GRIPS - long-term measurements at the mesopause near 87 km altitude

GRIPS: Long-term trends

Since the middle of the year 1987 the temperature in the Mesopause is measured 87 km height continuously. Temperature time series of this length and in this altitude range are very rare worldwide and therefore the Wuppertal time series is of great importance for the investigation of the long-term development of the temperatures. All the findings regarding the temperature development of the last decades are very important in view of the climate change discussions. Often, however, the entire picture of the development only comes after many years of measurement, because only then can long-term trends be distinguished from other variations.

The time series of night-time temperatures since 1987 are shown in Figure 1 above and the annual mean values below (the years 1991, 2012 and 2013 are missing due to large measurement gaps). The local maxima in the annual mean values occur at the same time as the maxima of the solar flux (red arrows), which is a measure of the activity of the sun and has an 11-year cycle. The investigation then showed a temperature dependence of the solar flux of about 4 K / (100 solar flux units). This value is an additional confirmation of the relationship between the temperature and the activity of the sun and already has been confirmed by many other measurements.

In addition to the influence of the sun, the temperature time series shows an oscillation with a period of about 25 years and an amplitude of about 2 K (see Figure 2 black curve). Thus, no long-term trend is needed to describe the time series. This new insight became possible only through a re-examination of the data series with measurements up to and including 2015. In the previous study (Offermann et al. (2010)) for the period 1988-2008, a negative long-term trend of about 2 K / decade was determined. However, this strong mean decrease in temperature is in line with the 25-year oscillation as it reaches a minimum towards the end of the investigation period (see black graph in Figure and green trend line). Due to the 25-year oscillation, different long-term trends result depending on the study period. It shows, e.g. a significantly weaker mean temperature decrease for the period 1988 - 2015 of less than 1 K / decade (red trend line in Figure 2), which is due to the increase in temperature since 2005/06. An assumed long-term trend from 1975 to 2015 would even be slightly positive (blue trend line in Figure 2). The whole picture of the development and the different possible trends became recognizable only after many years of measurement.

Further information on long-term development studies can be found in Kalicinsky et al. (2016) and (2018).

Figure 1: Night mean temperatures (above) and annual mean temperatures (below) since 1987. The red arrows mark the maximums of solar activity (graph changed and taken from by Kalicinsky et al. (2016))

 

Figure 2: Derived 25-year wave (black curve) and trend lines for different time periods (red: 1988 - 2015, green: 1988 - 2008, blue: 1975 - 2015) (Graph added by Kalicinsky et al. (2016))