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Variability and feedbacks in the middle atmosphere
Stockholm University, Faculty of Science, Department of Meteorology . (Atmospheric Physics)ORCID iD: 0000-0003-4064-9624
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The importance of the middle atmosphere for the weather and climate on Earth is increasingly realized. Variability and feedback processes in the middle atmosphere need to be better understood and form the subject of this thesis. Initially, the focus has been on the variability of the summer polar mesopause, which is the coldest place in the Earth's system. The variability of this region is driven by a variety of atmospheric processes, such as atmospheric waves and the solar cycle and is even coupled to the atmosphere on other side of the globe through interhemispheric coupling. The low temperatures in the summer polar mesopause allow for thin ice clouds to form: noctilucent clouds (NLCs). It is investigated how well the Canadian Middle Atmosphere Model (CMAM30), in which the NLCs are represented in terms of a simple model, can be used to study zonal mean NLC variability.  Comparing to satellite data, it is shown that the basic NLC characteristics, such as seasonal onsets and development, interannual variability and interhemispheric differences, are well captured by the model. The role of the winter residual circulation in shaping the conditions of the summer polar mesopause is also investigated, using the Whole Atmosphere Community Climate Model (WACCM). It is found that without the gravity waves in winter, the summer mesopause region would be significantly warmer. This means that the interhemispheric coupling mechanism has a net cooling effect on the summer mesopause regions. In addition, the effect of the solar cycle on the summer polar mesopause is studied. In CMAM30, there is no substantial temperature change due to the solar cycle. It is shown that there is an enhanced circulation in this region during solar maximum as compared to solar minimum, which causes adiabatic cooling counteracting the direct effect of the solar cycle. Finally, feedbacks in the middle atmosphere are studied using WACCM. The Climate Feedback Response Analysis Method (CFRAM) is used to examine the middle atmosphere response to a doubling of the CO2-concentration with respect the pre-industrial state. It was found that the temperature response to direct CO2 forcing would be approximately -9 K in the middle atmosphere. This cooling is being mitigated by the combined effect of the different feedbacks processes, the strongest of which being the ozone feedback. The dynamical feedback has large effects on the temperatures locally, while the role of the cloud, albedo and water vapor feedback are small in the middle atmosphere.

Place, publisher, year, edition, pages
Stockholm: Department of Meteorology, Stockholm University , 2019. , p. 42
Keywords [en]
Middle atmosphere dynamics, noctilucent clouds, solar cycle effects, climate feedbacks
National Category
Meteorology and Atmospheric Sciences Climate Research
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-170302ISBN: 978-91-7797-674-5 (print)ISBN: 978-91-7797-675-2 (electronic)OAI: oai:DiVA.org:su-170302DiVA, id: diva2:1330525
Public defence
2019-09-19, Ahlmannsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Manuscript.

Available from: 2019-08-27 Created: 2019-06-25 Last updated: 2019-08-07Bibliographically approved
List of papers
1. Exploring noctilucent cloud variability using the nudged and extended version of the Canadian Middle Atmosphere Model
Open this publication in new window or tab >>Exploring noctilucent cloud variability using the nudged and extended version of the Canadian Middle Atmosphere Model
2017 (English)In: Journal of Atmospheric and Solar-Terrestrial Physics, ISSN 1364-6826, E-ISSN 1879-1824, Vol. 164, p. 276-288Article in journal (Refereed) Published
Abstract [en]

Ice particles in the summer mesosphere-such as those connected to noctilucent clouds and polar mesospheric summer echoes-have since their discovery contributed to the uncovering of atmospheric processes on various scales ranging from interactions on molecular levels to global scale circulation patterns. While there are numerous model studies on mesospheric ice microphysics and how the clouds relate to the background atmosphere, there are at this point few studies using comprehensive global climate models to investigate observed variability and climatology of noctilucent clouds. In this study it is explored to what extent the large-scale inter-annual characteristics of noctilucent clouds are captured in a 30-year run-extending from 1979 to 2009-of the nudged and extended version of the Canadian Middle Atmosphere Model (CMAM30). To construct and investigate zonal mean inter-seasonal variability in noctilucent cloud occurrence frequency and ice mass density in both hemispheres, a simple cloud model is applied in which it is assumed that the ice content is solely controlled by the local temperature and water vapor volume mixing ratio. The model results are compared to satellite observations, each having an instrument-specific sensitivity when it comes to detecting noctilucent clouds. It is found that the model is able to capture the onset dates of the NLC seasons in both hemispheres as well as the hemispheric differences in NLCs, such as weaker NLCs in the SH than in the NH and differences in cloud height. We conclude that the observed cloud climatology and zonal mean variability are well captured by the model.

