Long-term changes in temperature and wave activity in the mesopause region according to observations in Zvenigorod

     Long-term observations of characteristics of various regions of the atmosphere are crucial for understanding climate change that exhibits both quasi-periodic oscillations and long-term trends caused by natural processes or/and anthropogenic influences. For the prolonged monitoring of the thermal state in the mesopause region (80−100 km), measurements of the rotational temperature of vibrationally excited hydroxyl (OH*) are widely used. The emitting layer of OH* molecules peaks in intensity at an altitude of approximately 87 km with a half-width of about 9 km. The height-weighted average temperature of OH* can be relatively easily observed from the surface of the Earth at nighttime under clear weather conditions. This work presents observations made at the Zvenigorod Scientific Station of the A.M. Obukhov Institute of Atmospheric Physics of the Russian Academy of Sciences from 2000 to 2024. The spectral equipment allowed for the determination of the OH* temperature based on its rotational structure in the (6-2) band. A temperature series consisting of 100334 values of 10-minute measurements was analyzed. Based on this data, estimations of long-term trends were made for the annual average temperature (−0.13±0.03 K/year) as well as for seasonal averages (May−July, −0.18±0.06 K/year, and October−March, −0.10±0.06 K/year). The dependence on solar activity was also determined.

     It is supposed that the cooling of the middle and upper layers of the atmosphere is primarily due to the increase in carbon dioxide concentration in the atmosphere. However, model calculations predict that radiative cooling in the region of the mesopause resulting from the rise in carbon dioxide should not exceed −0.1 K/year. One of the reasons for the discrepancy between experimental and model results may be due to underestimation of long-term changes in atmospheric dynamics, which are primarily governed by wave processes in the mesopause region. To address this, annual and seasonal average standard deviations of temperature were analyzed using the method of differential digital frequency filtering. The examined standard deviations made it possible to assess long-term trends in wave activity across various frequency ranges of mesoscale variations, which turned out to be positive in all cases, with the magnitude depending on the frequency of the variations.

     This work was carried out within the framework under state assignments 125021001819-8 and 17.1.