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We investigate the impacts of increased CO.sub.2 concentration on migrating diurnal tide (DW1). A future climate simulation is conducted using a WACCM-X model, with surface CO.sub.2 levels increasing according to the RCP8.5 scenario. The DW1 (1,1) mode, a propagating tide peaking near the equator, exhibits a statistically significant positive trend in a range of 20-70 km, and a significant negative trend in a range of 90-110 km. The positive trend is likely driven by a reduction in atmospheric density in the mesosphere and enhanced equatorial convective activity, while the negative trend appears in the mesosphere, which overwhelms the positive trend. Two potential mechanisms may explain the negative trend. First, increasing CO.sub.2 enhances mesospheric stability, reducing tidal vertical wavelengths. In our simulation, equatorial temperatures around â¼50-70 km become cooler than those in â¼70-90 km. This strong cooling could be linked to CO.sub.2 mixing and transport, as well as the contraction of the mesospheric ozone layer due to atmospheric descent induced by CO.sub.2 -driven cooling. Second, stronger convective activity intensifies gravity wave generation, increasing gravity wave diffusion in the mesosphere. This strong convective activity also likely intensifies the tide below â¼70 km. While our positive DW1 trend is consistent with McLandress and Fomichev (2006), the negative trend in the lower thermosphere contrasts with their results. This discrepancy might arise because their model used a time-independent diffusion coefficient, whereas WACCM-X accounts for CO.sub.2 -driven changes in gravity wave diffusion. The negative trend is confirmed in SABER observation for the last two decades, while the positive trend is not verified.

Water vapor (H.sub.2 O) is the source of reactive hydrogen radicals in the middle atmosphere, whereas carbon monoxide (CO), being formed by CO.sub.2 photolysis, is suitable as a dynamical tracer. In the mesosphere, both H.sub.2 O and CO are sensitive to solar irradiance (SI) variability because of their destruction/production by solar radiation. This enables us to analyze the solar signal in both models and observed data. Here, we evaluate the mesospheric H.sub.2 O and CO response to solar irradiance variability using the Chemistry-Climate Model Initiative (CCMI-1) simulations and satellite observations. We analyzed the results of four CCMI models (CMAM, EMAC-L90MA, SOCOLv3, and CESM1-WACCM 3.5) operated in CCMI reference simulation REF-C1SD in specified dynamics mode, covering the period from 1984-2017. Multiple linear regression analyses show a pronounced and statistically robust response of H.sub.2 O and CO to solar irradiance variability and to the annual and semiannual cycles. For periods with available satellite data, we compared the simulated solar signal against satellite observations, namely the GOZCARDS composite for 1992-2017 for H.sub.2 O and Aura/MLS measurements for 2005-2017 for CO. The model results generally agree with observations and reproduce an expected negative and positive correlation for H.sub.2 O and CO, respectively, with solar irradiance. However, the magnitude of the response and patterns of the solar signal varies among the considered models, indicating differences in the applied chemical reaction and dynamical schemes, including the representation of photolyzes. We suggest that there is no dominating thermospheric influence of solar irradiance in CO, as reported in previous studies, because the response to solar variability is comparable with observations in both low-top and high-top models. We stress the importance of this work for improving our understanding of the current ability and limitations of state-of-the-art models to simulate a solar signal in the chemistry and dynamics of the middle atmosphere.

