An iterative semi-implicit scheme with robust damping

An efficient, iterative semi-implicit (SI) numerical method for the time integration of stiff wave systems is presented. Physics-based assumptions are used to derive a convergent iterative formulation of the SI scheme which enables the monitoring and control of the error introduced by the SI operator. This iteration essentially turns a semi-implicit method into a fully implicit method. Accuracy, rather than stability, determines the timestep. The scheme is second-order accurate and shown to be equivalent to a simple preconditioning method. We show how the diffusion operators can be handled so as to yield the property of robust damping, i.e., dissipating the solution at all values of the parameter \mathcal D\dt, where $\mathcal D$ is a diffusion operator and \dt the timestep. The overall scheme remains second-order accurate even if the advection and diffusion operators do not commute. In the limit of no physical dissipation, and for a linear test wave problem, the method is shown to be symplectic. The method is tested on the problem of Kinetic Alfv\'en wave mediated magnetic reconnection. A Fourier (pseudo-spectral) representation is used. A 2-field gyrofluid model is used and an efficacious k-space SI operator for this problem is demonstrated. CPU speed-up factors over a CFL-limited explicit algorithm ranging from $\sim20$ to several hundreds are obtained, while accurately capturing the results of an explicit integration. Possible extension of these results to a real-space (grid) discretization is discussed.Comment: Submitted to the Journal of Computational Physics. Clarifications and caveats in response to referees, numerical demonstration of convergence rate, generalized symplectic proo

A community-based geological reconstruction of Antarctic Ice Sheet deglaciation since the Last Glacial Maximum

A robust understanding of Antarctic Ice Sheet deglacial history since the Last Glacial Maximum is important in order to constrain ice sheet and glacial-isostatic adjustment models, and to explore the forcing mechanisms responsible for ice sheet retreat. Such understanding can be derived from a broad range of geological and glaciological datasets and recent decades have seen an upsurge in such data gathering around the continent and Sub-Antarctic islands. Here, we report a new synthesis of those datasets, based on an accompanying series of reviews of the geological data, organised by sector. We present a series of timeslice maps for 20 ka, 15 ka, 10 ka and 5 ka, including grounding line position and ice sheet thickness changes, along with a clear assessment of levels of confidence. The reconstruction shows that the Antarctic Ice sheet did not everywhere reach the continental shelf edge at its maximum, that initial retreat was asynchronous, and that the spatial pattern of deglaciation was highly variable, particularly on the inner shelf. The deglacial reconstruction is consistent with a moderate overall excess ice volume and with a relatively small Antarctic contribution to meltwater pulse 1a. We discuss key areas of uncertainty both around the continent and by time interval, and we highlight potential priorities for future work. The synthesis is intended to be a resource for the modelling and glacial geological community

Ice Sheet Thinning Over 54.2 ka, Patagonia, South America

No abstract available

Empirical Ice Thickness Constraints from the Cerro Fitz Roy Massif, Southern Patagonia, Argentina

Southern Patagonia is the only landmass intersecting the southern westerly winds (SWW), which critically shape southern hemispheric and global climate. At present palaeo-climatic datasets stretching the 3000 km of SWW influence are in latitudinal disagreement suggesting a complex picture of SWW paleo-behaviour. In order to unravel the nature of the SWW fluctuations over the last glacial cycle, a tighter grid of palaeo-proxy studies in the region is required. The work presented here is based on three mountain valleys in close proximity to the Fitz Roy massive (49°S, 72°W), which is located just east of the southern Patagonian Ice Field. At this site the Patagonian cordillera provides a topographic barrier to the SWW, hence the dominant limiting factor for glacier expansion on the lee side is precipitation. Palaeo-ice surface, thickness and thinning rates spanning MIS3 to Termination I are presented based on 18 Be-10 exposure ages along three vertical transects over ~700m of relief. The results indicate a maximum ice thickness over MIS3 with progressive ice-lowering over MIS2 and small scale thickening at the end of the globally defined LGM. The palaeo-climatic implications will be discussed particularly with focus on SWW positioning and behaviour

MIS3 To Late Holocene Palaeo-Ice Thickness Constraints of Glacier Viedma, Hielo Patagonico Sur, Southern South America

Southern South America is the only landmass intersecting the southern westerly winds (SWW), which critically shape southern hemispheric and global climate. At present palaeo-climatic datasets stretching the 3000 km of SWW influence are in latitudinal disagreement, suggesting a complex picture of SWW palaeo-behavior. In order to unravel the nature of the SWW over the last glacial cycle, a tighter grid of sensitive palaeo-proxies in the region is required. The work presented here is based on the palaeo-glaciology of the Viedma outlet (49°S, 72°W), the second largest glacier draining the Hielo Patagonico Sur (HPS). This locality corresponds to the present day center of the precipitation bearing SWW in southern South America, which are a key factor in maintaining the HPS. Any variation in SWW strength or location has a direct impact on glacial expansion in the region, hence palaeo-glacial geometries can provide key insights into SWW dynamics. Palaeo-ice surface, thickness and thinning rates spanning MIS3 to the late Holocene are presented based on 16 Be-10 exposure ages along three vertical transects covering the accumulation and ablation areas of glacier Viedma. Results indicate a maximum ice thickness over MIS3, a thickening over the Antarctic Cold Reversal (ACR), which overprinted the vertical expression of the LGM, and progressive ice surface lowering from the middle to the late Holocene. The palaeo-climatic implications will be discussed, particularly with focus on SWW positioning and behavior

The geomorphology and sedimentology of the 'Tempanos' moraine at Laguna San Refael, Chile

The San Rafael Glacier is one of the largest and most dynamic outlet glaciers of the North Patagonian Icefield, Chile. The contemporary glacier calves into a large tidal laguna, which is partially impounded by a large arcuate moraine. This moraine, termed the Témpanos moraine, marks the former extent of an expanded San Rafael Glacier and is of an unknown age. Here we describe the geomorphology and sedimentology of the Témpanos moraine and relate these to styles of glacier advance and recession. The Témpanos moraine attains a maximum height in excess of 40 m and is either single-crested with a gentle ice-proximal face and steep distal face, or consists of multiple crests superimposed on a gentle slope. The internal composition of the moraine is variable, comprising diamicton, sorted sedimentary facies (silts, sands and gravel) and laminites. We interpret these sediments as evidence that the San Rafael Glacier advanced over a former proglacial area, culminating in the formation of the Témpanos moraine. This advance deformed a carpet of lacustrine or marine mud, which was eroded from its original location, transported and smeared over the glacier bed as a subglacial deposit. We use these sedimentary descriptions as the basis for a reconstruction of the Holocene fluctuations of the glacier