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  • 51. Bocquet, Florence
    et al.
    Balsley, Ben
    Tjernström, Michael
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Svensson, Gunilla
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Comparing Estimates of Turbulence Based on Near-Surface Measurements in the Nocturnal Stable Boundary Layer2011Inngår i: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 138, nr 1, s. 43-60Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Tethered Lifting System (TLS) estimates of the dissipation rate of turbulent kinetic energy (epsilon) are reasonably well correlated with concurrent measurements of vertical velocity variance (sigma(2)(w)) obtained from sonic anemometers located on a nearby 60-m tower during the CASES-99 field experiment. Additional results in the first 100m of the nocturnal stable boundary layer confirm our earlier claim that the presence of weak but persistent background turbulence exists even during the most stable atmospheric conditions, where e can exhibit values as low as 10(-7) m(2) s(-3). We also present a set of empirical equations that incorporates TLS measurements of temperature, horizontal wind speed, and e to provide a proxy measurement for sigma(2)(w) at altitudes higher than tower heights.

  • 52. Bodin, Svante
    A predictive numerical model of the atmospheric boundary layer based on the turbulent energy equation1979Doktoravhandling, monografi (Annet vitenskapelig)
  • 53. Bolmgren, K.
    et al.
    Vanhoenacker, Didrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Botaniska institutionen. Swedish Natural History Museum.
    Miller-Rushing, A. J.
    One man, 73 years, and 25 species. Evaluating phenological responses using a lifelong study of first flowering dates2013Inngår i: International journal of biometeorology, ISSN 0020-7128, E-ISSN 1432-1254, Vol. 57, nr 3, s. 367-375Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Phenological shifts linked to global warming reflect the ability of organisms to track changing climatic conditions. However, different organisms track global warming differently and there is an increasing interest in the link between phenological traits and plant abundance and distribution. Long-term data sets are often used to estimate phenological traits to climate change, but so far little has been done to evaluate the quality of these estimates. Here, we use a 73-year long data series of first flowering dates for 25 species from north-temperate Sweden to evaluate (i) correlations between first flowering dates and year for different time periods and (ii) linear regression models between first flowering date and mean monthly temperatures in preceding months. Furthermore, we evaluate the potential of this kind of data to estimate the phenological temperature sensitivities (i.e. number of days phenological change per degree temperature change, beta(60)) in such models. The sign of the correlations between first flowering dates and year were highly inconsistent among different time periods, highlighting that estimates of phenological change are sensitive to the specific time period used. The first flowering dates of all species were correlated with temperature, but with large differences in both the strength of the response and the period(s) of the year that were most strongly associated with phenological variation. Finally, our analyses indicated that legacy data sets need to be relatively long-term to be useful for estimating phenological temperature sensitivities (beta(60)) for inter-specific comparisons. In 10-year long observation series only one out of 24 species reached a parts per thousand yen80 % probability of estimating temperature sensitivity (beta(60)) within a +/- 1 range, and 17 out of 24 species reached a parts per thousand yen80 % probability when observation series were 20 years or shorter. The standard error for beta(60) ranged from 0.6 to 2.0 for 10-year long observation series, and 19 out of 24 species reached SE < 1 after 15 years. In general, late flowering species will require longer time series than early flowering species.

  • 54. Bosveld, Fred C.
    et al.
    Baas, Peter
    Steeneveld, Gert-Jan
    Holtslag, Albert A. M.
    Angevine, Wayne M.
    Bazile, Eric
    de Bruijn, Evert I. F.
    Deacu, Daniel
    Edwards, John M.
    Ek, Michael
    Larson, Vincent E.
    Pleim, Jonathan E.
    Raschendorfer, Matthias
    Svensson, Gunilla
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    The Third GABLS Intercomparison Case for Evaluation Studies of Boundary-Layer Models. Part B: Results and Process Understanding2014Inngår i: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 152, nr 2, s. 157-187Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We describe and analyze the results of the third global energy and water cycle experiment atmospheric boundary layer Study intercomparison and evaluation study for single-column models. Each of the nineteen participating models was operated with its own physics package, including land-surface, radiation and turbulent mixing schemes, for a full diurnal cycle selected from the Cabauw observatory archive. By carefully prescribing the temporal evolution of the forcings on the vertical column, the models could be evaluated against observations. We focus on the gross features of the stable boundary layer (SBL), such as the onset of evening momentum decoupling, the 2-m minimum temperature, the evolution of the inertial oscillation and the morning transition. New process diagrams are introduced to interpret the variety of model results and the relative importance of processes in the SBL; the diagrams include the results of a number of sensitivity runs performed with one of the models. The models are characterized in terms of thermal coupling to the soil, longwave radiation and turbulent mixing. It is shown that differences in longwave radiation schemes among the models have only a small effect on the simulations; however, there are significant variations in downward radiation due to different boundary-layer profiles of temperature and humidity. The differences in modelled thermal coupling to the land surface are large and explain most of the variations in 2-m air temperature and longwave incoming radiation among models. Models with strong turbulent mixing overestimate the boundary-layer height, underestimate the wind speed at 200 m, and give a relatively large downward sensible heat flux. The result is that 2-m air temperature is relatively insensitive to turbulent mixing intensity. Evening transition times spread 1.5 h around the observed time of transition, with later transitions for models with coarse resolution. Time of onset in the morning transition spreads 2 h around the observed transition time. With this case, the morning transition appeared to be difficult to study, no relation could be found between the studied processes, and the variation in the time of the morning transition among the models.

  • 55. Bothe, O.
    et al.
    Jungclaus, J. H.
    Zanchettin, D.
    Zorita, E.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för material- och miljökemi (MMK).
    Climate of the last millennium: ensemble consistency of simulations and reconstructions2013Inngår i: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 9, nr 3, s. 1089-1110Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Are simulations and reconstructions of past climate and its variability consistent with each other? We assess the consistency of simulations and reconstructions for the climate of the last millennium under the paradigm of a statistically indistinguishable ensemble. In this type of analysis, the null hypothesis is that reconstructions and simulations are statistically indistinguishable and, therefore, are exchangeable with each other. Ensemble consistency is assessed for Northern Hemisphere mean temperature, Central European mean temperature and for global temperature fields. Reconstructions available for these regions serve as verification data for a set of simulations of the climate of the last millennium performed at the Max Planck Institute for Meteorology. Consistency is generally limited to some sub-domains and some sub-periods. Only the ensemble simulated and reconstructed annual Central European mean temperatures for the second half of the last millennium demonstrates unambiguous consistency. Furthermore, we cannot exclude consistency of an ensemble of reconstructions of Northern Hemisphere temperature with the simulation ensemble mean. If we treat simulations and reconstructions as equitable hypotheses about past climate variability, the found general lack of their consistency weakens our confidence in inferences about past climate evolutions on the considered spatial and temporal scales. That is, our available estimates of past climate evolutions are on an equal footing but, as shown here, inconsistent with each other.

  • 56.
    Bourgeois, Quentin
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Ekman, Annica M. L.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Igel, Matthew R.
    Krejci, Radovan
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för miljövetenskap och analytisk kemi.
    Ubiquity and impact of thin mid-level clouds in the tropics2016Inngår i: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 7, artikkel-id 12432Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Clouds are crucial for Earth's climate and radiation budget. Great attention has been paid to low, high and vertically thick tropospheric clouds such as stratus, cirrus and deep convective clouds. However, much less is known about tropospheric mid-level clouds as these clouds are challenging to observe in situ and difficult to detect by remote sensing techniques. Here we use Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) satellite observations to show that thin mid-level clouds (TMLCs) are ubiquitous in the tropics. Supported by high-resolution regional model simulations, we find that TMLCs are formed by detrainment from convective clouds near the zero-degree isotherm. Calculations using a radiative transfer model indicate that tropical TMLCs have a cooling effect on climate that could be as large in magnitude as the warming effect of cirrus. We conclude that more effort has to be made to understand TMLCs, as their influence on cloud feedbacks, heat and moisture transport, and climate sensitivity could be substantial.

  • 57.
    Boyd, Emily
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Stockholm Resilience Centre. University of Reading, England.
    Cornforth, Rosalind J.
    Lamb, Peter J.
    Tarhule, Aondover
    Lele, M. Issa
    Brouder, Alan
    Building resilience to face recurring environmental crisis in African Sahel2013Inngår i: Nature Climate Change, ISSN 1758-678X, E-ISSN 1758-6798, Vol. 3, nr 7, s. 631-637Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The present food shortages in the Horn of Africa and the West African Sahel are affecting 31 million people. Such continuing and future crises require that people in the region adapt to an increasing and potentially irreversible global sustainability challenge. Given this situation and that short-term weather and seasonal climate forecasting have limited skill for West Africa, the Rainwatch project illustrates the value of near real-time monitoring and improved communication for the unfavourable 2011 West African monsoon, the resulting severe drought-induced humanitarian impacts continuing into 2012, and their exacerbation by flooding in 2012. Rainwatch is now coupled with a boundary organization (Africa Climate Exchange, AfClix) with the aim of integrating the expertise and actions of relevant institutions, agencies and stakeholders to broker ground-based dialogue to promote resilience in the face of recurring crisis.

