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  • 1.
    Acosta Navarro, Juan C.
    et al.
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Pausata, Francesco S. R.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Lewinschal, Anna
    Stockholm University, Faculty of Science, Department of Meteorology .
    Varma, Vidya
    Seland, Øyvind
    Gauss, Michael
    Iversen, Trond
    Kirkevåg, Alf
    Riipinen, Ilona
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Hansson, Hans Christen
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Future response of temperature and precipitation to reduced aerosol emissions as compared with increased greenhouse gas concentrations2017In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 30, no 3, p. 939-954Article in journal (Refereed)
    Abstract [en]

    Experiments with a climate model (NorESM1) were performed to isolate the effects of aerosol particles and greenhouse gases on surface temperature and precipitation in simulations of future climate. The simulations show that by 2025-2049, a reduction of aerosol emissions from fossil fuels following a maximum technically feasible reduction (MFR) scenario could lead to a global and Arctic warming of 0.26 K and 0.84 K, respectively; as compared with a simulation with fixed aerosol emissions at the level of 2005. If fossil fuel emissions of aerosols follow a current legislation emissions (CLE) scenario, the NorESM1 model simulations yield a non-significant change in global and Arctic average surface temperature as compared with aerosol emissions fixed at year 2005. The corresponding greenhouse gas effect following the RCP4.5 emission scenario leads to a global and Arctic warming of 0.35 K and 0.94 K, respectively.

    The model yields a marked annual average northward shift in the inter-tropical convergence zone with decreasing aerosol emissions and subsequent warming of the northern hemisphere. The shift is most pronounced in the MFR scenario but also visible in the CLE scenario. The modeled temperature response to a change in greenhouse gas concentrations is relatively symmetric between the hemispheres and there is no marked shift in the annual average position of the inter-tropical convergence zone. The strong reduction in aerosol emissions in MFR also leads to a net southward cross-hemispheric energy transport anomaly both in the atmosphere and ocean, and enhanced monsoon circulation in Southeast and East Asia causing an increase in precipitation over a large part of this region.

  • 2. Chung, Chul Eddy
    et al.
    Lewinschal, Anna
    Stockholm University, Faculty of Science, Department of Meteorology .
    Wilcox, Eric
    Relationship between low-cloud presence and the amount of overlying aerosols2016In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 16, no 9, p. 5781-5792Article in journal (Refereed)
    Abstract [en]

    Aerosols are often advected above cloud decks, and the amount of aerosols over cloud has been assumed to be similar to that at the same heights in nearby clear sky. In this assumption, cloud and aerosol above cloud-top height are considered randomly located with respect to each other. The Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) data are analyzed here to investigate this assumption on global scales. The CALIPSO data reveal that the aerosol optical depth (AOD) above low cloud tends to be smaller than in nearby clear sky during the daytime, and the opposite is true during the nighttime. In particular, over oceanic regions with wide-spread low cloud, such as the tropical southeastern Atlantic Ocean and northeastern Pacific Ocean, the daytime AOD above low cloud is often 40aEuro-% smaller than in surrounding clear skies.

  • 3.
    Ekman, Annica M. L.
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Lewinschal, Anna
    Stockholm University, Faculty of Science, Department of Meteorology .
    Struthers, Hamish
    Can an influence of changing aerosol emissions be detected in thepattern of surface temperature change between 1970 and 2000?Manuscript (preprint) (Other academic)
    Abstract [en]

    The general circulation model CAM-Oslo was used to examine the influence of varyingaerosol and greenhouse gas emissions on the pattern of surface temperature change betweenthe years 1970 and 2000, and whether the temperature response over different regions wasgoverned by local (due to changes in energy fluxes) or far-field (due to changes in large-scale circulation) processes. Circulation changes, originating from precipitation anomaliesmainly over the west/central Pacific and off the east coast of North America, influenced asubstantial part of the northern hemisphere temperature change pattern in CAM-Oslo, inparticular over southern North America, but also over Europe and Asia. The result highlightsthe importance of better understanding zonally asymmetric precipitation changes due todifferent forcing agents. A local response in surface temperature due to net surface radiativeflux (RF) anomalies could also be detected over Europe and Asia, where the differencein all-sky net surface RF was mainly driven by aerosol- or circulation-induced changes inliquid water path and cloud cover. A local anthropogenic aerosol effect on the cloud dropletsize and subsequent short-wave (SW) RF was found over Europe and Asia, but only whenexcluding a change in the greenhouse gas concentration. For clear skies, the SW RF patternwas well-correlated with the aerosol optical depth anomalies. However, this correlation wasat least partly governed by relative humidity fluctuations. Overall, the greenhouse andaerosol effects on surface temperature were in the simulations found to be non-linear with asignificant feedback on the aerosol population from a warming climate.

