Abstract. A global climatology of diurnal tides in the mesosphere and lower thermosphere (MLT) is constructed using multiyear observations from fifteen meteor radars distributed worldwide. The results show that diurnal tidal amplitudes are strongest at low and mid-latitudes (10°–50° N/S), with peak values of about 60 m s-1 near 20°–30° N/S, and are comparatively weak near the Equator and at polar latitudes. The seasonal variations of the diurnal tide are characterized by maxima around the equinoxes and minima during the solstices. In addition to these global climatological features, we identify a clear modulation of the vertical structure of diurnal tidal amplitude and phase by seasonal variations in solar forcing, represented here by the solar zenith angle (SZA). This modulation is particularly evident at northern low and mid-latitudes, but is much weaker in the Southern Hemisphere. The hemispheric asymmetry suggests that the tidal response to solar forcing is not globally uniform. To further explore the possible cause of this asymmetry, we examine the meridional fluxes of zonal tidal momentum. The results suggest that background zonal winds can influence tidal propagation through filtering effects and momentum drag, thereby contributing to the observed hemispheric differences in tidal structure. These results provide new observational evidence for the coupling between solar forcing and diurnal tides in the MLT region and offer useful constraints for the evaluation of general circulation models. They also improve our understanding of tidal propagation and variability in the middle and upper atmosphere.

Long‐term variability and tendencies in monthly mean semidiurnal tide (12‐hr) in zonal (U 12 ) and meridional (V 12 ) winds are investigated in northern polar mesosphere and lower thermosphere (MLT; ∼80–100 km) using meteor radar observations during 1999–2022 over Esrange (67.9°N, 21.1°E). The climatological mean of U 12 and V 12 amplitudes peak (up to ∼35 m/s) in winter (December–February) above ∼90–95 km with secondary maxima in late summer/early autumn (August–September), however the amplitude of V 12 is larger than U 12 . The U 12 and V 12 exhibit strong interannual variability that varies with altitude and month/season. The responses of U 12 and V 12 anomalies (from 1999–2003) to solar cycle (SC), Quasi Biennial Oscillation (QBO) at 10 hPa and 30 hPa, El Niño‐Southern Oscillation (ENSO), North Atlantic Oscillation (NAO), ozone (O 3 ) and carbon dioxide (CO 2 ) are analyzed using multiple linear regression. From the analysis, significant correlations are found between monthly tidal amplitudes and the above potential drivers, and the correlations vary with altitude and month. The U 12 and V 12 responses to O 3 are positive and significantly large (∼60–80 m/s/ppmv) below ∼85–90 km in February–March and above ∼95 km in January‐March. The tidal response to ENSO is significantly negative during August‐October (above ∼90 km) and positive in November‐December (above ∼85 km) in both components. The cumulative trend in U 12 is positive below ∼93 km and negative above this height peaking at ∼97 km. A positive trend in V 12 increases above ∼93 km and maximize at ∼98 km. The significant monthly trends vary with altitude in both tidal components.

In this study, we report the response of migrating diurnal tides (DW1) in the mesosphere and lower thermosphere (MLT) region to the Madden‒Julian oscillation (MJO). The DW1 amplitudes are decomposed from the neutral horizontal wind observed by four meteor radars in the equatorial region. The DW1 and zonal wind in the equatorial MLT region show consistent intraseasonal variations, implying that the upward propagating DW1 can affect the mesospheric zonal wind. By jointly analyzing the real‐time multivariate MJO (RMM) indices and mesospheric DW1, we found that DW1 responds strongly to the MJO during both boreal winter (difference relative to seasonal means: ∼20%–25%) and summer (∼25%). The MJO convection affects the mesospheric DW1 by modulating the solar radiative absorption by water vapor and latent heat release in the troposphere. The seasonal difference in DW1–MJO response can be attributed to a slight weakening of the DW1 tidal heating sources during MJO phases 1–4 and a significant enhancement during phases 6–7 during summer, driven by the moisture transport from the Indian Ocean and Pacific to the East Asian continent. This finding provides an opportunity to the understanding of the coupling between the troposphere and the mesosphere through migrating tides.

