ALBERT

Description: Conglomeratic alluvial sediments (sand–gravel–cobbles) are common in fluvial, periglacial, and tectonically active regions but have received little attention with respect to unsaturated flow, specifically moisture–tension–conductivity relationships, due to difficulty in making measurements in the field or laboratory and lack of agricultural value. We used a field-scale infiltration experiment, a one-dimensional layered forward model, and parameter estimation modeling to examine in situ flow behavior between residual and partial saturation in a four-layer system under steady infiltration (0.84 cm h –1 ) for 19 h. Prior information from ground-penetrating radar, grain-size distributions from core samples, and long-term tension () and moisture () monitoring were used to define geologic structure, simulate test behavior, and provide initial parameter estimates. Vertically distributed measurements of ( t ) and ( t ) from the experiment were matched using four parameters ( s , α, n , and K s ) of the van Genuchten–Mualem (VGM) relationships for each material layer and a Metropolis–Hastings (MH) search with multiple, independent-chain runs to 10 6 samples each. Scale reduction factors indicated convergence of independent chains for 11 of 16 parameters. Final distributions of individual parameters varied from normal to nearly uniform distributions, and some parameter pairs showed high cross-correlation ( R 2 〉 0.9). Results showed that (i) VGM relationships can be applied to these coarse, conglomeratic soils to characterize unsaturated flow behavior across the natural range of partial saturation, (ii) even under high sustained infiltration rates, these coarse conglomeratic soils remain well drained, despite relatively low porosity and significant cobble fraction, and (iii) high uncertainty and nonconvergence of MH chains does not lead to significant misfit of the observed data. These findings imply that a significant cobble fraction does not markedly reduce infiltration at low saturation levels that develop under natural recharge conditions.

Description: There is currently no doubt about the existence of a widespread hydrological network under the Antarctic Ice Sheet, which lubricates the ice base and thus leads to increased ice velocities. Consequently, ice models should incorporate basal hydrology to obtain meaningful results for future ice dynamics and their contribution to global sea level rise. Here, we introduce the balanced water layer concept, covering two prominent subglacial hydrological features for ice sheet modeling on a continental scale: the evolution of subglacial lakes and balance water fluxes. We couple it to the thermomechanical ice-flow model RIMBAY and apply it to a synthetic model domain. In our experiments we demonstrate the dynamic generation of subglacial lakes and their impact on the velocity field of the overlaying ice sheet, resulting in a negative ice mass balance. Furthermore, we introduce an elementary parametrization of the water flux–basal sliding coupling and reveal the predominance of the ice loss through the resulting ice streams against the stabilizing influence of less hydrologically active areas. We point out that established balance flux schemes quantify these effects only partially as their ability to store subglacial water is lacking.

Description: Subglacial lakes in Antarctica influence to a large extent the flow of the ice sheet. In this study we use an idealised lake geometry to study this impact. We employ a) an improved three-dimensional full-Stokes ice flow model with a nonlinear rheology, b) a three-dimensional fluid dynamics model with eddy diffusion to simulate the basal mass balance at the lake-ice interface, and c) a newly developed coupler to exchange boundary conditions between the two individual models. Different boundary conditions are applied over grounded ice and floating ice. This results in significantly increased temperatures within the ice on top of the lake, compared to ice at the same depth outside the lake area. Basal melting of the ice sheet increases this lateral temperature gradient. Upstream the ice flow converges towards the lake and accelerates by about 10% whenever basal melting at the ice-lake boundary is present. Above and downstream of the lake, where the ice flow diverges, a velocity decrease of about 10% is simulated.

Description: There is currently no doubt about the existence of a wide-spread hydrological network under the Antarctic ice sheet, which lubricates the ice base and thus leads to increased ice velocities. Consequently, ice models should incorporate basal hydrology to obtain meaningful results for future ice dynamics and their contribution to global sea level rise. Here, we introduce the balanced water layer concept, covering two prominent subglacial hydrological features for ice sheet modeling on a continental scale: the evolution of subglacial lakes and balance water fluxes. We couple it to the thermomechanical ice-flow model RIMBAY and apply it to a synthetic model domain inspired by the Gamburtsev Mountains, Antarctica. In our experiments we demonstrate the dynamic generation of subglacial lakes and their impact on the velocity field of the overlaying ice sheet, resulting in a negative ice mass balance. Furthermore, we introduce an elementary parametrization of the water flux–basal sliding coupling and reveal the predominance of the ice loss through the resulting ice streams against the stabilizing influence of less hydrologically active areas. We point out, that established balance flux schemes quantify these effects only partially as their ability to store subglacial water is lacking.

Description: The mass export of the West Antarctic Ice Sheet (WAIS) is dominated by fast flowing ice streams. Understanding their dynamics is a key to estimate the future integrity of the WAIS and its contributions to global sea level rise. This study focuses on the Ross Ice Streams (RIS) at the Siple Coast. In this sector, observations reveal a high variability of ice stream pathways and velocities which is assumed to be driven by subglacial hydrology. We compute subglacial water pathways for the present-day ice sheet and verify this assumption by finding high correlations between areas of enhanced basal water flow and the locations of the RIS. Moreover, we reveal that the ice flow velocities of the individual ice streams are correlated with the sizes of the water catchment areas draining underneath. The future development of the subglacial hydraulic environment is estimated by applying ice surface elevation change rates observed by ICESat and CryoSat-2 to the present-day ice sheet geometry and thus assessing prognostic basal pressure conditions. Our simulations consistently indicate that a major hydraulic tributary of the Kamb and Whillans Ice Stream (KIS and WIS) will be redirected underneath the Bindschadler Ice Stream (BIS) within the next two centuries. The water catchment area feeding underneath the BIS is estimated to grow by about 50 % while the lower part of the stagnated KIS becomes increasingly separated from its upper hydraulic tributaries. We conclude, that this might be a continuation of the subglacial hydraulic processes which caused the past stagnation of the KIS. The simulated hydraulic rerouting is also capable to explain the observed deceleration of the WIS and indicates a possible future acceleration of the BIS accompanied by an increased ice drainage of the corresponding ice sheet interior.

