Reconstructing with numerical Ice Sheet Models the post-LGM decay of the Eurasian Ice Sheets: data-model comparison and focus on the Storfjorden (Svalbard) ice stream dynamics history

The challenge of reconstructing the growth and decay of palaeo-ice sheets represents a critical task to better understand mechanisms of global climate change and associated sea-level fluctuations in the present and future. The Eurasian palaeo-Ice Sheet (EIS) at its maximum volume and extent (between 21,000 and 20,000 years ago) comprised the Scandinavian (SIS), Svalbard/Barents/Kara seas (SBKSIS), and British-Irish (BIIS) palaeo-ice sheets. The SBKSIS was a so called marine based ice sheet, as it rested several hundreds of meters below the sea level in the Barents and Kara seas. From a bathymetric and topographic point of view, there is a strong similarity between the SBKSIS and the present-day West Antarctic Ice Sheet (WAIS). Consequently, to reconstruct the dynamic processes driving the last deglaciation of the SBKSIS may represent an important task to better understand the WAIS present and future evolution. In this study, the evolution of the EIS during the last deglaciation is reconstructed with two hybrid Shallow-Ice/Shallow-Shelf-Approximation numerical Ice Sheet Models (ISMs), namely PSU and GRISLI. These two ISMs differ mainly in the ice stream parametrization and in the complexity with which grounding line migration is treated. A particular focus in this study is given to the Storfjorden glacial system in the north-western Barents Sea. In fact, several palaeo-data from this area furnish insights on the Storfjorden ice stream dynamics history, providing a good testing ground for ISMs. The ISMs are forced with macro-regional indexes representative of the climate evolution over Siberia/Kara Sea, Svalbard/Barents Sea and Fennoscandia during the last deglaciation. The climate in- dexes are based on TraCE-21ka, a transient climate simulation of the last 21,000 years carried out with the Atmosphere-Ocean General Circulation Model CCSM3. Two different ocean basal melting parametrizations based on ice-ocean heat fluxes are tested. The ocean basal melting parametrizations are forced with time-varying ocean temperature and salinity from TraCE-21ka transient climate simulation. Ocean temperature and salinity representative of Arctic Ocean, Norwegian Sea, north-western and south- western Barents Sea are employed. In order to deal with ISMs poorly constrained model parameters, a statistical approach is adopted. A Latin Hypercube Sampling (LHS) of five GRISLI parameters is performed. Due to large computational costs it is not possible to perform such a statistical approach with PSU ISM. Therefore, a restricted number of simulations performed with PSU ISM employ model parameter values from GRISLI significant simulations. GRISLI-simulated ice streams dynamics has a strong control on the deglaciation of the EIS, in particular in the Southern SIS and in the Western SBKSIS. In terms of total ice volume there is a good agreement between GRISLI simulations, ICE-5G reconstruction and global Eustatic Sea Level data. In contrast, GRISLI-simulated ice sheet extent evolution presents discrepancies with geological observation in the Southern SIS and in the Eastern SBKSIS. The use of different basal melting parametrizations in GRISLI has a strong impact both on the deglaciation of the SBKSIS and on the retreat of Storfjorden ice stream. The Storfjorden ice stream dynamics history in GRISLI simulations is in good agreement with palaeo-data in terms of timing of the ice stream retreat, Grounding Zone Wedges formation and response to Meltwater Pulse 1A. The different treatment in PSU of ice streams and Grounding Line migration has a remarkable effect on the EIS deglaciation. In particular, in PSU the Grounding Line dynamics plays a primary role with respect to ocean basal melting, thus exerting a strong control on the decay of the marine-based SBKSIS. Finally, an inverse-type approach is adopted to match PSU simulations with ICE-5G reconstruction and the other palaeo-data