The mesosphere and lower thermosphere (MLT) comprise a highly variable region that forms the transition region between the middle and upper atmosphere. The variability of this region is driven by atmospheric waves transporting energy and momentum from the lower and middle atmosphere to MLT altitudes. These waves cover a wide range of temporal (minutes to days) and spatial (kilometers to planetary) scales. The upward propagation of atmospheric gravity waves and tides is one of the key processes at all latitudes that alters the state of the ionosphere–thermosphere system, and their vertical propagation depends crucially on the background mean winds. The TIMED Doppler Interferometer (TIDI) on board the Thermosphere-Ionosphere-Mesosphere-Energetics and Dynamics (TIMED) satellite observes neutral winds at the MLT using airglow emissions. We establish a TIDI mean wind climatology, compare our results with existing climatologies derived from local meteor radar observations, and discuss similarities and differences depending on local time and geographical latitude.

The Hunga Tonga–Hunga Ha′apai volcano erupted on 15 January 2022, launching Lamb waves and gravity waves into the atmosphere. In this study, we present results using 13 globally distributed meteor radars and identify the volcanogenic gravity waves in the mesospheric/lower thermospheric winds. Leveraging the High-Altitude Mechanistic general Circulation Model (HIAMCM), we compare the global propagation of these gravity waves. We observed an eastward-propagating gravity wave packet with an observed phase speed of 240 ± 5.7 m s−1 and a westward-propagating gravity wave with an observed phase speed of 166.5 ± 6.4 m s−1. We identified these waves in HIAMCM and obtained very good agreement of the observed phase speeds of 239.5 ± 4.3 and 162.2 ± 6.1 m s−1 for the eastward the westward waves, respectively. Considering that HIAMCM perturbations in the mesosphere/lower thermosphere were the result of the secondary waves generated by the dissipation of the primary gravity waves from the volcanic eruption, this affirms the importance of higher-order wave generation. Furthermore, based on meteor radar observations of the gravity wave propagation around the globe, we estimate the eruption time to be within 6 min of the nominal value of 15 January 2022 04:15 UTC, and we localized the volcanic eruption to be within 78 km relative to the World Geodetic System 84 coordinates of the volcano, confirming our estimates to be realistic.

The mesosphere and lower thermosphere (MLT) plays a critical role in linking the middle and upper atmosphere. However, many General Circulation Models do not model the MLT and those that do remain poorly constrained. We use long-term meteor radar observations (2005-2021) from Rothera (67 degrees S, 68 degrees W) on the Antarctic Peninsula to evaluate the Whole Atmosphere Community Climate Model with thermosphere-ionosphere eXtension (WACCM-X) and investigate interannual variability. We find some significant differences between WACCM-X and observations. In particular, at upper heights, observations reveal eastwards wintertime (April-September) winds, whereas the model predicts westwards winds. In summer (October-March), the observed winds are northwards but predictions are southwards. Both the model and observations reveal significant interannual variability. We characterize the trend and the correlation between the winds and key phenomena: (a) the 11-year solar cycle, (b) El Nino Southern Oscillation, (c) Quasi-Biennial Oscillation and (d) Southern Annular Mode using a linear regression method. Observations of the zonal wind show significant changes with time. The summertime westwards wind near 80 km is weakening by up to 4-5 ms-1 per decade, whilst the eastward wintertime winds around 85-95 km are strengthening at by around 7 ms-1 per decade. We find that at some times of year there are significant correlations between the phenomena and the observed/modeled winds. The significance of this work lies in quantifying the biases in a leading General Circulation Model and demonstrating notable interannual variability in both modeled and observed winds. The mesosphere and lower thermosphere (MLT), at heights of 80-100 km is an important region for the coupling of the middle and upper atmosphere. We carry out a study of the winds above Rothera (Antarctic Peninsula) for the years 2005-2021. We use observations from a meteor radar which measures winds at heights of 80-100 km and compare with the eXtended version of the Whole Atmosphere Community Climate Model (WACCM-X), a leading general circulation model. We find that although most of the seasonal cycle in the winds is captured well, WACCM-X exhibits biases in the winds at upper heights. In wintertime, the zonal winds are westwards whereas in observations they are eastward. In summertime WACCM-X model meridional winds at 90-100 km are southwards but observations northwards. The observed and modeled winds also display significant interannual variability. We characterize the trends of the winds and the correlation with various drivers (the 11-year solar cycle, El Nino Southern Oscillation, the Quasi-Biennial Oscillation and the Southern Annular Mode), using a multi-linear regression method. The study uses a uniquely long data set of Antarctic MLT winds to test and further develop general circulation models and quantifies the relationship between these winds and drivers such as the solar cycle. We characterize the variability of monthly mean winds in the mesosphere and lower thermosphere (MLT) over 17 years at Rothera using meteor radar observations and the eXtended version of the Whole Atmosphere Community Climate Model (WACCM-X) WACCM-X displays biases in the wintertime winds in the upper MLT. Observed winds are eastwards whilst WACCM-X winds are westwards Significant variability, trends and intermittent correlations with the solar cycle, Quasi-Biennial Oscillation and Southern Annular Mode are found in the observed and modeled winds