Description: Subglacial lakes in Antarctica influence to a large extent the flow of the ice sheet. In this study we use an idealised lake geometry to study this impact. We employ a) an improved three-dimensional full Stokes ice flow model with a nonlinear rheology, b) a three-dimensional fluid dynamics model with eddy diffusion to simulate basal mass balance, and c) a newly developed coupler to exchange boundary conditions between individual models. Different boundary conditions are applied over grounded ice and floating ice. This results in significantly increased temperatures within the ice on top of the lake, compared to ice at the same depth outside the lake area. Basal melting of the ice sheet increases this lateral temperature gradient. Upstream the ice flow converges towards the lake and accelerates by about 10% whenever basal melting at the ice–lake boundary is present. Above and downstream of the lake, where the ice flow diverges, a velocity decrease of about 10% is simulated.

Description: We present results from new geophysical data allowing modelling of the water flow within Subglacial Lake Ellsworth (SLE), West Antarctica. Our simulations indicate that this lake has a novel temperature distribution due to significantly thinner ice than other surveyed subglacial lakes. The critical pressure boundary (tipping depth), established from the semi-empirical Equation of State, defines whether the lake's flow regime is convective or stratified. It passes through SLE and separates different temperature (and flow) regimes on either side of the lake. Our results have implications for the location of proposed access holes into SLE, the choice of which will depend on scientific or operational priorities. If an understanding of subglacial lake water properties and dynamics is the priority, holes are required in a basal freezing area at the North end of the lake. This would be the preferred priority suggested by this paper, requiring temperature and salinity profiles in the water column. A location near the Southern end, where bottom currents are lowest, is optimum for detecting the record of life in the bed sediments; to minimise operational risk and maximise the time span of a bed sediment core, a location close to the middle of the lake, where the basal interface is melting and the lake bed is at its deepest, remains the best choice. Considering potential lake-water salinity and ice-density variations, we estimate the critical tipping depth, separating different temperature regimes within subglacial lakes, to be in about 2900 to 3045 m depth.

Description: We present results from new geophysical data allowing 3-D modelling of the water flow within Subglacial Lake Ellsworth (SLE), West Antarctica. Our simulations indicate that this lake has a novel temperature distribution due to significantly thinner ice than other surveyed subglacial lakes. The critical pressure boundary (tipping depth), established from the semi-empirical Equation of State, defines whether the lake's flow regime is convective or stratified. It passes through SLE and separates different temperature (and flow) regimes on either side of the lake. Our results have implications for the location of proposed access holes into SLE, the choice of which will depend on scientific or operational priorities. If an understanding of subglacial lake water properties and dynamics is the priority, holes are required in a basal freezing area at the North end of the lake. This would be the preferred priority suggested by this paper, requiring temperature and salinity profiles in the water column. A location near the Southern end, where bottom currents are lowest, is optimum for detecting the record of life in the bed sediments; to minimise operational risk and maximise the time span of a bed sediment core, a location close to the middle of the lake, where the basal interface is melting and the lake bed is at its deepest, remains the best choice. Considering potential lake-water salinity and ice-density variations, we estimate the critical tipping depth, separating different temperature regimes within subglacial lakes, to be in about 2900 to 3045 m depth.

Description: Large-scale fully coupled Earth system models (ESMs) are usually applied in climate projections like the IPCC (Intergovernmental Panel on Climate Change) reports. In these models internal variability is often within the correct order of magnitude compared with the observed climate, but due to internal variability and arbitrary initial conditions they are not able to reproduce the observed timing of climate events or shifts as for instance observed in the El Niño–Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), or the Atlantic Meridional Overturning Circulation (AMOC). Additional information about the real climate history is necessary to constrain ESMs; not only to emulate the past climate, but also to introduce a potential forecast skill into these models through a proper initialisation. We attempt to do this by extending the fully coupled climate model Max Planck Institute Earth System Model (MPI-ESM) using a partial coupling technique (Modini-MPI-ESM). This method is implemented by adding reanalysis wind-field anomalies to the MPI-ESM's inherent climatological wind field when computing the surface wind stress that is used to drive the ocean and sea ice model. Using anomalies instead of the full wind field reduces potential model drifts, because of different mean climate states of the unconstrained MPI-ESM and the partially coupled Modini-MPI-ESM, that could arise if total observed wind stress was used. We apply two different reanalysis wind products (National Centers for Environmental Prediction, Climate Forecast System Reanalysis (NCEPcsfr) and ERA-Interim reanalysis (ERAI)) and analyse the skill of Modini-MPI-ESM with respect to several observed oceanic, atmospheric, and sea ice indices. We demonstrate that Modini-MPI-ESM has a significant skill over the time period 1980–2013 in reproducing historical climate fluctuations, indicating the potential of the method for initialising seasonal to decadal forecasts. Additionally, our comparison of the results achieved with the two reanalysis wind products NCEPcsfr and ERAI indicates that in general applying NCEPcsfr results in a better reconstruction of climate variability since 1980.