|Paper title||Interaction between the general circulation of the middle atmosphere and solar forcing in the long-term development of UMLT temperatures over Europe|
|Form of presentation||Poster|
Since October 2008 hydroxyl (OH) airglow observations are performed at the environmental research station ‘Schneefernerhaus’ (UFS, 47.42°N, 10.98°E). Further observation sites at Catania (CAT, 37.51°N, 15.04°E), the Observatoire de Haute-Provence (OHP, 43.93°N, 5.71°E) and the Georgian National Observatory (ABA, 41.75°N, 42.82°E) were equipped with identical instrumentation in 2011 and 2012. These airglow emissions originate in the upper mesosphere lower thermosphere (UMLT) and provide an efficient approach to derive atmospheric temperatures of the emission layer between approximately 80 to 100km height.
At UFS on timescales from weeks to several years, temperatures at this height are strongly influenced by both, the general circulation of the middle atmosphere and the variability of the solar forcing. The strongest component is given by the annual cycle (caused by the residual meridional circulation of the mesosphere) with a yearly amplitude varying between 16.5 K to 18.5 K. The 11-year solar cycle provides the second strongest forcing mechanism with approximately 5.9 ±0.6 K/100 sfu during solar cycle 24. The uncertainty of the solar forcing term is governed by the question whether or not a lag term is to be applied in the cross-correlation of the two parameters. Concerning annual means of both parameters, a correlation of up to R²=0.91 is achieved, if solar flux is assumed to lead OH temperatures by 110 days. The phases of the quasi-biennial oscillation (QBO) originating in the tropical stratosphere play an important role in the explanation of this (potential) lag: the QBO-related signal of ca. 1 K, which is best observed from 2011 until 2015 during solar maximum conditions, strongly decreases after 2016 complicating the interpretation of the solar forcing term.
Recently, the semi-annual oscillation and its variability (between 2 K and 4.5 K), which appears to be decoupled from the variation of the annual oscillation comes more and more into focus. So far, UMLT airglow emissions have been considered to be only part of neutral atmospheric physics and chemistry. However, the emissions originate in the ionospheric D-region and potential feedback mechanisms from the strong semi-annual oscillations observed in the ionosphere should be considered.
In summary, the individual contributions to the temperature development at this height make it still difficult to decide whether this region is subject to the expected long-term cooling of mesospheric temperatures or the heating of the thermosphere. The best estimate for the 10-year change between 2010 and 2019 amounts to -0.14 K/dec ±0.59 K/dec. Observations at further sites of the Network for the Detection of Mesospheric Change (NDMC) help discriminating between local and large-scale effects in the data.