Bert Bolin Center for Climate Research

Stockholm, Sweden

Bert Bolin Center for Climate Research

Stockholm, Sweden
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Caron L.-P.,University of Stockholm | Caron L.-P.,Bert Bolin Center for Climate Research | Jones C.G.,Swedish Meteorological and Hydrological Institute | Vaillancourt P.A.,Environment Canada | Winger K.,University of Quebec at Montréal
Climate Dynamics | Year: 2013

We compare two 28-year simulations performed with two versions of the Global Environmental Multiscale model run in variable-resolution mode. The two versions differ only by small differences in their radiation scheme. The most significant modification introduced is a reduction in the ice effective radius, which is observed to increase absorption of upwelling infrared radiation and increase temperature in the upper troposphere. The resulting change in vertical lapse rate is then observed to drive a resolution-dependent response of convection, which in turn modifies the zonal circulation and induces significant changes in simulated Atlantic tropical cyclone activity. The resulting change in vertical lapse rate and its implication in the context of anthropogenic climate change are discussed. © 2012 The Author(s).

Giesler R.,Climate Impacts Research Center | Lyon S.W.,Bert Bolin Center for Climate Research | Karlsson J.,Climate Impacts Research Center | Karlsson E.M.,Purdue University | And 2 more authors.
Biogeosciences | Year: 2014

Climatic change is currently enhancing permafrost thawing and the flow of water through the landscape in subarctic and arctic catchments, with major consequences for the carbon export to aquatic ecosystems. We studied stream water carbon export in several tundra-dominated catchments in northern Sweden. There were clear seasonal differences in both dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) concentrations. The highest DOC concentrations occurred during the spring freshet while the highest DIC concentrations were always observed during winter baseflow conditions for the six catchments considered in this study. Long-term trends for the period 1982 to 2010 for one of the streams showed that DIC concentrations has increased by 9% during the 28 yr of measurement while no clear trend was found for DOC. Similar increasing trends were also found for conductivity, Ca and Mg. When trends were discretized into individual months, we found a significant linear increase in DIC concentrations with time for September, November and December. In these subarctic catchments, the annual mass of C exported as DIC was in the same order of magnitude as DOC; the average proportion of DIC to the total dissolved C exported was 61% for the six streams. Furthermore, there was a direct relationship between total runoff and annual dissolved carbon fluxes for these six catchments. These relationships were more prevalent for annual DIC exports than annual DOC exports in this region. Our results also highlight that both DOC and DIC can be important in high-latitude ecosystems. This is particularly relevant in environments where thawing permafrost and changes to subsurface ice due to global warming can influence stream water fluxes of C. The large proportion of stream water DIC flux also has implications on regional C budgets and needs to be considered in order to understand climate-induced feedback mechanisms across the landscape. © 2014 Author(s).

Chiacchio M.,University of Stockholm | Chiacchio M.,Bert Bolin Center for Climate Research | Pausata F.S.R.,University of Stockholm | Pausata F.S.R.,Bert Bolin Center for Climate Research | And 9 more authors.
Journal of Geophysical Research: Atmospheres | Year: 2017

A generalized linear model based on Poisson regression has been used to assess the impact of environmental variables modulating tropical cyclone frequency in six main cyclone development areas: the East Pacific, West Pacific, North Atlantic, North Indian, South Indian, and South Pacific. The analysis covers the period 1980–2009 and focuses on widely used meteorological parameters including wind shear, sea surface temperature, and relative humidity from different reanalyses as well as aerosol optical depth for different compounds simulated by the Goddard Chemistry Aerosol Radiation and Transport model. Circulation indices are also included. Cyclone frequency is obtained from the International Best Track Archive for Climate Stewardship. A strong link is found between cyclone frequency and the relative sea surface temperature, Atlantic Meridional Mode, and wind shear with significant explained log likelihoods in the North Atlantic of 37%, 27%, and 28%, respectively. A significant impact of black carbon and organic aerosols on cyclone frequency is found over the North Indian Ocean, with explained log likelihoods of 27%. A weaker but still significant impact is found for observed dust aerosols in the North Atlantic with an explained log likelihood of 11%. Changes in lower stratospheric temperatures explain 28% of the log likelihood in the North Atlantic. Lower stratospheric temperatures from a subset of Coupled Model Intercomparison Project Phase 5 models properly simulate the warming and subsequent cooling of the lower stratosphere that follows a volcanic eruption but underestimates the cooling by about 0.5°C. ©2016. American Geophysical Union. All Rights Reserved.

