Description: Surface mass balance (SMB) is the builder of the Greenland ice sheet and the driver of ice dynamics. Quantifying the past, present and future state of SMB is important to understand the drivers and climatic processes that control SMB, and to both initialize and run ice sheet models which will help clarify sea level rise, and how likely changes in ice sheet extent feedback within the climate system. Regional climate models (RCMs) and climate reanalysis are used to quantify SMB estimates. Although different models have different spatial and temporal biases and may include different processes giving significant uncertainty in both SMB and the ice sheet dynamic response to it, all RCMs show a recent declining trend in SMB from the Greenland ice sheet, driven primarily by enhanced melt rates. Here, we present new simulations of the Greenland ice sheet SMB at 5 km resolution from the RCM HIRHAM5. The RCM is driven by the ERA-Interim reanalysis and the global climate model (GCM) EC-Earth v2.3 to make future projections for climate scenarios RCP8.5 and RCP4.5. Future estimates of SMB are affected by biases in driving global climate models, and feedbacks between the ice sheet surface and the global and regional climate system are neglected, likely resulting in significant underestimates of melt and precipitation over the ice sheet. These challenges will need to be met to better estimate the role climate change will have in modulating the surface mass balance of the Greenland ice sheet.

Description: Abstract. Various observational estimates indicate growing mass loss at Antarctica's margins as well as heavier precipitation across the continent. Simulated future projections reveal that heavier precipitation, falling on Antarctica, may counteract amplified iceberg discharge and increased basal melting of floating ice shelves driven by a warming ocean. Here, we test how the ansatz (implementation in a mathematical framework) of the precipitation boundary condition shapes Antarctica's sea level contribution in an ensemble of ice sheet simulations. We test two precipitation conditions: we either apply the precipitation anomalies from CMIP5 models directly or scale the precipitation by the air temperature anomalies from the CMIP5 models. In the scaling approach, it is common to use a relative precipitation increment per degree warming as an invariant scaling constant. We use future climate projections from nine CMIP5 models, ranging from strong mitigation efforts to business-as-usual scenarios, to perform simulations from 1850 to 5000. We take advantage of individual climate projections by exploiting their full temporal and spatial structure. The CMIP5 projections beyond 2100 are prolonged with reiterated forcing that includes decadal variability; hence, our study may underestimate ice loss after 2100. In contrast to various former studies that apply an evolving temporal forcing that is spatially averaged across the entire Antarctic Ice Sheet, our simulations consider the spatial structure in the forcing stemming from various climate patterns. This fundamental difference reproduces regions of decreasing precipitation despite general warming. Regardless of the boundary and forcing conditions applied, our ensemble study suggests that some areas, such as the glaciers from the West Antarctic Ice Sheet draining into the Amundsen Sea, will lose ice in the future. In general, the simulated ice sheet thickness grows along the coast, where incoming storms deliver topographically controlled precipitation. In this region, the ice thickness differences are largest between the applied precipitation methods. On average, Antarctica shrinks for all future scenarios if the air temperature anomalies scale the precipitation. In contrast, Antarctica gains mass in our simulations if we apply the simulated precipitation anomalies directly. The analysis reveals that the mean scaling inferred from climate models is larger than the commonly used values deduced from ice cores; moreover, it varies spatially: the highest scaling is across the East Antarctic Ice Sheet, and the lowest scaling is around the Siple Coast, east of the Ross Ice Shelf. The discrepancies in response to both precipitation ansatzes illustrate the principal uncertainty in projections of Antarctica's sea level contribution.

Description: What are the benefits of limiting the global warming to 1.5 degree with respect to pre-industrial conditions for the vulnerable region of West Antarctica which might be prone to positive feedback mechanisms between ocean circulation, melting of shelf ice and instabilities of the ice sheet? There are indications that West Antarctic ice sheet instabilities have occurred in the Last Interglacial around 125.000 years ago. At that time the polar surface temperature was about 2K warmer than today. The question under which circumstances a tipping point may be reached and if this may happen again is therefore highly relevant, especially since a disintegration of the West Antarctic ice sheet could cause a global sea level rise between 3 and 5 m. Here we address this question with variable resolution, global coupled ice sheet - shelf ice - ocean - atmosphere multi-century simulations. With our innovative ocean modelling approach in the Finite Element Sea-ice Ocean Model FESOM it is possible to refine the ocean resolution to up to 3 km in the Amundsen Sea and 10 km around the whole Antarctica while keeping it relatively coarse in the order of a couple of hundred km in dynamically not very active regions such as the subtropical regions. This means that we can simulate the feedback between ocean and ice in the relevant regions highly resolved given that the ice sheet model runs at a resolution of 5 to 10 km. Three different emission scenarios are applied up to 2100, two of them limiting the global mean temperature increase to 1.5 ◦ C and 2 ◦ C respectively and one of them assuming business-as-usual conditions (IPCC SRES RCP8.5 scenario). The simulations are extended to 2400 with the greenhouse gas and aerosol concentrations kept constant at 2100 levels, respectively, to be able to simulate the long-term implications of different global warming levels.