Long-term variabilities of monthly zonal (U) and meridional winds (V) in northern polar mesosphere and lower thermosphere (MLT, similar to 80-100 km) are investigated using meteor radar observations during 1999-2022 over Esrange (67.9 degrees N, 21.1 degrees E). The summer (June-August) mean zonal winds are characterized by westward flow up to similar to 88-90 km and eastward flow above this height. The summer mean meridional winds are equatorward with strong jet at similar to 85-90 km and it weakens above this height. The U and V exhibit strong interannual variability that varies with altitude and month or season. The responses of U and V anomalies (from 1999 to 2003) to solar cycle (SC), Quasi Biennial Oscillation at 10 and 30 hPa, El Nino-Southern Oscillation, North Atlantic Oscillation, ozone (O3) and carbon dioxide (CO2) are analyzed using multiple linear regression. From analysis, significant regions of correlations between MLT winds and above potential drivers vary with altitude and month. The positive responses of U and V to SC (up to 15 m/s/100 sfu) indicates the strengthening of eastward winds in mid-late winter, and poleward winds in late autumn and early winter. The O3 likely intensifies the eastward and poleward winds (similar to 100 m/s/ppmv) in winter and early spring. The CO2 significantly influence the eastward flow in late winter and summer (above similar to 90-95 km) and strengthen the meridional circulation. The significant positive trend in U peaks in summer, late autumn and early winter (similar to 0.6 m/s/year), the negative trend in V is more prominent in summer above similar to 90-95 km. The transition region between middle atmosphere and thermosphere is known as the mesosphere and lower thermosphere (MLT). The dynamics and circulation in this region are significant for global transport of important trace chemical species. Further the MLT winds play crucial role for the dynamical coupling of the middle and upper atmosphere. In the present study, the long-term variability and tendencies in monthly mean zonal and meridional winds are investigated in the Arctic MLT between similar to 80 and similar to 100 km from meteor radar observations during 1999-2022 over Esrange (67.9 degrees N, 21.1 degrees E) in Sweden. The ability of radar provided the unique, consistent, and long-term data set of polar MLT winds for duration of about two solar cycles. The MLT winds show important characteristic features and significant interannual variability that vary with altitude and month or season. In addition, the possible influence of climate forcings viz., solar activity, Quasi Biennial Oscillation (at 10 and 30 hPa), El Nino-Southern Oscillation, North Atlantic Oscillation, ozone, and carbon dioxide on the variabilities of polar MLT winds has been analyzed using multiple linear regression. The significant interannual variabilities and tendencies in northern polar MLT zonal and meridional winds can be attributed to the above potential drivers. The long-term variabilities in arctic mesosphere and lower thermosphere winds in response to potential climate forcings have been investigated for 1999-2022 over Esrange The variability in U and V significantly correlated with O3 in winter and early spring, and with CO2 in summer based on altitude The interannual variability in U and V is found to vary with altitude and month or season

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.

