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 observed
from Wuppertal, Germany, since 1988 with respect to periodic fluctuations (2 to 60 d) using the
Lomb-Scargle periodogram. The main type of fluctuation observed in the last decades shows
a period of about 28 d. Other periods which are frequently detected in the observations lie in
the period ranges around 2 d, from 5 to 6 d, from 8 to 12 d, and around 15 d and can likely
be 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 different
wave filtering in summer and winter. This winter to summer difference holds for waves with
longer periods, but it breaks off in the case of shorter periods below about 20 d. The occurrence
frequency of these waves (< 20 d) exhibit two smaller maxima around the equinoxes. Thereby
the waves with periods below 10 d account for the majority of observations in the months from
April to September, whereby the waves with periods between 10 d and 20 d were more equally
observed in the whole year except for the late spring and summer, where almost no events were
observed.
The long-term behaviour of the wave activity indicates a quasi-bidecadal oscillation, which is
seen in different proxies for the wave activity. A further comparison of these proxies indicates
that this long-term oscillation is likely driven by the amplitude of the waves, i.e. the strength
of the events and not the duration of the events.