The dynamics of dense water cascades: from laboratory scales to the Arctic Ocean

The sinking of dense shelf waters down the continental slope (or “cascading”) contributes to oceanic water mass formation and carbon cycling. Cascading is therefore of significant importance for the global overturning circulation and thus climate. The occurrence of cascades is highly intermittent in space and time and observations of the process itself (rather than its outcomes) are scarce. Global climate models do not typically resolve cascading owing to numerical challenges concerning turbulence, mixing and faithful representation of bottom boundary layer dynamics. This work was motivated by the need to improve the representation of cascading in numerical ocean circulation models. Typical 3-D hydrostatic ocean circulation models are employed in a series of numerical experiments to investigate the process of dense water cascading in both idealised and realistic model setups. Cascading on steep bottom topography is modelled using POLCOMS, a 3-D ocean circulation model using a terrain-following s-coordinate system. The model setup is based on a laboratory experiment of a continuous dense water flow from a central source on a conical slope in a rotating tank. The descent of the dense flow as characterised by the length of the plume as a function of time is studied for a range of parameters, such as density difference, speed of rotation, flow rate and (in the model) diffusivity and viscosity. Very good agreement between the model and the laboratory results is shown in dimensional and non-dimensional variables. It is confirmed that a hydrostatic model is capable of reproducing the essential physics of cascading on a very steep slope if the model correctly resolves velocity veering in the bottom boundary layer. Experiments changing the height of the bottom Ekman layer (by changing viscosity) and modifying the plume from a 2-layer system to a stratified regime (by enhancing diapycnal diffusion) confirm previous theories, demonstrate their limitations and offer new insights into the dynamics of cascading outside of the controlled laboratory conditions. In further numerical experiments, the idealised geometry of the conical slope is retained but up-scaled to oceanic dimensions. The NEMO-SHELF model is used to study the fate of a dense water plume of similar properties to the overflow of brine-enriched shelf waters from the Storfjorden in Svalbard. The overflow plume, resulting from sea ice formation in the Storfjorden polynya, cascades into the ambient stratification resembling the predominant water masses of Fram Strait. At intermediate depths between 200-500m the plume encounters a layer of warm, saline AtlanticWater. In some years the plume ‘pierces’ the Atlantic Layer and sinks into the deep Fram Strait while in other years it remains ‘arrested’ at Atlantic Layer depths. It has been unclear what parameters control whether the plume pierces the Atlantic Layer or not. In a series of experiments we vary the salinity ‘S’ and the flow rate ‘Q’ of the simulated Storfjorden overflow to investigate both strong and weak cascading conditions. Results show that the cascading regime (piercing, arrested or ‘shaving’ - an intermediate case) can be predicted from the initial values of S and Q. In those model experiments where the initial density of the overflow water is considerably greater than of the deepest ambient water mass we find that a cascade with high initial S does not necessarily reach the bottom if Q is low. Conversely, cascades with an initial density just slightly higher than the deepest ambient layer may flow to the bottom if the flow rate Q is high. A functional relationship between S/Q and the final depth level of plume waters is explained by the flux of potential energy (arising from the introduction of dense water at shallow depth) which, in our idealised setting, represents the only energy source for downslope descent and mixing. Lastly, the influence of tides on the propagation of a dense water plume is investigated using a regional NEMO-SHELF model with realistic bathymetry, atmospheric forcing, open boundary conditions and tides. The model has 3 km horizontal resolution and 50 vertical levels in the sh-coordinate system which is specially designed to resolve bottom boundary layer processes. Tidal effects are isolated by comparing results from model runs with and without tides. A hotspot of tidally-induced horizontal diffusion leading to the lateral dispersion of the plume is identified at the southernmost headland of Spitsbergen which is in close proximity to the plume path. As a result the lighter fractions in the diluted upper layer of the plume are drawn into the shallow coastal current that carries Storfjorden water onto the Western Svalbard Shelf, while the dense bottom layer continues to sink down the slope. This bifurcation of the plume into a diluted shelf branch and a dense downslope branch is enhanced by tidally-induced shear dispersion at the headland. Tidal effects at the headland are shown to cause a net reduction in the downslope flux of Storfjorden water into deep Fram Strait. This finding contrasts previous results from observations of a dense plume on a different shelf without abrupt topography. The dispersive mechanism which is induced by the tides is identified as a mechanism by which tides may cause a relative reduction in downslope transport, thus adding to existing understanding of tidal effects on dense water overflows.NER

GNSS transpolar earth reflectometry exploriNg system (G-TERN): mission concept

The global navigation satellite system (GNSS) Transpolar Earth Reflectometry exploriNg system (G-TERN) was proposed in response to ESA's Earth Explorer 9 revised call by a team of 33 multi-disciplinary scientists. The primary objective of the mission is to quantify at high spatio-temporal resolution crucial characteristics, processes and interactions between sea ice, and other Earth system components in order to advance the understanding and prediction of climate change and its impacts on the environment and society. The objective is articulated through three key questions. 1) In a rapidly changing Arctic regime and under the resilient Antarctic sea ice trend, how will highly dynamic forcings and couplings between the various components of the ocean, atmosphere, and cryosphere modify or influence the processes governing the characteristics of the sea ice cover (ice production, growth, deformation, and melt)? 2) What are the impacts of extreme events and feedback mechanisms on sea ice evolution? 3) What are the effects of the cryosphere behaviors, either rapidly changing or resiliently stable, on the global oceanic and atmospheric circulation and mid-latitude extreme events? To contribute answering these questions, G-TERN will measure key parameters of the sea ice, the oceans, and the atmosphere with frequent and dense coverage over polar areas, becoming a “dynamic mapper”of the ice conditions, the ice production, and the loss in multiple time and space scales, and surrounding environment. Over polar areas, the G-TERN will measure sea ice surface elevation (<;10 cm precision), roughness, and polarimetry aspects at 30-km resolution and 3-days full coverage. G-TERN will implement the interferometric GNSS reflectometry concept, from a single satellite in near-polar orbit with capability for 12 simultaneous observations. Unlike currently orbiting GNSS reflectometry missions, the G-TERN uses the full GNSS available bandwidth to improve its ranging measurements. The lifetime would be 2025-2030 or optimally 2025-2035, covering key stages of the transition toward a nearly ice-free Arctic Ocean in summer. This paper describes the mission objectives, it reviews its measurement techniques, summarizes the suggested implementation, and finally, it estimates the expected performance.Peer ReviewedPostprint (published version