradients as a function of local elevation variations (offline correction). This method is often used to represent the melt-elevation feedback and avoid a coupling expensive in computation time. In the fully-coupled MAR simulation, the ice sheet morphology evolution (changing slope and reducing the orographic barrier) induces changes in local atmospheric circulation. More specifically, wind regimes are modified which influences the melt rate at the ice sheet margins. We highlighted a mitigation of the melt lapse rate on the margins by modifying the surface morphology. The lapse rates considered by the offline correction are no longer valid at the ice sheet margins. If used (1-way simulation), this correction implies an overestimation of the sea level rise contribution of 2.5 %. The mitigation of the melt lapse rate on the margins can only be corrected by using a full coupling between an ice-sheet model and an atmospheric model.