| 1. |
- Liakka, Johan, et al.
(author)
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Interactions between stationary waves and ice sheets : linear versus nonlinear atmospheric response
- 2012
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In: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 38:5-6, s. 1249-1262
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Journal article (peer-reviewed)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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| 2. |
- Löfverström, Marcus, et al.
(author)
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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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In: Climate of the Past. - : Copernicus GmbH. - 1814-9324 .- 1814-9332. ; 10:4, s. 1453-1471
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Journal article (peer-reviewed)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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| 3. |
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| 4. |
- Löfverström, Marcus, 1981-
(author)
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On the interaction between ice sheets and the large-scale atmospheric circulation over the last glacial cycle
- 2014
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Doctoral thesis (other academic/artistic)abstract
- The last glacial cycle (c. 115-12 kyr BP) was the most recent in a series of recurring glaciations of the subpolar continents. Massive ice sheets evolved in Eurasia and North America, which, at their maximum, were of continental scale and together lowered the global sea-level by approximately 100 m. The paleo-modelling community has focused on the last glacial maximum (LGM, ~ 20 kyr BP), leaving the longer period when the ice sheets evolved to their LGM configurations largely unexplored.In this thesis we study the mutual interaction between the time-mean atmospheric circulation and the evolution of the Northern Hemisphere ice sheets over the build-up phase of the last glacial cycle. Experiments are conducted with coupled atmosphere-ice-sheet models and a circulation model forced by geologically consistent reconstructions of the ice-sheet topography at key stages of the glacial cycle.The main findings from these studies are that the ice evolution in North America may have been controlled by circulation anomalies induced by the background topography in conjunction with the ice sheets themselves. A geologically consistent pre-LGM ice sheet could only be obtained when including the North American Cordillera. However, the ice sheets' influence on the local climate conditions is also found to be paramount for this configuration. We further suggest that the incipient ice sheets may have had a limited influence on the large-scale winter circulation as a result of their location relative the westerly mean flow. The LGM Laurentide Ice Sheet (LIS) was, however, different because of its continent-wide extent, and it may therefore have had a large influence on the planetary-scale circulation, especially in the Atlantic sector. We find that the planetary waves forced by the LIS were considerably larger than at earlier times, and, as a result of a more frequent planetary wave reflection over the Atlantic Ocean basin, an altered stationary wave field and a zonalised winter jet.
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| 5. |
- Löfverström, Marcus, et al.
(author)
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The North American Cordillera-An Impediment to Growing the Continent-Wide Laurentide Ice Sheet
- 2015
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In: Journal of Climate. - 0894-8755 .- 1520-0442. ; 28:23, s. 9433-9450
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Journal article (peer-reviewed)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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| 7. |
- Pausata, Francesco S. R., et al.
(author)
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On the enigmatic similarity in Greenland delta O-18 between the Oldest and Younger Dryas
- 2015
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In: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 42:23
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Journal article (peer-reviewed)abstract
- The last deglaciation (20.0-10.0 kyr B.P.) was punctuated by two major cooling events affecting the Northern Hemisphere: the Oldest Dryas (OD; 18.0-14.7 kyr B.P.) and the Younger Dryas (YD; 12.8-11.5 kyr B.P.). Greenland ice core delta O-18 temperature reconstructions suggest that the YD was as cold as the OD, despite a 50 ppmv increase in atmospheric CO2, while modeling studies suggest that the YD was approximately 4-5 degrees C warmer than the OD. This discrepancy has been surmised to result from changes in the origin of the water vapor delivered to Greenland; however, this hypothesis has not been hitherto tested. Here we use an atmospheric circulation model with an embedded moisture-tracing module to investigate atmospheric processes that may have been responsible for the similar delta O-18 values during the OD and YD. Our results show that the summer-to-winter precipitation ratio over central Greenland in the OD is twice as high as in the YD experiment, which shifts the delta O-18 signal toward warmer (summer) temperatures (enriched delta O-18 values and it accounts for similar to 45% of the expected YD-OD delta O-18 difference). A change in the inversion (cloud) temperature relationship between the two climate states further contributes (similar to 20%) to altering the delta O-18-temperature-relation model. Our experiments also show a 7% decrease of delta O-18-depleted precipitation from distant regions (e.g., the Pacific Ocean) in the OD, hence further contributing (15-20%) in masking the actual temperature difference. All together, these changes provide a physical explanation for the ostensible similarity in the ice core delta O-18 temperature reconstructions in Greenland during OD and YD.
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