| 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. |
- Liakka, Johan, et al.
(författare)
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Interactions between stationary waves and ice sheets : linear versus nonlinear atmospheric response
- 2012
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Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 38:5-6, s. 1249-1262
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Tidskriftsartikel (refereegranskat)abstract
- This study examines the mutual interaction between topographically-forced atmospheric stationary waves and continental-scale ice sheets using a thermomechanical ice-sheet model coupled to a linear as well as a fully-nonlinear dry atmospheric primitive equation model. The focus is on how the stationary-wave induced ablation feeds back on the ice sheet. Simulations are conducted in which an embryonal ice mass, on an idealised “North American” continent, evolves to an equilibrium ice sheet. Under the coupling to the linear atmospheric model, the equilibrium ice sheet is primarily controlled by the ratio between the wavelength of the stationary waves and the zonal continental extent. When this ratio is near two, the ice sheet has its center of mass shifted far eastward and its shape is broadly reminiscent of the Laurentide ice sheet at LGM. For wavelengths comparable to the continental extent, however, the ice margin extends far equatorward on the central continent but is displaced poleward near the eastern coast. Remarkably, the coupling to the nonlinear atmospheric model yields equilibrium ice sheets that are virtually identical to the ones obtained in uncoupled simulations, i.e. a symmetric ice sheet with a zonal southern margin. Thus, the degree of linearity of the atmospheric response should control to what extent topographically-forced stationary waves can reorganise the structure of ice sheets. If the stationary-wave response is linear, the present results suggest that spatial reconstructions of past ice sheets can provide some information on the zonal-mean atmospheric circulation that prevailed.
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| 3. |
- Liakka, Johan, et al.
(författare)
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The impact of topographically forced stationary waves on local ice-sheet climate
- 2010
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Ingår i: Journal of Glaciology. - : International Glaciological Society. - 0022-1430 .- 1727-5652. ; 56:197, s. 534-544
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Tidskriftsartikel (refereegranskat)abstract
- A linear two-level atmospheric model is employed to study the influence of ice-sheet topography on atmospheric stationary waves. In particular, the stationary-wave-induced temperature anomaly is considered locally over a single ice-sheet topography, which is computed using the plastic approximation. It is found that stationary waves induce a local cooling which increases linearly with the ice volume for ice sheets of horizontal extents smaller than 1400 km. Beyond this horizontal scale, the dependence of stationary-wave-induced cooling on the ice volume becomes gradually weaker. For a certain ice-sheet size, and for small changes of the surface zonal wind, it is further shown that the strength of the local stationary-wave-induced cooling is proportional to the basic state meridional temperature gradient multiplied by the vertical stratification in the atmosphere. These results are of importance for the nature of the feedback between ice sheets and stationary waves, and may also serve as a basis for parameterizing this feedback in ice-sheet model simulations (e.g. through the Pleistocene glacial/interglacial cycles).
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| 4. |
- 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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| 5. |
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| 6. |
- Colleoni, Florence, et al.
(författare)
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An East Siberian ice shelf during the Late Pleistocene glaciations : Numerical reconstructions
- 2016
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Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791 .- 1873-457X. ; 147:SI, s. 148-163
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Tidskriftsartikel (refereegranskat)abstract
- A recent data campaign in the East Siberian Sea has revealed evidence of grounded and floating ice dynamics in regions of up to 1000 m water depth, and which are attributed to glaciations older than the Last Glacial Maximum (21 kyrs BP). The main hypothesis based on this evidence is that a small ice cap developed over Beringia and expanded over the East Siberian continental margin during some of the Late Pleistocene glaciations. Other similar evidence of ice dynamics that have been previously collected on the shallow continental shelves of the Arctic Ocean have been attributed to the penultimate glaciation, i.e. Marine Isotopes Stage 6 (approximate to 140 kyrs BP). We use an ice sheet model, forced by two previously simulated MIS 6 glacial maximum climates, to carry out a series of sensitivity experiments testing the impact of dynamics and mass-balance related parameters on the geometry of the East Siberian ice cap and ice shelf. Results show that the ice cap developing over Beringia connects to the Eurasian ice sheet in all simulations and that its volume ranges between 6 and 14 m SLE, depending on the climate forcing. This ice cap generates an ice shelf of dimensions comparable with or larger than the present-day Ross ice shelf in West Antarctica. Although the ice shelf extent strongly depends on the ice flux through the grounding line, it is particularly sensitive to the choice of the calving and basal melting parameters. Finally, inhibiting a merging of the Beringia ice cap with the Eurasian ice sheet affects the expansion of the ice shelf only in the simulations where the ice cap fluxes are not large enough to compensate for the fluxes coming from the Eurasian ice sheet.
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| 7. |
- Colleoni, Florence, et al.
