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Interactions between aerosols and large-scale circulation systems in the atmosphere
Stockholm University, Faculty of Science, Department of Meteorology .
2013 (English)Doctoral 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.

Place, publisher, year, edition, pages
Stockholm: Department of Meteorology, Stockholm University , 2013. , 34 p.
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-93872ISBN: 978-91-7447-763-4 (print)OAI: oai:DiVA.org:su-93872DiVA: diva2:649442
Public defence
2013-10-18, Föreläsningssalen, Institutionen för ekologi, miljö och botanik, Lilla Frescativägen 5, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Mistra - The Swedish Foundation for Strategic Environmental Research
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Manuscript. Paper 3: Manuscript. Paper 4: Manuscript.

Available from: 2013-09-26 Created: 2013-09-18 Last updated: 2013-09-19Bibliographically approved
List of papers
1. The role of precipitation in aerosol-induced changes in northern hemisphere wintertime stationary waves
Open this publication in new window or tab >>The role of precipitation in aerosol-induced changes in northern hemisphere wintertime stationary waves
2013 (English)In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 41, no 3-4, 647-661 p.Article in journal (Refereed) Published
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.

Keyword
Aerosol, Stationary waves, Convective precipitation, EC-Earth
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-86514 (URN)10.1007/s00382-012-1622-7 (DOI)000322619500007 ()
Note

AuthorCount: 3

 

Available from: 2013-01-14 Created: 2013-01-14 Last updated: 2017-12-06Bibliographically approved
2. Diverse effects of aerosol forcing distribution and magnitude on tropical zonal circulation
Open this publication in new window or tab >>Diverse effects of aerosol forcing distribution and magnitude on tropical zonal circulation
(English)Manuscript (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.

National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-93875 (URN)
Available from: 2013-09-18 Created: 2013-09-18 Last updated: 2013-09-18Bibliographically approved
3. Can an influence of changing aerosol emissions be detected in thepattern of surface temperature change between 1970 and 2000?
Open this publication in new window or tab >>Can an influence of changing aerosol emissions be detected in thepattern of surface temperature change between 1970 and 2000?
(English)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.

National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-93874 (URN)
Available from: 2013-09-18 Created: 2013-09-18 Last updated: 2013-09-18Bibliographically approved
4. The Impact of Oceanic Boundary Conditions on the Climate Effect of Aerosols in ECHAM5-HAM and CAM-Oslo
Open this publication in new window or tab >>The Impact of Oceanic Boundary Conditions on the Climate Effect of Aerosols in ECHAM5-HAM and CAM-Oslo
Show others...
(English)Manuscript (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.

National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-93873 (URN)
Available from: 2013-09-18 Created: 2013-09-18 Last updated: 2013-09-18Bibliographically approved

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