Long-term variations in the mesopause region derived from OH*(3,1) rotational temperature observations at Wuppertal, Germany, from 1988 – 2022

Abstract

We analysed the time series of OH*(3,1) rotational temperatures observed from Wuppertal in the time interval 1988 – 2022. The long-term evolution of the time series is characterised by two components. Firstly, we derived a significant correlation of the temperatures and the F10.7 cm solar radio flux. The sensitivity to the 11-year cycle of solar activity is 4.8 ± 0.7 K (100SFU)−1. Then we show that the second major component in addition to this solar influence is a significant long-period oscillation. The oscillation has a period of P = 22.2 ± 1.5 years with an amplitude of A = 1.8 ± 0.4 K. A significant linear trend cannot be derived for the time series of OH*(3,1) rotational temperatures. Due to the long-period oscillation the derivation of linear trends heavily depends on the analysed time interval. This explains the different linear trends that have been derived in past studies for this time series. All of the previous results for this quantity are in good agreement with the long-period oscillation determined here, because different parts of the time series and, therefore, different parts of the oscillation (e.g mainly downswing or a complete cycle) have been analysed which led to largely different linear trends.