  • 58. Brandefelt, J.
    et al.
    Kjellstrom, E.
    Naslund, J. -O
    Strandberg, G.
    Voelker, A. H. L.
    Wohlfarth, Barbara
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för geologiska vetenskaper.
    A coupled climate model simulation of Marine Isotope Stage 3 stadial climate2011Inngår i: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 7, nr 2, s. 649-670Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present a coupled global climate model (CGCM) simulation, integrated for 1500 yr to quasi-equilibrium, of a stadial (cold period) within Marine Isotope Stage 3 (MIS 3). The simulated Greenland stadial 12 (GS12; similar to 44 ka BP) annual global mean surface temperature (T(s)) is 5.5 degrees C lower than in the simulated recent past (RP) climate and 1.3 degrees C higher than in the simulated Last Glacial Maximum (LGM; 21 ka BP) climate. The simulated GS12 is evaluated against proxy data and previous modelling studies of MIS3 stadial climate. We show that the simulated MIS 3 climate, and hence conclusions drawn regarding the dynamics of this climate, is highly model-dependent. The main findings are: (i) Proxy sea surface temperatures (SSTs) are higher than simulated SSTs in the central North Atlantic, in contrast to earlier simulations of MIS 3 stadial climate in which proxy SSTs were found to be lower than simulated SST. (ii) The Atlantic Meridional Overturning Circulation (AMOC) slows down by 50% in the GS12 climate as compared to the RP climate. This slowdown is attained without freshwater forcing in the North Atlantic region, a method used in other studies to force an AMOC shutdown. (iii) El-Nino-Southern Oscillation (ENSO) teleconnections in mean sea level pressure (MSLP) are significantly modified by GS12 and LGM forcing and boundary conditions. (iv) Both the mean state and variability of the simulated GS12 is dependent on the equilibration. The annual global mean T(s) only changes by 0.10 degrees C from model years 500-599 to the last century of the simulation, indicating that the climate system may be close to equilibrium already after 500 yr of integration. However, significant regional differences between the last century of the simulation and model years 500-599 exist. Further, the difference between simulated and proxy SST is reduced from model years 500-599 to the last century of the simulation. The results of the ENSO variability analysis is also shown to depend on the equilibration.

  • 59.
    Brandefelt, Jenny
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Atmospheric circulation regimes and climate change2005Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    The Earth's atmosphere is expected to warm in response to increasing atmospheric concentrations of greenhouse gases (GHG). The response of the Earth's complex and chaotic climate system to the GHG emissions is, however, difficult to assess. In this thesis, two issues of importance for the assessment of this response are studied. The first concerns the magnitude of the natural and anthropogenic emissions of CO2. An atmospheric transport model is used, combined with inventories of anthropogenic CO2 emissions and estimates of natural emissions, to compare modelled and observed variations in the concentration of CO2 at an Arctic monitoring site. The anthropogenic and natural emissions are shown to exert approximately equal influence on Arctic CO2 variations during winter.

    The primary focus of this thesis is the response of the climate system to the enhanced GHG forcing. It has been proposed that this response may project onto the leading modes of variability. In the present thesis, this hypothesis is tested against the alternative that the spatial patterns of variability change in response to the enhanced forcing. The response of the atmospheric circulation to the enhanced GHG forcing as simulated by a specific coupled global climate model (CGCM) is studied. The response projects strongly onto the leading modes of present-day variability. The spatial patterns of the leading modes are however changed in response to the enhanced GHG forcing. These changes in the spatial patterns are associated with a strengthening of the waveguide for barotropic Rossby waves in the Southern Hemisphere. The Northern Hemisphere waveguide is however unchanged.

    The magnitude of the global mean responses to an enhanced GHG forcing as simulated by CGCMs vary. Moreover, the regional responses vary considerably among CGCMs. In this thesis, it is hypothesised that the inter-CGCM differences in the spatial patterns of the response to the enhanced GHG forcing are partially explained by inter-CGCM differences in zonal-mean properties of the atmospheric flow. In order to isolate the effect of these differences in the zonal-mean background state from the effects of other sensitivities, a simplified model with idealised forcing is employed. The model used is a global three-level quasi-geostrophic model. The sensitivity of the stationary wave pattern (SWP) to changes in the zonal-mean wind and tropopause height of similar magnitude as those found in response to the enhanced GHG forcing in CGCMs is investigated. The SWP in the simplified model shows a sensitivity of comparable magnitude to the analogous response in CGCMs. These results indicate that the CGCM-simulated response is sensitive to relatively small differences in the zonal-mean background state. To assess the uncertainties in the regional response to the enhanced forcing associated with this sensitivity, ensemble simulations of climate change are of great importance.

  • 60.
    Brodeau, Laurent
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Barnier, Bernard
    Gulev, Sergey K.
    Woods, Cian
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Climatologically significant effects of some approximations in the bulk parameterizations of turbulent air-sea fluxes2017Inngår i: Journal of Physical Oceanography, ISSN 0022-3670, E-ISSN 1520-0485, Vol. 47, nr 1, s. 5-28Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This paper quantifies the impacts of approximations and assumptions in the parameterization of bulk formulae on the exchange of momentum, heat, and freshwater computed between the ocean and atmosphere. An ensemble of sensitivity experiments are examined. Climatologies of wind stress, turbulent heat flux, and evaporation for the 1982-2014 period are computed using SST and surface meteorological state variables from the ERA-Interim reanalysis. Each experiment differs from the defined control experiment in only one aspect of the parameterization of the bulk formulae. The wind stress is most sensitive to the closure used to relate the neutral drag coefficient to the wind speed in the bulk algorithm, which mainly involves the value of the Charnock parameter. The disagreement between the state-of-the-art algorithms examined is typically of the order of 10%. The largest uncertainties in turbulent heat flux and evaporation are also related to the choice of the algorithm (typically 15%), but also emerge in experiments examining approximations related to the surface temperature and saturation humidity. Thus, approximations for the skin temperature and the salt-related reduction of saturation humidity have a substantial impact on the heat flux and evaporation (typically 10%). Approximations such as the use of a fixed air density, sea level pressure, or simplified formula for the saturation humidity, lead to errors no larger than 4% when tested individually. The impacts of these approximations combine linearly when implemented together, yielding errors up to 20% over mid- and subpolar latitudes.

  • 61. Bromley, T.
    et al.
    Allan, W.
    Martin, R.
    Fletcher, S. E. Mikaloff
    Lowe, D. C.
    Struthers, Hamish
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU). Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Moss, R.
    Shipboard measurements and modeling of the distribution of CH4 and (CH4)-C-13 in the western Pacific2012Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 117, s. D04307-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present observations of methane (CH4) mixing ratio and C-13/C-12 isotopic ratios in CH4 (delta C-13) data from a collaborative shipboard project using bulk carrier ships sailing between Nelson, New Zealand, and Osaka, Japan, in the western Pacific Ocean. Measurements of the CH4 mixing ratio and delta C-13 in CH4 were obtained from large clean-air samples collected in each 2.5 degrees to 5 degrees of latitude between 30 degrees S and 30 degrees N on eight voyages from 2004 to 2007. The data show large variations in CH4 mixing ratio in the tropical western Pacific, and data analysis suggests that these large variations are related to the positions and strengths of the South Pacific Convergence Zone and the Intertropical Convergence Zone, with variability in the sources playing a much smaller role. These measurements are compared with results from a modified version of the Unified Model (UMeth) general circulation model along two transects, one similar to the ship transects and another 18.75 degrees to the east. Although UMeth was run to a steady state with the same sources and sinks each year, the gradient structures varied considerably from year to year, supporting our conclusion that variability in transport is a major driver for the observed variations in CH4. Simulations forced with an idealized representation of the El Nino-Southern Oscillation (ENSO) suggest that a large component of the observed variability in latitudinal gradients of CH4 and its delta C-13 arises from intrinsic variability in the climate system that does not occur on ENSO time scales.

  • 62.
    Broomé, Sara
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU). Stockholm University.
    Atlantic Water in the Nordic Seas: A satellite altimetry perspective on ocean circulation2020Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    The Atlantic Water in the Nordic Seas contributes to the mild climate of Northern Europe and is the main oceanic source of heat for the Arctic. The northward bound transport of the warm and saline Atlantic Water is mediated by a topographically constrained cyclonic boundary current along the Norwegian continental slope. The analysis within this thesis is based on satellite observations of dynamic Sea Surface Heights (SSH) from 1993 to the recent present, combined with both hydrographic observations and modelling. It provides some new perspectives and results, as well as corroborates the essential role of bottom topography for the circulation in the Nordic Seas.

    In the first part of the thesis, the topographic constraint is used in the analysis by examining the satellite-derived SSH along topographic contours. We find stationary along-contour anomalies that indicate deviations from strict topographic steering. However, we show that these deviations are dynamically consistent with, and can be explained by, potential vorticity conservation in an adiabatic steady-state model for flow over a topographic slope. The analysis along topographic contours is further developed to study northward-propagating, low-frequency ocean temperature signals. These signals have an expression in the SSH and their propagation speed is remarkably slow compared to the current speed. We propose a conceptual model of shear dispersion effects, in which the effective advection speed of a tracer is determined not only by the rapid current core, but by a mean velocity taken over the cross-flow extent of Atlantic Water. The model predicts a reduced effective tracer advection velocity, comparable to the one observed.

