TROIS-AS

The largest uncertainty on the 21st century sea level rise is related to the Antarctic ice sheet, where dynamical ice discharge is projected to increase, being only partly compensated by the effect of increased snowfall. A very large part of the increased discharge will likely occur through Pine Island and Thwaites glaciers that have an ice-shelf termination in the Amundsen Sea. These glaciers may already be engaged in a dynamical instability triggered by increased melt beneath their ice shelves. The later results from the Amundsen Sea warming that is likely related to wind changes at both regional and global scales. Several significant feedbacks between atmosphere, ocean and glacier dynamics have been suggested in the literature. Current Earth System Models have many biases in the Antarctic region, and they do not represent the ocean beneath ice shelves and the glacier dynamics. The capability of representing the ocean in static ice-shelf cavities has been introduced in a few ocean models very recently. The ice-sheet models have also much improved over the last few years, and a few three-dimensional ice models are now able to capture the fast acceleration of glaciers with a marine termination. However, these models are still forced by an imposed and idealized melt rate beneath their ice shelves, which hides feedbacks and makes future projections overly uncertain. Concerning the atmosphere, only a few regional models designed for the polar environment are currently able to simulate the complex surface mass balance and coastal winds of Antarctica. The TROIS-AS project aims to build a regional modeling system enabling the representation of physical processes and feedbacks at the ocean/ice-sheet and atmosphere/ice-sheet interfaces. The objective is to quantify the contribution of Pine Island and Thwaites glaciers to global sea level rise over the next 100 years. The project will cover 48 months and will be conducted at the Laboratoire de Glaciologie et de Géophysique de l’Environnement (LGGE, Grenoble, France). It offers a great opportunity to strengthen interactions between two teams at LGGE. It is based on three models: Elmer/Ice for the continental ice, NEMO for the ocean and sea ice, and MAR for the atmosphere. Key developers or coordinators of these models are based at LGGE, and are involved in this project. TROIS-AS is divided into three tasks. Task 1 aims to better understand and simulate melting at the ocean/ice-shelf interface. The interannual melting rate beneath ice-shelves will be estimated in NEMO simulations. Then it will be used to constrain Elmer/Ice simulations of Pine Island and Thwaites glaciers over 1979-2015. Task 2 aims to better understand the evolution of the atmosphere/ice-shelf interface in a global warming scenario. In particular, the direct response of the surface mass balance to anthropogenic forcing will be compared to the indirect response resulting from a change in glacier thickness. The impact of a change in winds and heat fluxes at the surface of the Amundsen Sea will also be investigated to assess potential feedbacks on subglacial melting. The main objective of Task 3 is to develop a modeling system able to estimate the contribution of Pine Island and Thwaites glaciers to sea level rise in ~2100. For this, NEMO will be modified to allow growth of oceanic meshes towards continental meshes, and the coupling interfaces will be developed. The budget for this proposal is 346 keuros. This includes a 3-year PhD fellowship for Task 2, and a 2-year position for a research engineer who will undertake the technical developments of Task 3. The PI N. Jourdain will be 83% involved (40 months), and five other permanent researchers from LGGE will be involved for a total of 40 more months. This project addresses questions of direct interest for the society and the media. Our results will therefore be communicated to the public, including through web pages dedicated to popular science.