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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)


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.


Orre S.,Nansen Environmental and Remote Sensing Center and Bjerknes Center for Climate Research | Orre S.,Badger Explorer ASA | Smith J.N.,Bedford Institute of Oceanography | Alfimov V.,ETH Zurich | Bentsen M.,Nansen Environmental and Remote Sensing Center and Bjerknes Center for Climate Research
Environmental Fluid Mechanics | Year: 2010

Transport of the radioactive tracer Iodine-129 (129I, T1/2 = 15.7 Myr) in the northern North Atlantic Ocean has been investigated using a global isopycnic Ocean General Circulation Model (OGCM) and observed data. 129I originates mainly from the nuclear fuel reprocessing plants in Sellafield (UK) and La Hague (France), and is transported northwards along the Norwegian coast, and then into surface and intermediate layers in the Arctic Ocean through the Barents Sea and the Fram Strait, but also partly recirculating south along the eastern coast of Greenland. In the North Atlantic Subpolar Seas, 129I is mainly found in dense overflow waters from the Nordic Seas being exported southwards in the Deep Western Boundary Current, and to a lesser extent in surface and intermediate layers circulating cyclonically within the Subpolar Gyre. Observed concentration of 129I along a surface transect in the eastern Nordic Seas in 2001 is captured by the OGCM, while in the Nansen Basin of the Arctic Ocean the OGCM overestimates the observed values by a factor of two. The vertical profile of 129I in the Labrador Sea, repeatedly observed since 1997 to present, is fairly realistically reproduced by the OGCM. This indicates that the applied model system has potential for predicting the magnitude and depth of overflow waters from the Nordic Seas into the North Atlantic Subpolar Seas. To supplement the information obtained from the 129I distribution, we have conducted a number of idealized tracer experiments with the OGCM, including tracers mimicking pure water masses, and instantaneous pulse releases. New insight into time-scales of tracer transport in this region is obtained by utilizing a few recently developed methods based on the theory of Transit Time Distribution (TTD) and age of tracers. Implications for other types of "anomalies" in the northern North Atlantic Ocean, being anomalous hydrography or chemical tracers, and how they are interpreted, are discussed. © Springer Science+Business Media B.V. 2009.


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.


Counillon F.,Nansen Environmental and Remote Sensing Center and Bjerknes Center for Climate Research | Bethke I.,University of Bergen | Keenlyside N.,University of Bergen | Bentsen M.,University of Bergen | And 2 more authors.
Tellus, Series A: Dynamic Meteorology and Oceanography | Year: 2014

Here, we firstly demonstrate the potential of an advanced flow dependent data assimilation method for performing seasonal-to-decadal prediction and secondly, reassess the use of sea surface temperature (SST) for initialisation of these forecasts. We use the Norwegian Climate Prediction Model (NorCPM), which is based on the Norwegian Earth System Model (NorESM) and uses the deterministic ensemble Kalman filter to assimilate observations. NorESM is a fully coupled system based on the Community Earth System Model version 1, which includes an ocean, an atmosphere, a sea ice and a land model. A numerically efficient coarse resolution version of NorESM is used. We employ a twin experiment methodology to provide an upper estimate of predictability in our model framework (i.e. without considering model bias) of NorCPM that assimilates synthetic monthly SST data (EnKF-SST). The accuracy of EnKF-SST is compared to an unconstrained ensemble run (FREE) and ensemble predictions made with near perfect (i.e. microscopic SST perturbation) initial conditions (PERFECT). We perform 10 cycles, each consisting of a 10-yr assimilation phase, followed by a 10-yr prediction. The results indicate that EnKF-SST improves sea level, ice concentration, 2 m atmospheric temperature, precipitation and 3-D hydrography compared to FREE. Improvements for the hydrography are largest near the surface and are retained for longer periods at depth. Benefits in salinity are retained for longer periods compared to temperature. Near-surface improvements are largest in the tropics, while improvements at intermediate depths are found in regions of large-scale currents, regions of deep convection, and at the Mediterranean Sea outflow. However, the benefits are often small compared to PERFECT, in particular, at depth suggesting that more observations should be assimilated in addition to SST. The EnKFSST system is also tested for standard ocean circulation indices and demonstrates decadal predictability for Atlantic overturning and sub-polar gyre circulations, and heat content in the Nordic Seas. The system beats persistence forecast and shows skill for heat content in the Nordic Seas that is close to PERFECT. © 2014 F. Counillon et al.


