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Diverse effects of aerosol forcing distribution and magnitude on tropical zonal circulation
Stockholm University, Faculty of Science, Department of Meteorology .
Stockholm University, Faculty of Science, Department of Meteorology .
(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: urn:nbn:se:su:diva-93875OAI: oai:DiVA.org:su-93875DiVA: diva2:649431
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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