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Kimmritz M.,Alfred Wegener Institute for Polar and Marine Research | Kimmritz M.,Nansen Environmental and Remote Sensing Center and Bjerknes Center for Climate Research | Danilov S.,Alfred Wegener Institute for Polar and Marine Research | Danilov S.,Russian Academy of Sciences | Losch M.,Alfred Wegener Institute for Polar and Marine Research
Ocean Modelling | Year: 2016

Stability and convergence of the modified EVP implementation of the visco-plastic sea ice rheology by Bouillon et al., Ocean Modell., 2013, is analyzed on B- and C-grids. It is shown that the implementation on a B-grid is less restrictive with respect to stability requirements than on a C-grid. On C-grids convergence is sensitive to the discretization of the viscosities. We suggest to adaptively vary the parameters of pseudotime subcycling of the modified EVP scheme in time and space to satisfy local stability constraints. This new approach generally improves the convergence of the modified EVP scheme and hence its numerical efficiency. The performance of the new "adaptive EVP" approach is illustrated in a series of experiments with the sea ice component of the MIT general circulation model (MITgcm) that is formulated on a C-grid. © 2016 Elsevier Ltd.

Vasskog K.,University of Bergen | Langebroek P.M.,University of Bergen | Andrews J.T.,University of Colorado at Boulder | Nilsen J.E.O.,Nansen Environmental and Remote Sensing Center and Bjerknes Center for Climate Research | Nesje A.,University of Bergen
Earth-Science Reviews | Year: 2015

During the Last Interglacial (LIG) period, between 130 and 116 thousand years before present (ka BP), the Greenland Ice Sheet (GrIS) was considerably reduced in size, contributing to a global mean sea-level (MSL) rise of 0.5-4.2m relative to the present. This is not sufficient to explain the 6-9m MSL rise estimated for the LIG, which implies that a significant contribution to the LIG highstand came from Antarctica. Following the LIG the GrIS grew and attained its maximum volume of about 12m global sea-level equivalents (SLEs) between 18 and 16ka BP. Since then the GrIS margins have retreated on an order of several hundred km, following a general pattern of lagged response to changes in high-latitude summer insolation and global greenhouse gas concentrations. On shorter timescales and over smaller spatial scales the combination of internal ice-sheet dynamics and external climate dynamics has often resulted in a more complex and asynchronous behaviour of the ice-sheet margin. The GrIS probably reached its minimum Holocene extent around 4ka BP, and modelling studies suggest that it contributed to a rise in global MSL of less than 0.2m relative to the present at this time. A period of steady growth followed the Holocene minimum and in many areas the ice sheet advanced beyond its present limits during the 'Little Ice Age' (i.e. the last few centuries). Currently the GrIS occupies an area of ~1.7×106km2 and features a volume of ~2.96×106km3, which amounts to ~7.4m SLE. Observations show that the rate of mass loss from the GrIS has been accelerating over the past few decades, and ice sheet modelling indicates that we have to go back to the last deglaciation (around 10ka BP) in order to find sustained melt rates that were higher than those experienced over the last decade. The future development of the GrIS will have a profound influence on sea level, both globally and regionally, but there are large uncertainties related to how the GrIS will respond to future global warming. Based on a range of modelling studies employing different emission scenarios, the GrIS is expected to contribute about 0.1-0.3m to global MSL rise by the end of the 21st century. In a longer time perspective, modelling suggests that melting of the GrIS may replace ocean thermal expansion as the most important factor in future sea-level rise, potentially contributing with 0.7-2.6m SLE within the next 500years. Multi-millennial simulations show that the entire ice sheet (~7.4m SLE) might disappear completely within less than three thousand years under high-emission scenarios, i.e. with atmospheric CO2 reaching four times preindustrial levels. © 2015 Elsevier B.V.

