The results of an analysis of temperature variations in the mesopause region based on long-term measurements of hydroxyl airglow at the Zvenigorod Scientific Station of the A.M. Obukhov Institute of Atmospheric Physics RAS (ZSS IAP RAS) in 1960-2024 in comparison with variations of surface temperature characterizing global-scale climate changes are presented. Along with temperature variations in the mesopause region, two versions of temperature variations in the mesopause region, normalized to the same level of solar activity, were analyzed. Quantitative estimates of a strong decrease in temperature in the mesopause region over the past decades in winter against the background of a global increase in surface temperature have been obtained. It was noted that significant coherence of long-term variations for temperature in the mesopause region with the surface temperature in the Northern Hemisphere with the use of cross-wavelet analysis, what was not previously evident in data for a shorter time interval. The possibility of such coherence was predicted in (Mokhov et al., 2017) under the continuation of global warming based on the results of model simulations for the 2021 centuries, taking into account anthropogenic forcing. It was not previously manifested from observational data for a shorter time interval. Along with long-term trends, features of a sharp decrease in temperature in the mesopause region in the 1970s with its synchronicity with the known shift in surface climate regimes associated with El Nino events were analyzed. The results of cross-wavelet analysis using data obtained at the ZSS IAP RAS for the time interval 1960-2024 indicate a more significant connection between temperature variations in the mesopause region and El Nino indices in recent decades. The problem of climate change is one of the key modern problems. To adequately understand ongoing climate change, it is necessary to determine the relative role of natural and anthropogenic factors. The analysis of vertical profiles of temperature trends in the atmosphere is of particular importance. Under global warming at the surface and in the troposphere in recent decades, a general cooling of the stratosphere and mesosphere has been revealed. The general cooling of the middle and upper atmosphere under general warming at the surface, noted from the results of various measurements, is an important indicator of the potential role of anthropogenic mechanisms of global climate change. The results of model simulations indicate significant differences in temperature changes in different atmospheric layers, in particular in the stratosphere and mesosphere, depending on the type of external forcing on the climate system: natural (including solar and volcanic activity) and anthropogenic (including changes in the content of greenhouse gases and aerosol in the atmosphere) ones [119]. According to model simulations, under the increase in the content of greenhouse gases (carbon dioxide and others) in the atmosphere against the background of general warming at the surface and in the troposphere, a cooling of the atmospheric layers above the tropopause is expected . At the same time, due to natural variations in insolation, a general synphase of temperature variations in the lower and middle atmosphere appears (see, for example, [8,20,21]). In the Earth's climate system, a special temperature regime is distinguished in the mesopause region, i.e. at the boundary between the mesosphere and thermosphere (at altitudes of 80-100 km) with the lowest temperature values. It is no coincidence that a global network of stations has been created that monitors changes in the mesopause region, so called NDMC (Network for the Detection of Mesopause Change, The key argument for the creation of the international NDMC project is the possibility of early detection of global changes in the temperature regime of the atmosphere and its composition from measurements in the upper atmosphere, since the reaction of rarefied layers of the upper atmosphere, in particular in the mesopause region, is more sensitive to both the effects of solar radiation and other forcings, including anthropogenic ones, compared with the response for the lower atmosphere. It should be noted that the lower layers of the atmosphere are significantly influenced by various dynamic processes that arise when interacting with inhomogeneities on the Earth's surface [19]. Within the framework of the NDMC system, the longest time series of data for atmospheric temperature in the mesopause region is based on measurements at the Zvenigorod Scientific Station of the A.M. Obukhov Institute of Atmospheric Physics RAS (ZSS IAP RAS) since the 1950s [7]. The data for the temperature Tms in the mesopause region by measurements at the Zvenigorod Scientific Station (56 deg N, 37 deg E) of the A.M. Obukhov Institute of Atmospheric Physics RAS in 1959-2024 were analyzed. The values of Tms were determined (with an accuracy of 2K) from the spectrometric measurements of hydroxyl airglow with the emitting maximum at an altitude of about 87 km [7] (see also [14,16,19]). In 2015, a series of spectrophotometric measurements of temperature around the mesopause at the ZSS IAP RAS together with measurements in Moscow region of vertical distributions of temperature and ozone content in the atmosphere using a multifunctional highaltitude sensing lidar, were carried out [22]. During lidar soundings, altitude distributions of atmospheric temperature were determined up to an altitude of about 100 km. Along with this, temperature profiles obtained from lidar measurements were compared with satellite data (AURA, TIMED/SABER) and the CIRA model. The obtained comparison results indicate a fairly good agreement between the various data, in particular for the temperature in the mesopause region. We analyzed also the variations in the temperature in the mesopause region, Tm, obtained by the reduction of the Tms values to the constant level of solar activity characterized by the index of the flux density of solar radiation F10.7 ( act/flux10.7/daily/) . In this case Tms = Tm + Tms, where the contribution of solar radiation flux variations to temperature variations in the mesopause region Tms was estimated similar to [7]. Temperature values in the mesopause region Tm(1) were analyzed, taking into account Tms = 25 log [F10.7 (t  0.42)/150], where t is time in years.
