Atomic hydrogen (H) is crucial for understanding photochemistry and the energy budget in the mesopause region. However, there is still no consensus on the H abundance in this region. This study presents a new hydrogen data set derived from Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) OH(9–6) band spectra, collocated with temperature and ozone profiles from other remote sensing instruments. H number densities peak at 82–87 km and range from to , depending on season and latitude. Two other H data sets obtained from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) are presented for comparison: those from Mlynczak et al. (2018, https://doi.org/10.1029/2018GL077377 ) and Panka et al. (2021, https://doi.org/10.1029/2020GL091053 ) are approximately 30% lower and 50% higher than the SCIAMACHY data at peak altitudes, respectively. Additionally, the H number density retrieved in this study partly shows better agreement with the only direct rocket in situ measurements than those from SABER.

We investigate interhemispheric coupling (IHC) during the austral winter by examining global characteristics of the dynamical fields initiated by stratospheric warming in the Southern Hemisphere using output of the Whole Atmosphere Community Climate Model version 6 covering 195 simulation years. About 100 events were extracted and analyzed, focusing on the seasonal and interannual variation of the IHC response in the Northern Hemisphere. The seasonal variation can be explained by differences in the background atmosphere. The upward shift and weakening of the gravity wave forcing during the IHC in September are opposite to the characteristics of July and August. However, the mean zonal wind and the momentum deposition of gravity waves in the northern middle atmosphere in September are also opposite to those in July and August. Together, these result in the same sign of the anomaly. Additionally, an interannual variation in the altitude of the warming in the northern polar region is also observed, with warmings split at 100 km. A warming that appears above 100 km is explained by an additional mechanism of gravity wave modulation, which drives the winter-to-summer circulation in the lower thermosphere, while a warming below 100 km corresponds to the weakening of the summer-to-winter circulation in the upper mesosphere. Correspondingly, no warming is observed near 100 km, where the circulation and its modulation are absent. This interannual variation of the IHC is influenced by the intraseasonal oscillation, which represents a similar meridional temperature structure in the global middle atmosphere.

In the mesosphere and lower thermosphere, diurnal tides are responsible for the dynamics and structures at low latitudes since they have largest amplitudes there. Based on the 20-year (2002-2021) observations from Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics Doppler interferometer (TIDI), we investigate the seasonal variations of three diurnal tidal components (DW1, DE3, DW2), and their responses to stratospheric quasi-biennial oscillation (SQBO) and solar cycle globally. The results show that: (a) DW1, DE3 and DW2 show prominent semiannual (SAO) and annual oscillations (AO) in their peak regions where the ratio of their annual mean amplitude to their maximum annual mean amplitude is larger than 0.8. DW1 also exhibit strong terannual oscillations (TAO) especially in meridional wind. (b) The responses of the amplitudes of seasonal variations of DW1, DE3 and DW2 to SQBO and solar cycle are comparable in magnitude to those of their annual mean amplitudes (for response to solar cycle, even stronger in some cases), and thus cannot be neglected. (c) In their respective peak regions, the responses of annual mean amplitudes of these diurnal tidal components to SQBO are uniformly positive except DW2, and their responses to solar cycle are uniformly negative. For these diurnal tides, the amplitudes of the dominant seasonal variations exhibit consistent response patterns with those of annual means to the SQBO/solar cycle. (d) Empirical formulas are given, which well describe the seasonal and interannual variations of dominant diurnal tidal components in their peak regions.

The representation of the semi-annual oscillation (SAO) in climate models shows a common easterly bias of several tens of metres per second compared to observations. These biases could be due to deficiencies in eastward tropical wave forcing, the position or strength of the climatological summertime jet or the strength/timing of the Brewer-Dobson circulation. This motivates further analysis of the momentum budget of the upper stratosphere within models and a more detailed comparison with reanalyses to determine the origin of the bias. In this study, the transformed Eulerian mean momentum equation is used to evaluate the different forcing terms that contribute to the SAO in the MERRA2 reanalysis dataset. This is then compared with the equivalent analysis using data from a climate simulation of the Whole Atmosphere Community Climate Model (WACCM). The comparison shows that WACCM underestimates eastward forcing by both resolved and parameterised waves at equatorial latitudes when compared with MERRA2 and also has a weaker tropical upwelling above 1 hPa. Using a sample climate model, we have investigated the reason behind an easterly bias of the semi-annual oscillation (SAO) in climate models. The strength of the driving processes of the modelled SAO is compared with the corresponding terms calculated from the Modern-Era Retrospective Analysis for Research and Applications, Version 2 (MERRA2) reanalysis data. Our results show that the CESM2-Whole Atmosphere Community Climate Model (WACCM) model under-represents eastward forcing by both resolved and parameterised waves at equatorial latitudes when compared with MERRA2 and also has a weaker tropical upwelling above 1 hPa. image