Sudden stratospheric warmings (SSWs) are large‐scale phenomena characterized by dramatic dynamic disruptions in the stratospheric winter polar regions. Previous studies, especially those employing whole atmosphere models, indicate that SSWs have strong impacts on the circulation of the mesosphere lower thermosphere (MLT) and drive a reversal in the mean meridional circulation (MMC) near 90–125 km altitude. However, the robustness of these effects and the roles of SSW‐induced changes in global‐scale wave activity to drive the reversal have been difficult to observe simultaneously. This work employs horizontal lower thermospheric (∼93–106 km altitude) winds near 10°S‐40°N latitude from the Michelson Interferometer for Global High‐resolution Thermospheric Imaging instrument onboard the Ionospheric Connection Explorer (ICON) to present observational evidence of a prominent MLT MMC reversal associated with the January 2021 major SSW event and to demonstrate connections to semidiurnal tidal activity and possible associations with a ∼3‐day ultra‐fast Kevin wave. Plain Language Summary The winds in the mesosphere lower thermosphere (MLT) are strongly impacted by dramatic changes in the stratospheric winter polar regions associated with Sudden stratospheric warmings (SSWs). Models have shown that the climatological direction of the MLT north‐south and vertical circulation, characterized by equatorward flow near ∼100–120 km and poleward flow near ∼80–100 km, reverses following the onset of SSWs. Yet, the impacts and causes of these dynamical effects are not well established observationally due to the lack of comprehensive global measurements of the MLT region. This study evaluates the evolution of MLT winds and associated tidal and ultra‐fast Kevin wave variations during the January 2021 SSW using horizontal wind observations from the Michelson Interferometer for Global High‐resolution Thermospheric Imaging instrument onboard the Ionospheric Connection Explorer to present observational evidence of a large MLT north‐south wind reversal due to the SSW and associated global‐scale wave influences. Key Points A prominent (∼30 m/s) reversal in the mesosphere lower thermosphere (MLT) mean meridional circulation during the January 2021 major sudden stratospheric warming (SSW) is observed in Michelson Interferometer for Global High‐resolution Thermospheric Imaging winds Strong (∼45 m/s) MLT westward flow enhancements and latitudinal (10S–40N) dependencies in the wind response occur as a result of the SSW Amplification in MLT SW2 and ultra‐fast Kevin wave1 zonal wind amplitudes are consistent with the observed westward flow enhancements

Observational monitoring of the stratospheric transport circulation, the Brewer-Dobson circulation (BDC), is crucial to estimate any decadal to long-term changes therein, a prerequisite to interpret trends in stratospheric composition and to constrain the consequential impacts on climate. The transport time along the BDC (i.e. the mean stratospheric age of air, AoA) can best be deduced from trace gas measurements of tracers which increase linearly with time and are chemically passive. The gas sulfur hexafluoride (SF.sub.6) is often used to deduce AoA because it has been increasing monotonically since the â¼1950s, and previously its chemical sinks in the mesosphere have been assumed to be negligible for AoA estimates. However, recent studies have shown that the chemical sinks of SF.sub.6 are stronger than assumed and become increasingly relevant with rising SF.sub.6 concentrations.

Using a network of meteor radar observations, observational evidence of polar-to-tropical mesospheric coupling during the 2018 major sudden stratosphere warming (SSW) event in the northern hemisphere is presented. In the tropical lower mesosphere, a maximum zonal wind reversal (−24 m/s) is noted and compared with that identified in the extra-tropical regions. Moreover, a time delay in the wind reversal between the tropical/polar stations and the mid-latitudes is detected. A wide spectrum of waves with periods of 2 to 16 days and 30–60 days were observed. The wind reversal in the mesosphere is due to the propagation of dominant intra-seasonal oscillations (ISOs) of 30–60 days and the presence and superposition of 8-day period planetary waves (PWs). The ISO phase propagation is observed from high to low latitudes (60° N to 20° N) in contrast to the 8-day PW phase propagation, indicating the change in the meridional propagation of winds during SSW, hence the change in the meridional circulation. The superposition of dominant ISOs and weak 8-day PWs could be responsible for the delay of the wind reversal in the tropical mesosphere. Therefore, this study has strong implications for understanding the reversed (polar to tropical) mesospheric meridional circulation by considering the ISOs during SSW.

