| 1. |
- Liakka, Johan, 1981-
(författare)
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The mutual interaction between the time-mean atmospheric circulation and continental-scale ice sheets
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Geomorphological evidence of glaciations exist for the Last Glacial Maximum (about 20 kyr ago). At this time, both North America and Eurasia were covered by extensive ice sheets which are both absent today. However, the temporal and spatial evolution of the ice sheets from the previous interglacial up to the fully-glaciated conditions at LGM is still unresolved and remains a vexing question in climate dynamics. The evolution of ice sheets is essentially controlled by the prevailing climate conditions. On glacial time-scales, the climate is shaped the by the orbital variations of the Earth, but also by internal feedbacks within the climate system. In particular, the ice sheets themselves have the potential to change the climate within they evolve. This thesis focuses on the interactions between ice sheets and the time-mean atmospheric circulation (stationary waves). It is studied how the stationary waves, which are forced by the ice-sheet topography, influence ice-sheet evolution through changing the near-surface air temperature. In this thesis, it is shown that the degree of linearity of the atmospheric response controls to what extent the stationary waves can reorganise the structure of ice sheet. Provided that the response is linear, the stationary waves constitute a leading-order feedback, which serves to increase the volume and deform the shape of ice sheets. If the stationary-wave response to ice-sheet topography is nonlinear in character, the impact on the ice-sheet evolution tends to be weak. However, it is further shown that the amplitude of the nonlinear topographical response, and hence its effect on the ice-sheet evolution, can be significantly enhanced if thermal cooling over the ice sheets is taken into account.
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| 2. |
- Kleman, Johan, et al.
(författare)
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Pre-LGM Northern Hemisphere ice sheet topography
- 2013
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Ingår i: Climate of the Past. - : Copernicus GmbH. - 1814-9324 .- 1814-9332. ; 9, s. 2365-2378
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Tidskriftsartikel (refereegranskat)abstract
- We here reconstruct the paleotopography of Northern Hemisphere ice sheets during the glacial maxima of marine isotope stages (MIS) 5b and 4.We employ a combined approach, blending geologically based reconstruction and numerical modeling, to arrive at probable ice sheet extents and topographies for each of these two time slices. For a physically based 3-D calculation based on geologically derived 2-D constraints, we use the University of Maine Ice Sheet Model (UMISM) to calculate ice sheet thickness and topography. The approach and ice sheet modeling strategy is designed to provide robust data sets of sufficient resolution for atmospheric circulation experiments for these previously elusive time periods. Two tunable parameters, a temperature scaling function applied to a spliced Vostok–GRIP record, and spatial adjustment of the climatic pole position, were employed iteratively to achieve a good fit to geological constraints where such were available. The model credibly reproduces the first-order pattern of size and location of geologically indicated ice sheets during marine isotope stages (MIS) 5b (86.2 kyr model age) and 4 (64 kyr model age). From the interglacial state of two north–south obstacles to atmospheric circulation (Rocky Mountains and Greenland), by MIS 5b the emergence of combined Quebec–central Arctic and Scandinavian–Barents-Kara ice sheets had increased the number of such highland obstacles to four. The number of major ice sheets remained constant through MIS 4, but the merging of the Cordilleran and the proto-Laurentide Ice Sheet produced a single continent-wide North American ice sheet at the LGM.
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| 3. |
- Kleman, Johan, et al.
(författare)
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Rubbat förtroende för forskarna
- 2010
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Ingår i: Svenska Dagbladet. - 1101-2412. ; :25 maj
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Tidskriftsartikel (populärvet., debatt m.m.)
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| 4. |
- Löfverström, Marcus, et al.
(författare)
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Evolution of the large-scale atmospheric circulation in response to changing ice sheets over the last glacial cycle
- 2014
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Ingår i: Climate of the Past. - : Copernicus GmbH. - 1814-9324 .- 1814-9332. ; 10:4, s. 1453-1471
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Tidskriftsartikel (refereegranskat)abstract
- We present modelling results of the atmospheric circulation at the cold periods of marine isotope stage 5b (MIS 5b), MIS 4 and the Last Glacial Maximum (LGM), as well as the interglacial. The palaeosimulations are forced by ice-sheet reconstructions consistent with geological evidence and by appropriate insolation and greenhouse gas concentrations. The results suggest that the large-scale atmospheric winter circulation remained largely similar to the interglacial for a significant part of the glacial cycle. The proposed explanation is that the ice sheets were located in areas where their interaction with the mean flow is limited. However, the LGM Laurentide Ice Sheet induces a much larger planetary wave that leads to a zonalisation of the Atlantic jet. In summer, the ice-sheet topography dynamically induces warm temperatures in Alaska and central Asia that inhibits the expansion of the ice sheets into these regions. The warm temperatures may also serve as an explanation for westward propagation of the Eurasian Ice Sheet from MIS 4 to the LGM.