    The close connection between anomalies in SSH and heat content is further used to study decadal variability in the Nordic Seas. There is a shift in decadal trends in the mid-2000s, from a period of strong increase in SSH and heat content to a more stagnant period. We find this variability to be forced remotely, rather than by local air-sea heat fluxes. By developing a conceptual model of ocean heat convergence, we are able to explain the broad features of the decadal changes with the temperature variability of the inflowing Atlantic Water from the subpolar North Atlantic.

    In the final part of the thesis, satellite-derived surface geostrophic velocity fields are used as input to a Lagrangian trajectory model. Based on this, we study the fractionation of the Atlantic Water in the Nordic Seas between the two straits towards the Arctic Ocean: the Barents Sea Opening and the Fram Strait. This Lagrangian approach also provides insights on the origin of the water that reach the straits. We find that it is the frontal current branch, rather than the slope current, that contributes to the variability of the Barents Sea Opening inflow of warm Atlantic Water, and thus potentially to the climate of the Barents Sea and its sea ice cover.

  • 63. Browse, J.
    et al.
    Carslaw, K. S.
    Mann, G. W.
    Birch, C. E.
    Arnold, S. R.
    Leck, Caroline
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    The complex response of Arctic aerosol to sea-ice retreat2014Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 14, nr 14, s. 7543-7557Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Loss of summertime Arctic sea ice will lead to a large increase in the emission of aerosols and precursor gases from the ocean surface. It has been suggested that these enhanced emissions will exert substantial aerosol radiative forcings, dominated by the indirect effect of aerosol on clouds. Here, we investigate the potential for these indirect forcings using a global aerosol microphysics model evaluated against aerosol observations from the Arctic Summer Cloud Ocean Study (ASCOS) campaign to examine the response of Arctic cloud condensation nuclei (CCN) to sea-ice retreat. In response to a complete loss of summer ice, we find that north of 70 degrees N emission fluxes of sea salt, marine primary organic aerosol (OA) and dimethyl sulfide increase by a factor of similar to 10, similar to 4 and similar to 15 respectively. However, the CCN response is weak, with negative changes over the central Arctic Ocean. The weak response is due to the efficient scavenging of aerosol by extensive drizzling stratocumulus clouds. In the scavenging-dominated Arctic environment, the production of condensable vapour from oxidation of dimethyl sulfide grows particles to sizes where they can be scavenged. This loss is not sufficiently compensated by new particle formation, due to the suppression of nucleation by the large condensation sink resulting from sea-salt and primary OA emissions. Thus, our results suggest that increased aerosol emissions will not cause a climate feedback through changes in cloud microphysical and radiative properties.

  • 64. Budhavant, K. B.
    et al.
    Rao, P. S. P.
    Safai, P. D.
    Granat, L.
    Rodhe, Henning
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Chemical composition of the inorganic fraction of cloud-water at a high altitude site in West India2014Inngår i: Atmospheric Environment, ISSN 1352-2310, E-ISSN 1873-2844, Vol. 88, s. 59-65Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Data from a ground-based cloud-water collection system intercepting water from clouds at a mountain field station, Sinhagad near Pune in India are presented. This study was part of an Indo-Swedish Collaboration Project on Atmospheric Brown Cloud-Asia (ABC-A). Cloud-water and rainwater (wet-only) samples were collected during June 2007-Dec. 2010. Concentrations of major anions and cations were determined. Ion concentrations were generally higher (NO3-, about 8 times; SO42- and K+, 5 times; NH4+ times and Cl-, Na+, Ca2+, Mg2+ 3 times) in cloud-water samples than in rainwater samples collected during the same days. The average pH of cloud-water samples was 6.0 with about 20% of the values below 5.6 and only 4% less than 5.0. Despite high concentrations of SO42- and NO3- the cloud water samples were on average not more acidic than rainwater samples. This is different from most of the other studies of cloud-water composition which have noted a substantially higher acidity (i.e. lower pH) in cloud-water than in rainwater. The slightly alkaline (pH > 5.6) nature of the cloud-water samples is mainly due to the presence of soil derived calcium carbonate in quantities more than enough to neutralize the acids or their precursors. A separation of the cloud-water data into trajectory groups showed that samples in air-masses having spent the last few days over the Indian sub-continent were in general more acidic (due to anthropogenic emissions) than those collected during days with air-masses of marine origin. A high correlation mutually between Ca2+, Na+, NO3- and SO42- makes it difficult to estimate the contribution to SO42- from different sources. Anthropogenic SO2- emissions and soil dust may both give important contributions.

  • 65.
    Caballero, Rodrigo
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Carlson, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Surface superrotation2018Inngår i: Journal of the Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 75, nr 10, s. 3671-3689Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Equatorial superrotation is commonly observed in simulations of Earth and planetary climates, but is almost without exception found to occur only at upper levels, with zero or easterly winds at the surface. Surface superrotation—a state with climatological zonal-mean westerlies at the equatorial surface—would lead to a major reorganization of the tropical ocean circulation with important consequences for global climate. Here, we examine the mechanisms that give rise to surface superrotation. We identify four theoretical scenarios under which surface superrotation may be achieved. Using an axisymmetric model forced by prescribed zonal-mean torques, we provide concrete examples of surface superrotation under all four scenarios. We also find that we can induce surface superrotation in a full-complexity atmospheric general circulation model, albeit in an extreme parameter range (in particular, convective momentum transport is artificially increased by almost an order of magnitude). We conclude that a transition to surface superrotation is unlikely in Earth-like climates, including ancient or future warm climates, though this conclusion is subject to the currently large uncertainties in the parameterization of convective momentum transport.

  • 66.
    Caballero, Rodrigo
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Hanley, John
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Midlatitude Eddies, Storm-Track Diffusivity, and Poleward Moisture Transport in Warm Climates2012Inngår i: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 69, nr 11, s. 3237-3250Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Recent work using both simplified and comprehensive GCMs has shown that poleward moisture transport across midlatitudes follows Clausius-Clapeyron scaling at temperatures close to modern, but that it reaches a maximum at sufficiently elevated temperatures and then decreases with further warming. This study explores the reasons for this nonmonotonic behavior using a sequence of NCAR Community Atmosphere Model, version 3 (CAM3) simulations in an aquaplanet configuration spanning a broad range of climates. No significant change is found in the scale, structure, or organization of midlatitude eddies across these simulations. Instead, the high-temperature decrease in poleward moisture transport is attributed to the combined effect of decreasing eddy velocities and contracting mixing lengths. The contraction in mixing length is, in turn, a consequence of the decreasing eddy velocities in combination with constant eddy decorrelation time scales.

  • 67.
    Caballero, Rodrigo
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Huber, Matthew
    State-dependent climate sensitivity in past warm climates and its implications for future climate projections2013Inngår i: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 110, nr 35, s. 14162-14167Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Projections of future climate depend critically on refined estimates of climate sensitivity. Recent progress in temperature proxies dramatically increases the magnitude of warming reconstructed from early Paleogene greenhouse climates and demands a close examination of the forcing and feedback mechanisms that maintained this warmth and the broad dynamic range that these paleoclimate records attest to. Here, we show that several complementary resolutions to these questions are possible in the context of model simulations using modern and early Paleogene configurations. We find that (i) changes in boundary conditions representative of slow Earth system feedbacks play an important role in maintaining elevated early Paleogene temperatures, (ii) radiative forcing by carbon dioxide deviates significantly from pure logarithmic behavior at concentrations relevant for simulation of the early Paleogene, and (iii) fast or Charney climate sensitivity in this model increases sharply as the climate warms. Thus, increased forcing and increased slow and fast sensitivity can all play a substantial role in maintaining early Paleogene warmth. This poses an equifinality problem: The same climate can be maintained by a different mix of these ingredients; however, at present, the mix cannot be constrained directly from climate proxy data. The implications of strongly state-dependent fast sensitivity reach far beyond the early Paleogene. The study of past warm climates may not narrow uncertainty in future climate projections in coming centuries because fast climate sensitivity may itself be state-dependent, but proxies and models are both consistent with significant increases in fast sensitivity with increasing temperature.

  • 68. Carleer, M. R.
    et al.
    Boone, C. D.
    Walker, K. A.
    Bernath, P. F.
    Strong, K.
    Sica, R. J.
    Randall, C. E.
    Vömel, H.
    Kar, J.
    Höpfner, M.
    Milz, M.
    von Clarmann, T.
    Kivi, R.
    Valverde-Canossa, J.
    Sioris, C. E.
    Izawa, M. R. M.
    Dupuy, E.
    McElroy, C. T.
    Drummond, J. R.
    Nowlan, C. R.
    Zou, J.
    Nichitiu, F.
    Lossow, Stefan
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Urban, J.
    Murtagh, D.
    Dufour, D. G.
    Validation of water vapour profiles from the Atmospheric Chemistry Experiment (ACE)2008Inngår i: Atmospheric Chemistry and Physics Discussion: An Interactive Open Access Journal of the European Geosciences Union, Vol. 8, nr 2, s. 4499-4559Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The Atmospheric Chemistry Experiment (ACE) mission was launched in August 2003 to sound the atmosphere by solar occultation. Water vapour (H2O), one of the most important molecules for climate and atmospheric chemistry, is one of the key species provided by the two principal instruments, the infrared Fourier Transform Spectrometer (ACE-FTS) and the MAESTRO UV-Visible spectrometer (ACE-MAESTRO). The first instrument performs measurements on several lines in the 1362–2137 cm−1 range, from which vertically resolved H2O concentration profiles are retrieved, from 7 to 90 km altitude. ACE-MAESTRO measures profiles using the water absorption band in the near infrared part of the spectrum at 926.0–969.7 nm. This paper presents a comprehensive validation of the ACE-FTS profiles. We have compared the H2O volume mixing ratio profiles with space-borne (SAGE II, HALOE, POAM III, MIPAS, SMR) observations and measurements from balloon-borne frostpoint hygrometers and a ground based lidar. We show that the ACE-FTS measurements provide H2O profiles with small retrieval uncertainties in the stratosphere (better than 5% from 15 to 70 km, gradually increasing above). The situation is unclear in the upper troposphere, due mainly to the high variability of the water vapour volume mixing ratio in this region. A new water vapour data product from the ACE-MAESTRO (Measurement of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation) is also presented and initial comparisons with ACE-FTS are discussed.

  • 69.
    Carlson, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Atmospheric dynamics and the hydrologic cycle in warm climates2018Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Past warm climates represent one extreme of Earth's known climate states. Here, we study warm climates in both idealized simulations and full-complexity general circulation model (GCM) simulations of the early Eocene epoch, approximately 50 million years ago.

    In increasingly warmer idealized aquaplanet simulations, the amplitude of intra-seasonal tropical variability is enhanced. The anomalies propagate eastward in the tropics and resemble the observed Madden-Julian Oscillation (MJO). The strong MJO anomalies drive a momentum convergence on the equator that causes westerly winds in the troposphere, a state known as superrotation. The results in this thesis show that superrotation further enhances the MJO by affecting the penetration of midlatitude eddies into the deep tropics. An additional question is how a super-rotating atmosphere, a dramatically different general circulation regime compared to today, will affect the climate, potentially via changes in cloud distributions and ocean circulation. If the superrotation extends down to the surface near the equator, surface westerly winds will drive equatorial downwelling in the eastern equatorial Pacific Ocean, rather than upwelling as in the present climate. Here, we show that surface superrotation is unlikely in past warm climates, although this in part depends on the intensity of the vertical momentum transfer associated with cumulus convection and how this process is represented in a specific GCM. 

    There is, currently, no consensus on what the specific mix of forcings was that caused the warm climates of the early Eocene. High greenhouse gases likely played a significant role, but simulations with reasonable greenhouse gas concentrations cannot reproduce the high temperatures estimated by proxy data. Here, we investigate both an early Eocene climate forced by high greenhouse gas concentrations and one forced by optically thinner clouds, with artificially increased cloud droplet radius that causes increased solar radiation at the surface. Both alternative warming scenarios produce nearly identical zonal mean temperatures, but the hydrologic cycle differs; the thinner clouds scenario has 11% larger global mean precipitation. Moreover, the results in this thesis indicate that a reasonable estimate of vegetation, based on the model simulation, is likely necessary to evaluate alternative warming scenarios with proxy data.

  • 70.
    Caron, Louis-Philippe
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Jones, Colin G.
    Understanding and simulating the link between African easterly waves and Atlantic tropical cyclones using a regional climate model: the role of domain size and lateral boundary conditions2012Inngår i: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 39, nr 1-2, s. 113-135Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Using a suite of lateral boundary conditions, we investigate the impact of domain size and boundary conditions on the Atlantic tropical cyclone and african easterly Wave activity simulated by a regional climate model. Irrespective of boundary conditions, simulations closest to observed climatology are obtained using a domain covering both the entire tropical Atlantic and northern African region. There is a clear degradation when the high-resolution model domain is diminished to cover only part of the African continent or only the tropical Atlantic. This is found to be the result of biases in the boundary data, which for the smaller domains, have a large impact on TC activity. In this series of simulations, the large-scale Atlantic atmospheric environment appears to be the primary control on simulated TC activity. Weaker wave activity is usually accompanied by a shift in cyclogenesis location, from the MDR to the subtropics. All ERA40-driven integrations manage to capture the observed interannual variability and to reproduce most of the upward trend in tropical cyclone activity observed during that period. When driven by low-resolution global climate model (GCM) integrations, the regional climate model captures interannual variability (albeit with lower correlation coefficients) only if tropical cyclones form in sufficient numbers in the main development region. However, all GCM-driven integrations fail to capture the upward trend in Atlantic tropical cyclone activity. In most integrations, variations in Atlantic tropical cyclone activity appear uncorrelated with variations in African easterly wave activity.

  • 71.
    Caron, Louis-Philippe
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Jones, Colin G.
    Doblas-Reyes, Francisco
    Multi-year prediction skill of Atlantic hurricane activity in CMIP5 decadal hindcasts2014Inngår i: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 42, nr 9-10, s. 2675-2690Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Using a statistical relationship between simulated sea surface temperature and Atlantic hurricane activity, we estimate the skill of a CMIP5 multi-model ensemble at predicting multi-annual level of Atlantic hurricane activity. The series of yearly-initialized hindcasts show positive skill compared to simpler forecasts such as persistence and climatology as well as non-initialized forecasts and return anomaly correlation coefficients of similar to 0.6 and similar to 0.8 for five and nine year forecasts, respectively. Some skill is shown to remain in the later years and making use of those later years to create a lagged-ensemble yields, for individual models, results that approach that obtained by the multi-model ensemble. Some of the skill is shown to come from persisting rather than predicting the climate shift that occur in 1994-1995. After accounting for that shift, the anomaly correlation coefficient for five-year forecasts is estimated to drop to 0.4, but remains statistically significant up to lead years 3-7. Most of the skill is shown to come from the ability of the forecast systems at capturing change in Atlantic sea surface temperature, although the failure of most systems at reproducing the observed slow down in warming over the tropics in recent years leads to an underestimation of hurricane activity in the later period.

  • 72.
    Caron, Louis-Philippe
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Jones, Colin G.
    Vaillancourt, Paul A.
    Winger, Katja
    On the relationship between cloud-radiation interaction, atmospheric stability and Atlantic tropical cyclones in a variable-resolution climate model2013Inngår i: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 40, nr 5-6, s. 1257-1269Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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.

  • 73. Casini, Michele
    et al.
    Blenckner, Thorsten
    Stockholms universitet, Stockholm Resilience Centre, Baltic Nest Institute.
    Moellmann, Christian
    Gardmark, Anna
    Lindegren, Martin
    Llope, Marcos
    Kornilovs, Georgs
    Plikshs, Maris
    Stenseth, Nils Christian
    Predator transitory spillover induces trophic cascades in ecological sinks2012Inngår i: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 109, nr 21, s. 8185-8189Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Understanding the effects of cross-system fluxes is fundamental in ecosystem ecology and biological conservation. Source-sink dynamics and spillover processes may link adjacent ecosystems by movement of organisms across system boundaries. However, effects of temporal variability in these cross-system fluxes on a whole marine ecosystem structure have not yet been presented. Here we show, using 35 y of multitrophic data series from the Baltic Sea, that transitory spillover of the top-predator cod from its main distribution area produces cascading effects in the whole food web of an adjacent and semi-isolated ecosystem. At varying population size, cod expand/contract their distribution range and invade/retreat from the neighboring Gulf of Riga, thereby affecting the local prey population of herring and, indirectly, zooplankton and phytoplankton via top-down control. The Gulf of Riga can be considered for cod a true sink habitat, where in the absence of immigration from the source areas of the central Baltic Sea the cod population goes extinct due to the absence of suitable spawning grounds. Our results add a metaecosystem perspective to the ongoing intense scientific debate on the key role of top predators in structuring natural systems. The integration of regional and local processes is central to predict species and ecosystem responses to future climate changes and ongoing anthropogenic disturbances.

  • 74.
    Chafik, Léon
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Dynamics and Variability of the Circulation in the North-Atlantic Subpolar Seas2014Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    This thesis deals with the dynamics and circulation in the northern North Atlantic and the Nordic Seas, processes of crucial importance for the mild climate of Scandinavia and Northern Europe. High-resolution ADCP scans of currents from Greenland to Scotland in the top 400 m demonstrate that the Reykjanes Ridge is a very effective separator of flow towards the Nordic and Labrador Seas, respectively. It was found that the meridional overturning circulation has weakened by ~1.7 Sv (1 Sv = 106 m3 s-1) during the 18-year period when altimetric data were available. This trend may be an effect of the Atlantic Multidecadal Oscillation, but is certainly not due to the North Atlantic Oscillation (NAO). By studying the circulation in the Faroe-Shetland Channel, which is an important choke point for the global thermohaline circulation, it was concluded that the contraction of the Norwegian-Sea gyre during low NAO periods plays an important role for disturbing the flow pattern. This specifically affects the regional ocean climate by leading to an accumulation of warm and saline Atlantic waters in the channel. During high NAO phases the circulation is strongly topographically controlled. The Norwegian Atlantic Slope Current (NwASC) is the main flow branch linking the North Atlantic to the Arctic and Barents Sea. It was found that the NwASC is largely coherent over seasonal to interannual time-scales. However, on shorter time-scales the coherency of the flow shows a sustained and pronounced weakening downstream of Lofoten. Intense eddy-shedding from the slope into the Lofoten Basin damps the coherent structure of the flow. The eddies take about two months to propagate to and to merge with the semi-permanent anticyclonic vortex above the deepest part of the Lofoten Basin. These results have implications for how flow/hydrographic anomalies are transferred through the Nordic Seas towards the Arctic. Anomalous transports of warm water into the Arctic and Barents Sea via the NwASC are found to be driven by a combination of the NAO and the other two leading modes of atmospheric variability in the North Atlantic. The results reported in the thesis may be of importance for achieving a correct representation of the heat conveyed polewards in climate models.

  • 75.
    Chafik, Léon
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Nilsson, Johan
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Lundberg, Peter
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Skagseth, Øystein
    Hannachi, Abdelwahab
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Atmospheric circulation patterns control the variability of the oceanic transport towards the Arctic regionManuskript (preprint) (Annet vitenskapelig)
  • 76. Chang, R. Y. -W
    et al.
    Leck, Caroline
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Graus, M.
    Mueller, M.
    Paatero, J.
    Burkhart, J. F.
    Stohl, A.
    Orr, L. H.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Hayden, K.
    Li, S. -M
    Hansel, A.
    Tjernström, Michael
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Leaitch, W. R.
    Abbatt, J. P. D.
    Aerosol composition and sources in the central Arctic Ocean during ASCOS2011Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 11, nr 20, s. 10619-10636Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Measurements of submicron aerosol chemical composition were made over the central Arctic Ocean from 5 August to 8 September 2008 as a part of the Arctic Summer Cloud Ocean Study (ASCOS) using an aerosol mass spectrometer (AMS). The median levels of sulphate and organics for the entire study were 0.051 and 0.055 mu gm(-3), respectively. Positive matrix factorisation was performed on the entire mass spectral time series and this enabled marine biogenic and continental sources of particles to be separated. These factors accounted for 33% and 36% of the sampled ambient aerosol mass, respectively, and they were both predominantly composed of sulphate, with 47% of the sulphate apportioned to marine biogenic sources and 48% to continental sources, by mass. Within the marine biogenic factor, the ratio of methane sulphonate to sulphate was 0.25+/-0.02, consistent with values reported in the literature. The organic component of the continental factor was more oxidised than that of the marine biogenic factor, suggesting that it had a longer photochemical lifetime than the organics in the marine biogenic factor. The remaining ambient aerosol mass was apportioned to an organic-rich factor that could have arisen from a combination of marine and continental sources. In particular, given that the factor does not correlate with common tracers of continental influence, we cannot rule out that the organic factor arises from a primary marine source.

  • 77. Charlson, Robert J.
    et al.
    Ackerman, Andrew S.
    Bender, Frida
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Anderson, Theodore L.
    Liu, Zhaoyan
    On the climate forcing consequences of the albedo continuum between cloudy and clear air2007Inngår i: Tellus. Series B: Chemical and Physical Meteorology, ISSN 0280-0889, Vol. 59, nr 4, s. 715-727Artikkel i tidsskrift (Fagfellevurdert)
  • 78.
    Charpentier Ljungqvist, Fredrik
    Stockholms universitet, Humanistiska fakulteten, Historiska institutionen. Stockholms universitet, Humanistiska fakulteten, Centrum för medeltidsstudier.
    A new reconstruction of temperature variability in the extra-tropical Northern Hemisphere during the last two millennia2010Inngår i: Geografiska Annaler. Series A, Physical Geography, ISSN 0435-3676, E-ISSN 1468-0459, Vol. 92A, nr 3, s. 339-351Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A new temperature reconstruction with decadal resolution, covering the last two millennia, is presented for the extra-tropical Northern Hemisphere (90–30°N), utilizing many palaeotemperature proxy records never previously included in any large-scale temperature reconstruction. The amplitude of the reconstructed temperature variability on centennial time-scales exceeds 0.6°C. This reconstruction is the first to show a distinct Roman Warm Period c. AD 1–300, reaching up to the 1961–1990 mean temperature level, followed by the Dark Age Cold Period c. AD 300–800. The Medieval Warm Period is seen c. AD 800–1300 and the Little Ice Age is clearly visible c. AD 1300–1900, followed by a rapid temperature increase in the twentieth century. The highest average temperatures in the reconstruction are encountered in the mid to late tenth century and the lowest in the late seventeenth century. Decadal mean temperatures seem to have reached or exceeded the 1961–1990 mean temperature level during substantial parts of the Roman Warm Period and the Medieval Warm Period. The temperature of the last two decades, however, is possibly higher than during any previous time in the past two millennia, although this is only seen in the instrumental temperature data and not in the multi-proxy reconstruction itself. Our temperature reconstruction agrees well with the reconstructions by Moberg et al. (2005) and Mann et al. (2008) with regard to the amplitude of the variability as well as the timing of warm and cold periods, except for the period c. AD 300–800, despite significant differences in both data coverage and methodology.

  • 79.
    Charpentier Ljungqvist, Fredrik
    Stockholms universitet, Humanistiska fakulteten, Historiska institutionen. Stockholms universitet, Humanistiska fakulteten, Centrum för medeltidsstudier.
    A regional approach to the Medieval Warm Period and the Little Ice Age2010Inngår i: Climate Change and Variability / [ed] Suzanne W. Simard & Mary E. Austin, Rijeka: Sciyo , 2010, s. 1-25Kapittel i bok, del av antologi (Annet vitenskapelig)
  • 80.
    Charpentier Ljungqvist, Fredrik
    Stockholms universitet, Humanistiska fakulteten, Historiska institutionen.
    Global nedkylning: klimatet och människan under 10 000 år2009Bok (Annet (populærvitenskap, debatt, mm))
    Abstract [sv]

    Klimatet har förändrats både regionalt och globalt sedan senaste istiden tog slut, ofta med dramatiska konsekvenser för naturen och människan. Fastän det talas så mycket om klimatförändringar idag är det få som vet särskilt mycket om hur klimatet har varierat förr.

    Det är först under de senaste åren som forskningen börjat kunna beskriva vad som faktiskt hänt med klimatet under olika tider, på olika platser. Historikern Fredrik Charpentier Ljungqvist har tagit ett samlat grepp på den senaste forskningen och resultatet är en resa i vått och torrt, i hetta och kyla, jorden runt under 10 000 år. Vi får stifta bekantskap med många olika folk och kulturer – babylonier, romare, mayaindianer och vikingar – som alla under historiens gång varit utsatta för klimatförändringar.

  • 81.
    Charpentier Ljungqvist, Fredrik
    Stockholms universitet, Humanistiska fakulteten, Historiska institutionen.
    Klimat, missväxt och extremt väder 1830–19202012Inngår i: Sveriges historia : 1830–1920 / [ed] Bo Stråth, Stockholm: Norstedts Förlag, 2012, s. 289-292Kapittel i bok, del av antologi (Annet (populærvitenskap, debatt, mm))
  • 82.
    Charpentier Ljungqvist, Fredrik
    Stockholms universitet, Humanistiska fakulteten, Historiska institutionen.
    Temperature proxy records covering the last two millennia: a tabular and visual overview2009Inngår i: Geografiska Annaler: Physical Geography, ISSN 0435-3676, Vol. 91A, nr 1, s. 11-29Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Proxy data are our only source to knowledge of temperature variability in the period prior to instrumental temperature measurements. Until recently, very few quantitative palaeotemperature records extended back a millennium or more but the number is now increasing. Here, the first systematic survey is presented, with graphic representations, of most quantitative temperature proxy data records covering the last two millennia that have been published in the peer-reviewed literature. In total, 71 series are presented together with basic essential information on each record. This overview will hopefully assist future palaeoclimatical research by facilitating an orientation among available palaeotemperature records and thus reduce the risk of missing less well-known proxy series. The records show an amplitude between maximum and minimum temperatures during the past two millennia on centennial time scales ranging from c. 0.5 to 4 °C and averaging c. 1.5–2 °C for both high and low latitudes although these variations are not always occurring synchronically. Both the Medieval Warm Period, the Little Ice Age and the 20th century warming are clearly visible in most records, whereas the Roman Warm Period and the Dark Age Cold Period are less clearly discernible.

  • 83.
    Charpentier Ljungqvist, Fredrik
    et al.
    Stockholms universitet, Humanistiska fakulteten, Historiska institutionen.
    Grudd, Håkan
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Arctic summer temperature variability since AD 8002009Inngår i: European climate of the last millennium: Millennium milestone meeting 3: Book of abstract, 2009, s. 8-9Konferansepaper (Annet vitenskapelig)
  • 84.
    Chiacchio, Marc
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU). Abdus Salam International Centre for Theoretical Physics, Italy.
    Solmon, Fabien
    Giorgi, Filippo
    Stackhouse, Paul, Jr.
    Wild, Martin
    Evaluation of the radiation budget with a regional climate model over Europe and inspection of dimming and brightening2015Inngår i: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 120, nr 5, s. 1951-1971Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Shortwave (SW) and longwave (LW) components of the radiation budget at the surface and top of atmosphere (TOA) are evaluated in the regional climate model RegCM version 4 driven by European Centre for Medium-Range Weather Forecasts Reanalysis over Europe. The simulated radiative components were evaluated with those from satellite-based products and reanalysis. At the surface the model overestimated the absorbed solar radiation but was compensated by a greater loss of thermal energy while both SW and LW TOA net fluxes were underestimated representing too little solar energy absorbed and too little outgoing thermal energy. Averaged biases in radiative parameters were generally within 25 Wm(-2), were dependent on differences by as much as 0.2 in cloud fraction, surface, and planetary albedo and less dependent on surface temperature associated with the surface longwave parameters, and are in line with other studies. Clear-sky fluxes showed better results when cloud cover differences had no influence. We also found a clear distinction between land versus water with smaller biases over land at the surface and over water at the TOA due to differences in cloud fraction and albedo. Finally, we inspected dimming and brightening for the period 1979-2010 with an indication for dimming early in the time series (i.e., 1979-1987) and brightening after, which agrees with surface-based observations. After 2000, however, a decrease in the brightening by more than 1 order of magnitude was evident which is in contrast to the continued brightening found in surface records and satellite-derived estimates.

  • 85.
    Colleoni, Florence
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för geologiska vetenskaper. Joseph Fourier University, France.
    Liakka, Johan
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Krinner, Gerhard
    Jakobsson, Martin
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för geologiska vetenskaper.
    Masina, Simona
    Peyaud, Vincent
    The sensitivity of the Late Saalian (140 ka) and LGM (21 ka) Eurasian ice sheets to sea surface conditions2011Inngår i: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 37, nr 3-4, s. 531-553Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This work focuses on the Late Saalian (140 ka) Eurasian ice sheets’ surface mass balance (SMB) sensitivity to changes in sea surface temperatures (SST). An Atmospheric General Circulation Model (AGCM), forced with two preexisting Last Glacial Maximum (LGM, 21 ka) SST reconstructions, is used to compute climate at 140 and 21 ka (reference glaciation). Contrary to the LGM, the ablation almost stopped at 140 ka due to the climatic cooling effect from the large ice sheet topography. Late Saalian SST are simulated using an AGCM coupled with a mixed layer ocean. Compared to the LGM, these 140 ka SST show an inter-hemispheric asymmetry caused by the larger ice-albedo feedback, cooling climate. The resulting Late Saalian ice sheet SMB is smaller due to the extensive simulated sea ice reducing the precipitation. In conclusion, SST are important for the stability and growth of the Late Saalian Eurasian ice sheet.

  • 86. Cook, Edward R.
    et al.
    Krusic, Paul J.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Anchukaitis, Kevin J.
    Buckley, Brendan M.
    Nakatsuka, Takeshi
    Sano, Masaki
    Tree-ring reconstructed summer temperature anomalies for temperate East Asia since 800 CE2013Inngår i: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 41, nr 11-12, s. 2957-2972Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We develop a summer temperature reconstruction for temperate East Asia based on a network of annual tree-ring chronologies covering the period 800-1989 C.E. The East Asia reconstruction is the regional average of 585 individual grid point summer temperature reconstructions produced using an ensemble version of point-by-point regression. Statistical calibration and validation tests indicate that the regional average possesses sufficient overall skill to allow it to be used to study the causes of temperature variability and change over the region. The reconstruction suggests a moderately warm early medieval epoch (ca. 850-1050 C.E.), followed by generally cooler 'Little Ice Age' conditions (ca. 1350-1880 C.E.) and 20th century warming up to the present time. Since 1990, average temperature has exceeded past warm epochs of comparable duration, but it is not statistically unprecedented. Superposed epoch analysis reveals a volcanic forcing signal in the East Asia summer temperature reconstruction, resulting in pulses of cooler summer conditions that may persist for several years. Substantial uncertainties remain, however, particularly at lower frequencies, thus requiring caution and scientific prudence in the interpretation of this record.

  • 87.
    Corell, Hanna
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Moksnes, Per-Olav
    Engqvist, Anders
    Jonsson, Per R.
    Larval depth distribution critically affects dispersal and the efficiency of marine protected areasInngår i: Marine Ecology Progress Series, ISSN 0171-8630, E-ISSN 1616-1599Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This study aims to improve estimates of dispersal by including information on larval traits, and in particular to explore how larval depth distribution affects connectivity and MPA functionality in the Baltic Sea. A field survey showed that both invertebrates and fish differed in their larval depth distribution ranging from surface waters to more than 100 m. A biophysical model of larval dispersal in the Baltic Sea showed that decreased depthdistribution increased average dispersal distance 2.5 times, decreased coastal retention and local recruitment, and increased connectivity substantially. Together with pelagic larval duration (PLD), depth distribution explained 80% of total variation in dispersal distance, whereas spawning season, geographic and annual variations in circulation had only marginal effects. Median dispersal distances varied between 8 and 46 km, with 10% of simulated trajectories dispersing beyond 30-160 km depending on drift depth and PLD. In the Baltic Sea, the majority of shallow Natura 2000 MPAs are smaller than 8 km. In the present study, only one of the 11 assessed larval taxa would have a local recruitment >10% within MPAs of this size. Connectivity between MPAs was expected to be low for most larval trait combinations. Our simulations and the empirical data suggest that the MPA size within the Natura2000 system is considerably below what is required for local recruitment of most sessile invertebrates and sedentary fish. Future designs of MPA networks would benefit from spatially explicit biophysical models that consider dispersal and connectivity for complex circulation patterns and informed larval traits.

  • 88.
    Coz Diego, Esther
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Leck, Caroline
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Morphology and state of mixture of atmospheric soot aggregates during the winter season over Southern Asia-a quantitative approach2011Inngår i: Tellus. Series B, Chemical and physical meteorology, ISSN 0280-6509, E-ISSN 1600-0889, Vol. 63, nr 1, s. 107-116Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The atmospheric brown cloud phenomena characterized by a high content of soot and a large impact on the solar radiative heating especially affects the tropical Indian Ocean during the winter season. The present study focuses on morphological characteristics and state of mixture of soot aggregates during the winter season over India. Given are quantitative measures of size, morphology and texture on aggregates collected in air at two different sites: Sinhagad near Pune in India and Hanimaadhoo in Maldives. For the latter site two different synoptic patterns prevailed: advection of air from the Arabian region and from the Indian subcontinent, respectively. Aggregates collected at Sinhagad, were associated with open branched structures, characteristic of fresh emission and diameters between 220 and 460 nm. The Hanimaadhoo aggregates were associated with aged closed structures, smaller sizes (130-360 nm) and frequently contained inorganic inclusions. Those arriving from the Indian subcontinent were characterized by the presence of an additional organic layer that covered the aggregate structure. These organic coatings might be a reasonable explanation of the low average wash-out ratios of soot two to seven times lower than that of nss-SO(4)2- that have been reported for air flow arriving at Hanimaadhoo from the Indian subcontinent in winter.

  • 89.
    Craig, Stephens
    Stockholms universitet.
    Uncertainties and variability in the global carbon cycle1998Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Anthropogenic emissions of carbon dioxide (CO2) are perturbing the radiation balance of the atmosphere that ultimately determines the climate for all life on Earth. The prediction of future climate change and the explanation of observed changes are contingent on us quantitatively understanding the link between CO2 emissions and atmospheric concentrations that is provided by the biogeochemical carbon cycle. Our knowledge of key processes is currently incomplete and there is a perceived imbalance in the atmospheric carbon budget. In this thesis, the imbalance and its uncertainty are calculated from preindustrial times to 1990 and it is shown to be statistically significant from the 1950s to the present day. The implications for future CO2 projections are demonstrated by calculating atmospheric concentrations to 2100 assuming that different mechanisms (CO2 fertilisation and temperate forest regrowth) are responsible for the budget imbalance. The different balancing mechanisms result in divergent concentration projections, thereby illustrating the importance of resolving the issue. A step in this direction is taken by simulating terrestrial carbon exchange and atmospheric CO2 concentrations on-line in an atmospheric general circulation model, whose advanced land surface parameterisation calculates carbon fluxes at each time step. After identifying important strengths and weaknesses in an evaluation of the model, the climate induced interannual variability in terrestrial carbon exchange and atmospheric CO2 concentrations is examined. This variability represents carbon cycle "noise" that is superimposed on the long-term anthropogenic trend. It is shown that fluctuations driven by unforced variability in the atmosphere are likely to be non-negligible relative to perturbations from specific phenomena like El Niño and volcanic eruptions. Continental regions exhibit large variations, and yet they are poorly sampled by the current CO2 monitoring program. The effects of El Niño are investigated and isolated from these unforced variations in an ensemble integration. Land areas are simulated to release CO2 to the atmosphere during El Niño events as a result of meteorological perturbations driven by the sea surface temperature anomalies. The release is sufficiently large to explain why atmospheric CO2 concentrations are observed to increase during El Niño, even though the usual CO2 outgassing from the equatorial Pacific is diminished.

  • 90. Croft, B.
    et al.
    Wentworth, G. R.
    Martin, R. V.
    Leaitch, W. R.
    Murphy, J. G.
    Murphy, Benjamin N.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för miljövetenskap och analytisk kemi. U.S. Environmental Protection Agency, USA.
    Kodros, J. K.
    Abbatt, J. P. D.
    Pierce, J. R.
    Contribution of Arctic seabird-colony ammonia to atmospheric particles and cloud-albedo radiative effect2016Inngår i: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 7, artikkel-id 13444Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The Arctic region is vulnerable to climate change and able to affect global climate. The summertime Arctic atmosphere is pristine and strongly influenced by natural regional emissions, which have poorly understood climate impacts related to atmospheric particles and clouds. Here we show that ammonia from seabird-colony guano is a key factor contributing to bursts of newly formed particles, which are observed every summer in the near-surface atmosphere at Alert, Nunavut, Canada. Our chemical-transport model simulations indicate that the pan-Arctic seabird-influenced particles can grow by sulfuric acid and organic vapour condensation to diameters sufficiently large to promote pan-Arctic cloud-droplet formation in the clean Arctic summertime. We calculate that the resultant cooling tendencies could be large (about -0.5Wm(-2) pan-Arctic-mean cooling), exceeding -1Wm(-2) near the largest seabird colonies due to the effects of seabird-influenced particles on cloud albedo. These coupled ecological-chemical processes may be susceptible to Arctic warming and industrialization.

  • 91.
    Crona, Beatrice
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Stockholm Resilience Centre.
    Wutich, Amber
    Brewis, Alexandra
    Gartin, Meredith
    Perceptions of climate change: Linking local and global perceptions through a cultural knowledge approach2013Inngår i: Climatic Change, ISSN 0165-0009, E-ISSN 1573-1480, Vol. 119, nr 2, s. 519-531Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Understanding public perceptions of climate change is fundamental to both climate science and policy because it defines local and global socio-political contexts within which policy makers and scientists operate. To date, most studies addressing climate change perceptions have been place-based. While such research is informative, comparative studies across sites are important for building generalized theory around why and how people understand and interpret climate change and associated risks. This paper presents a cross-sectional study from six different country contexts to illustrate a novel comparative approach to unraveling the complexities of local vs global perceptions around climate change. We extract and compare 'cultural knowledge' regarding climate change using the theory of 'culture as consensus'. To demonstrate the value of this approach, we examine cross-national data to see if people within specific and diverse places share ideas about global climate change. Findings show that although data was collected using ethnographically derived items collected through place-based methods we still find evidence of a shared cultural model of climate change which spans the diverse sites in the six countries. Moreover, there are specific signs of climate change which appear to be recognized cross-culturally. In addition, results show that being female and having a higher education are both likely to have a positive effect on global cultural competency of individuals. We discuss these result in the context of literature on environmental perceptions and propose that people with higher education are more likely to share common perceptions about climate change across cultures and tentatively suggest that we appear to see the emergence of a 'global', cross-cultural mental model around climate change and its potential impacts which in itself is linked to higher education.

  • 92. Cui, Z
    et al.
    Tjernström, Michael
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Grisogono, Bramko
    Idealized simulations of atmospheric coastal flow along the central coast of California1998Inngår i: Journal of applied meteorology (1988), ISSN 0894-8763, E-ISSN 1520-0450, Vol. 37, s. 1332-1363Artikkel i tidsskrift (Fagfellevurdert)
  • 93.
    Dalirian, Maryam
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för miljövetenskap och analytisk kemi.
    Investigating parameters governing liquid-phase cloud activation of atmospheric particles2018Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Aerosol-cloud interactions are one of the main sources of uncertainties in modeling and predicting the Earth’s climate. To overcome this uncertainty, we need to improve the understanding about the processes and parameters defining how aerosol particles turn into cloud condensation nuclei (CCN) or ice nuclei (IN) to produce cloud droplets or ice crystals. The focus of this dissertation is on liquid phase cloud droplets. The thesis investigates the effect of water solubility and surface tension on the CCN activity of atmospheric aerosol particles. These parameters are among the key properties defining how an aerosol particle can turn into a cloud droplet. The main goals of this thesis are to investigate 1) the CCN activity of aerosol particles containing both water soluble and insoluble substances and 2) the contribution of molecular-scale surface structure to the surface tension and CCN activity of atmospherically relevant aqueous mixtures.

    In the first part of this thesis, the CCN activity of water-insoluble aerosol constituents coated by water-soluble or sparingly soluble species was investigated. The results showed that the CCN activity of the insoluble silica and black carbon particles, with sizes between 100 and 300 nm, increased with the amount of the coating on the insoluble cores and at thick enough coating approached the CCN activity of the soluble species. Moreover, controlled dry coating of the insoluble BC cores yielded a size-independent distribution of the coating material on the insoluble cores, which was not achieved by wet coating of the silica particles. The results also confirmed that by knowing the fraction of soluble material (coating thicknesses), the existing theories gave a reasonable estimate of the CCN activity for the mixed soluble-insoluble particles. Finally, the results highlight the need for including the impacts of co-emitted or later condensed compounds in estimates of the climate impacts of atmospheric insoluble aerosol species.

    In the second part of the thesis, surface propensity of succinic acid, pure or mixed with soluble inorganic salts in the aqueous droplets, were quantified via molecular-level surface composition measurement by X-ray Photoelectron Spectroscopy (XPS). The XPS and molecular dynamic (MD) simulations of succinic acid aqueous solutions showed strong enrichment of the succinic acid at the surface of the liquid droplets compared to the bulk solution. This effect was more pronounced in the presence of the highly soluble inorganic salts like NaCl and (NH4)2SO4 in the system. The modeled surface tension of the pure organic or mixture of organic and inorganic substances, using surface enrichment factors derived from the XPS experiments were in good agreement with the experimental surface tension data. This demonstrates the high potential of XPS for direct measurements of the surface composition of atmospherically relevant aqueous mixtures. The results suggest that for modeling the phase-state and water content of the atmospheric particles, the contribution by the surface layer needs to be considered, because aqueous droplet can contain larger amounts of organic compounds than the bulk solubility limit of the solutions. However, the effect of the aqueous surface composition on the CCN activation of particles consisting of the studied mixtures was estimated to be very small.

    The results presented in this thesis provide new insights into the relationship between aerosol particle composition and cloud condensation nuclei activity. However, the effect of more realistic complex mixtures will require more research. The results showed that for modeling semi-volatile species, the partitioning between the gas and condensed phase needs to be considered. In addition, along with the liquid-phase cloud activation, the ice nucleation ability of the particles made of soluble and insoluble species requires to be further investigated.

  • 94.
    Dalirian, Maryam
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för miljövetenskap och analytisk kemi.
    Keskinen, H.
    Ahlm, Lars
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för miljövetenskap och analytisk kemi.
    Ylisirniö, A.
    Romakkaniemi, S.
    Laaksonen, A.
    Virtanen, A.
    Riipinen, Ilona
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för miljövetenskap och analytisk kemi.
    CCN activation of fumed silica aerosols mixed with soluble pollutants2015Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 15, nr 7, s. 3815-3829Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Particle-water interactions of completely soluble or insoluble particles are fairly well understood but less is known of aerosols consisting of mixtures of soluble and insoluble components. In this study, laboratory measurements were performed to investigate cloud condensation nuclei (CCN) activity of silica particles mixed with ammonium sulfate (a salt), sucrose (a sugar) and bovine serum albumin known as BSA (a protein). The agglomerated structure of the silica particles was investigated using measurements with a differential mobility analyser (DMA) and an aerosol particle mass analyser (APM). Based on these data, the particles were assumed to be compact agglomerates when studying their CCN activation capabilities. Furthermore, the critical super-saturations of particles consisting of pure and mixed soluble and insoluble compounds were explored using existing theoretical frameworks. These results showed that the CCN activation of single-component particles was in good agreement with Kohler- and adsorption theory based models when the agglomerated structure was accounted for. For mixed particles the CCN activation was governed by the soluble components, and the soluble fraction varied considerably with particle size for our wet-generated aerosols. Our results confirm the hypothesis that knowing the soluble fraction is the key parameter needed for describing the CCN activation of mixed aerosols, and highlight the importance of controlled coating techniques for acquiring a detailed understanding of the CCN activation of atmospheric insoluble particles mixed with soluble pollutants.

  • 95.
    Dalirian, Maryam
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för miljövetenskap och analytisk kemi.
    Ylisirniö, Arttu
    Buchholz, Angela
    Schlesinger, Daniel
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för miljövetenskap och analytisk kemi.
    Ström, Johan
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för miljövetenskap och analytisk kemi.
    Virtanen, Annele
    Riipinen, Ilona
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för miljövetenskap och analytisk kemi.
    Cloud droplet activation of black carbon particles coated with organic compounds of varying solubility2018Inngår i: Atmospheric Chemistry and Physics Discussions, ISSN 1680-7367, E-ISSN 1680-7375, Vol. 18, nr 16, s. 12477-12489Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Atmospheric black carbon (BC) particles are a concern due to their impact on air quality and climate. Their net climate effect 15 is, however, still uncertain. This uncertainty is partly related to the contribution of coated BC-particles to the global CCN budgets. In this study, laboratory measurements were performed to investigate cloud condensation nuclei (CCN) activity of BC (Regal black) particles, in pure state or coated through evaporating and subsequent condensation of glutaric acid, levoglucosan (both water-soluble organics) or oleic acid (an organic compound with low solubility). A combination of Soot Particle Aerosol Mass Spectrometer (SP-AMS) measurements and size distribution measurements with Scanning Mobility 20 Particle Sizer (SMPS) showed that the studied BC particles were nearly spherical agglomerates with a fractal dimension of 2.79 and that they were coated evenly by the organic species. The CCN activity of BC particles increased after coating with all the studied compounds and was governed by the fraction of organic material. The CCN activation of the BC particles coated by glutaric acid and levoglucosan were in good agreement with the theoretical calculations using shell-and-core model, which is based on a combination of the CCN activities of the pure compounds. The oleic acid coating enhanced the CCN 25 activity of the BC particles, even though the pure oleic acid particles were CCN inactive. The surprising effect of oleic acid might be related to the arrangement of the oleic acid molecules on the surface of the BC cores or other surface phenomena facilitating water condensation onto the coated particles. Our results show potential in accurately predicting the CCN activity of atmospheric BC coated with organic species by present theories, given that the identities and amount of the coating species are known. Furthermore, our results suggest that even relatively thin soluble coatings (around 2 nm for the compounds studied here) are enough to make the insoluble BC particles CCN active at typical atmospheric supersaturations and thus be efficiently taken up by cloud droplets. This highlights the need of an accurate description of the composition of atmospheric particles containing BC to unravel their net impact on climate.

