Would El Niño enhance or suppress the migrating diurnal tide in the MLT region?

Previous observations and simulations are controversial as to whether El Niño will increase or decrease the diurnal tide (DW1) in the upper mesosphere and lower thermosphere (MLT) region. This study revisited the linear response of the MLT DW1 to El Niño during the winter (December-January-February) based on 19-year satellite observations of Sounding of the Atmosphere using Broadband Emission Radiometry (SABER). The MLT DW1 temperature amplitudes decreased by ~10 % during four El Niño winters from 2002 to 2020, consistent with the results from the simulation of the Specified-Dynamics version of the Whole Atmosphere Community Climate Model (SD-WACCM). According to the multiple linear regression analysis, the linear effects of El Niño-Southern Oscillation (ENSO) on tropical MLT DW1 are negative in both SABER observations and SD-WACCM simulations. In the SD-WACCM simulation, Hough mode (1, 1) dominates the DW1 tidal variation in the tropical MLT region. The consistency between the (1, 1) mode in the tropopause region and in the MLT region, as well as the downward phase progression from 15 to 100 km, indicates the direct upward propagation of DW1 from the excitation source in the troposphere. During 7 of 8 El Niño winters from 1979 to 2014, the anomalous amplitudes of the (1, 1) mode are negative in both the tropopause region and MLT region. The suppressed DW1 heating rates in the tropical troposphere (average over ~0–16 km and 35° S–35° N) during the El Niño events contribute to the decreased DW1 tide. The mesospheric latitudinal zonal wind shear anomalies during El Niño winters would lead to a narrower waveguide and prevent the vertical propagation of the DW1 tide. The gravity wave drag excited by convection also plays a role in modulating the MLT DW1 amplitude.

Roles of Rossby Waves, Rossby-Gravity Waves, and Gravity Waves Generated in the Middle Atmosphere for Interhemispheric Coupling

It has often been reported that warming at high latitudes in the Southern Hemisphere (SH) summer mesosphere and lower thermosphere (MLT) appears during Arctic sudden stratospheric warming (SSW) events. This phenomenon, which is called “interhemispheric coupling (IHC)”, has been thought to occur because of the modulation of mesospheric meridional circulation driven by forcing of gravity waves (GWs) originating in the troposphere. However, quasi-two-day waves (QTDWs) develop during SSWs and result in strong wave forcing in the SH mesosphere. Thus, this study revisits IHC following Arctic SSWs from the viewpoint of wave forcing, not only by GWs and Rossby waves (RWs) originating in the troposphere but also by GWs, RWs, and Rossby-gravity waves generated in situ in the middle atmosphere, and elucidates the causes of warm anomalies in the SH MLT region. During SSWs, westward wind anomaly forms because of cold equatorial stratosphere, GW forcing is then modulated, and barotropic/baroclinic and shear instabilities are strengthened in the SH mesosphere. These instabilities generate QTDWs and GWs, respectively, which cause significant anomalous westward wave forcing, forming a warm anomaly in the SH MLT region. The intra-seasonal variation in QTDW activity can explain seasonal dependence of the time lag in IHC. Moreover, it is revealed that the cold equatorial stratosphere is formed by middle-atmosphere Hadley circulation, which is strengthened by wave forcing associated with stationary RW breaking leading to SSWs. The IHC mechanism revealed in this study indicates that waves generated in the middle atmosphere contribute significantly to the meridional circulation, especially during SSWs.

Trends in the Airglow Temperatures in the MLT Region—Part 3: Ground-Based and SABER Measurements

Ground-based temperature measurements at Svalbard, Wuppertal, and Hohenpeissenberg were analyzed to obtain F10.7, Ap index, and Dst index trends. The trends were then compared to those obtained from Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) temperature measurements at the same locations. Trend analysis was carried out for overlapped time periods, full range of available data, and the CO2-detrended full range of available data. The Svalbard meteor radar (SABER) temperature showed a weak (moderate) correlation with F10.7 and a moderate (weak) correlation with Ap and Dst indices. The trends in the Wuppertal OH* temperature compare well with the SABER temperature when a full range of data is used in the analysis. Both temperatures had a similar F10.7 trend with the same level of correlation coefficient. The F10.7 trend in the Hohenpeissenberg OH* temperature compared well with that obtained by SABER, but the former displayed a weak correlation. The Hohenpeissenberg data displayed a very weak correlation with Ap and Dst indices. Our study clearly shows that a longer dataset would better capture trends in temperature, as was evidenced by the results of Wuppertal data. The CO2-detrended temperatures overall showed slightly larger trend values with a slightly better correlation.

