Long-term oscillations and trends of the mesosphere derived from 60 Years of standard phase-heights measurements over Europe: An update

The time series of standard phase-height (SPH) and plasma scale-height (PSH) have been updated from a 60-year long-radio-wave measurement of the broadcasting station Allouis (France, 162 kHz). The signal was received at Kühlungsborn (54° N, 12° E, Mecklenburg, Northern Germany).

The statistical analysis of the SPH series shows a significant overall trend with a decrease of 116 m/decade indicating a subsidence of the long-radio wave reflection height of about 700 m. With consideration of a stratopause altitude trend (-70 m/decade) follows an overall mesospheric shrinking of about 300 m over Western Europe.

As expected the time series of SPH shows in its spectrum dominant modes which are typical for the solar cycle, ENSO and for QBO bands indicating solar and lower atmospheric influences. Solar cycle and ENSO (-QBO)-like band-pass show a growing increase of SPH up to 1987, followed by a decrease afterward. We found a strong reduction in the amplitude of the solar cycle band due to the weak solar cycle 24, but an increase in the ENSO band.

For summer months during solar minimum years, and without stratopause altitude trend, a thickness temperature trend of the mesosphere is significant with a trend value of −0.47 ± 0.43 K/decade. The long-term solar variability and the stratopause altitude trend were excluded to determine a more realistic intrinsic mesospheric thickness temperature trend. The overall cooling of the intrinsic mesospheric temperature during 60 years of observation is in the order of 3 K.

The long-term solar variability including the decreasing maximum of last solar cycle, and the stratopause altitude trend have to be excluded in order to determine an intrinsic mesospheric temperature trend, which may be caused by greenhouse gas increase in the middle atmosphere.