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Response of the middle atmosphere to CO2 doubling: Results from the Canadian Middle Atmosphere Model
Stockholm University, Faculty of Science, Department of Meteorology .
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2007 (English)In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 20, no 7Article in journal (Refereed) Published
Abstract [en]

The Canadian Middle Atmosphere Model (CMAM) has been used to examine the middle atmosphere response to CO2 doubling. The radiative-photochemical response induced by doubling CO2 alone and the response produced by changes in prescribed SSTs are found to be approximately additive, with the former effect dominating throughout the middle atmosphere. The paper discusses the overall response, with emphasis on the effects of SST changes, which allow a tropospheric response to the CO2 forcing. The overall response is a cooling of the middle atmosphere accompanied by significant increases in the ozone and water vapor abundances. The ozone radiative feedback occurs through both an increase in solar heating and a decrease in infrared cooling, with the latter accounting for up to 15% of the total effect. Changes in global mean water vapor cooling are negligible above 30 hPa. Near the polar summer mesopause, the temperature response is weak and not statistically significant. The main effects of SST changes are a warmer troposphere, a warmer and higher tropopause, cell-like structures of heating and cooling at low and middlelatitudes in the middle atmosphere, warming in the summer mesosphere, water vapor increase throughout the domain, and O3 decrease in the lower tropical stratosphere. No noticeable change in upward-propagating planetary wave activity in the extratropical winter–spring stratosphere and no significant temperature response in the polar winter–spring stratosphere have been detected. Increased upwelling in the tropical stratosphere has been found to be linked to changed wave driving at low latitudes.

Place, publisher, year, edition, pages
2007. Vol. 20, no 7
Keyword [en]
Carbon dioxide, Ozone, Chemistry, atmospheric, Sea surface temperature
National Category
Climate Research
URN: urn:nbn:se:su:diva-25094DOI: 10.1175/JCLI4030.1OAI: diva2:198862
Part of urn:nbn:se:su:diva-769Available from: 2005-12-12 Created: 2005-12-12 Last updated: 2010-08-09Bibliographically approved
In thesis
1. Modelling the middle atmosphere and its sensitivity to climate change
Open this publication in new window or tab >>Modelling the middle atmosphere and its sensitivity to climate change
2005 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The Earth's middle atmosphere at about 10-100 km has shown a substantial sensitivity to human activities. First, the ozone layer has been reduced since the the early 1980s due to man-made emissions of halogenated hydrocarbons. Second, the middle atmosphere has been identified as a region showing clear evidence of climate change due to increased emissions of greenhouse gases. While increased CO2 abundances are expected to lead to a warmer climate near the Earth's surface, observations show that the middle atmosphere has been cooling by up to 2-3 degrees per decade over the past few decades. This is partly due to CO2 increases and partly due to ozone depletion.

Predicting the future development of the middle atmosphere is problematic because of strong feedbacks between temperature and ozone. Ozone absorbs solar ultraviolet radiation and thus warms middle atmosphere, and also, ozone chemistry is temperature dependent, so that temperature changes are modulated by ozone changes.

This thesis examines the middle atmospheric response to a doubling of the atmospheric CO2 content using a coupled chemistry-climate model. The effects can be separated in the intrinsic CO2-induced radiative response, the radiative feedback through ozone changes and the response due to changes in the climate of the underlying atmosphere and surface. The results show, as expected, a substantial cooling throughout the middle atmosphere, mainly due to the radiative impact of the CO2 increase. Model simulations with and without coupled chemistry show that the ozone feedback reduces the temperature response by up to 40%. Further analyses show that the ozone changes are caused primarily by the temperature dependency of the reaction O+O2+M->O3+M. The impact of changes in the surface climate on the middle atmosphere is generally small. In particular, no noticeable change in upward propagating planetary wave flux from the lower atmosphere is found. The temperature response in the polar regions is non-robust and thus, for the model used here, polar ozone loss does not appear to be sensitive to climate change in the lower atmosphere as has been suggested recently. The large interannual variability in the polar regions suggests that simulations longer than 30 years will be necessary for further analysis of the effects in this region.

The thesis also addresses the long-standing dilemma that models tend to underestimate the ozone concentration at altitudes 40-75 km, which has important implications for climate change studies in this region. A photochemical box model is used to examine the photochemical aspects of this problem. At 40-55 km, the model reproduces satellite observations to within 10%, thus showing a substantial reduction in the ozone deficit problem. At 60-75 km, however, the model underestimates the observations by up to 35%, suggesting a significant lack of understanding of the chemistry and radiation in this region.

Place, publisher, year, edition, pages
Stockholm: Meteorologiska institutionen (MISU), 2005. 30 p.
climate change, middle atmosphere, ozone, carbon dioxide, stratosphere, mesosphere, chemical kinetics, atmospheric tides
National Category
Meteorology and Atmospheric Sciences
urn:nbn:se:su:diva-769 (URN)91-7155-185-9 (ISBN)
Public defence
2006-01-13, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 8 C, Stockholm, 10:00
Available from: 2005-12-12 Created: 2005-12-12Bibliographically approved

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