Abstract. Temperature trends in the upper stratosphere, particularly above ~45 km are difficult to quantify due to a deficit of long-term observational data in this region. The recent v7.3 upper stratospheric (35–60 km) temperature data product from the Optical Spectrograph and InfraRed Imager System (OSIRIS) includes over 22 years of observations that can be used to estimate temperature trends. The trends in OSIRIS temperatures over 2005–2021 are compared to those from two other satellite limb instruments: SABER and MLS. We find that the upper stratosphere cooled by ~0.5 to 1 K/decade during this period. Results from the three instruments are generally in agreement. By merging the OSIRIS observations with those from channel 3 of the Stratospheric Sounding Unit (SSU), we find that the stratosphere cooled at a rate of approximately -0.6 K/decade between 1979 and 2021 near 45 km, in agreement with earlier results based on SSU and MLS. The similarity between OSIRIS temperature trends and those from other records improves confidence in observed upper stratospheric temperature changes over the last several decades.

Temperature trends in the upper stratosphere, particularly above similar to 45 km, are difficult to quantify due to a lack of observational data with high vertical resolution in this region that span multiple decades. The recent v7.3 upper-stratospheric (35-60 km) temperature data product from the Optical Spectrograph and InfraRed Imager System (OSIRIS) includes over 22 years of observations that can be used to estimate temperature trends. The trends in OSIRIS temperatures over 2005-2021 are compared to those from two other satellite limb instruments: Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) and Microwave Limb Sounder (MLS). We find that the upper stratosphere cooled by similar to 0.5 to 1 K per decade during this period. Results from the three instruments are generally in agreement. By merging the OSIRIS observations with those from channel 3 of the Stratospheric Sounding Unit (SSU), we find that the stratosphere cooled at a rate of approximately -0.6 K per decade between 1979 and 2021 near 45 km, in agreement with earlier results based on SSU and MLS. The similarity between OSIRIS temperature trends and those from other records improves confidence in observed upper-stratospheric temperature changes over the last several decades.

How is Geospace changing?This fundamental scientific question is motivated by the need to understand the effects of increasing carbon dioxide and ongoing solar variability on the Geospace environment.The scientific understanding of Geospace change will ultimately inform future space operations, the developing space economy, and issues in space policy and space law.However, the architecture of the measurement system to accurately determine the changes in Geospace is fundamentally different from that of a classic NASA science mission.This is because the rate of change (i.e., the trends) in key quantities (e.g., temperature and total density) is small relative to the natural variability of Geospace.Significant advancement in measurement accuracy over current practice and measurement continuity are essential to provide scientific knowledge with sufficient statistical confidence to inform future decisions across a spectrum of space science disciplines and societal concerns. Background:The last 30 years have seen tremendous growth in understanding of the terrestrial geospace environment, nominally the region covering Earth's Ionosphere, Thermosphere, and Mesosphere (the ITM, 50 to 500 km).Once referred to as the "ignorosphere", the geospace environment is now understood as a highly coupled system both within itself and to the space environment above and to the stratosphere and troposphere below.Exploration of the ITM began in the late 1970's and early 1980's with the Atmospheric Explorer (AE), the Dynamics Explorer (DE), and the Solar-Mesosphere Explorer (SME) missions.In subsequent decades a host of missions and instruments were launched worldwide to continue the investigation of this scientifically interesting and societally relevant portion of Earth's atmosphere.Today geospace science stands at a crossroads: forty years of measurements point to an incredibly complex system that is also undergoing long-term change due to increasing carbon dioxide.The problem of space debris mitigation and the burgeoning space economy are dependent on understanding the evolution of geospace.Unfortunately, the missions and instruments launched mostly in the early 2000's are advanced in age with no defined successors.A gap in key measurements seems inevitable.Mlynczak et al., (2021) andBruinsma et al., (2021) called attention to this problem and they emphasized the scientific and societal needs for continued observations of the geospace system specifically for the detection of long-term trends.

<p>The seasonal and interannual variations of global tides of neutral winds in the mesosphere and lower thermosphere (MLT) are investigated based on the neutral horizontal wind data measured by TIMED Doppler interferometer (TIDI). The particular focus is on how the seasonal variation of tidal amplitude varies in response to solar cycle (SC) and to the quasi-biennial oscillation in winds in the lower stratosphere (SQBO). We find that the responses of seasonal variations of tides to SQBO and SC are comparable in magnitude to those of their corresponding annual means. Further, we show that the response patterns of seasonal variations of tides to SQBO and SC are not always similar to those of their corresponding annual means, which indicates that the tidal responses differ at different times of the year. In addition, we reveal that migrating tides show strong terannual oscillations (TAO) especially in meridional wind, whereas nonmigrating tides do not show obvious TAO.</p>

