Necessary conditions for warm inflow towards the Filchner Ice Shelf, Weddell Sea

Understanding changes in Antarctic ice shelf basal melting is a major challenge for predicting future sea level. Currently, warm Circumpolar Deep Water surrounding Antarctica has limited access to the Weddell Sea continental shelf; consequently, melt‐rates at Filchner‐Ronne Ice Shelf are low. However, large‐scale model projections suggest that changes to the Antarctic Slope Front and the coastal circulation may enhance warm inflows within this century. We use a regional high‐resolution ice shelf cavity and ocean circulation model to explore forcing changes that may trigger this regime shift. Our results suggest two necessary conditions for supporting a sustained warm inflow into the Filchner Ice Shelf cavity; (i) an extreme relaxation of the Antarctic Slope Front density gradient, and (ii) substantial freshening of the dense shelf water. We also find that the on‐shelf transport over the western Weddell Sea shelf is sensitive to the Filchner Trough overflow characteristics

Projecting Antarctic ice discharge using response functions from SeaRISE ice-sheet models

The largest uncertainty in projections of future sea-level change results from the potentially changing dynamical ice discharge from Antarctica. Basal ice-shelf melting induced by a warming ocean has been identified as a major cause for additional ice flow across the grounding line. Here we attempt to estimate the uncertainty range of future ice discharge from Antarctica by combining uncertainty in the climatic forcing, the oceanic response and the ice-sheet model response. The uncertainty in the global mean temperature increase is obtained from historically constrained emulations with the MAGICC-6.0 (Model for the Assessment of Greenhouse gas Induced Climate Change) model. The oceanic forcing is derived from scaling of the subsurface with the atmospheric warming from 19 comprehensive climate models of the Coupled Model Intercomparison Project (CMIP-5) and two ocean models from the EU-project Ice2Sea. The dynamic ice-sheet response is derived from linear response functions for basal ice-shelf melting for four different Antarctic drainage regions using experiments from the Sea-level Response to Ice Sheet Evolution (SeaRISE) intercomparison project with five different Antarctic ice-sheet models. The resulting uncertainty range for the historic Antarctic contribution to global sea-level rise from 1992 to 2011 agrees with the observed contribution for this period if we use the three ice-sheet models with an explicit representation of ice-shelf dynamics and account for the time-delayed warming of the oceanic subsurface compared to the surface air temperature. The median of the additional ice loss for the 21st century is computed to 0.07 m (66% range: 0.02–0.14 m; 90% range: 0.0–0.23 m) of global sea-level equivalent for the low-emission RCP-2.6 (Representative Concentration Pathway) scenario and 0.09 m (66% range: 0.04–0.21 m; 90% range: 0.01–0.37 m) for the strongest RCP-8.5. Assuming no time delay between the atmospheric warming and the oceanic subsurface, these values increase to 0.09 m (66% range: 0.04–0.17 m; 90% range: 0.02–0.25 m) for RCP-2.6 and 0.15 m (66% range: 0.07–0.28 m; 90% range: 0.04–0.43 m) for RCP-8.5. All probability distributions are highly skewed towards high values. The applied ice-sheet models are coarse resolution with limitations in the representation of grounding-line motion. Within the constraints of the applied methods, the uncertainty induced from different ice-sheet models is smaller than that induced by the external forcing to the ice sheets

A multi-disciplinary perspective on climate model evaluation for Antarctica

A workshop was organized by Antarctic Climate 21 (AntClim21), with the topic 'evaluation of climate models' representation of Antarctic climate from the perspective of long-term twenty-first-century climate change.' The suggested approach for evaluating whether climate models over- or underestimate the effects of ozone depletion is to diagnose simulated historical trends in lower-stratospheric temperature and compare these to observational estimates. With regard to more regional changes over Antarctica, such as West Antarctic warming, the simulation of teleconnection patterns to the tropical Pacific was highlighted. To improve the evaluation of low-frequency variability and trends in climate models, the use and development of approaches to emulate ice-core proxies in models was recommended. It is recommended that effort be put into improving datasets of ice thickness, motion, and composition to allow for a more complete evaluation of sea ice in climate models. One process that was highlighted in particular is the representation of Antarctic clouds and resulting precipitation. It is recommended that increased effort be put into observations of clouds over Antarctica, such as the use of instruments that can detect cloud-base height or the use of remote sensing resources