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| 5. |
- Löfverström, Marcus, et al.
(författare)
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The North American Cordillera-An Impediment to Growing the Continent-Wide Laurentide Ice Sheet
- 2015
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Ingår i: Journal of Climate. - 0894-8755 .- 1520-0442. ; 28:23, s. 9433-9450
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Tidskriftsartikel (refereegranskat)abstract
- This study examines the evolution of a continental-scale ice sheet on a triangular representation of North America, with and without the influence of the Cordilleran region. Simulations are conducted using a comprehensive atmospheric general circulation model asynchronously coupled to a three-dimensional thermomechanical ice-sheet model. The atmospheric state is updated for every 2 x 10(6) km(3) increase in ice volume, and the coupled model is integrated to steady state. In the first experiment a flat continent with no background topography is used. The ice sheet evolves fairly zonally symmetric, and the equilibrium state is continent-wide and has the highest point in the center of the continent. This equilibrium ice sheet forces an anticyclonic circulation that results in relatively warmer (cooler) summer surface temperatures in the northwest (southeast), owing to warm (cold) air advection and radiative heating due to reduced cloudiness. The second experiment includes a simplified representation of the Cordilleran region. The ice sheet's equilibrium state is here structurally different from the flat continent case; the center of mass is strongly shifted to the east and the interior of the continent remains ice freean outline broadly resembling the geologically determined ice margin in Marine Isotope Stage 4. The limited glaciation in the continental interior is the result of warm summer surface temperatures primarily due to stationary waves and radiative feedbacks.
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| 6. |
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| 7. |
- De Angelis, Hernán, et al.
(författare)
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Palaeo-ice stream onsets: examples from the north-eastern Laurentide Ice Sheet
- 2008
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Ingår i: Earth Surface Processes and Landforms. - : Wiley. - 0197-9337 .- 1096-9837. ; 33:4, s. 560-572
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we report on observations of glacial landscapes at the head of geomorphologically interpreted palaeo-ice streams, i.e. palaeo-ice-stream onsets. Our work is based on the results of systematic palaeoglaciological mapping of the portion of north-eastern Canada formerly covered by the Laurentide Ice Sheet. Four different cases are considered and analysed in detail: one is the Dubawnt Lake Ice Stream and the others are smaller (<70 km long) palaeo-ice streams. We found that in this region onset zones are generally characterized by a heterogeneous landform record and are more easily recognized when landscapes typical of partially frozen beds are present. Based on publicly available topographical and geological data as well as theoretical developments, we discuss the conditions and controls on the location of these onset zones and their potential stability. Except for the Dubawnt Lake Ice Stream, we found that, although topography and geology are important conditioning factors, the geomorphological imprint of palaeo-ice-stream onsets cannot be explained without invoking the role of the basal thermal boundary between cold- and warm-based ice. Finally, we observed that, except for the Dubawnt Lake Ice Stream, the onset zones of large ice streams are rarely preserved, and we suggest two possible explanations for this. We conclude that although the same qualitative processes are present under the majority of ice streams, conditions for the preservation of the onset zones are more likely to be met in the case of relatively small ice streams because these are more rapidly deglaciated.
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| 8. |
- De Angelis, Hernán, et al.
