The aim of this research was to demonstrate the relationship between variations in summertime surface mass balance of the McMurdo Ice Shelf and atmospheric processes. The approach encompassed a broad range of techniques. An existing energy balance mass balance model was adapted to deal with debris-covered ice surfaces and modified to produce distributed output. Point based surface energy and mass balance for two key surfaces of the ice shelf were linked to different synoptic types that were identified using a manual synoptic classification. The distributed model was initialised with distributed parameters derived from satellite remote sensing and forced with data from a regional climate model. Patterns of summertime surface mass balance produced by the distributed model were assessed against stake measurements and with respect to atmospheric processes. During the summers of 2003-2004 and 2004-2005 an automatic weather station (AWS) was operated on bare and debris-covered ice surfaces of the McMurdo Ice shelf, Antarctica. Surface mass balance was calculated using the energy balance model driven by the data from the AWS and additional data from permanent climate stations. Net mass balance for the measurement period was reproduced reasonably well when validated against directly measured turbulent fluxes, stake measurements, and continuously measured surface height at the AWS. For the bare ice surface net radiation provided the major energy input for ablation, whereas sensible heat flux was a second heat source. Ablation was by both melt (70%) and sublimation (30%). At the debris-covered ice site investigated, it is inferred that the debris cover is sufficient to insulate the underlying ice from ablation. Synoptic weather situations were analysed based on AVHRR composite images and surface pressure charts. Three distinct synoptic situations were found to occur during the summers, these were defined as Type A, low pressure system residing in the Ross Sea Embayment; Type B, anticyclonic conditions across region; and Type C, a trough of low pressure extending into the Ross Sea Embayment. A dependence of surface energy fluxes and mass balance on synoptic situation was identified for the bare ice surface. The distributed model was found to produce spatial patterns of mass balance which compared well with stake measurements. Mass balance patterns show that the McMurdo Ice Shelf was generally ablating in the west, and accumulating in the east during summer. Areas of enhanced ablation were found which were likely to be caused by the surface conditions and topographic effects on the wind field. The mean summertime surface mass balance across the entire ice shelf for the 2003-2004 and 2004-2005 summers were –2.5 mm w.e. and –6.7 mm w.e. respectively. The differences between the two summers are inferred to be a result of more frequent type A conditions occurring during the summer of 2004-2005.

The aim of this research was to demonstrate the relationship between variations in summertime surface mass balance of the McMurdo Ice Shelf and atmospheric processes. The approach encompassed a broad range of techniques. An existing energy balance mass balance model was adapted to deal with debris-covered ice surfaces and modified to produce distributed output. Point based surface energy and mass balance for two key surfaces of the ice shelf were linked to different synoptic types that were identified using a manual synoptic classification. The distributed model was initialised with distributed parameters derived from satellite remote sensing and forced with data from a regional climate model. Patterns of summertime surface mass balance produced by the distributed model were assessed against stake measurements and with respect to atmospheric processes. During the summers of 2003-2004 and 2004-2005 an automatic weather station (AWS) was operated on bare and debris-covered ice surfaces of the McMurdo Ice shelf, Antarctica. Surface mass balance was calculated using the energy balance model driven by the data from the AWS and additional data from permanent climate stations. Net mass balance for the measurement period was reproduced reasonably well when validated against directly measured turbulent fluxes, stake measurements, and continuously measured surface height at the AWS. For the bare ice surface net radiation provided the major energy input for ablation, whereas sensible heat flux was a second heat source. Ablation was by both melt (70%) and sublimation (30%). At the debris-covered ice site investigated, it is inferred that the debris cover is sufficient to insulate the underlying ice from ablation. Synoptic weather situations were analysed based on AVHRR composite images and surface pressure charts. Three distinct synoptic situations were found to occur during the summers, these were defined as Type A, low pressure system residing in the Ross Sea Embayment; Type B, anticyclonic conditions across region; and Type C, a trough of low pressure extending into the Ross Sea Embayment. A dependence of surface energy fluxes and mass balance on synoptic situation was identified for the bare ice surface. The distributed model was found to produce spatial patterns of mass balance which compared well with stake measurements. Mass balance patterns show that the McMurdo Ice Shelf was generally ablating in the west, and accumulating in the east during summer. Areas of enhanced ablation were found which were likely to be caused by the surface conditions and topographic effects on the wind field. The mean summertime surface mass balance across the entire ice shelf for the 2003-2004 and 2004-2005 summers were –2.5 mm w.e. and –6.7 mm w.e. respectively. The differences between the two summers are inferred to be a result of more frequent type A conditions occurring during the summer of 2004-2005.

Western Antarctica has been experiencing significant warming for at least the past fifty years. While higher Net Surface Mass Balance (SMB) over West Antarctica during this period of warming is expected, SMB reconstructions from ice cores reveal a more complex pattern during the period of warming. The mechanisms giving rise to SMB variability over the West Antarctic Ice Sheet (WAIS) are not well understood due to lack of instrumental data. The Southern Annular Mode (SAM) and El Niño Southern Oscillation (ENSO) are believed to contribute to WAIS SMB variability but the assumption has not been rigorously tested. SMB during years where SAM and ENSO are in extreme phases is compared to average SMB from the period 1979-2010. Additionally, atmospheric circulation anomalies are used to assess what circulation patterns accompany extreme modes of climate during the same period. The results suggest that significantly lower SMB occurs when SAM is in an extremely positive phase or ENSO is in an extremely negative phase. Additionally, atmospheric circulation anomalies show that certain circulation patterns accompany extreme modes of climate, which contribute to SMB variability over the WAIS. Ultimately, the location of low and high pressure cells is the best predictor for extreme accumulation events over the WAIS. These results are verified by assessing observed net SMB trends from a network of firn cores located from the central WAIS. Seven new firn cores are added to improve the spatial network of regional net SMB measurements. Reconstructed net SMB is calculated from new firn core records, and compared to the existing cores. The new suite of preliminary firn core records show the same significant decreasing trend that is observed in existing cores. This represents a negative region-wide SMB trend that is likely in part due to trends in SAM and ENSO.

This thesis focuses on the glaciation of the Late Saalian period (160 -140 ka) over Eurasia. The Quaternary Environment of the Eurasian North (QUEEN) project determined that during this period, the Eurasian ice sheet was substantially larger than during the entire Weichselian cycle and especially that of the Last Glacial Maximum (21 ka, LGM). The Late Saalian astronomical forcing was different than during the LGM while greenhouse gas concentrations were similar. To understand how this ice sheet could have grown so large over Eurasia during the Late Saalian, we use an Atmospherical General Circulation Model (AGCM) coupled to an oceanic mixed layer and a vegetation model to explore the influence of regional parameters, sea surface temperatures (SST) and orbital parameters on the surface mass balance (SMB) of the Late Saalian Eurasian ice sheet. At140 ka, proglacial lakes, vegetation and simulated Late Saalian SST cool the Eurasian climate, which reduce the ablation along the southern ice sheet margins. Dust deposition on snow has the opposite effect. The presence of a Canada Basin ice-shelf during MIS6 in the Arctic Ocean, does not affect the mass balance of the ice sheet. According to geological evidence, the Late Saalian Eurasian ice sheet reached its maximum extent before 160 ka. Northern Hemisphere high latitude summer insolation shows a large insolation peak near 150 ka. The simulated climate prior to 140 ka is milder and ablation is larger along the southern margins of the Eurasian ice sheet although the mean annual SMB is positive. The Late Saalian Eurasian ice sheet may have been large enough to generate its own cooling, thus maintaining itself over Eurasia. / Joint PhD Degree between Stockholm University and Université Joseph FourierAt the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Submitted. Paper 4: Submitted. Paper 5: Manuscript.

Changes to ice sheet surface mass balance (SMB) are going to play a significant role in future sea level rise (SLR), particularly for the Greenland ice sheet. The Coupled Model Intercomparison Project Phase 5 (CMIP5) found that Greenland ice sheet (GIS) response to changes in SMB is expected to contribute 9 ± 4 cm to sea level by 2100 (Fettweis et al 2013), though other estimates suggest the possibility of an even larger response. Modern ice sheet geometry and surface velocities are common metrics for determining a model’s predictability of future climate. However, care must be taken to robustly quantify prediction uncertainty because errors in boundary conditions such as SMB can be compensated by (and therefore practically inseparable from) errors in other aspects of the model, complicating calculations of total uncertainty. We find that SMB calculated using the Community Earth System Model (CESM) differs from established standards due to errors in the CESM SMB boundary condition. During the long ice sheet initialization process, small SMB errors such as these have an opportunity to amplify into larger uncertainties in GIS sensitivity to climate change. These uncertainties manifest themselves in ice sheet surface geometry changes, ice mass loss, and subsequent SLR. While any bias in SMB is not desirable, it is not yet clear how sensitive SLR projections are to boundary condition forcing errors. We explore several levels of SMB forcing bias in order to analyze their influence on future SLR. We evaluate ensembles of ice sheets forced by 4 different levels of SMB forcing error, covering a range of errors similar to SMB biases between CESM and RACMO SMB. We find that GIS SMB biases on the order of 1 m/yr result in 7.8 ± 3.4 cm SLR between 1850 and 2100, corresponding to 100% uncertainty at the 2σ level. However, we find unexpected feedbacks between SMB and surface geometry in the northern GIS. We propose that the use of elevation classes may be incorrectly altering the feedback mechanisms in that part of the ice sheet. / text

Antarctic mass balance is mainly controlled by surface mass balance (SMB, i.e. the net effect of precipitations at the surface of the ice sheet) and ice discharge at its margins, mostly through ice shelves. These floating ice bodies made from ice flowing from the continent to the ocean are buttressed by ice rises (elevation of the sea floor on which ice shelf re-grounds) such as the Derwael Ice Rise (DIR) in Dronning Maud Land (DML). In addition to this role important to consider in the future contribution of Antarctica to sea level rise, ice rises are also “climate dipsticks” helping to reconstruct the climate of the past centuries to millennia at high resolution. Due to their coastal location, they witness the changes happening there more rapidly than inland. Furthermore, their internal stratigraphy forms arches that allow to assess their stability, to date their own formation and therefore, in some cases, to constrain the past extension of the ice sheet at the scale of several millennia. As part of the IceCon project :Constraining ice mass changes in Antarctica, this thesis aimed to drill a 120 m ice core (named IC12 for the IceCon project, 2012) at the divide of the DIR and perform physico-chemical analyses to study its density and its internal annual layering with the aim of reconstructing SMB of the last two centuries. We also recorded a virtual image of the borehole using an optical televiewer (OPTV) to assess the ability of this instrument to reconstruct a density profile and measure vertical strain rates when the logging is repeated in the same borehole after a sufficient period of time (here, 2 years).The results show a general increase in snow accumulation rates (SMB) of 30-40% during the 20th century, particularly marked during the last 20-50 years. SMB variability is governed to a large extent by atmospheric circulation and to a lesser extent by variations in sea ice cover. The vertical velocity profile measured from repeat borehole OPTV was applied to refine SMB correction and the results fall in the error range of the corrections made using a model previously developed to study the DIR’s stability. This thesis also contributed to characterizing the spatial variability of SMB across the DIR by dating internal reflection horizons (IRHs), former surfaces of the DIR buried under subsequent snow layers and detected using radio-echo-sounding, and by measuring the density profile of IC12. SMB is found to be 2.5 times higher on the upwind slope than on the downwind slope due to the orographic effect. This pattern is regularly observed on ice rises in DML and stresses the importance of adopting a sufficient spatial resolution (5 km) in climate models.Finally, the technical developments allowing to rapidly reconstruct a density profile from the OPTV image of a borehole contributed to improving our knowledge of two features of Antarctic ice shelves, namely melt ponds, influencing surface mass balance and subglacial channels, influencing basal mass balance. Specifically, the results show that density is 5 % higher in surface trenches associated with subglacial channels, and that ice below melt ponds can reach the density of bubble-free ice due to melting and refreezing processes, with implications on ice shelf viscosity. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

No description available.

Résumé : La région de l'archipel canadien, située en Arctique, connaît actuellement d'importants changements climatiques, se traduisant notamment par une augmentation des températures, une réduction de l'étendue de la banquise marine et du couvert nival terrestre ou encore une perte de masse significative des calottes glaciaires disséminées sur les îles de l'archipel. Parmi ces calottes glaciaires, la calotte Barnes, située en Terre de Baffin, ne fait pas exception comme le montrent les observations satellitaires qui témoignent d'une importante perte de masse ainsi que d'une régression de ses marges, sur les dernières décennies. Bien que les calottes glaciaires de l'archipel canadien ne représentent que quelques dizaines de centimètres d'élévation potentielle du niveau des mers, leur perte de masse est une composante non négligeable de l'augmentation actuelle du niveau des mers. Les projections climatiques laissent à penser que cette contribution pourrait rester significative dans les décennies à venir. Cependant, afin d'estimer les évolutions futures de ces calottes glaciaires et leur impact sur le climat ou le niveau des mers, il est nécessaire de caractériser les processus physiques tels que les modifications du bilan de masse de surface. Cette connaissance est actuellement très limitée du fait notamment du sous-échantillonnage des régions arctiques en terme de stations météorologiques permanentes. Une autre particularité de certaines calottes de l'archipel canadien, et de la calotte Barnes en particulier, est de présenter un processus d'accumulation de type glace surimposée, ce phénomène étant à prendre en compte dans l'étude des processus de surface. Pour pallier au manque de données, l'approche retenue a été d'utiliser des données de télédétection, qui offrent l'avantage d'une couverture spatiale globale ainsi qu'une bonne répétitivité temporelle. En particulier les données acquises dans le domaine des micro-ondes passives sont d'un grand intérêt pour l'étude de surfaces enneigées. En complément de ces données, la modélisation du manteau neigeux, tant d'un point de vue des processus physiques que de l'émission électromagnétique permet d'avoir accès à une compréhension fine des processus de surface tels que l'accumulation de la neige, la fonte, les transferts d'énergie et de matière à la surface, etc. Ces différents termes sont regroupés sous la notion de bilan de masse de surface. L'ensemble du travail présenté dans ce manuscrit a donc consisté à développer des outils permettant d'améliorer la connaissance des processus de surface des calottes glaciaires du type de celles que l'on rencontre dans l'archipel canadien, l'ensemble du développement méthodologique ayant été réalisé sur la calotte Barnes à l'aide du schéma de surface SURFEX-CROCUS pour la modélisation physique et du modèle DMRT-ML pour la partie électromagnétique. Les résultats ont tout d'abord permis de mettre en évidence une augmentation significative de la durée de fonte de surface sur la calotte Barnes (augmentation de plus de 30% sur la période 1979-2010), mais aussi sur la calotte Penny, elle aussi située en Terre de Baffin et qui présente la même tendance (augmentation de l'ordre de 50% sur la même période). Ensuite, l'application d'une chaîne de modélisation physique contrainte par diverses données de télédétection a permis de modéliser de manière réaliste le bilan de masse de surface de la dernière décennie, qui est de +6,8 cm/an en moyenne sur la zone sommitale de la calotte, qui est une zone d'accumulation. Enfin, des tests de sensibilité climatique sur ce bilan de masse ont permis de mettre en évidence un seuil à partir duquel cette calotte voit disparaître sa zone d'accumulation. Les modélisations effectuées suggèrent que ce seuil a de fortes chances d'être atteint très prochainement, pour une augmentation de température moyenne inférieure à 1°C, ce qui aurait pour conséquence une accélération de la perte de masse de la calotte. // Abstract : Significant climate change is curently monitored in the Arctic, and especially in the region of the canadian arctic archipellago. This climate warming leads to recession of seaice extent and seasonnal snow cover, and also to large mass loss of the archipellago’s ice caps. One of the most southern ice cap, the Barnes Ice Cap, located on the Baffin Island, is no exception to significant mass loss and margins recession as satellite observations exhibited over the last decades. Despite the relative low sea level potential of the small ice caps located in the canadian arctic achipellago in regards to major ice sheets, Antarctica and Greenland, their contribution to the current sea level rise is significant. Climate projections show that this contribution could accelerate significant over the next decades. However, to estimate the future evolution of these ice caps and their impact on climate or sea level rise, a better characterisation of the surface processes such as the evolution of the surface mass balance is needed. This knowledge is currently very limited, mainly due to the sparse covering of automatic weather stations or in-situ measurements over the Arctic. Furthermore, several ice caps, among with the Barnes Ice Cap, present a superimposed ice accumulation area which particularities have to be taken into account in the surface processes studies. Given the lack of in-situ data, the approach choosen in this work is to use remote sensing data, that have the advantage to offer a good spatial and temporal coverage. In particular, passive microwave data are very suitable for snowy surfaces studies. To complement these data, physical and electromagnetic snowpack modeling provide a fine characterisation of surface processes such as snow accumulation. The whole work presented in this manuscript thus consisted in developping specific tools to improve the understanding of surface processes of small arctic ice caps. This methodological development was performed and applied on the Barnes Ice Cap using the surface scheme SURFEX-CROCUS and the electromagnetic model DMRT-ML. First results highlight a significant increase in surface melt duration over the past 3 decades on the Barnes Ice Cap (increase of more than 30% over 1979-2010 period). A similar trend is also monitored over the Penny Ice Cap, located in the south part of the Baffin Island (increase of more than 50% over the same period). Then, the surface mass balance over the last decade was modeled by using a physical based modeling chain constrained by remote sensing data. The results give a mean net accumulation of +6,8 cm y−1 on the summit area of the ice cap. Finaly, sensitivity tests, performed to investigate the climatic sensitivity of the surface mass balance, highlight a threshold effect that may lead to a complete disapearence of the accumulation area of the Barnes Ice Cap. With a temperature increase less than 1°C, modeling results suggest it is likely that the threshold will be reached rapidly leading to an increase in mass loss from the ice cap.

Le Bilan de Masse de Surface (BMS, c'est-à-dire les précipitations de neige auxquelles est retranchée l'ablation par sublimation, ruissellement ou érosion) de la calotte polaire Antarctique représente une contribution majeure et encore mal connue à l'évolution actuelle du niveau des mers. Le stockage d'eau douce par accumulation de neige sur la calotte posée est supposé s'intensifier au cours du 21eme siècle, modérant l'élévation du niveau des mers et modifiant la dynamique glaciaire. Les trois-quarts du bilan de masse de surface Antarctique sont concentrés au dessous de 2000 m d'altitude alors que cette zone ne représente que 40% de la surface de la calotte posée. Les précipitations orographiques sont une contribution majeure à l'accumulation dans cette région, il est donc crucial d'estimer précisément ce terme. La modélisation de ce processus est fortement dépendant de la résolution des modèles, car les pentes de la calotte influencent l'intensité des précipitations orographiques. La sublimation et la fonte de la neige sont eux aussi fortement dépendant de l'élévation. Bien qu'ils contribuent actuellement peu au bilan de masse de surface de l'Antarctique, ils sont susceptibles de subir des changements importants au cours des prochains siècles. Les modèles atmosphériques de climat, globaux ou régionaux, ne peuvent pas atteindre une résolution allant au delà de 40 km sur l'Antarctique pour des simulations à l'échelle du siècle du fait de coûts de calcul importants. A ces résolutions, la topographie des zones côtières Antarctique n'est pas correctement représentée. C'est pourquoi nous avons développé le modèle de régionalisation SMHiL (Surface Mass balance High-resolution downscaLing) qui permet d'estimer les composantes du bilan de masse de surface Antarctique à haute résolution (~15 km) à partir de champs atmosphériques de plus grande échelle. Nous calculons l'effet de la topographie fine sur les précipitations orographiques et sur les processus de couche limite menant à la sublimation, la fonte et le regel. SMHiL est validé pour la période actuelle à partir d'un jeu de données inédit constitué de plus de 2700 observations de qualité contrôlée. Cependant, les observations représentatives du BMS de la zone côtière Antarctique y sont sous-représentées. Dans ce contexte, nous montrons que la ligne de balise mise en place par l'observatoire GLACIOCLIM-SAMBA en bordure de calotte constitue une référence pour estimer les performances des modèles. Enfin, nous utilisons SMHiL à l'aval du modèle de circulation générale LMDZ4 pour évaluer les variations de BMS au cours du 21eme et du 22eme siècles. Le BMS à haute résolution est significativement différent de celui de LMDZ4 et est plus proche du BMS observé pour la période actuelle. Les résultats suggèrent que les précédentes estimations d'augmentation du BMS au cours du prochain siècle étaient sous-estimées de près de 30% par LMDZ4. Les changements de BMS à faible élévation résulteront d'une compétition entre l'augmentation d'accumulation de neige et de ruissellement. SMHiL est un outil destiné à être appliqué à l'aval d'autres modèles de climat, globaux ou régionaux, pour une meilleure estimation des variations futures du niveau des mers. / The Antarctic Surface Mass Balance (SMB, i.e. the snow accumulation from which we subtract ablation by sublimation, run-off or erosion) is a major yet badly known contribution to changes in the present-day sea level. Water storage by snow accumulation on the Antarctic continent is expected to increase in the 21st century, which would moderate the rise in sea level and impact the ice dynamic response of the ice sheet. Three-quarters of the Antarctic SMB are concentrated below 2000 m above sea level whereas this area represents only 40% of the grounded ice sheet area. Orographic precipitation is a major contributor to snow accumulation in this region, which is why a better estimation of this term is important. The representation of this process by models depends to a great extent on the resolution of the model, since precipitation amounts depend on the ice sheet slopes. Sublimation and snowmelt also depend on elevation, and although they are presently minor contributors to the Antarctic SMB, their role is expected to become more important in the coming centuries. Global and regional atmospheric climate models are unable to achieve a 40-km resolution over Antarctica at a century time scale, due to their computing cost. At this resolution, the Antarctic coastal area is still badly represented. This is why we developed the downscaling model SMHiL (Surface mass balance high-resolution downscaling) to estimate the Antarctic SMB components at a high resolution (~15 km) from large-scale atmospheric forcings. We computed the impact of the high-resolution topography on orographic precipitation amounts and the boundary layer processes that lead to sublimation, melting and refreezing. SMHiL has been validated for the present period with a dataset composed of more than 2700 quality-controlled observations. However, very few of these observations are representative of the Antarctic coastal area. In this context, we show that the GLACIOCLIM-SAMBA stake lines located on the ice sheet coast-to-plateau area is an appropriate reference to evaluate model performance. Finally, we used SMHiL to estimate the SMB changes during the 21st and 22nd centuries, by downscaling the atmospheric global climate model LMDZ4. The high-resolution SMB is significantly different from the SMB given by LMDZ4 and is closer to the observed one for the present period. Our results suggest that previous studies using the LMDZ4 models underestimated the future increase in SMB in Antarctica by about 30%. Future changes in the Antarctic SMB at low elevations will result from the conflict between higher snow accumulation and runoff. The downscaling model is a powerful tool that can be applied to climate models for a better assessment of a future rise in sea level.