Ice shelf basal melting in a global finite-element sea ice/ice shelf/ocean model

The Finite Element Sea ice Ocean Model (FESOM) has been augmented by an ice-shelf component with a three-equation system for diagnostic computation of boundary layer temperature and salinity. Ice shelf geometry and global ocean bathymetry have been derived from the RTopo-1 dataset. A global domain with a triangular mesh and a hybrid vertical coordinate is used. To evaluate sub-ice shelf circulation and melt rates for present-day climate, the model is forced with NCEP reanalysis data. Basal mass fluxes are mostly realistic with maximum melt rates in the deepest parts near the grounding lines and marine ice formation in the northern sectors of Ross Ice Shelf and Filchner-Ronne Ice Shelf. Total basal mass loss for the ten largest ice shelves reflects the importance of the Amundsen Sea ice shelves; Getz Ice Shelf is shown to be a major melt water contributor to the Southern Ocean. Despite their modest melt rates, the ``cold water' ice shelves in the Weddell Sea are still substantial sinks of continental ice in Antarctica. Discrepancies between the model and observations can partly be attributed to deficiencies in the forcing data or to (sometimes unavoidable) smoothing of ice shelf and bottom topographies

Precursors of Antarctic Bottom Water formed on the continental shelf off Larsen Ice Shelf

The dense water flowing out from the Weddell Sea significantly contributes to Antarctic Bottom Water (AABW) and plays an important role in the Meridional Overturning Circulation. The relative importance of the two major source regions, the continental shelves in front of Filchner-Ronne Ice Shelf and Larsen Ice Shelf, however, remains unclear. Several studies focused on the contribution of the Filchner-Ronne Ice Shelf region for the deep and bottom water production within the Weddell Gyre, but the role of the Larsen Ice Shelf region for this process, especially the formation of deep water, remains speculative. Measurements made during the Polarstern cruise ANT XXIX-3 (2013) add evidence to the importance of the source in the western Weddell Sea. Using Optimum Multiparameter analysis we show that the dense water found on the continental shelf in front of the former Larsen A and B together with a very dense water originating from Larsen C increases the thickness and changes the θ/S characteristics of the layer that leaves the Weddell Sea to contribute to AABW

The Filchner Trough / Filchner Ice Shelf cavity system

Since austral summer 2013/14 AWI maintains a mooring array on the eastern slope of Filchner Trough at 76°S to monitor any flow of warm waters of open ocean origin towards the Filchner Ice Shelf (FIS) cavity. During the austral summers 2015/16 and 2016/17, seven oceanographic moorings were deployed beneath FIS through hot-water-drilled access holes to investigate and monitor the processes controlling the supply of ocean heat to the ice shelf base. This data, transferred to AWI via satellite link, shows that two ‘regimes’ exist beneath FIS. Dense High Salinity Shelf Water (HSSW), formed in front of the Ronne Ice Shelf, dominates the southern cavity and exits as Ice Shelf Water (ISW) the cavity along the western flank of the Filchner Trough. Less dense HSSW with a local origin in front of FIS enters the cavity on the eastern side of the Filchner Trough during parts of the year but seems to be trapped at depth, interacting laterally with derivatives of the Ronne-sourced HSSW. No evidence exists that it penetrates to the deep southern FIS grounding line. At 76°S, the flow of warm waters towards FIS is seasonal, limited to late summer/early winter, being replaced by ISW for the rest of the year. The link of the two sub-ice shelf circulation regimes to different regions of dense water formation on the continental shelf, and its sensitivity to the inflow of warm waters need to be investigated further to reduce the uncertainty of estimates on the FIS mass balance for today and the future

The Southern Ocean A survey of oceanographic and marine meteorological research work

SIGLETIB: RN 9219 (26) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman