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The mesopause is the temperature minimum at the boundary between the mesosphere and the thermosphere atmospheric regions. Due to the lack of solar heating and very strong cooling by radiative emission from CO2, the mesopause is the coldest place on Earth with temperatures as low as -100°C (-146°F or 173 K) [1]. The altitude of the mesopause for many years was assumed to be at around 85km, but observations to higher altitudes and modeling studies in the last 10 years have shown that in fact the mesopause consists of two minima - one at about 85 km and a stronger minimum at about 100km [2].
An interesting feature is that the summer mesopause is cooler than the winter. This is sometimes referred to as the mesopause anomaly. It is due to a summer-to-winter circulation giving rise to upwelling at the summer pole and downwelling at the winter. Air rising will expand and cool resulting in a cold summer mesopause and conversely downwelling air results in compression and associated increase in temperature at the winter mesopause. It should be noted that in the mesosphere the summer-to-winter circulation is due to gravity wave dissipation, which deposits momentum against the mean east-west flow resulting in a small north-south circulation [7].
In recent years the mesopause has also been the focus for studies on global climate change associated with increases in CO2. Unlike the troposphere, where greenhouse gases result in the atmosphere heating up, increased CO2 in the mesosphere acts to cool the atmosphere due to increased radiative emission by CO2. This results in a measurable effect - the mesopause should become cooler with increased CO2. Observations do show a decrease of temperature of the mesopause, though the magnitude of this decrease varies and is subject to further study [3]. It should be noted that modeling studies of this phenomenon have also been carried out [4][5][6].
References
1. International Union of Pure and Applied Chemistry. "mesosphere". Compendium of Chemical Terminology Internet edition.
2. Xu, Jiyao; Liu, H.-L.; Yuan, W.; Smith, A. K.; Roble, R. G.; Mertens, C. J.; Russell, J. M.; Mlynczak, M. G., "Mesopause structure from Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics (TIMED)/Sounding of the Atmosphere Using Broadband Emission Radiometry (SABER)", Journal of Geophysical Research, Volume 112, Issue D9
3. Beig, G., Keckhut, P., Lowe, R.P., et al., 2003. Review of mesospheric temperature trends. Rev. Geophys. 41 (4), 1015.
4. Roble, R.G., Dickinson, R.E., 1989. How will changes in carbon-dioxide and methane modify the mean structure of the mesosphere and thermosphere? Geophys. Res. Lett. 16 (12), 1441-1444.
5. Akmaev, R.A., Fomichev, V.I., Zhu, X., 2006. Impact of middle-atmospheric composition changes on greenhouse cooling in the upper atmosphere. J. Atmos. Solar-Terr. Phys. 68 (17), 1879-1889.
6. Ingrid Cnossen, Matthew J. Harris, Neil F. Arnold and Erdal Yiğit, "Modelled effect of changes in the CO2 concentration on the middle and upper atmosphere: sensitivity to gravity wave parameterization", Journal of Atmospheric and Solar-Terrestrial Physics (accepted October 2008 - in Press)
7. The Physics of Atmospheres, John Theodore Houghton, section and references therein of The general circulation of the middle atmosphere
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