Marzeion B.,Massachusetts Institute of Technology | Marzeion B.,Nansen Environmental and Remote Sensing Center and Bjerknes Center for Climate Research | Marzeion B.,University of Innsbruck | Levermann A.,Potsdam Institute for Climate Impact Research | Mignot J.,University Pierre and Marie Curie
Climate Dynamics | Year: 2010

We use a reduced complexity climate model with a three-dimensional ocean component and realistic topography to investigate the effect of stratification-dependent mixing on the sensitivity of the North Atlantic subpolar gyre (SPG), and the Atlantic meridional overturning circulation (AMOC), to idealized CO2 increase and peaking scenarios. The vertical diffusivity of the ocean interior is parameterized as κ ~ N-α, where N is the local buoyancy frequency. For all parameter values 0 ≤ α ≤ 3, we find the SPG, and subsequently the AMOC, to weaken in response to increasing CO2 concentrations. The weakening is significantly stronger for α ≥ αcr ≈ 1.5. Depending on the value of α, two separate model states develop. These states remain different after the CO2 concentration is stabilized, and in some cases even after the CO2 concentration has been decreased again to the pre-industrial level. This behaviour is explained by a positive feedback between stratification and mixing anomalies in the Nordic Seas, causing a persistent weakening of the SPG. © Springer-Verlag 2009.

Langehaug H.R.,Nansen Environmental and Remote Sensing Center and Bjerknes Center for Climate Research | Matei D.,Max Planck Institute for Meteorology | Eldevik T.,University of Bergen | Lohmann K.,Max Planck Institute for Meteorology | And 2 more authors.
Climate Dynamics | Year: 2016

The Nordic Seas and the Barents Sea is the Atlantic Ocean’s gateway to the Arctic Ocean, and the Gulf Stream’s northern extension brings large amounts of heat into this region and modulates climate in northwestern Europe. We have investigated the predictive skill of initialized hindcast simulations performed with three state-of-the-art climate prediction models within the CMIP5-framework, focusing on sea surface temperature (SST) in the Nordic Seas and Barents Sea, but also on sea ice extent, and the subpolar North Atlantic upstream. The hindcasts are compared with observation-based SST for the period 1961–2010. All models have significant predictive skill in specific regions at certain lead times. However, among the three models there is little consistency concerning which regions that display predictive skill and at what lead times. For instance, in the eastern Nordic Seas, only one model has significant skill in predicting observed SST variability at longer lead times (7–10 years). This region is of particular promise in terms of predictability, as observed thermohaline anomalies progress from the subpolar North Atlantic to the Fram Strait within the time frame of a couple of years. In the same model, predictive skill appears to move northward along a similar route as forecast time progresses. We attribute this to the northward advection of SST anomalies, contributing to skill at longer lead times in the eastern Nordic Seas. The skill at these lead times in particular beats that of persistence forecast, again indicating the potential role of ocean circulation as a source for skill. Furthermore, we discuss possible explanations for the difference in skill among models, such as different model resolutions, initialization techniques, and model climatologies and variance. © 2016 The Author(s)

Li F.,CAS Institute of Atmospheric Physics | Li F.,Chinese Academy of Sciences | Li F.,University of Chinese Academy of Sciences | Wang H.,CAS Institute of Atmospheric Physics | And 3 more authors.
Climate Dynamics | Year: 2015

We use both the National Centers for Environmental Prediction (NCEP)–National Center for Atmospheric Research (NCAR) reanalysis data (1979–2013) and the Community Atmospheric Model Version 3 to explore the modulation of El Niño–Southern Oscillation (ENSO) on the co-variability of the Aleutian Low (AL) and the Antarctic Oscillation (AAO). The empirical orthogonal function analysis on the NCEP–NCAR reanalysis data indicates that after the late-1990s the global sea level pressure (SLP) and 300-hPa geopotential height (Z300) in boreal January are characterized by the concurrence of the AL and the negative phase of the AAO (−AAO). Associated with this AL–AAO co-variation is a sea surface temperature anomaly that resembles the ENSO cycle. Further analyses reveal that the interdecadal change in ENSO signal (westward extension and more La Niña events) is responsible for the co-variability of AL and AAO after the late-1990s. Correspondingly, the El Niño-related anomalous heating and upward motion over the eastern–central equatorial Pacific can lead to the upper-tropospheric divergence in the western–central Pacific. This upper-tropospheric divergence plays an essential role in coupling the equatorial heat anomalies with the variation of the subtropical westerly jet of both hemispheres, and therefore results in the enhanced meridional circulation of the three cells. It thus implies that ENSO might act as a bridge linking AL and AAO after the late-1990s, causing their consistent co-variability. The numerical experiment also supports this hypothesis. © 2014, Springer-Verlag Berlin Heidelberg.

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