The parameterization (1) was obtained based on observations without taking into account seasonality (see [7]). It was recently noted that winter and summer temperatures in the mesopause region have different responses to changes in solar activity [23]. It should be also noted, that for the winter season the response is linear and without time lag: sfu is the unit of measurement of F10.7 solar flux. Along with Tm(1), the temperature values in the mesopause region Tm(2) were analyzed, reduced to the level of solar activity F10.7=130 taking into account (2). It is necessary to note that due to gaps in measurements (associated with cloudiness) in the time interval 19602024 the winter mean (December to February) temperature values in the mesopause region were analyzed using approximations for annual time intervals (from July 1 of the previous year to June 30 of the next year), taking into account annual and semi-annual harmonics. Along with estimates of the temperature trends in the mesopause region dTm(1)/dt, dTm(2)/dt and dTms(1)/dt, the sensitivity parameters of temperature in the mesopause region to changes in the NH surface temperature TNHs were determined using the corresponding linear regressions: dTms/dTNHs, dTm(1)/dTNHs, dTm(2)/dTNHs. We used monthly-mean values of the NH surface temperature anomalies (and for the Earth as a whole) relative to its base conditions for the period 1961-1990 ( For winter 20232024 the TNHs value was determined by the average value for December 2023 and January 2024. The cross-wavelet analysis [24] was also used, in particular, the local coherence of temperature in the mesopause region with surface temperature, solar activity indices and El Nino phenomena indices, including the Nino3, Nino3.4, Nino4 indices (https:/ /psl.noaa.gov/data/climateindices/). The results of analysis of observational data were compared with results of model simulations for the 20th-21st centuries [19]. Figure 2 characterizes the cross-wavelet coherence of winter variations in hemispheric surface temperature TNHs and temperature in the mesopause region (at the level of 0.003 hPa) at latitude 56 deg N from model simulations taking into account anthropogenic forcings [19]. According to Fig. 3 there is a shift since the end of the 20th century in the boundary of significant coherence (limited by the thick curve) towards shorter-period variations, namely to periods of about 20 years in the second half of the 21st century, and by the end of the century to periods of less than 10 years. According to model estimates, under continuation of global warming at the surface (and in the troposphere) with cooling of the strato-mesosphere, we have to expect in the next decades the manifestation of significant long-term coherence of temperature variations at the level of the mesopause and TNHs from the analysis of observational data. According to Table 2 for shorter time subintervals, along with negative estimates dTm(1)/dTNHs and dTm(2)/dTNHs, in particular for the subinterval 19761992, positive estimates dTm(1)/dTNHs and dTm(2)/dTNHs, in particular for the 19601976 subinterval, were obtained. The results obtained are consistent with the conclusions made in [19] about significant differences, up to the sign, between the parameters of the relationship of temperature variations in the mesopause region and near the surface on longer time scales of several decades and on shorter shorter time scales (from interannual to interdecadal). Abrupt change in temperature regime near mesopause in the 1970s also manifests itself when analyzing the connection between winter variations in Tm based on 65 years of measurement data at the ZSS IAP RAS with the corresponding variations in the hemispheric surface temperature TNHs in Analysis of distribution functions for winter Tms values for the entire analyzed period 1960-2024 indicates the manifestation of different temperature regimes in the mesopause region before and after the noted temperature jump in the 1970s. This was previously identified for a shorter time interval of 19602015 [19]. Distribution function for winter Tms values for the entire analyzed period 1960-2024 is characterized by two maxima: in the ranges of 230-235 K and 210-215 K, with a minimum between them in the range of 220-225 K . The minimum of the distribution function corresponds to the years of temperature jump in the 1970s. The maximum with an average Tms value in the range of 230-235 K corresponds to the first time subinterval. A maximum in the range of 210-215 K characterizes Tms after the temperature jump in the 1970s and transition to a new regime. In [14,19], the synchronicity of the noted jump in the temperature regime in the mesopause region according to measurements at the ZSS IAP RAS with the well-known climate shift in the 1970s, noted in various near-surface climatic features, including the intensification of the Aleutian atmospheric center of action, was noted (see, for example, [19]). Climate shift 1976-1977 in the Pacific sector is associated with qualitative changes in the processes of formation of El Nino phenomena [25][26][27]. In particular, in [25], the manifestation of more frequent El Nino events and rarer La Nina events has been noted since the second half of the 1970s. A more significant connection at the end of the 20th century with the El Nino phenomena was also noted for the characteristics of the winter North Pacific centers of atmospheric action, the Aleutian minimum and the Hawaiian maximum [28] (see also [29,30]). In [31], significant changes are noted in the frequency of various interphase transitions for El Nino phenomena (to which the strongest interannual variations in global surface temperature are associated) against the background of longer-period (with a period of several decades) variations and, in general, positive secular trend in global surface temperature. In [32,33], a tendency is noted for the intensification and frequency of El Nino phenomena with global warming. As for long-term tendencies, no significant coherence of the longest-period variations in Tms and Tm with TNHs was detected in [19] from observations for the time interval 19602015 with the use of cross-wavelet analysis.
