Ground-based observations of periodic temperaturefluctuations in the mesopause region with periods larger than 2 days

We analysed more than 30 years of OH*(3,1) rotational temperatures that have been observedfrom Wuppertal, Germany, since 1988 with respect to periodic fluctuations (2 to 60 d) using theLomb-Scargle periodogram. The main type of fluctuation observed in the last decades showsa period of about 28 d. Other periods which are frequently detected in the observations lie inthe period ranges around 2 d, from 5 to 6 d, from 8 to 12 d, and around 15 d and can likelybe assigned to the quasi-2-day, the quasi-5-day, the quasi-10-day , and the quasi-16-day wave,respectively.The wave activity is typically larger in winter time than in summer time because of the differentwave filtering in summer and winter. This winter to summer difference holds for waves withlonger periods, but it breaks off in the case of shorter periods below about 20 d. The occurrencefrequency of these waves (< 20 d) exhibit two smaller maxima around the equinoxes. Therebythe waves with periods below 10 d account for the majority of observations in the months fromApril to September, whereby the waves with periods between 10 d and 20 d were more equallyobserved in the whole year except for the late spring and summer, where almost no events wereobserved.The long-term behaviour of the wave activity indicates a quasi-bidecadal oscillation, which isseen in different proxies for the wave activity. A further comparison of these proxies indicatesthat this long-term oscillation is likely driven by the amplitude of the waves, i.e. the strengthof the events and not the duration of the events.