  • 4. Kirkevag, Alf
    et al.
    Grini, Alf
    Olivie, Dirk
    Seland, Oyvind
    Alterskjaer, Kari
    Hummel, Matthias
    Karset, Inger H. H.
    Lewinschal, Anna
    Stockholm University, Faculty of Science, Department of Meteorology .
    Liu, Xiaohong
    Makkonen, Risto
    Bethke, Ingo
    Griesfeller, Jan
    Schulz, Michael
    Iversen, Trond
    A production-tagged aerosol module for Earth system models, OsloAero5.3-extensions and updates for CAM5.3-Oslo2018In: Geoscientific Model Development, ISSN 1991-959X, E-ISSN 1991-9603, Vol. 11, no 10, p. 3945-3982Article in journal (Refereed)
    Abstract [en]

    We document model updates and present and discuss modeling and validation results from a further developed production-tagged aerosol module, OsloAero5.3, for use in Earth system models. The aerosol module has in this study been implemented and applied in CAM5.3-Oslo. This model is based on CAM5.3-CESM1.2 and its own predecessor model version CAM4-Oslo. OsloAero5.3 has improved treatment of emissions, aerosol chemistry, particle life cycle, and aerosol-cloud interactions compared to its predecessor OsloAero4.0 in CAM4-Oslo. The main new features consist of improved aerosol sources; the module now explicitly accounts for aerosol particle nucleation and secondary organic aerosol production, with new emissions schemes also for sea salt, dimethyl sulfide (DMS), and marine primary organics. Mineral dust emissions are updated as well, adopting the formulation of CESM1.2. The improved model representation of aerosol-cloud interactions now resolves heterogeneous ice nucleation based on black carbon (BC) and mineral dust calculated by the model and treats the activation of cloud condensation nuclei (CCN) as in CAM5.3. Compared to OsloAero4.0 in CAM4-Oslo, the black carbon (BC) mass concentrations are less excessive aloft, with a better fit to observations. Near-surface mass concentrations of BC and sea salt aerosols are also less biased, while sulfate and mineral dust are slightly more biased. Although appearing quite similar for CAM5.3-Oslo and CAM4-Oslo, the validation results for organic matter (OM) are inconclusive, since both of the respective versions of OsloAero are equipped with a limited number of OM tracers for the sake of computational efficiency. Any information about the assumed mass ratios of OM to organic carbon (OC) for different types of OM sources is lost in the transport module. Assuming that observed OC concentrations scaled by 1.4 are representative for the modeled OM concentrations, CAM5.3-Oslo with OsloAero5.3 is slightly inferior for the very sparsely available observation data. Comparing clear-sky column-integrated optical properties with data from ground-based remote sensing, we find a negative bias in optical depth globally; however, it is not as strong as in CAM4-Oslo, but has positive biases in some areas typically dominated by mineral dust emissions. Aerosol absorption has a larger negative bias than the optical depth globally. This is reflected in a lower positive bias in areas where mineral dust is the main contributor to absorption. Globally, the low bias in absorption is smaller than in CAM4-Oslo. The Angstrom parameter exhibits small biases both globally and regionally, suggesting that the aerosol particle sizes are reasonably well represented. Cloud-top droplet number concentrations over oceans are generally underestimated compared to satellite retrievals, but seem to be overestimated downwind of major emissions of dust and biomass burning sources. Finally, we find small changes in direct radiative forcing at the top of the atmosphere, while the cloud radiative forcing due to anthropogenic aerosols is now more negative than in CAM4-Oslo, being on the strong side compared to the multi-model estimate in IPCC AR5. Although not all validation results in this study show improvement for the present CAM5.3-Oslo version, the extended and updated aerosol module OsloAero5.3 is more advanced and applicable than its predecessor OsloAero4.0, as it includes new parameterizations that more readily facilitate sensitivity and process studies and use in climate and Earth system model studies in general.

