Projections of ice-sheet mass balance require regional ocean warming projections derived from atmosphere-ocean general circulation models (AOGCMs). However, the coarse resolution of AOGCMs: (1) may lead to systematic or AOGCM-specific biases and (2) makes it difficult to identify relevant water masses. Here, we employ a large-scale metric of Antarctic Shelf Bottom Water (ASBW) to investigate circum-Antarctic temperature biases and warming projections in 19 different Coupled Model Intercomparison Project Phase 5 (CMIP5) AOGCMs forced with two different ‘representative concentration pathways’ (RCPs). For high-emissions RCP 8.5, the ensemble mean 21st century ASBW warming is 0.66, 0.74 and 0.58°C for the Amundsen, Ross and Weddell Seas (AS, RS and WS), respectively. RCP 2.6 ensemble mean projections are substantially lower: 0.21, 0.26, and 0.19°C. All distributions of regional ASBW warming are positively skewed; for RCP 8.5, four AOGCMs project warming of greater than 1.8°C in the RS. Across the ensemble, there is a strong, RCP-independent, correlation between WS and RS warming. AS warming is more closely linked to warming in the Southern Ocean. We discuss possible physical mechanisms underlying the spatial patterns of warming and highlight implications of these results on strategies for forcing ice-sheet mass balance projections.

To reduce the uncertainty in the calculation of Antarctic solid ice fluxes, the firn depth correction (Δh) in Antarctica is inferred from a steady-state firn densification model forced by a regional atmospheric climate model. The modelled density agrees well with observations from firn cores, apart from a site at the origin of fast flowing West Antarctic ice stream (Upstream B), where densification is anomalously rapid. The spatial distribution of Δh over Antarctica shows large variations, especially in the grounding line zone where large climate gradients exist. In places where the grounding line crosses ablation areas, Δh is zero. Along the remainder of the grounding line, Δh values range from typically 13 m in dry coastal areas (e.g. Dronning Maud Land) to 19 m in wet coastal areas (e.g. West Antarctica).