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The Impact of Oceanic Boundary Conditions on the Climate Effect of Aerosols in ECHAM5-HAM and CAM-Oslo
Stockholm University, Faculty of Science, Department of Meteorology .
Stockholm University, Faculty of Science, Department of Meteorology .
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(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: urn:nbn:se:su:diva-93873OAI: oai:DiVA.org:su-93873DiVA: diva2:649433
Available from: 2013-09-18 Created: 2013-09-18 Last updated: 2013-09-18Bibliographically approved
In thesis
1. Interactions between aerosols and large-scale circulation systems in the atmosphere
Open this publication in new window or tab >>Interactions between aerosols and large-scale circulation systems in the atmosphere
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:nbn:se:su:diva-93872 (URN)978-91-7447-763-4 (ISBN)
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

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