  • 5.
    Lewinschal, Anna
    Stockholm University, Faculty of Science, Department of Meteorology .
    Interactions between aerosols and large-scale circulation systems in the atmosphere2013Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Anthropogenic aerosol emissions have increased during the last century. The higher atmospheric aerosol burden is believed to partly have masked the enhanced greenhouse gas warming during the same period. However, the many different types of aerosols, and the uncertainties regarding their effect on clouds, makes it difficult to estimate their total climate impact. With their strong effect on atmospheric radiation and their varying spatial and temporal distribution, aerosols may also affect the atmospheric circulation. This thesis focuses on aspects of aerosol-induced circulation changes as represented in general circulations models.

    Anthropogenic aerosol forcing is believed to generally cool the earth system, but model simulations show that the strongest cooling is not necessarily co-located with the strongest aerosol radiative forcing. It is shown that aerosol forcing can cause anomalies in the stationary wave pattern, which affects surface temperatures far from the region of aerosol forcing. In absence of a substantial global mean aerosol-induced cooling, the anomalous stationary wave pattern has a large influence on the simulated temperature-response pattern. The waves are primarily generated by aerosol-induced precipitation changes in the tropics, showing an important connection between aerosol emissions at low latitudes and surface temperate changes in the extra-tropics.

    It is also demonstrated that the aerosol climate response differs depending on how the ocean surface is represented in a model, i.e. if a sea surface temperature response is permitted or not. The anthropogenic aerosol forcing generates a stronger cooling of the northern hemisphere when the sea surface temperatures can change compared to when they are fixed. The stronger inter-hemispheric temperature gradient affects both the tropical and extra-tropical zonal mean circulation. Thus, aerosol-induced circulation changes are dependent on the simulated surface temperature response.

  • 6.
    Lewinschal, Anna
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Hansson, Hans-Christen
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Sand, Maria
    Berntsen, Terje K.
    Langner, Joakim
    Local and remote temperature response of regional SO2 emissions2019In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 19, no 4, p. 2385-2403Article in journal (Refereed)
    Abstract [en]

    Short-lived anthropogenic climate forcers (SLCFs), such as sulfate aerosols, affect both climate and air quality. Despite being short-lived, these forcers do not affect temperatures only locally; regions far away from the emission sources are also affected. Climate metrics are often used in a policy context to compare the climate impact of different anthropogenic forcing agents. These metrics typically relate a forcing change in a certain region with a temperature change in another region and thus often require a separate model to convert emission changes to radiative forcing (RF) changes. In this study, we used a coupled Earth system model, NorESM (Norwegian Earth System Model), to calculate emission-to-temperature-response metrics for sulfur dioxide (SO2) emission changes in four different policy-relevant regions: Europe (EU), North America (NA), East Asia (EA) and South Asia (SA). We first increased the SO2 emissions in each individual region by an amount giving approximately the same global average radiative forcing change (-0.45 Wm(-2)). The global mean temperature change per unit sulfur emission compared to the control experiment was independent of emission region and equal to similar to 0.006 K(TgSyr(-1))(-1). On a regional scale, the Arctic showed the largest temperature response in all experiments. The second largest temperature change occurred in the region of the imposed emission increase, except when South Asian emissions were changed; in this experiment, the temperature response was approximately the same in South Asia and East Asia. We also examined the non-linearity of the temperature response by removing all anthropogenic SO2 emissions over Europe in one experiment. In this case, the temperature response (both global and regional) was twice that in the corresponding experiment with a European emission increase. This non-linearity in the temperature response is one of many uncertainties associated with the use of simplified climate metrics.