Vihma T.,Finnish Meteorological Institute | Vihma T.,University Center in Svalbard | Pirazzini R.,Finnish Meteorological Institute | Fer I.,University of Bergen | And 15 more authors.
Atmospheric Chemistry and Physics | Year: 2014

The Arctic climate system includes numerous highly interactive small-scale physical processes in the atmosphere, sea ice, and ocean. During and since the International Polar Year 2007-2009, significant advances have been made in understanding these processes. Here, these recent advances are reviewed, synthesized, and discussed. In atmospheric physics, the primary advances have been in cloud physics, radiative transfer, mesoscale cyclones, coastal, and fjordic processes as well as in boundary layer processes and surface fluxes. In sea ice and its snow cover, advances have been made in understanding of the surface albedo and its relationships with snow properties, the internal structure of sea ice, the heat and salt transfer in ice, the formation of superimposed ice and snow ice, and the small-scale dynamics of sea ice. For the ocean, significant advances have been related to exchange processes at the ice-ocean interface, diapycnal mixing, double-diffusive convection, tidal currents and diurnal resonance. Despite this recent progress, some of these small-scale physical processes are still not sufficiently understood: these include wave-turbulence interactions in the atmosphere and ocean, the exchange of heat and salt at the ice-ocean interface, and the mechanical weakening of sea ice. Many other processes are reasonably well understood as stand-alone processes but the challenge is to understand their interactions with and impacts and feedbacks on other processes. Uncertainty in the parameterization of small-scale processes continues to be among the greatest challenges facing climate modelling, particularly in high latitudes. Further improvements in parameterization require new year-round field campaigns on the Arctic sea ice, closely combined with satellite remote sensing studies and numerical model experiments. © Author(s) 2014. CC Attribution 3.0 License.

Mauritsen T.,Max Planck Institute for Meteorology | Sedlar J.,Bert Bolin Center for Climate Research | Sedlar J.,University of Stockholm | TjernstrAm M.,Bert Bolin Center for Climate Research | And 10 more authors.
Atmospheric Chemistry and Physics | Year: 2011

On average, airborne aerosol particles cool the Earth's surface directly by absorbing and scattering sunlight and indirectly by influencing cloud reflectivity, life time, thickness or extent. Here we show that over the central Arctic Ocean, where there is frequently a lack of aerosol particles upon which clouds may form, a small increase in aerosol loading may enhance cloudiness thereby likely causing a climatologically significant warming at the ice-covered Arctic surface. Under these low concentration conditions cloud droplets grow to drizzle sizes and fall, even in the absence of collisions and coalescence, thereby diminishing cloud water. Evidence from a case study suggests that interactions between aerosol, clouds and precipitation could be responsible for attaining the observed low aerosol concentrations. © 2011 Author(s).

Graham R.M.,University of East Anglia | Graham R.M.,Bert Bolin Center for Climate Research | Graham R.M.,University of Stockholm | De Boer A.M.,Bert Bolin Center for Climate Research | And 4 more authors.
Journal of Geophysical Research: Oceans | Year: 2012

The location of fronts has a direct influence on both the physical and biological processes in the Southern Ocean. Here we explore the relative importance of bottom topography and winds for the location of Southern Ocean fronts, using 100 years of a control and climate change simulation from the high resolution coupled climate model HiGEM. Topography has primary control on the number and intensity of fronts at each longitude. However, there is no strong relationship between the position or spacing of jets and underlying topographic gradients because of the effects of upstream and downstream topography. The Southern Hemisphere Westerlies intensify and shift south by 1.3 in the climate change simulation, but there is no comparable meridional displacement of the Antarctic Circumpolar Current's (ACC) path or the fronts within its boundaries, even over flat topography. Instead, the current contracts meridionally and weakens. North of the ACC, the Subtropical Front (STF) shifts south gradually, even over steep topographic ridges. We suggest the STF reacts more strongly to the wind shift because it is strongly surface intensified. In contrast, fronts within the ACC are more barotropic and are therefore more sensitive to the underlying topography. An assessment of different methods for identifying jets reveals that maxima of gradients in the sea surface height field are the most reliable. Approximating the position of fronts using sea surface temperature gradients is ineffective at high latitudes while using sea surface height contours can give misleading results when studying the temporal variability of front locations. © 2012. American Geophysical Union. All Rights Reserved.

Tjiputra J.F.,University of Bergen | Tjiputra J.F.,Bjerknes Center for Climate Research | Assmann K.,University of Bergen | Assmann K.,Bjerknes Center for Climate Research | And 9 more authors.
Geoscientific Model Development | Year: 2010

We developed a complex Earth system model by coupling terrestrial and oceanic carbon cycle components into the Bergen Climate Model. For this study, we have generated two model simulations (one with climate change inclusions and the other without) to study the large scale climate and carbon cycle variability as well as its feedback for the period 1850-2100. The simulations are performed based on historical and future IPCC CO2 emission scenarios. Globally, a pronounced positive climate-carbon cycle feedback is simulated by the terrestrial carbon cycle model, but smaller signals are shown by the oceanic counterpart. Over land, the regional climate-carbon cycle feedback is highlighted by increased soil respiration, which exceeds the enhanced production due to the atmospheric CO2 fertilization effect, in the equatorial and northern hemisphere mid-latitude regions. For the ocean, our analysis indicates that there are substantial temporal and spatial variations in climate impact on the air-sea CO2 fluxes. This implies feedback mechanisms act inhomogeneously in different ocean regions. In the North Atlantic subpolar gyre, the simulated future cooling of SST improves the CO2 gas solubility in seawater and, hence, reduces the strength of positive climate carbon cycle feedback in this region. In most ocean regions, the changes in the Revelle factor is dominated by changes in surface pCO2, and not by the warming of SST. Therefore, the solubility-associated positive feedback is more prominent than the buffer capacity feedback. In our climate change simulation, the retreat of Southern Ocean sea ice due to melting allows an additional ∼20 Pg C uptake as compared to the simulation without climate change. © Author(s) 2010.

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