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.


Suo L.,Nansen Environmental and Remote Sensing Center and Bjerknes Center for Climate Research | Gao Y.,Nansen Environmental and Remote Sensing Center and Bjerknes Center for Climate Research | Gao Y.,CAS Institute of Atmospheric Physics | Guo D.,Chinese Academy of Sciences | And 4 more authors.
Climate Dynamics | Year: 2015

We have used an Atmospheric General Circulation Model with a large ensemble (300) to explore the atmospheric responses during the autumn–winter (September to February) to the projected sea-ice free Arctic in autumn (September to November). The detectability of the responses against the internal variability has also been studied. Three ensemble experiments have been performed, the control (CONT) forced by the simulated present-day Arctic sea-ice concentration (SIC) and sea surface temperature (SST), the second forced by the projected autumn Arctic SIC free and present-day SSTs (SENSICE) and the third forced by the projected autumn Arctic SIC free and projected SSTs (SENS). The results show that the disappearance of autumn Arctic sea-ice can cause significant synchronous near-surface warming and increased precipitation over the regions where the sea-ice is removed. The changes in autumn surface heat flux (sensible plus latent), surface air temperature (SAT) and precipitation averaged over the sea-ice reduction region between the SENS and the CONT are about 46, 43 and 50 % more respectively than the changes between the SENSICE and the CONT, which is consistent with the prescribed boundary setting: the surface temperature warming averaged over the sea-ice reduction region in the SENS relative to the CONT is 48 % higher than that in the SENSICE relative to the CONT. The response shows a significant negative Arctic Oscillation (AO) in the troposphere during autumn and December. However, the negative AO does not persist into January–February (JF). Instead, 500 hPa geopotential height (GH) response presents a wave train like pattern in JF which is related to the downstream propagation of the planetary wave perturbations during November–December. The SAT increases over northern Eurasia in JF in accordance with the atmosphere circulation changes. The comparison of the atmosphere response with the atmosphere internal variability (AIV) shows that the responses of SAT and precipitation in the Arctic far exceed the AIV in autumn and the response of the 500 hPa GH is comparable to the AIV in autumn, but none of the responses during JF exceeds the AIV. © 2015 The Author(s)


Wang Y.,Nansen Environmental and Remote Sensing Center and Bjerknes Center for Climate Research | Counillon F.,Nansen Environmental and Remote Sensing Center and Bjerknes Center for Climate Research | Bertino L.,Nansen Environmental and Remote Sensing Center and Bjerknes Center for Climate Research
Quarterly Journal of the Royal Meteorological Society | Year: 2016

This work is based on the Norwegian Climate Prediction Model (NorCPM), which applies the ensemble Kalman filter (EnKF) to a fully coupled Earth System Model (NorESM) with an isopycnic ocean model (MICOM) for climate predictions. An idealized assimilation framework is first developed to identify the origin of assimilation-created bias in the current version of NorCPM. It is found that the bias in ocean heat and salt contents is introduced by the non-negativity constraint of isopycnic layer thickness values. Secondly, a new and computationally efficient method (referred to as upscaling) is proposed and tested in the idealized framework. In the upscaling method, layers for which analysis yields negative values are grouped iteratively with neighbouring layers, resulting in a probability density function with a larger mean and smaller standard deviation that prevents the appearance of negative layer thickness values. Analysis increments of the grouped layer are then distributed proportionally, which also prevents empty layers from becoming filled and vice versa. The upscaling method acts as a moderation in the location where the non-negativity constraint is not satisfied and, as such, is suboptimal. The upscaling method shows a reduction in heat and salt contents and sea-surface height bias by a factor of 10. A small bias remains, due to the update of ensemble anomalies, but the upscaling method would be unbiased for the heat and salt contents with data assimilation methods that utilize a static forecast-error covariance matrix (e.g. EnOI). Finally, the upscaling method is demonstrated in a realistic framework with NorCPM by assimilating sea-surface temperature observations. Over a 25year analysis period, the new method does not impair the predictive skill of the system but corrects the assimilation-created bias in steric sea-level rise and provides an estimation in better agreement with the Intergovernmental Panel on Climate Change. © 2016 Royal Meteorological Society.

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