<p>Atmospheric Gravity Waves (AGWs) forced in the lower atmosphere are known to have a significant impact on the mesosphere and lower thermosphere (MLT) region. In the ionosphere, they can generate Medium-Scale Traveling Ionospheric Disturbances (MSTIDs). These disturbances roughly occur on time scales of 15−80 min and are therefore often parametrized rather than directly resolved in ionosphere models. The energy and momentum transport by AGW-TIDs strongly depends on their wave parameters. Measurements of AGW-TIDs in the MLT region and determination of the wave parameters (vertical and horizontal wavelength, wave period and propagation direction) are therefore an essential step to improve ionosphere modelling. However, measurements that provide a good resolution in the vertical dimension (≲ 10 km) and time (≲ 10 min) as well as a large enough coverage in the horizontal dimension (≳ 300 × 300 km) are difficult at MLT altitudes. We show, that combined measurements of the EISCAT VHF incoherent scatter radar and the Nordic Meteor Radar Cluster allow to determine the wave parameters of AGW-TIDs across the whole MLT region. Fourier filter methods are used to separate wave modes by wavelength, period and propagation direction. The extracted wave modes are fitted with wave functions in time-altitude and horizontal cross sections which gives the wave parameters. The coverage regions of the two applied instruments are separated only by approximately 10 km in altitude, which allows to identify a single wave mode in both measurements. We present the developed techniques on the example of a strongly pronounced AGW-TID measured on July 7, 2020. As a first application, two measurement campaigns have been conducted in early September and mid-October 2022 to study possible changes in AGW-TID parameters due to the MLT fall transition occurring around equinox. Another possible application of our method is to infer thermospheric neutral winds from the observed waves. We demonstrate this process under the assumption of the anelastic dissipative gravity wave dispersion relation.</p>

Atmospheric gravity waves and traveling ionospheric disturbances can be observed in the neutral atmosphere and the ionosphere at a wide range of spatial and temporal scales. Especially at medium scales, these oscillations are often not resolved in general circulation models and are parameterized. We show that ionospheric disturbances forced by upward-propagating atmospheric gravity waves can be simultaneously observed with the EISCAT very high frequency incoherent scatter radar and the Nordic Meteor Radar Cluster. From combined multi-static measurements, both vertical and horizontal wave parameters can be determined by applying a specially developed Fourier filter analysis method. This method is demonstrated using the example of a strongly pronounced wave mode that occurred during the EISCAT experiment on 7 July 2020. Leveraging the developed technique, we show that the wave characteristics of traveling ionospheric disturbances are notably impacted by the fall transition of the mesosphere and lower thermosphere. We also demonstrate the application of using the determined wave parameters to infer the thermospheric neutral wind velocities. Applying the dissipative anelastic gravity wave dispersion relation, we obtain vertical wind profiles in the lower thermosphere.

The Hunga Tonga–Hunga Ha′apai volcano eruption was a unique event that caused many atmospheric phenomena around the globe. In this study, we investigate the atmospheric gravity waves in the mesosphere/lower-thermosphere (MLT) launched by the volcanic explosion in the Pacific, leveraging multistatic meteor radar observations from the Chilean Observation Network De Meteor Radars (CONDOR) and the Nordic Meteor Radar Cluster in Fennoscandia. MLT winds are computed using a recently developed 3DVAR+DIV algorithm. We found eastward- and westward-traveling gravity waves in the CONDOR zonal and meridional wind measurements, which arrived 12 and 48 h after the eruption, and we found one in the Nordic Meteor Radar Cluster that arrived 27.5 h after the volcanic detonation. We obtained observed phase speeds for the eastward great circle path at both locations of about 250 m s−1, and they were 170–150 m s−1 for the opposite propagation direction. The intrinsic phase speed was estimated to be 200–212 m s−1. Furthermore, we identified a potential lamb wave signature in the MLT winds using 5 min resolved 3DVAR+DIV retrievals.

