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Abstract. The Antarctic Continental Shelf seas (ACSS) are a critical, rapidly changingelement of the Earth system. Analyses of global-scale general circulationmodel (GCM) simulations, including those available through the Coupled ModelIntercomparison Project, Phase 6 (CMIP6), can help reveal the origins ofobserved changes and predict the future evolution of the ACSS. However, anevaluation of ACSS hydrography in GCMs is vital: previous CMIP ensemblesexhibit substantial mean-state biases (reflecting, for example, misplacedwater masses) with a wide inter-model spread. Because the ACSS are also asparely sampled region, grid-point-based model assessments are of limitedvalue. Our goal is to demonstrate the utility of clustering tools foridentifying hydrographic regimes that are common to different source fields(model or data), while allowing for biases in other metrics (e.g., water masscore properties) and shifts in region boundaries. We apply K-meansclustering to hydrographic metrics based on the stratification from one GCM(Community Earth System Model version 2; CESM2) and one observation-basedproduct (World Ocean Atlas 2018; WOA), focusing on the Amundsen,Bellingshausen and Ross seas. When applied to WOA temperature and salinityprofiles, clustering identifies “primary” and “mixed” regimes that havephysically interpretable bases. For example, meltwater-freshened coastalcurrents in the Amundsen Sea and a region of high-salinity shelf waterformation in the southwestern Ross Sea emerge naturally from the algorithm.Both regions also exhibit clearly differentiated inner- and outer-shelfregimes. The same analysis applied to CESM2 demonstrates that, althoughmean-state model biases in water mass T–S characteristics can be substantial,using a clustering approach highlights that the relative differences betweenregimes and the locations where each regime dominates are well representedin the model. CESM2 is generally fresher and warmer than WOA and has a limitedfresh-water-enriched coastal regimes. Given the sparsity of observations ofthe ACSS, this technique is a promising tool for the evaluation of a largermodel ensemble (e.g., CMIP6) on a circum-Antarctic basis. 

Abstract. The ice sheet model intercomparison project for CMIP6 (ISMIP6) effort brings together the ice sheet and climate modeling communities to gain understanding of the ice sheet contribution to sea level rise. ISMIP6 conducts stand-alone ice sheet experiments that use space- and time-varying forcing derived from atmosphere–ocean coupled global climate models (AOGCMs) to reflect plausible trajectories for climate projections. The goal of this study is to recommend a subset of CMIP5 AOGCMs (three core and three targeted) to produce forcing for ISMIP6 stand-alone ice sheet simulations, based on (i) their representation of current climate near Antarctica and Greenland relative to observations and (ii) their ability to sample a diversity of projected atmosphere and ocean changes over the 21st century. The selection is performed separately for Greenland and Antarctica. Model evaluation over the historical period focuses on variables used to generate ice sheet forcing. For stage (i), we combine metrics of atmosphere and surface ocean state (annual- and seasonal-mean variables over large spatial domains) with metrics of time-mean subsurface ocean temperature biases averaged over sectors of the continental shelf. For stage (ii), we maximize the diversity of climate projections among the best-performing models. Model selection is also constrained by technical limitations, such as availability of required data from RCP2.6 and RCP8.5 projections. The selected top three CMIP5 climate models are CCSM4, MIROC-ESM-CHEM, and NorESM1-M for Antarctica and HadGEM2-ES, MIROC5, and NorESM1-M for Greenland. This model selection was designed specifically for ISMIP6 but can be adapted for other applications. 

Abstract. Projection of the contribution of ice sheets to sea level change as part ofthe Coupled Model Intercomparison Project Phase 6 (CMIP6) takes the formof simulations from coupled ice sheet–climate models and stand-alone icesheet models, overseen by the Ice Sheet Model Intercomparison Project forCMIP6 (ISMIP6). This paper describes the experimental setup forprocess-based sea level change projections to be performed with stand-aloneGreenland and Antarctic ice sheet models in the context of ISMIP6. TheISMIP6 protocol relies on a suite of polar atmospheric and oceanicCMIP-based forcing for ice sheet models, in order to explore the uncertaintyin projected sea level change due to future emissions scenarios, CMIPmodels, ice sheet models, and parameterizations for ice–ocean interactions.We describe here the approach taken for defining the suite of ISMIP6stand-alone ice sheet simulations, document the experimental framework andimplementation, and present an overview of the ISMIP6 forcing to beused by participating ice sheet modeling groups. 

Abstract. Ice flow models of the Antarctic ice sheet are commonly used to simulate its future evolution inresponse to different climate scenarios and assess the mass loss that would contribute tofuture sea level rise. However, there is currently no consensus on estimates of the future massbalance of the ice sheet, primarily because of differences in the representation of physicalprocesses, forcings employed and initial states of ice sheet models. This study presentsresults from ice flow model simulations from 13 international groups focusing on the evolutionof the Antarctic ice sheet during the period 2015–2100 as part of the Ice Sheet ModelIntercomparison for CMIP6 (ISMIP6). They are forced with outputs from a subset of models from theCoupled Model Intercomparison Project Phase 5 (CMIP5), representative of the spread in climatemodel results. Simulations of the Antarctic ice sheet contribution to sea level rise in responseto increased warming during this period varies between −7.8 and 30.0 cm of sea level equivalent(SLE) under Representative ConcentrationPathway (RCP) 8.5 scenario forcing. These numbers are relative to a control experiment withconstant climate conditions and should therefore be added to the mass loss contribution underclimate conditions similar to present-day conditions over the same period. The simulated evolution of theWest Antarctic ice sheet varies widely among models, with an overall mass loss, up to 18.0 cm SLE, in response to changes in oceanic conditions. East Antarctica mass change varies between −6.1 and8.3 cm SLE in the simulations, with a significant increase in surface mass balance outweighingthe increased ice discharge under most RCP 8.5 scenario forcings. The inclusion of ice shelfcollapse, here assumed to be caused by large amounts of liquid water ponding at the surface ofice shelves, yields an additional simulated mass loss of 28 mm compared to simulations without iceshelf collapse. The largest sources of uncertainty come from the climate forcing, the ocean-induced melt rates, thecalibration of these melt rates based on oceanic conditions taken outside of ice shelf cavitiesand the ice sheet dynamic response to these oceanic changes. Results under RCP 2.6 scenario basedon two CMIP5 climate models show an additional mass loss of 0 and 3 cm of SLE on average compared tosimulations done under present-day conditions for the two CMIP5 forcings used and displaylimited mass gain in East Antarctica.