The storm‐time temperature difference with respect to its quiet‐time expectation (ΔT) in the mesosphere and lower thermosphere were studied during the extreme storms on 2024 Mother's Day and 2003 Halloween Day. The storm‐time ΔT were determined by performing daily zonal running mean on the temperature profiles in the ascending and descending nodes separately. The storm‐time ΔT had peak values of ≥25 K and extended downward to ∼100 km globally. Above 105 km, the global mean ΔT had values of ≥20 K in the early morning and of ≥15 K in the late afternoon during storm‐time. At high latitudes, the storm‐time ΔT was larger in the late afternoon than in the early morning. This is opposite to that at middle and low latitudes. Adiabatic warming/cooling caused by the heating‐induced circulation changes outside of the auroral oval is likely responsible for the local time and latitude dependence of the storm‐time ΔT. Plain Language Summary The storm‐time energy input in the auroral oval plays an important role in changing the dynamics and electrodynamics of the neutral atmosphere and ionosphere. Although the energy input due to Joule and particle heating is the strongest at high latitudes, its influences are global. The mesosphere and lower thermosphere are transition regions between the middle atmosphere and the ionosphere, which are affected by the lower atmosphere, the ionosphere, and the solar and geomagnetic activities. This complicates the physics and dynamical structures of the mesosphere and lower thermosphere during the storm‐time, especially the two extreme storms in the past 20 years. The temperature measured by SABER is used to study the local time and latitude dependence of the storm‐time temperature difference with respect to its quiet‐time expectation. The extreme storms are rare with average occurrence frequency of about 4 days per 11‐year. The storm‐time temperature difference was larger in the late afternoon than in the early morning at high latitudes, which is opposite to that at middle and low latitudes. This highlight that the extreme storms induce much larger and observable temperature changes as compared with those associated tides. Key Points The temperature increased globally and depended on local time during extreme storms on the 2024 Mother's Day and 2003 Halloween Day The storm‐time temperature difference (ΔT) had global means of ≥20 K in the early morning and of ≥15 K in the late afternoon The ΔT is larger in the late afternoon than in the early morning at high latitudes, but the reverse is true at middle and low latitudes

An analytical expression is derived which shows that the release of chemical heat from exothermic chemical reactions is a negative nonlinear feedback mechanism relative to temperature in the mesopause region. This role is confirmed through model simulations. This feedback mechanism stabilizes the thermal regime of the mesopause region. The simulations show that a 1–2 K/day change (8%–18% relative variation) in the chemical heating rate occurs as a response to a 10 K decrease in temperature.

For space-based atmospheric wind measurements, full-link simulation is critical for the optimization of the instrument indicators and the evaluation of the measurements' performance. This paper presents observation simulations and error verification of the mesosphere wind measurement with four emission lines of the O[sub.2] (0-1) band by using the space-based Doppler Asymmetric Spatial Heterodyne (DASH), named the Mesosphere Wind Image Interferometer (MWII). The passive wind measurement principle and the DASH concept are first described. The full-link simulation consists of radiation simulation, the instrument forward model, and the wind retrieval model. The four emission lines at about 866.5 nm of the O[sub.2] (0-1) band were selected as the observation targets. The radiation characteristics of the target lines were studied and calculated, as well as the background radiation. Based on the LOS radiation integral model, a numerical simulation of the raw observation data was carried out using the instrument model. The interference fringe priority strategy and joint wind decision method were proposed to achieve multiple-emission-line wind retrieval with higher precision. In the simulation, multiple-line retrieval could improve the precision by more than 30% compared to single-line retrieval under the same conditions. The error simulation indicated that the wind profile precision was 3-9 m/s in the altitude range of 50-110 km, with an average accuracy of about 1 m/s, proving that the scheme of MWII has good altitude coverage of the whole mesosphere and a part of the lower thermosphere.

Mean vertical velocity measurements obtained from radars at polar latitudes using polar mesosphere summer echoes (PMSEs) as an inert tracer have been considered to be non-representative of the mean vertical winds over the last couple of decades. We used PMSEs observed with the Middle Atmosphere Alomar Radar System (MAARSY) over Andøya, Norway (69.30∘ N, 16.04∘ E), during summers of 2016 and 2017 to derive mean vertical winds in the upper mesosphere. The 3-D vector wind components (zonal, meridional and vertical) are based on a Doppler beam swinging experiment using five beam directions (one vertical and four oblique). The 3-D wind components are computed using a recently developed wind retrieval technique. The method includes full non-linear error propagation, spatial and temporal regularisation, and beam pointing corrections and angular pointing uncertainties. Measurement uncertainties are used as weights to obtain seasonal weighted averages and characterise seasonal mean vertical velocities. Weighted average values of vertical velocities reveal a weak upward behaviour at altitudes ∼84–87 km after eliminating the influence of the speed of falling ice. At the same time, a sharp decrease (increase) in the mean vertical velocities at the lower (upper) edges of the summer mean altitude profile, which are attributed to the sampling issues of the PMSE due to disappearance of the target corresponding to the certain regions of motions and temperatures, prevails. Thus the mean vertical velocities can be biased downwards at the lower edge, and the mean vertical velocities can be biased upwards at the upper edge, while at the main central region the obtained mean vertical velocities are consistent with expected upward values of mean vertical winds after considering ice particle sedimentation.