  • 96. D'Andrea, S. D.
    et al.
    Hakkinen, S. A. K.
    Westervelt, D. M.
    Kuang, C.
    Levin, E. J. T.
    Kanawade, V. P.
    Leaitch, W. R.
    Spracklen, D. V.
    Riipinen, Ilona
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Pierce, J. R.
    Understanding global secondary organic aerosol amount and size-resolved condensational behavior2013Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 13, nr 22, s. 11519-11534Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Recent research has shown that secondary organic aerosols (SOA) are major contributors to ultrafine particle growth to climatically relevant sizes, increasing global cloud condensation nuclei (CCN) concentrations within the continental boundary layer (BL). However, there are three recent developments regarding the condensation of SOA that lead to uncertainties in the contribution of SOA to particle growth and CCN concentrations: (1) while many global models contain only biogenic sources of SOA (with annual production rates generally 10-30 Tg yr(-1)), recent studies have shown that an additional source of SOA around 100 Tg yr(-1) correlated with anthropogenic carbon monoxide (CO) emissions may be required to match measurements. (2) Many models treat SOA solely as semi-volatile, which leads to condensation of SOA proportional to the aerosol mass distribution; however, recent closure studies with field measurements show nucleation mode growth can be captured only if it is assumed that a significant fraction of SOA condenses proportional to the Fuchs-corrected aerosol surface area. This suggests a very low volatility of the condensing vapors. (3) Other recent studies of particle growth show that SOA con-densation at sizes smaller than 10 nm and that size-dependent growth rate parameterizations (GRP) are needed to match measurements. We explore the significance of these three findings using GEOS-Chem-TOMAS global aerosol microphysics model and observations of aerosol size distributions around the globe. The change in the concentration of particles of size D-p > 40 nm (N40) within the BL assuming surface-area condensation compared to mass-distribution net condensation yielded a global increase of 11% but exceeded 100% in biogenically active regions. The percent change in N40 within the BL with the inclusion of the additional 100 Tg SOAyr(-1) compared to the base simulation solely with biogenic SOA emissions (19 Tg yr-1) both using surface area condensation yielded a global increase of 13.7 %, but exceeded 50% in regions with large CO emissions. The inclusion of two different GRPs in the additional-SOA case both yielded a global increase in N40 of < 1 %, however exceeded 5% in some locations in the most extreme case. All of the model simulations were compared to measured data obtained from diverse locations around the globe and the results confirmed a decrease in the model-measurement bias and improved slope for comparing modeled to measured CCN number concentration when non-volatile SOA was assumed and the extra SOA was included.

  • 97.
    Das, Ruby
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Granat, Lennart
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Leck, Caroline
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Praveen, P. S.
    Rodhe, Henning
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Chemical composition of rainwater at Maldives Climate Observatory at Hanimaadhoo (MCOH)2011Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 11, nr 8, s. 3743-3755Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Water-soluble inorganic components in rain deposited at the Maldives Climate Observatory Hanimaadhoo (MCOH) were examined to determine seasonality and possible source regions. The study, which is part of the Atmospheric Brown Cloud (ABC) project, covers the period June 2005 to December 2007. Air mass trajectories were used to separate the data into situations with transport of air from India and adjacent parts of the Asian continent during the months December and January (Indian group) and those with southerly flow from the Indian Ocean during the summer monsoon season June to September (Marine group). A third trajectory group was identified with transport from the northern parts of the Arabian Sea and adjacent land areas during the months March, April and October (Arabian Sea group). The concentrations of nss-SO(4)(2-), NH(4)(+) and NO(3)(-) were more than a factor of 4 higher in the Indian group than in the Marine group. The average rainwater pH was significantly lower in the Indian group (4.7) than in the Marine group (6.0). This shows a pronounced influence of continental pollutants during December and January. The origin of the very high concentration of nss-Ca(2+) found in the Marine group - a factor of 7 higher than in the Indian group - is unclear. We discuss various possibilities including long-range transport from the African or Australian continents, local dust from nearby islands and calcareous plankton debris and exopolymer gels emitted from the ocean surface. The occurrence of NO(3)(-) and NH(4)(+) in the Marine group suggests emissions from the ocean surface. Part of the NO(3)(-) could also be associated with lightning over the ocean. Despite the fact that the concentrations of nss-SO(4)(2-), NO(3)(-), and NH(4)(+) were highest in the Indian group the wet deposition was at least as big in the Marine group reflecting the larger amount of rainfall during the monsoon season. The annual wet deposition of NO(3)(-), NH(4)(+) and nss-SO(4)(2-) at MCOH is about a factor of three lower than observed at rural sites in India.

  • 98. Davies, Frazer J.
    et al.
    Renssen, Hans
    Blaschek, Michael
    Muschitiello, Francesco
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för geologiska vetenskaper.
    The impact of Sahara desertification on Arctic cooling during the Holocene2015Inngår i: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 11, s. 571-586Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Since the start of the Holocene, temperatures in the Arctic have steadily declined. This has been accredited to the orbitally forced decrease in summer insolation reconstructed over the same period. However, here we present climate modelling results from an Earth model of intermediate complexity (EMIC) that indicate that 17–40% of the cooling in the Arctic, over the period 9–0 ka, was a direct result of the desertification that occurred in the Sahara after the termination of the African Humid Period. We have performed a suite of sensitivity experiments to analyse the impact of different combinations of forcings, including various vegetation covers in the Sahara. Our simulations suggest that over the course of the Holocene, a strong increase in surface albedo in the Sahara as a result of desertification led to a regional increase in surface pressure, a weakening of the trade winds, the westerlies and the polar easterlies, which in turn reduced the meridional heat transported by the atmosphere to the Arctic. We conclude that during interglacials, the climate of the Northern Hemisphere is sensitive to changes in Sahara vegetation type.

  • 99.
    de Boer, Agatha M.
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för geologiska vetenskaper.
    Collier, Andrew B.
    Caballero, Rodrigo
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Processes driving thunderstorms over the Agulhas Current2013Inngår i: JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, ISSN 2169-897X, Vol. 118, nr 5, s. 2220-2228Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Lightning occurs predominantly over land and is not common over the open ocean. We study here one oceanic region in which thunderstorms are frequently found, namely the warm Agulhas Current off the southeast coast of South Africa. The seasonal and interannual lightning variability is derived from satellite and terrestrial data sets. Favorable climatic conditions for lightning are investigated using both ERA-Interim and NCEP/NCAR reanalysis data. We find peak lightning in austral autumn over the Agulhas Current but with low seasonality (i.e., there is considerable lightning throughout the year). While the climatological wind direction varies strongly with latitude and season, the wind direction is predominantly northerly throughout the region during thunderstorms. A composite of sea level pressure during thunderstorm days indicates that thunderstorms are related to eastward-propagating synoptic-scale wave trains passing through the Agulhas Current region. The strong convective activity during thunderstorms occur in the warm sector of a cyclone and is associated with horizontal convergence and lifting of warm, moist surface air originating over the warm Agulhas Current.

  • 100. de Boer, G.
    et al.
    Shupe, M. D.
    Caldwell, P. M.
    Bauer, S. E.
    Persson, O.
    Boyle, J. S.
    Kelley, M.
    Klein, S. A.
    Tjernström, Michael
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Near-surface meteorology during the Arctic Summer Cloud Ocean Study (ASCOS): evaluation of reanalyses and global climate models2014Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 14, nr 1, s. 427-445Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Atmospheric measurements from the Arctic Summer Cloud Ocean Study (ASCOS) are used to evaluate the performance of three atmospheric reanalyses (European Centre for Medium Range Weather Forecasting (ECMWF)-Interim reanalysis, National Center for Environmental Prediction (NCEP)-National Center for Atmospheric Research (NCAR) reanalysis, and NCEP-DOE (Department of Energy) reanalysis) and two global climate models (CAM5 (Community Atmosphere Model 5) and NASA GISS (Goddard Institute for Space Studies) ModelE2) in simulation of the high Arctic environment. Quantities analyzed include near surface meteorological variables such as temperature, pressure, humidity and winds, surface-based estimates of cloud and precipitation properties, the surface energy budget, and lower atmospheric temperature structure. In general, the models perform well in simulating large-scale dynamical quantities such as pressure and winds. Near-surface temperature and lower atmospheric stability, along with surface energy budget terms, are not as well represented due largely to errors in simulation of cloud occurrence, phase and altitude. Additionally, a development version of CAMS, which features improved handling of cloud macro physics, has demonstrated to improve simulation of cloud properties and liquid water amount. The ASCOS period additionally provides an excellent example of the benefits gained by evaluating individual budget terms, rather than simply evaluating the net end product, with large compensating errors between individual surface energy budget terms that result in the best net energy budget.

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