(författare)
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The Late Saalian period (160 - 140 ka): insight on an unusual glaciation
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- This work focuses on the climate evolution over the Late Saalian period (160 - 140 ka) over Eurasia. At this time, the Eurasian ice sheet was larger and higher than during the Last Glacial Maximum. June insolation over the high latitudes presents a large fluctuation over this period: two glacial minima toward 160 and 140 ka and a large insolation peak toward 150 ka. From the geological evidence chronology, it seems clear that the large Eurasian ice sheet already reached its maximum extent at 160 ka. To understand how this ice sheet could survive the 150 ka June insolation maximum, we use several numerical models to simulate the evolution of the vegetation cover, the surface ocean temperatures and finally the evolution of the Late Saalian climate over the three time slices 140, 150 and 160 ka. Results show that the Late Saalian climate variations are dominated by orbital forcings, responding to a large eccentricity enhancing the precession effect especially at 140 ka. From 160 to 150 ka, the surface ocean exhibits open water conditions in the North At- lantic during summer while sea surface temperature at 140 ka are clearly colder with a large sea ice extent reaching 40◦ N in both the North Atlantic and the North Pacific. This corresponds to a milder climate before 140 ka inducing a larger positive surface mass balance despite the 150 ka insolation peak because of larger precipitation rates. On the contrary, the drastic cooling caused by the astronomical forcing at 140 ka leads to a drier climate cancelling ablation and reducing the accumulation over the ice sheet.
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| 8. |
- Colleoni, Florence, 1983-, et al.
(författare)
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The Late Saalian surface ocean (140 ka): sensitivity of the Late Saalian Eurasian ice sheet to sea surface conditions
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- This work focuses on the Late Saalian (140 ka) and LGM (21 ka) Eurasian ice sheets surface mass balance (SMB) sensitivity to changes in sea surface temperatures (SST). Since no global Late Saalian SST compilations exist, we test the sensitivity of the Late Saalian climate using an AGCM forced with two data-based LGM SST reconstructions. Furthermore, an attempt to reconstruct the Late Saalian SST is performed using an AGCM coupled to a mixed-layer ocean. The resulting Late Saalian SST are cooler than the LGM SST in the Northern Hemisphere and warmer in the Southern. The winter sea ice extends to 40◦ N in both North Atlantic and Pacific oceans. Changes in SST affect the SMB of the Eurasian ice sheet during both glaciations although the Late Saalian ice sheet is less sensitive to the prescribed SST changes than the LGM.
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| 9. |
- Colleoni, Florence, et al.
(författare)
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The sensitivity of the Late Saalian (140 ka) and LGM (21 ka) Eurasian ice sheets to sea surface conditions
- 2011
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Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 37:3-4, s. 531-553
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Tidskriftsartikel (refereegranskat)abstract
- This work focuses on the Late Saalian (140 ka) Eurasian ice sheets’ surface mass balance (SMB) sensitivity to changes in sea surface temperatures (SST). An Atmospheric General Circulation Model (AGCM), forced with two preexisting Last Glacial Maximum (LGM, 21 ka) SST reconstructions, is used to compute climate at 140 and 21 ka (reference glaciation). Contrary to the LGM, the ablation almost stopped at 140 ka due to the climatic cooling effect from the large ice sheet topography. Late Saalian SST are simulated using an AGCM coupled with a mixed layer ocean. Compared to the LGM, these 140 ka SST show an inter-hemispheric asymmetry caused by the larger ice-albedo feedback, cooling climate. The resulting Late Saalian ice sheet SMB is smaller due to the extensive simulated sea ice reducing the precipitation. In conclusion, SST are important for the stability and growth of the Late Saalian Eurasian ice sheet.
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| 10. |
- Feurdean, Angelica, et al.
(författare)
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Origin of the forest steppe and exceptional grassland diversity in Transylvania (central-eastern Europe)
- 2015
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Ingår i: Journal of Biogeography. - : Wiley. - 1365-2699 .- 0305-0270. ; 42:5, s. 951-963
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Tidskriftsartikel (refereegranskat)abstract
- Aim The forest steppe of the Transylvanian Plain is a landscape of exceptionally diverse steppe-like and semi-natural grasslands. Is this vegetation a remnant of a once continuous temperate forest extensively cleared by humans, or has the area, since the last glacial, always been a forest steppe? Understanding the processes that drive temperate grassland formation is important because effective management of this biome is critical to the conservation of the European cultural landscape. Location Lake Stiucii, north-western Romania, central-eastern Europe. Methods We analysed multi-proxy variables (pollen, coprophilous fungi, plant macroremains, macrocharcoal) from a 55,000year discontinuous sequence (c. 55,000-35,000; 13,000-0cal. yr bp), integrating models of pollen-based vegetation cover, biome reconstruction, global atmospheric simulations and archaeological records. Results Needleleaf woodland occurred during glacial Marine Isotope Stage (MIS) 3, but contracted at the end of this period. Forest coverage of c. 55% (early Holocene) and 65% (mid-Holocene) prevailed through the Holocene, but Bronze Age humans extensively cleared forests after 3700cal. yr bp. Forest coverage was most widespread between 8600 and 3700cal. yr bp, whereas grasses, steppe and xerothermic forbs were most extensive between 11,700 and 8600cal. yr bp and during the last 3700cal. yr bp. Cerealia pollen indicate the presence of arable agriculture by c. 7000cal. yr bp. Main conclusions We have provided the first unequivocal evidence for needleleaf woodland during glacial MIS 3 in this region. Extensive forests prevailed prior to 3700cal. yr bp, challenging the hypothesis that the Transylvanian lowlands were never wooded following the last glaciation. However, these forests were never fully closed either, reflecting dry growing season conditions, recurrent fires and anthropogenic impacts, which have favoured grassland persistence throughout the Holocene. The longevity of natural and semi-natural grasslands in the region may explain their current exceptional biodiversity. This longer-term perspective implies that future climatic warming and associated fire will maintain these grasslands.
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