Variation of CO/CO2 profiles in the Marian mesosphere and lower thermosphere retrieved from TGO/NOMAD

<p>CO is produced by the photodissociation of CO<sub>2</sub> and recycled to CO<sub>2</sub> by the catalytic cycle involving HOx in the Martian atmosphere [e.g., McElroy & Donahue, 1972]. In the mesosphere and lower thermosphere (MLT) region of Mars, the number density of CO is determined by photodissociation, diffusion, and atmospheric circulation. The increase of the CO mixing ratio in the MLT region and further enhancement in the polar region due to the transport of CO-enriched air via meridional circulation are predicted in the 3D models [Daerden et al., 2018; Holmes et al., 2019]. On the other hand, the decrease in the CO mixing ratio in the MLT region during a global dust storm is detected by TGO/ACS, which suggests that the increase in the hygropause altitude leads to the increase in the vertical range over which OH becomes available to convert into CO<sub>2</sub> [Olsen et al., 2021]. Additionally, a substantial variation of the homopause altitude has been investigated [Slipski et al., 2018; Jakosky et al., 2017; Yoshida et al., 2020], which suggests that the order of magnitude changes in the eddy diffusion coefficient at the homopause [Slipski et al., 2018], and then variations in the profile of CO mixing ratio in the MLT region. However, the effects of change in the eddy diffusion coefficient on the profile of CO mixing ratio have not been investigated. The variability of the CO mixing ratio profiles can be a clue for understanding the dynamical coupling between the lower and the upper atmospheres.</p> <p>To clarify the contributions of photochemistry, diffusion, and atmospheric circulation to the CO/CO<sub>2</sub> profiles in the MLT region, we use the Nadir and Occultation for MArs Discovery (NOMAD) instrument aboard Trace Gas Orbiter (TGO). NOMAD solar occultation is designed as the combination of the Acousto Optical Turnable Filter and echelle grating [Neefs et al., 2015; Thomas et al., 2016]. NOMAD solar occultation operates in the wavelength range of 2.2 - 4.3 μm (2320 to 4350 cm<sup>-1</sup>) with a high spectral resolution (λ/dλ = 20000) [Vandaele et al., 2018]. It provides us CO and CO<sub>2</sub> spectra below 100 km and 180 km altitudes, respectively.</p> <p>In this study, we applied the equivalent width technique [Chamberlain and Hunten, 1987; Krasnopolsky, 1986] to derive a new set of CO and CO<sub>2</sub> column densities, respectively, with the observed atmospheric transmittance spectra by NOMAD solar occultation. The absorption lines centered at 4285.0, 4288.2, and 4291.5 cm<sup>-1</sup> for CO (2-0) band and 3358.7, 3364.9, and 3366.4 cm<sup>-1</sup> for CO<sub>2</sub> (21102-00001) band are carefully selected for retrievals due to the contribution of nearby and central orders [cf. Liuzzi et al., 2019]. It is noted that the line strengths of the selected CO<sub>2</sub> have high sensitivity to the background temperature. In this study, we applied the vertical profiles of temperature simulated in the GEM-Mars model [Neary et al., 2018; Daerden et al., 2019]. We retrieve the CO and CO<sub>2</sub> slant column densities between 60 and ~100 km altitudes because those slant opacities are saturated below 60 km altitude. The CO and CO<sub>2</sub> spectra observed from April 2018 to September 2020, corresponding to from MY 34 Ls ~ 150 to MY 35 Ls ~ 280, are investigated.</p> <p>We found that the retrieved CO/CO<sub>2</sub> ratio between 60 and ~100 km increases with altitude. A behavior of the decrease in the CO/CO<sub>2</sub> ratio during the global dust storm corresponds to the previous observations [Olsen et al., 2021]. However, the CO/CO<sub>2</sub> profiles also vary with season and latitude. For interpretation, the 1D photochemical model will be compared with newly obtained CO/CO<sub>2</sub> profiles, especially in order to discuss the contributions from the variations in eddy diffusion coefficient and photochemistry in the MLT region on Mars.</p>