  • 7.
    Lewinschal, Anna
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Körnich, Heiner
    Diverse effects of aerosol forcing distribution and magnitude on tropical zonal circulationManuscript (preprint) (Other academic)
    Abstract [en]

    The effect of anthropogenic direct aerosol radiative forcing on tropical zonal circulation has been investigated using the climate-system model EC-Earth2.5 The relatively low forcing resulting from only modeling the direct aerosol effect led to a negligible tropical mean temperature and precipitation response. Nevertheless, the aerosol direct radiative forcing had a considerable local impact on the Indian Ocean Walker circulation cell, which experienced a decreased intensity during the fall, winter, and spring season. Partitioning the aerosol radiative forcing into a scattering and an absorbing part revealed that the scattering aerosol dominated the circulation response. Including the aerosol cloud albedo effect, on the other hand, led to a tropical-wide cooling and sub sequent precipitation reduction. The results indicate that relatively low aerosol direct radiative forcing can lead to substantial local effects on the tropical zonal overturning circulation and precipitation without necessarily relying on a tropical wide cooling and a thermodynamic scaling argument.

  • 8.
    Lewinschal, Anna
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Körnich, Heiner
    The role of precipitation in aerosol-induced changes in northern hemisphere wintertime stationary waves2013In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 41, no 3-4, p. 647-661Article in journal (Refereed)
    Abstract [en]

    The coupled climate model EC-Earth2 is used to investigate the impact of direct radiative effects of aerosols on stationary waves in the northern hemisphere wintertime circulation. The direct effect of aerosols is simulated by introducing prescribed mixing ratios of different aerosol compounds representing pre-industrial and present-day conditions, no indirect effects are included. In the EC-Earth2 results, the surface temperature response is uncorrelated with the highly asymmetric aerosol radiative forcing pattern. Instead, the anomalous extratropical temperature field bears a strong resemblance to the aerosol-induced changes in the stationary-wave pattern. It is demonstrated that the main features of the wave pattern of EC-Earth2 can be replicated by a linear, baroclinic model forced with latent heat changes corresponding to the anomalous convective precipitation generated by EC-Earth2. The tropical latent heat release is an effective means of generating stationary wave trains that propagate into the extratropics. Hence, the results of the present study indicate that aerosol-induced convective precipitation anomalies govern the extratropical wave-field changes, and that the far-field temperature response dominates over local effects of aerosol radiative forcing.

  • 9.
    Lewinschal, Anna
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Sousa-Santos, Gabriela
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Struthers, Hamish
    Bey, Isabelle
    The Impact of Oceanic Boundary Conditions on the Climate Effect of Aerosols in ECHAM5-HAM and CAM-OsloManuscript (preprint) (Other academic)
    Abstract [en]

    Two general circulation models with sophisticated aerosol treatments, ECHAM5-HAM and CAM-Oslo, have been used to investigate how the sea surface representation influences the simulated aerosol-climate response. Two experiments were performed with ECHAM5-HAM, one with fixed sea surface temperatures (SSTs) and one where a mixed-layer ocean (MLO) model was used. With CAM-Oslo, one experiment with an MLO was conducted. In all experiments equivalent emissions of anthropogenic aerosols and aerosol precursors were used, representing the increased emissions of these between the years 1850 and 2000.

    The different ocean surface representations had minor effects on the simulated anthropogenic aerosol distribution. The model-specific aerosol treatment had a larger influence on the simulated anthropogenic aerosol optical depth than the change in the ocean boundary condition. The natural aerosols distribution was, on the other hand, sensitive to the ocean surface representation in ECHAM5-HAM.

    The more substantial surface temperature change and altered atmospheric thermal structure supported in the MLO experiments influenced both the surface energy budget and the global circulation. Despite separating the analysis of the surface energy budget into land and ocean areas, the choice of ocean boundary condition influenced the surface energy-flux changes over land.

    The larger cooling of the northern hemisphere compared to the southern hemisphere in the MLO experiments led to a southward shift of the tropical Hadley circulation as well as the eddy-driven circulation in the northern hemisphere. In the experiment with fixed SSTs, the winter Hadley cell intensity decreased in each hemisphere. The most prominent change in the zonal mean zonal wind in the fixed SST experiment was an Arctic Oscillation-like southward shift of the mid-latitude jet in the northern hemisphere during the northern hemisphere winter.