Keywords
Noctilucent clouds, CMAM30, SOFIE, SBUV
National Category
Earth and Related Environmental Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-150991 (URN)10.1016/j.jastp.2017.08.019 (DOI)000417671000029 ()
Available from: 2018-01-11 Created: 2018-01-11 Last updated: 2019-06-26Bibliographically approved
2. On How the Middle Atmospheric Residual Circulation Responds to the Solar Cycle Close to the Solstices
Open this publication in new window or tab >>On How the Middle Atmospheric Residual Circulation Responds to the Solar Cycle Close to the Solstices
2018 (English)In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 31, no 1, p. 401-421Article in journal (Refereed) Published
Abstract [en]

During high solar activity, the atmosphere receives more energy from the sun, particularly in the form of shortwave radiation. Most notable is the effect in the middle and upper atmosphere, which in general shows a positive temperature response due to physical and chemical processes that are intensified at high solar activity. It is thus surprising that a clear solar cycle signal is absent in the summer polar mesosphere region in spite of it being illuminated around the clock. In this study, it is investigated how the circulation in the summer mesosphere is affected by changes in the solar flux using a 30-yr run from the nudged version of the Canadian Middle Atmosphere Model (CMAM30). It is found that-in July-the solar cycle signal from direct solar heating is counteracted by an enhanced residual circulation, which adiabatically cools the region at a higher rate when the solar activity is above average. The dynamical cooling is partly initiated in the Southern Hemisphere winter stratosphere.

Keywords
Atmospheric circulation, Middle atmosphere, Solar cycle
National Category
Earth and Related Environmental Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-156023 (URN)10.1175/JCLI-D-17-0202.1 (DOI)000429528800024 ()
Available from: 2018-05-04 Created: 2018-05-04 Last updated: 2019-06-26Bibliographically approved
3. The role of the winter residual circulation in the summer mesopause regions in WACCM
Open this publication in new window or tab >>The role of the winter residual circulation in the summer mesopause regions in WACCM
2018 (English)In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 18, no 6, p. 4217-4228Article in journal (Refereed) Published
Abstract [en]

High winter planetary wave activity warms the summer polar mesopause via a link between the two hemispheres. Complex wave-mean-flow interactions take place on a global scale, involving sharpening and weakening of the summer zonal flow. Changes in the wind shear occasionally generate flow instabilities. Additionally, an altering zonal wind modifies the breaking of vertically propagating gravity waves. A crucial component for changes in the summer zonal flow is the equatorial temperature, as it modifies latitudinal gradients. Since several mechanisms drive variability in the summer zonal flow, it can be hard to distinguish which one is dominant. In the mechanism coined interhemispheric coupling, the mesospheric zonal flow is suggested to be a key player for how the summer polar mesosphere responds to planetary wave activity in the winter hemisphere. We here use the Whole Atmosphere Community Climate Model (WACCM) to investigate the role of the summer stratosphere in shaping the conditions of the summer polar mesosphere. Using composite analyses, we show that in the absence of an anomalous summer mesospheric temperature gradient between the equator and the polar region, weak planetary wave forcing in the winter would lead to a warming of the summer mesosphere region instead of a cooling, and vice versa. This is opposing the temperature signal of the interhemispheric coupling that takes place in the mesosphere, in which a cold and calm winter stratosphere goes together with a cold summer mesopause. We hereby strengthen the evidence that the variability in the summer mesopause region is mainly driven by changes in the summer mesosphere rather than in the summer stratosphere.

National Category
Earth and Related Environmental Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-156079 (URN)10.5194/acp-18-4217-2018 (DOI)000428471900001 ()
Available from: 2018-05-21 Created: 2018-05-21 Last updated: 2019-06-26Bibliographically approved
4. Quantifying climate feedbacks in the middle atmosphere using WACCM
Open this publication in new window or tab >>Quantifying climate feedbacks in the middle atmosphere using WACCM
(English)Manuscript (preprint) (Other academic)
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-170301 (URN)
Available from: 2019-06-25 Created: 2019-06-25 Last updated: 2019-06-26Bibliographically approved

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