It was noted that although the series of data for variations in Tms and Tm from measurements at the ZSS IAP RAS is the longest in the world (since 1959), its duration so far makes it possible to detect the coherence of variations in Tms and Tm with TNHs only with characteristic times of less than two decades. Figure 8a, which characterizes the cross-wavelet coherence of winter temperature variations in the mesopause region Tms with variations of the hemispheric surface temperature TNHs according to 65-year data for 1960-2024, already reveals a significant coherence of their longterm (more than two decades) variations. The correlation of long-term variations is negative. For comparison, Fig. 8b presents the results of a similar analysis for 56 years of data (19602015), as in [19], without the manifestation of significant coherence of long-term variations of Tms and TNHs. 1) and TNHs indicate a more significant coherence of their long-term variations in recent years than for Tms and TNHs. The results obtained confirm the presence of a significant negative correlation of long-term temperature variations in the mesopause region with variations in surface temperature, which was expected in accordance with model prognostic simulations [19]. Results of measurements at the ZSS IAP RAS since the late 1950s indicate that, against the background of a general increase in surface temperature, the temperature at the mesopause level has decreased significantly, in particular in winter, with a sharp temperature shift in the 1970s and weakening cooling rates in recent decades. As part of the NDMC system, regular measurements of temperature in the mesopause region at other stations began to be carried out in the 1980s, i.e. after the sharp temperature changes in the mesopause region were noted in the 1970s according to measurements at the ZSS IAP RAS. Comparison of data obtained at the ZSS IAP RAS with available measurement data at other stations of the NDMC system indicates the general consistency of data for temperature in the mesopause region and its trend over the past decades (see, for example, [9,10,17]). Special comparisons in 2015 of temperature values in the mesopause region by measurements at the ZSS IAP RAS with those obtained from lidar measurements of temperature profiles in the Moscow region revealed their good agreement [22]. Multi-year increase in global surface temperature is accompanied by the temperature decrease in the mesopause region with a slowdown in recent years. At the same time, a sharp decrease in temperature in the mesopause region in the 1970s was revealed [14,19] synchronously with the well-known climate shift of the 1970s, previously identified in different climatic features near the surface. The noted climate change with the transition to a new regime is associated with changes in the El Nino phenomena. The results of cross-wavelet analysis using the results of measurements at the ZSS IAP RAS for the time interval 19602024 indicate a more significant connection between temperature variations in the mesopause region and El Nino indices in recent decades. Significant coherence of the long-term temperature variations in the mesopause region with the hemispheric surface temperature from observational data for the time interval 19602024 was revealed with the use of the cross-wavelet-analysis. Previously, such a relationship was not noted in [19] based on the corresponding data for a shorter time interval. At the same time, it was revealed by longer-period climate model simulations for the 2021st centuries, and it was assumed that to identify the corresponding significant coherence of long-term anti-phase variations at the mesopause level and at the surface according to observational data, longer measurements are necessary. The noted features of temperature changes in the mesopause region against the background of changes in surface temperature are an important indicator for quantitative assessment of the comparative role of natural and anthropogenic factors in the formation of modern climate changes. The work was carried out within the framework of the RSF project 24-17-00211.