    In the experiments presented in this study, the use of fixed SST and an MLO yielded distinctly different climate impacts despite the a small difference in the simulated anthropogenic aerosol burden.

  • 10. Voigt, Aiko
    et al.
    Pincus, Robert
    Stevens, Bjorn
    Bony, Sandrine
    Boucher, Olivier
    Bellouin, Nicolas
    Lewinschal, Anna
    Stockholm University, Faculty of Science, Department of Meteorology .
    Medeiros, Brian
    Wang, Zhili
    Zhang, Hua
    Fast and slow shifts of the zonal-mean intertropical convergence zone in response to an idealized anthropogenic aerosol2017In: Journal of Advances in Modeling Earth Systems, ISSN 1942-2466, Vol. 9, no 2, p. 870-892Article in journal (Refereed)
    Abstract [en]

    Previous modeling work showed that aerosol can affect the position of the tropical rain belt, i.e., the intertropical convergence zone (ITCZ). Yet it remains unclear which aspects of the aerosol impact are robust across models, and which are not. Here we present simulations with seven comprehensive atmosphere models that study the fast and slow impacts of an idealized anthropogenic aerosol on the zonalmean ITCZ position. The fast impact, which results from aerosol atmospheric heating and land cooling before sea-surface temperature (SST) has time to respond, causes a northward ITCZ shift. Yet the fast impact is compensated locally by decreased evaporation over the ocean, and a clear northward shift is only found for an unrealistically large aerosol forcing. The local compensation implies that while models differ in atmospheric aerosol heating, this does not contribute to model differences in the ITCZ shift. The slow impact includes the aerosol impact on the ocean surface energy balance and is mediated by SST changes. The slow impact is an order of magnitude more effective than the fast impact and causes a clear southward ITCZ shift for realistic aerosol forcing. Models agree well on the slow ITCZ shift when perturbed with the same SST pattern. However, an energetic analysis suggests that the slow ITCZ shifts would be substantially more modeldependent in interactive-SST setups due to model differences in clear-sky radiative transfer and clouds. We also discuss implications for the representation of aerosol in climate models and attributions of recent observed ITCZ shifts to aerosol.

  • 11. Wilcox, Laura J.
    et al.
    Dunstone, Nick
    Lewinschal, Anna
    Stockholm University, Faculty of Science, Department of Meteorology .
    Bollasina, Massimo
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Highwood, Eleanor J.
    Mechanisms for a remote response to Asian anthropogenic aerosol in boreal winter2019In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 19, no 14, p. 9081-9095Article in journal (Refereed)
    Abstract [en]

    Asian emissions of anthropogenic aerosols and their precursors have increased rapidly since 1980, with half of the increase since the pre-industrial era occurring in this period. Transient experiments with the HadGEM3-GC2 coupled model were designed to isolate the impact of Asian anthropogenic aerosols on global climate in boreal winter. It is found that this increase has resulted in local circulation changes, which in turn have driven decreases in precipitation over China, alongside an intensification of the offshore monsoon flow. No large temperature changes are seen over China. Over India, the opposite response is found, with decreasing temperatures and increasing precipitation. The dominant feature of the local circulation changes is an increase in low-level convergence, ascent, and precipitation over the Maritime Continent, which forms part of a tropical Pacific-wide La Nina-like response. HadGEM3-GC2 also simulates pronounced far-field responses. A decreased meridional temperature gradient in the North Pacific leads to a positive Pacific-North American circulation pattern, with associated temperature anomalies over the North Pacific and North America. Anomalous northeasterly flow over northeast Europe drives advection of cold air into central and western Europe, causing cooling in this region. An anomalous anticyclonic circulation over the North Atlantic causes drying over western Europe. Using a steady-state primitive equation model, LUMA, we demonstrate that these far-field midlatitude responses arise primarily as a result of Rossby waves generated over China, rather than in the equatorial Pacific.

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