Description: The AWI climate model AWI-CM consists of the Finite Element Sea-ice Ocean Model (FESOM) developed at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research in Bremerhaven and the ECHAM6 atmosphere land model developed at the Max Planck Institute for Meteorology in Hamburg. With the innovative flexible mesh structure of the FESOM model it is possible to highly resolve key ocean regions such as the western boundary currents or the Antarctic Circumpolar Current. In relatively coarse resolutions the model has proven to be of comparable quality as the CMIP5 models when measured with objective performance indices while in finer resolutions long-standing biases such as the North Atlantic deep ocean bias could be improved considerably. The AWI climate model will be part of CMIP6 including HighResMIP and production simulations have recently been started. Furthermore, at the moment the AWI-CM is being coupled to the Parallel Ice Sheet Model PISM to investigate the stability of the West Antarctic Ice Sheet when considering ice shelf – ocean interactions at very high resolutions (5 to 10 km) locally around Antarctica while leaving the ocean resolution in the order of 100 km in the dynamically less active subtropical areas.

Description: The knowledge of chlorofluorocarbon (CFC11, CFC12) concentrations in ocean surface waters is a prerequisite for deriving formation rates of, and water mass ages in, deep and bottom waters on the basis of CFC data. In the Antarctic coastal region, surface-layer data are sparse in time and space, primarily due to the limited accessibility of the region. To help filling this gap, we carried out CFC simulations using a regional ocean general circulation model (OGCM) for the Southern Ocean, which includes the ocean-ice shelf interaction. The simulated surface layer saturations, i.e. the actual surface concentrations relative to solubility-equilibrium values, are verified against available observations. The CFC input fluxes driven by concentration gradients between atmosphere and ocean are controlled mainly by the sea ice cover and sea surface temperature and salinity. However, no uniform explanation exists for the controlling mechanisms. Here we present simulated long-term trends and seasonal variations of surface-layer saturation at Southern Ocean deep and bottom water formation sites and other key regions, and we discuss differences between these regions. The amplitudes of the seasonal saturation cycle range from 22% to 66% and their long-term trends amount to rises of 0.1%/year to 0.9%/year. The seasonal saturation maximum lags the ice cover minimum by 2 months. We show that ignoring the trends and using instead the saturations actually observed can lead to systematic errors in deduced inventory-based formation rates by up to 10% and suggest an erroneous decline with time.

Description: Ocean/ice interaction at the base of deep drafted Antarctic ice shelves modifies the physical properties of inflowing shelf waters to become Ice Shelf Water (ISW).In contrast to the conditions at the atmosphere/ocean interface, the increased hydrostaticpressure at the glacial base causes gases embedded in the ice to solve completely after being released by melting. Helium and neon with an extremely low solubility are supersatured in glacial meltwater by more than 1000%. At the continentalslope in front of the large Antarctic caverns ISW mixes with ambient waters toform different precursors of Antarctic Bottom Water. A regional ocean circulation model is presented which uses an explicit formulation of the ocean/ice shelf interaction to describe for the first time the input of noble gases to the Southern Ocean. The results reveal a long-term variability of the basal mass loss solely controlled by the interaction between waters of the continental shelf and the ice shelf cavern. Modeled helium and neon supersaturations from the Filchner-Ronne Ice Shelf front reveal a "low-pass" filtering of the inflowing signal due to cavern processes. On circumpolar scales the helium and neon distributions in the Southern Ocean quantify the ISW contribution to bottom water which spreads with the coastal current connecting the major formation sites in Ross and Weddell Seas.