| 1. |
- Björk, Göran, 1956, et al.
(författare)
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On the Arctic Ocean ice thickness response to changes in the external forcing
- 2012
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Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 39:12, s. 3007-3018
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Tidskriftsartikel (refereegranskat)abstract
- Submarine and satellite observations show that the Arctic Ocean ice cover has undergone a large thickness reduction and a decrease in the areal extent during the last decades. Here the response of the Arctic Ocean ice cover to changes in the poleward atmospheric energy transport, F wall, is investigated using coupled atmosphere-ice-ocean column models. Two models with highly different complexity are used in order to illustrate the importance of different internal processes and the results highlight the dramatic effects of the negative ice thickness—ice volume export feedback and the positive surface albedo feedback. The steady state ice thickness as a function of F wall is determined for various model setups and defines what we call ice thickness response curves. When a variable surface albedo and snow precipitation is included, a complex response curve appears with two distinct regimes: a perennial ice cover regime with a fairly linear response and a less responsive seasonal ice cover regime. The two regimes are separated by a steep transition associated with surface albedo feedback. The associated hysteresis is however small, indicating that the Arctic climate system does not have an irreversible tipping point behaviour related to the surface albedo feedback. The results are discussed in the context of the recent reduction of the Arctic sea ice cover. A new mechanism related to regional and temporal variations of the ice divergence within the Arctic Ocean is presented as an explanation for the observed regional variation of the ice thickness reduction. Our results further suggest that the recent reduction in areal ice extent and loss of multiyear ice is related to the albedo dependent transition between seasonal and perennial ice i.e. large areas of the Arctic Ocean that has previously been dominated by multiyear ice might have been pushed below a critical mean ice thickness, corresponding to the above mentioned transition, and into a state dominated by seasonal ice.
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| 2. |
- Björk, Göran, 1956, et al.
(författare)
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The Sensitivity of the Arctic Ocean Sea Ice Thickness and Its Dependence on the Surface Albedo Parameterization
- 2013
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Ingår i: Journal of Climate. - : American Meteorological Society. - 0894-8755 .- 1520-0442. ; 26:4, s. 1355-1370
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Tidskriftsartikel (refereegranskat)abstract
- In this study, the response of sea ice thickness to changes in the external forcing is investigated and particularly how this response depends on the surface albedo formulation by means of a one-dimensional coupled ocean-ice-atmosphere model. The main focus is on the thickness response to the atmospheric heat advection F-wall, solar radiation F-SW, and amount of snow precipitation S-prec. Different albedo parameterization schemes [ECHAM5, CSIRO, and Community Climate System Model, version 3 (CCSM3)] representing albedos commonly used in global climate models are compared together with more simplified schemes. Using different albedo schemes with the same external forcing produces large differences in ice thickness. The ice thickness response is similar for all realistic albedo schemes with a nearly linear decrease with increasing F-wall in the perennial ice regime and with a steplike transition into seasonal ice when F-wall exceeds a certain threshold. This transition occurs at an annual-mean ice thickness of 1.7-2.0 m. Latitudinal differences in solar insolation generally leads to increasing ice thickness toward the North Pole. The snow response varies significantly depending on which albedo scheme is used. The ECHAM5 scheme yields thinner ice with S-prec, the CSIRO scheme gives ice thickness nearly independent of S-prec, and with the CCSM3 scheme the ice thickness decreases with S-prec. A general result is that the modeled ice cover is rather sensitive to positive perturbations of the external heat supply when it is close to the transition such that just a small increase of, for example, F-wall can force the ice cover into the seasonal regime.
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| 3. |
- Stranne, Christian, 1979, et al.
(författare)
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Analysis and modeling of hydrothermal plume data acquired from the 85°E segment of the Gakkel Ridge
- 2010
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Ingår i: Journal of Geophysical Research. - 0148-0227. ; 115:C06028
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Tidskriftsartikel (refereegranskat)abstract
- We use data from a CTD plume-mapping campaign conducted during the Arctic Gakkel Vents (AGAVE) expedition in 2007 to constrain the nature of hydrothermal processes on the Gakkel Ridge at 85 degrees E. Thermal and redox potential (Eh) anomalies were detected in two discrete depth intervals: 2400-2800 m (Interval 1) and 3000-3800 m (Interval 2). The spatial and temporal patterns of the signals indicate that the Interval 1 anomalies were most likely generated by a single large, high-temperature (T > 100 degrees C) vent field located on the fault terraces that form the NE axial valley wall. In contrast, the Interval 2 anomalies appear to have been generated by up to 7 spatially distinct vent fields associated with constructional volcanic features on the floor of the axial valley, many of which may be sites of diffuse, low-temperature (T < 10 degrees C) discharge. Numerical simulations of turbulent plumes rising in a weakly stratified Arctic Ocean water column indicate that the high-temperature field on the axial valley wall has a thermal power of 1.8 GW, similar to the Trans-Atlantic Geotraverse and Rainbow fields in the Atlantic Ocean, whereas the sites on the axial valley floor have values ranging from 5 to 110 MW.
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| 4. |
- Stranne, Christian, 1979, et al.