OSAS-B: a balloon-borne heterodyne spectrometer for sounding atomic oxygen in the MLT region

<p>The Oxygen Spectrometer for Atmospheric Science on a Balloon (OSAS-B) is dedicated to the remote sounding of atomic oxygen in the mesosphere and lower thermosphere (MLT) region of Earth's atmosphere, where atomic oxygen is the dominant species. Quantitative radiometry of atomic oxygen via its visible and near-infrared transitions has been difficult, due to the complex excitation physics involved. OSAS-B is a heterodyne spectrometer for the thermally excited ground state transition of atomic oxygen at 4.75 THz. It will enable spectrally resolved measurements of the line shape,  which in turn enables the determination of the concentration of atomic oxygen in the MLT. Due to water absorption, this line can only be observed from high-altitude platforms such as a high-flying airplanes, balloons or satellites. Recently the first spectrally resolved observation of the 4.75-THz line has been reported using a heterodyne spectrometer on SOFIA, the Stratospheric Observatory for Infrared Astronomy [1]. Compared to SOFIA a balloon-borne instrument has the advantage of not being hampered by atmospheric water vapor absorption. OSAS-B will comprise a hot-electron bolometer mixer and a quantum-cascade laser as local oscillator in a combined helium/nitrogen dewar. A turning mirror will allow for sounding at different vertical inclinations. The  first flight of OSAS-B is planned for autumn 2022 in the frame of the European HEMERA project [2].</p><p>[1] H. Richter et al., Direct measurements of atomic oxygen in the mesosphere and lower thermosphere using terahertz heterodyne spectroscopy, accepted for publication in Communications Earth & Environment (2021).</p><p>[2] https://www.hemera-h2020.eu/</p>

Combining satellite and lidar measurements to investigate the sodium nightglow

<p>Using a combination of different measurement techniques is important to understand the numerous processes happening in the MLT-region. One of those processes is the excitation of atomic sodium by reaction with ozone which leads to emission of electromagnetic radiation: a phenomenon called Airglow. Although the sodium excitation mechanism was already proposed in 1939 by Sidney Chapman and further investigation was done by a great number of scientists, there are still some key parameters that are not well-known today. One of those parameters is the branching ratio f<sub>A</sub> which determines the amount of sodium in the excited state. Exact knowledge of this value would offer the opportunity to use Na-nightglow measurements to determine sodium profiles in the MLT-region. In this study we used both, satellite measurements and ground-based Lidar measurements to help approach a more reliable branching ratio f<sub>A</sub>. By comparing measurements that were made by the two instruments OSIRIS on Odin (Satellite) and the Lidar of the Colorado State University (ground-based) we found a branching ratio f<sub>A</sub> of 0.064 +- 0.028.</p>

Burst of Unusual Quasi-10 day Wave During the 2019 Southern Sudden Stratospheric Warming

<div> <p>An unusual sudden stratospheric warming (SSW) occurred in the Southern hemisphere in September 2019. Ground-based and satellite observations show the presence of a transient westward-propagating quasi-10 day planetary wave with zonal wavenumber one during the SSW. The planetary wave activity maximizes in the MLT region approximately 10 days after the SSW onset. Analysis indicates the quasi-10 day planetary wave is symmetric about the equator which is contrary to theory for such planetary waves. </p> </div><div> <p>Observations from MLS and SABER provide a unique opportunity to study the global structure and evolution of the symmetric quasi-10 day wave with observations of both geopotential height and temperature during these unusual atmospheric conditions. The space-based measurements are combined with meteor radar wind measurements from Antarctica, providing a comprehensive view of the quasi-10 day wave activity in the southern hemisphere during this SSW. We will also present the results of our mesospheric and lower thermospheric analysis along with a preliminary analysis of the ionospheric response to these wave perturbations.</p> </div>