(författare)
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Palaeo-ice streams in the Foxe/Baffin sector of the Laurentide Ice Sheet
- 2007
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Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791. ; 26:9-10, s. 1313-1331
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we describe mapping of palaeo-ice streams in the Foxe/Baffin sector of the Laurentide Ice Sheet by means of geomorphological interpretation of high-resolution satellite images. Our interpretations were guided by a glaciological inversion scheme, aided by digital elevation models, publicly available sonar surveys and field studies. As a result, we produced a map depicting the location, geometry and relative temporal changes of palaeo-ice streams and analysed their palaeoglaciological implications for the Foxe/Baffin sector. We conclude that in the period between the Last Glacial Maximum and approximately 7.0 kyr BP, the Foxe/Baffin sector was largely drained by topographically controlled outlet glaciers and ice streams, which were organized in a relatively stable pattern. During this time, large areas of Melville Peninsula and central Baffin Island were subject to cold-based conditions. Between 7.0 and 6.0 kyr BP, the Foxe/Baffin sector collapsed catastrophically in the Foxe Basin, after which its remnant portion became confined to Baffin Island. During this collapse, rapid successions of small transient ice streams occurred along wide, loosely defined topographical corridors in two sectors of Baffin Island. The presently available landform archive on emerged land is not sufficient to conclusively support the existence of fully scaled ice streams along the whole length of Hudson Strait. As an alternative solution, we propose that topographically controlled ice streams might have occurred along the deepest parts of Hudson Strait, with attendant cold-based ice zones on marginal areas and islands at the head of the strait.
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| 9. |
- De Angelis, Hernán, 1974-
(författare)
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Palaeo-ice streams in the north-eastern Laurentide Ice Sheet
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis presents a palaeoglaciological study aimed to determine the location, geometry and temporal evolution of palaeo-ice streams of the north-easternmost Laurentide Ice Sheet. The work was accomplished through the geomorphological interpretation of satellite imagery over 3.19 x 106 km2 of the Canadian Arctic, using a glaciological inversion scheme. Ice streams were active in this region during most of the time between the Last Glacial Maximum and the last deglaciation. A web of ice streams and inter-ice stream areas existed. Three major ice stream networks are identified: the M'Clintock Channel, Gulf of Boothia – Lancaster Sound and Hudson Strait. The M'Clintock Channel bears the most complex landform record, comprising three generations of palaeo-ice streams. Their location was weakly controlled by the subglacial topography and their geometry was determined by frozen-bed portions of the ice sheet, thus providing evidence for pure ice streams in the Laurentide Ice Sheet. In contrast, the more pronounced relief of the Gulf of Boothia – Lancaster Sound corridor supported topographically controlled ice streams. The landform record on emerged land along Hudson Strait is insufficient to support the existence of ice streams. It is therefore proposed that ice streams were constrained within the deep parts of the strait while flanked by cold-based zones on the margins. Small transient ice streams on Baffin and Prince of Wales islands drained local remnant ice caps during the collapse of the ice sheet. Analysis of the controls on the location and flow of palaeo-ice streams suggests that the interaction between the subglacial topography and thermal state of the substrate plays a more fundamental role than the geology. It is concluded that the behaviour of ice streams cannot be explained in terms of environmental controls alone, but the complex dynamics of ice stream shear margins and onset zones must be considered.
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| 10. |
- De Angelis, Hernán, et al.
(författare)
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Palaeo-ice streams in the northern Keewatin sector of the Laurentide Ice Sheet
- 2005
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Ingår i: Annals of Glaciology. - : International Glaciological Society. - 0260-3055 .- 1727-5644. ; 42, s. 135-144
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Tidskriftsartikel (refereegranskat)abstract
- Evidence for ice streams in the Laurentide ice sheet is widespread. In the region of northern Keewatin and the Boothia Peninsula, Nunavut, Canada, palaeo-ice streams have been recognized, but their location, size and potential role in ice-sheet dynamics are poorly constrained. Based on the interpretation of satellite imagery, we produce a palaeo-ice-stream map of this region. Glacial directional landforms, eskers and moraines were mapped and integrated into landform assemblages using a glacial geological inversion model. Palaeo-frozen bed areas were also identified. Relative age of the geomorphic swarms was assessed by cross-cutting relationships and radiocarbon ages where available. Using this information we obtained a glaciologically plausible picture of ice-stream evolution within the northernmost Laurentide ice sheet. On the M'Clintock Channel corridor, three generations of pure ice streams are found. On Baffin Island and the Gulf of Boothia, glaciation was dominated by frozen-bed zones located on high plateaus and ice streams running along the troughs, i.e. topographic ice streams. A massive convergent pattern at the head of Committee Bay drained ice from both the Keewatin and Foxe sectors and was probably one of the main deglaciation channels of the Laurentide ice sheet. Finally, our results indicate that streaming flow was present in the deep interior of the Laurentide ice sheet, as recently shown for the Greenland and Antarctic ice sheets.
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