. The mesosphere and lower thermosphere (MLT), at heights of 80-100 km, is critical in the coupling of the middle and upper atmosphere and controls the momentum and energy transfer between these two regions. However, despite its importance, many General Circulation Models (GCMs) do not extend upwards into the MLT and those that do remain poorly constrained. In this study, we use a long-term meteor radar wind dataset from Rothera (67°S, 68°W) on the Antarctic Peninsula to test the Whole Atmosphere Community Climate Model with thermosphere-ionosphere eXtension (WACCM-X). This radar has 5 an interferometer to determine meteor heights and has been running since 2005. This unique combination yields a dataset ideally suited to investigate interannual variability. We find that although some characteristic features in monthly median winds are represented well in WACCM-X, the model exhibits significant biases. In particular, the observations reveal a ∼ 10 ms − 1 eastward wind at heights of 85-100 km in Antarctic winter, whereas the model predicts winds of the same magnitude but of opposite direction. We propose that this bias exists because WACCM-X is missing eastward momentum forcing in the MLT 10 from the breaking of secondary gravity waves. the observations significant in We the of particular key external phenomena the in this region. These phenomena are; i) variations in Solar activity, ii) the El Niño Southern Oscillation (ENSO), iii) the Quasi-Biennial Oscillation (QBO) and iv) the Southern Annular Mode (SAM). We use a linear regression method to investigate how the observed and modelled winds, and modelled gravity wave 15 tendencies in the Antarctic MLT vary in relation to the indices that quantify these phenomena. We find that there are some times of year and some height ranges at which there are significant correlations between the indices and the observed/modelled winds. In particular, in summer, there is a strong positive correlation in the modelled and observed zonal winds with the 11-year Solar cycle of magnitude up to 9 ms − 1 per 70 Solar flux units. However, there appears to be little significant influence of the ENSO on the winds observed by the radar although WACCM-X zonal winds display a 20 negative correlation throughout January-February and a positive correlation during March-May. Results from the QBO indices are varied and we find differing correlations in the model and observations. Finally, we find a positive correlation between observed summertime zonal winds and the SAM which has a magnitude of 9 ms − 1 per 2.5 hPa change in the SAM index. However, in WACCM-X zonal winds the summertime response is negative and around 10 ms − 1 per 2.5 hPa. The significance of this work lies in our quantifying the biases in a leading GCM and demonstrating there is significant interannual variability 25 in both modelled and observed winds, some of which are consistent with the proposal of external forcing. In this work we present the first long-term study (i.e. spanning over a Solar cycle) of the interannual variability of Antarctic winds from a meteor radar equipped with height resolving capabilities and compare these observations to the predictions of WACCM-X. Long-term Antarctic MLT winds have been explored in the past by radars without height resolving Portnyagin and Merzlyakov et al. (2009). Whilst some of these studies span a longer time period than that considered here, the MLT winds have strong variation with height which could not be addressed. Other long-term work has 65 used observations from MF radars Baumgaertner 2005; Dowdy 2007; Iimura et al., 2011; Portnyagin al., Merzlyakov 2009). However, MF radars have known and significant biases in winds measured at heights above 90 km 2004; Jacobi 2009; 2017). Here we investigate data recorded from 2005-2020 by the Rothera meteor radar and build upon work done earlier by Sandford et al. who reported first results from this radar and compared to winds in Esrange (68°N, 21°E). Cullens et al. also inspires the basis for this study wherein they used 70 WACCM to explore the influence of the 11-year Solar cycle on atmospheric winds globally. They found that in the southern hemisphere there are statistically significant changes in gravity wave drag and associated winds that are likely due to the Solar cycle. Our goal here is to determine the interannual variability found in this long radar dataset and compare the winds to the WACCM-X model. We use a linear regression method to explore the relationship between both observed and modelled MLT winds and modelled gravity wave tendencies with four particular potential external phenomena, namely, i) the Solar cycle, ii) 75 the ENSO, iii) the QBO and iv) zonal the is the zonal winds no significant in of the the model’s stratospheric polar vortex altering the critical level filtering of study was proposed and by TMG, NM, CW, SE, CC, PN. Data analysis was led by PN with radar hourly winds supplied by NH. Model data was processed by CC. Manuscript and all figures prepared by PN. Scientific interpretation led by PN and contributed to by all authors

We present the migrating tidal winds decomposed jointly from multiple meteor radars in four longitudinal sectors situated in the equatorial mesosphere and lower thermosphere. The radars are located in Cariri, Brazil (7.4°S, 36.5°W), Kototabang, Indonesia (0.2°S, 100.3°E), Ascension Island, United Kingdom (7.9° S, 14.4°W), and Darwin, Australia (12.3°S, 130.8°E). Harmonic analysis was used to obtain amplitudes and phases for diurnal and semidiurnal solar migrating tides between 82 and 98 km altitude during the period 2005–2008. To verify the reliability of the tidal components calculated by the four meteor radar wind measurements, we also present a similar analysis for the Whole Atmosphere Community Climate Model winds, which suggests that the migrating tides are well observed by the four different radars. The tides include the important tidal components of diurnal westward-propagating zonal wavenumber 1 and semidiurnal westward-propagating zonal wavenumber 2. In addition, the results based on observations were compared with the Climatological Tidal Model of the Thermosphere (CTMT). In general, in terms of climatic features, our results for the major components of migrating tides are qualitatively consistent with the CTMT models derived from satellite data. In addition, the tidal amplitudes are unusually stronger in January–February 2006. This result is probably because tides were enhanced by the 2006 Northern Hemisphere stratospheric sudden warming event.