(författare)
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Arctic Ocean perennial sea ice breakdown during the Early Holocene Insolation Maximum
- 2014
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Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791 .- 1873-457X. ; 92, s. 123-132
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Tidskriftsartikel (refereegranskat)abstract
- Arctic Ocean sea ice proxies generally suggest a reduction in sea ice during parts of the early and middle Holocene (similar to 6000-10,000 years BP) compared to present day conditions. This sea ice minimum has been attributed to the northern hemisphere Early Holocene Insolation Maximum (EHIM) associated with Earth's orbital cycles. Here we investigate the transient effect of insolation variations during the final part of the last glaciation and the Holocene by means of continuous climate simulations with the coupled atmosphere sea ice ocean column model CCAM. We show that the increased insolation during EHIM has the potential to push the Arctic Ocean sea ice cover into a regime dominated by seasonal ice, i.e. ice free summers. The strong sea ice thickness response is caused by the positive sea ice albedo feedback. Studies of the GRIP ice cores and high latitude North Atlantic sediment cores show that the Bolling Allerod period (c. 12,700-14,700 years BP) was a climatically unstable period in the northern high latitudes and we speculate that this instability may be linked to dual stability modes of the Arctic sea ice cover characterized by e.g. transitions between periods with and without perennial sea ice cover.
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| 5. |
- Stranne, Christian, 1979
(författare)
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Studies of the Arctic Ocean sea ice cover and hydrothermal heat fluxes
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Since reliable ice extent estimates from satellite data became available in 1979 the Arctic sea ice cover has followed a declining long term trend. Over the last decade the decline has accelerated with record low sea ice extents recorded in 2002, 2005, 2007 and 2012. In order to explain the mechanisms behind the observed ice cover decline it is critical to gain a better understanding of the climate system as a whole which in turn requires detailed studies of the involved processes. The main objective of the work presented in this thesis is to improve the knowledge of the Arctic ice cover sensitivity to climate change through detailed studies of some key processes involved. The observed sea ice cover reduction has not been homogeneous over the Arctic Ocean. A typical pattern is that the thinning has been larger in regions with thick ice compared to regions with thinner ice. It has been argued that the ice thickness - ice growth rate feedback mechanism is the dominating process explaining these regional variations. The sea ice thickness response to variations in the atmospheric forcing is studied with a succession of increased model complexity. When the model realism is increased by the inclusion of processes such as ice divergence and variable surface albedo, the ice cover response properties become more complex with e.g. a very high sensitivity close to the transition between perennial and seasonal ice. These results imply that other mechanisms than the ice thickness - ice growth rate feedback might be more important for explaining the observed regional variations of the sea ice cover decline. It is suggested that temporal variations in the local ice divergence is one such mechanism and supporting observational data are presented. Simulations of the present Arctic sea ice cover performed with coupled 3D models (both global and regional) show large inter-model scatter. Analyses of the mechanism behind this scatter point at differences in the surface albedo parameterization as one of the major factors. In the present thesis we address this problem by running a model with identical forcing and using a number of different albedo parameterizations taken from well-known global climate models. It is shown that the surface short wave radiation budget is strongly influenced by the choice of albedo parameterization. This means that for the same forcing the different parameterizations cannot all yield a realistic Arctic sea ice cover. Global climate models need then differences in e.g. the atmospheric composition in order to compensate for the different surface albedos such that they all produce ice covers that are fairly consistent with observations. The effect on the Arctic sea ice cover of variations in solar insolation associated with Earth’s orbital parameters is also studied. It is shown that the increased solar forcing in the Arctic during the early Holocene insolation maximum (~9500 years before present) had the potential to force the ice cover into a state dominated by seasonal ice. A compilation of available ice cover proxy data is also presented and the emerging picture is consistent with the model results. The thesis also includes a separate study on hydrothermal plume modeling based on data collected during the AGAVE 2007 Expedition. An indirect method for estimating the heat flux of hydrothermal vents using buoyant plume dynamics in combination with water column observations is presented. The results show that one of the plumes investigated likely stems from a hydrothermal vent with a heat flux comparable to the largest known vents on Earth. Keywords: Arctic Ocean, Sea ice, Sensitivity, Ice divergence, Albedo feedback, Ice export, Tipping point, Hysteresis, Milankovitch cycles, Holocene, Insolation, Climatic feedback, Gakkel Ridge, Hydrothermal vent, Hydrothermal plume, Hydrothermal heat flux
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| 6. |
- Wåhlin, Anna, 1970, et al.
(författare)
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Variability of Warm Deep Water Inflow in a Submarine Trough on the Amundsen Sea Shelf
- 2013
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Ingår i: Journal of Physical Oceanography. - 0022-3670 .- 1520-0485. ; 43:10, s. 2054-2070
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Tidskriftsartikel (refereegranskat)abstract
- The ice shelves in the Amundsen Sea are thinning rapidly, and the main reason for their decline appears to be warm ocean currents circulating below the ice shelves and melting these from below. Ocean currents transportwarm densewater ontothe shelf,channeledby bathymetric troughs leadingto the deep inner basins. A hydrographic mooring equipped with an upward-looking ADCP has been placed in one of these troughs on the central Amundsen shelf. The two years (2010/11) of mooring data are here used to characterize the inflow of warm deep water to the deep shelf basins. During both years, the warm layer thickness and temperature peaked in austral fall. The along-trough velocity is dominated by strong fluctuations that do not vary in the vertical. These fluctuations are correlated with the local wind, with eastward wind over the shelf and shelf break giving flow toward the ice shelves. In addition, there is a persistent flow of dense lower Circumpolar Deep Water (CDW) toward the ice shelves in the bottom layer. This bottom-intensified flow appears to be driven by buoyancy forces rather than the shelfbreak wind. The years of 2010 and 2011 were characterized by a comparatively stationary Amundsen Sea low, and hence there were no strong eastward winds during winter that could drive an upwelling of warm water along the shelf break. Regardless of this, there was a persistent flow of lower CDW in the bottom layer during the two years. The average heat transport toward the ice shelves in the trough was estimated from the mooring data to be 0.95 TW.
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