Analysis of the Long-term Evolution of the Mesopause region derived from OH(3,1) rotational temperatures

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Long-periodic temperature changes are a common feature of atmospheric temperatures. One prominent example is the 11-year cycle of solar activity that is observed in many different temperature time series from ground up to the lower thermosphere. Furthermore, also temperature fluctuations with even longer periods of about 20 years and about 70 years are observed in surface temperatures. The quasi-bidecadal oscillation was also observed in stratospheric temperatures. In previous studies we clearly observed this oscillation also in OH(3,1) rotational temperatures in the mesopause region that have been observed from Wuppertal using GRIPS (GRound-based Infrared P-branch Spectrometers) instruments. This oscillation has a large influence on any derived linear trend in the mesopause region, i.e. depending on the analysed time interval the linear trend largely differs, and is therefore of great importance. The oscillation surely also influences any temperature related quantity such as the equivalent summer duration (ESD). The ESD is defined as the time interval where the temperatures lie below 198 K. In former studies the ESD showed an increase with time. The new knowledge of the quasi-bidecadal oscillation and the much longer time series that is available now enables a new analysis of the ESD and the attribution of ESD changes to long-term temperature changes (e.g. 11-year cycle of solar activity; quasi-bidecadal oscillation) on the one hand and changes of the shape of the seasonal cycle on the other hand. Beside the temperatures themselves also other atmospheric quantities show a quasi-bidecadal oscillation or a trend-break, i.e. a reversal of the trend, in the mid 2000s. The wave activity of the planetary waves and also gravity waves belong to these quantities. Because these waves are the drivers of the residual circulation in the stratosphere and mesosphere they are of great importance. Former studies already showed that the planetary wave activity shows a quasi-bidecadal oscillation and the gravity wave activity shows a trend-break in about 2004. But the former studies were limited in time, as only much shorter time series were available compared to now. Furthermore, in the case of the gravity wave activity also a limitation of the considered period range during parts of the analysis were present. With the longer time series that is available now and with an improved technique a new and more detailed analysis of the long-term evolution of the wave activity can be obtained. The new technique is based on the Lomb-Scargle-Periodogram and can handle time series with unequal spacing which is very common for OH(3,1) rotational temperatures. In the former studies either the analysed time interval and therefore the period range was restricted (gravity waves study) or the data gaps have been interpolated in advance (planetary waves study). Thus, our new technique will overcome these drawbacks.