<p>Multistatic meteor radar observations offer the possibility to investigate the short-term variability at the mesosphere and lower thermosphere on regional scales. Here we present preliminary results of spatially resolved 3D winds and their corresponding horizontal wavelength spectra using the Nordic Meteor Radar Cluster and CONDOR in Chile with a recently developed 3DVAR+div retrieval. The new retrieval provides for the first time a physically consistent solution for the vertical winds that conform the continuity equation. Based on these spectra we can separate the spatial scales that are driven by rotational modes from those dominated by divergent gravity waves. Furthermore, we present the first results of momentum flux spectra derived from these observations on a daily basis.</p>

Realistic modeling of the winds and dynamical variations in the mesosphere and lower thermosphere (MLT) at Southern Hemisphere (SH) mid‐to‐high latitudes near 60°S where dramatic motions occur has been a challenge. This work presents an evaluation of the MLT zonal and meridional winds from ∼80 to 98 km altitude produced by the high‐altitude version of the Navy Global Environmental Model (NAVGEM‐HA) numerical weather prediction system during the Antarctic Sudden Stratospheric Warming (SSW) in September 2019. These results are compared with the coincident measurements by five meteor radars at Tierra del Fuego (TDF; 53.7°S, 67.7°W), King Edward Point (KEP; 54.3°S, 36.5°W), King Sejong Station (KSS; 62.2°S, 58.8°W), Rothera (ROT; 67.5°S, 68.0°W), and Davis (DAV; 68.6°S, 78.0°E) across SH mid‐to‐high latitudes. We find that the day‐to‐day variations in NAVGEM‐HA winds related to tidal motions are overall consistent with variations in the radar winds, and the daily mean winds have a correlation of 0.7–0.9 between them. Three‐hourly NAVGEM‐HA winds have a correlation of ∼0.5 and mean difference <10 m/s to the radar observations at most stations, and the Root Mean Square (RMS) error ranges from ∼25 to 35 m/s. Above 90 km altitude, the correlation coefficient decreases, and the difference and RMS error increase, indicating an upper limit to the validity of the NAVGEM‐HA results. Both the analyzed and observed winds reveal an enhancement in diurnal and semidiurnal tidal amplitude during this SH SSW. NAVGEM‐HA shows some evidence that nonmigrating tidal enhancements are produced through the interaction of migrating tides with planetary waves.

The solar tides of the mesosphere and lower thermosphere (MLT) show great variability on time scales of days to years, with significant variability at interannual time scales. However, the nature and causes of this variability remain poorly understood. Here, we present measurements made over the interval 2005–2020 of the interannual variability of the 12‐hr tide as measured at heights of 80–100 km by a meteor radar over Rothera (68°S, 68°W). We use a linear regression analysis to investigate correlations between the 12‐hr tidal amplitudes and several climate indices, specifically the solar cycle (as measured by F10.7 solar flux), El Niño Southern Oscillation (ENSO), the Quasi‐Biennial Oscillation (QBO) at 10 and 30 hPa and the Southern Annular Mode (SAM). Our observations reveal that the 12‐hr tide has a large amplitude and a clearly defined seasonal cycle with monthly mean values as large as 35 m s −1 . We observe substantial interannual variability, with monthly mean 12‐hr tidal amplitudes at 95 km exhibiting a two standard‐deviation range (2 σ ) in spring of 13.4 m s −1 , 11.2 m s −1 in summer, 18.6 m s −1 in autumn, and 7.0 m s −1 in winter. We find that F10.7, QBO10, QBO30, and SAM all have significant correlations to the 12‐hr tidal amplitudes at the 95% level, with a linear trend also present. Whereas we detect very minimal correlation with ENSO. These results suggest that variations in F10.7, the QBO and SAM may contribute significantly to the interannual variability of 12‐hr tidal amplitudes in the Antarctic MLT.

