We present the analysis of annual average OH* temperatures in the mesopause region derived from measurements of the GRound based Infrared P-branch Spectrometer (GRIPS) at Wuppertal (51° N, 7° E) in the time interval 1988 to 2015. The temperature time series shows a clear correlation with the solar radio flux F10.7cm (11-year cycle of solar activity) with a sensitivity of about 4 – 5 K/(100 SFU) . Beside this correlation we find a clear trend break in the temperature time series in middle of 2006.

This apparent trend break can be described as long periodic oscillation. A multiple linear regression using the solar radio flux and a 24-year oscillation as parameters leads to a sensitivity to the solar activity of (4.3 ±0.7) K/(100 SFU) and an amplitude of the 24-year oscillation A = (1.95 ±0.43) K. The most important finding here is that using these parameters for the multiple linear regression an additional linear trend is no longer needed. Moreover, with the knowledge of this 24-year oscillation the linear trends derived in this and in a former study of the Wuppertal data series can be reproduced by just fitting a line to the corresponding time interval of the oscillation. This actually means that depending on the analysed time interval completely different linear trends with respect to magnitude and sign can be observed. This fact is of essential importance for any comparison between different observations and model simulations.