The wind field in the mesosphere and lower thermosphere (MLT), at heights between 80 and 100 km, is dominated by the global scale oscillations of the atmospheric tides. The tides are crucial to the dynamics of the middle and upper atmosphere and hence to understanding the coupling between the lower atmosphere and space. The tides are known to show considerable variability on timescales of days to years, with significant variability at interannual timescales. However, the nature and causes of this variability remain poorly understood. Here, we present measurements made over the interval 2005 to 2020 of the interannual variability of the 12-hour tide as measured at heights of 80 – 100 km by a meteor radar over the British Antarctic Survey base at Rothera (68°S, 68°W). We use a linear regression analysis to investigate correlations between the 12-hour tidal amplitudes and several climate indices, specifically the solar cycle (as measured by F10.7 solar flux), El Niño Southern Oscillation (ENSO), the Quasi-Biennial Oscillation (QBO) at 10 hPa and 30 hPa, the Southern Annular Mode (SAM) and time. Our observations reveal that the 12-hour tide has a large amplitude and a clearly defined seasonal cycle with monthly mean values as large as 35 ms-1. We observe substantial interannual variability with monthly mean tidal amplitudes at 95 km exhibiting an interdecile range in spring of 17.2 ms-1, 12.6 ms-1 in summer, 23.6 ms-1 in autumn and 9.0 ms-1 in winter. We find that F10.7, QBO10, QBO30, SAM and time all have significant correlations at the 95% level, whereas we detect very minimal correlation with ENSO. For example, there is a significant negative correlation between F10.7 solar flux and tidal amplitudes in summer, implying an increase in solar flux is related to a decrease in monthly mean tidal amplitudes in the MLT. These results suggest that the amplitude of the polar 12-hour tide is modulated by the solar cycle, QBO and SAM.

Meteor radars have become widely used instruments to study atmospheric dynamics, particularly in the 70 to 110 km altitude region. These systems have been proven to provide reliable and continuous measurements of horizontal winds in the mesosphere and lower thermosphere. Recently, there have been many attempts to utilize specular and/or transverse scatter meteor measurements to estimate vertical winds and vertical wind variability. In this study we investigate potential biases in vertical wind estimation that are intrinsic to the meteor radar observation geometry and scattering mechanism, and we introduce a mathematical debiasing process to mitigate them. This process makes use of a spatiotemporal Laplace filter, which is based on a generalized Tikhonov regularization. Vertical winds obtained from this retrieval algorithm are compared to UA-ICON model data. This comparison reveals good agreement in the statistical moments of the vertical velocity distributions. Furthermore, we present the first observational indications of a forward scatter wind bias. It appears to be caused by the scattering center's apparent motion along the meteor trajectory when the meteoric plasma column is drifted by the wind. The hypothesis is tested by a radiant mapping of two meteor showers. Finally, we introduce a new retrieval algorithm providing a physically and mathematically sound solution to derive vertical winds and wind variability from multistatic meteor radar networks such as the Nordic Meteor Radar Cluster (NORDIC) and the Chilean Observation Network De meteOr Radars (CONDOR). The new retrieval is called 3DVAR+DIV and includes additional diagnostics such as the horizontal divergence and relative vorticity to ensure a physically consistent solution for all 3D winds in spatially resolved domains. Based on this new algorithm we obtained vertical velocities in the range of w = ± 1–2 m s−1 for most of the analyzed data during 2 years of collection, which is consistent with the values reported from general circulation models (GCMs) for this timescale and spatial resolution.

Using advanced meteor radar network observations along with ERA5 data, we report observational evidence of polar to tropical mesospheric teleconnections during the 2018 major sudden stratosphere warming (SSW) event in the northern hemisphere.A peak SSW on February 14, 2018, characterized by a ˜45 K rise in polar stratosphere temperature and a zonal wind reversal of ˜(-25) m/s at 60°N and 10 hPa, is observed.In the tropical lower mesosphere, a maximum zonal wind reversal (-24 m/s) compared with that identified in the extra-tropical regions was observed.Moreover, a time delay in the wind reversal between the tropical/polar stations and the mid-latitudes was detected.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 ISOs phase propagation is observed from the high-to low-latitudes (60 °N to 20 °N) in contrast to 8-day PWs phase propagation, indicating the change in the meridional propagation of winds during SSW.However, 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 during SSW. Hosted fileessoar.10510284.1.docx