Abstract Simulating whole atmosphere dynamics, chemistry, and physics is computationally expensive. It can require high vertical resolution throughout the middle and upper atmosphere, as well as a comprehensive chemistry and aerosol scheme coupled to radiation physics. An unintentional outcome of the development of one of the most sophisticated and hence computationally expensive model configurations is that it often excludes a broad community of users with limited computational resources. Here, we analyze two configurations of the Community Earth System Model Version 2, Whole Atmosphere Community Climate Model Version 6 (CESM2(WACCM6)) with simplified “middle atmosphere” chemistry at nominal 1 and 2° horizontal resolutions. Using observations, a reanalysis, and direct model comparisons, we find that these configurations generally reproduce the climate, variability, and climate sensitivity of the 1° nominal horizontal resolution configuration with comprehensive chemistry. While the background stratospheric aerosol optical depth is elevated in the middle atmosphere configurations as compared to the comprehensive chemistry configuration, it is comparable among all configurations during volcanic eruptions. For any purposes other than those needing an accurate representation of tropospheric organic chemistry and secondary organic aerosols, these simplified chemistry configurations deliver reliable simulations of the whole atmosphere that require 35% and 86% fewer computational resources at nominal 1 and 2° horizontal resolution, respectively.

The region known as the MLT, or mesosphere -lower thermosphere, is a key transition region between the lower and the upper atmosphere.The region spans the altitude range from 50 to 130 km: a zone of very rapid transitions in temperature, composition, and prevalent dynamical and physical processes.Waves and other perturbations generated in the troposphere and stratosphere must propagate through the MLT to reach the upper atmosphere.Likewise, upper atmospheric perturbations due to solar and geomagnetic activity propagate into and through the MLT to affect lower altitudes.The mean state of the MLT is not just a passive background for wave propagation but a highly dynamic region.The processes of wave breaking and dissipation contribute to the rapid changes in the mean state in both space (all dimensions) and on a broad range of timescales, from minutes to multi-decadal trends.To understand the coupling between the lower and upper atmosphere, we depend on a continual dialog among multidisciplinary theoreticians, modelers, and observationalists.Working together, we can determine how perturbations interact with the mean state and how these interactions affect the downward and upward propagation of perturbations at all scales.In this document we emphasize a growing concern among researchers about the significant upcoming gap in global measurement capabilities for the MLT.This is an acute situation for global observations.We outline the reasons for concern and give some recommendations for how the decadal survey can best address the pressing needs.Our first recommendation is for continued operation and support of existing satellite instruments for as long as they are providing quality observations.Related to this, we urge support for expanded or improved data products through improved or new retrieval algorithms.Finally, we recommend the accelerated development of new instruments and flight opportunities that can eliminate or shorten the expected gap in observations of the mean state of the MLT.

The Hunga Tonga Hunga-Ha'apai (HTHH) volcanic eruption on 15 January 2022 injected water vapor and SO2 into the stratosphere. Several months after the eruption, significantly stronger westerlies, and a weaker Brewer-Dobson circulation developed in the stratosphere of the Southern Hemisphere and were accompanied by unprecedented temperature anomalies in the stratosphere and mesosphere. In August 2022, the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) satellite instrument observed record-breaking temperature anomalies in the stratosphere and mesosphere that alternate signs with altitude. Ensemble simulations carried out with the Whole Atmosphere Community Climate Model (WACCM6) indicate that the strengthening of the stratospheric westerlies explains the mesospheric temperature changes. The stronger westerlies cause stronger westward gravity wave drag in the mesosphere. Although the enhanced gravity wave drag is partly balanced by a weakening of planetary wave forcing, the net result is an acceleration of the mesospheric mean meridional circulation. The stronger mesospheric circulation, in turn, plays a dominant role in driving the changes in mesospheric temperatures. This study highlights the impact of large volcanic eruptions on middle atmospheric dynamics and provides insight into their long-term effects in the mesosphere. On the other hand, we could not discern a clear mechanism for the observed changes in stratospheric circulation. In fact, an examination of the WACCM ensemble reveals that not every member reproduces the large changes observed by SABER. We conclude that there is a stochastic component to the stratospheric response to the HTHH eruption.

The climatology of earth's Na density over Fort Collins, CO (41°N, 105°W) based on nocturnal Na lidar observations between 1990 and 1999 was reported by She et al. (2000, https://doi.org/10.1029/2000gl003825 ). Based on a continued 28‐year data set between 1990 and 2017 with the latter part observed over Logan, UT (42N, 112W), we update the seasonal variations between 80 and 110 km. This data set is also used to deduce long‐term responses of Na density (profile) between 75 and 110 km, showing a positive linear trend between 75 and 93 km (with maximum ∼2.87 × 10 8 m −3 /decade at 87 km); it turns negative before approaching zero at 110 km (with minimum ∼−2.96 × 10 7 m −3 /decade at 100 km). The associated solar response is also positive for the altitude range in question (with maximum ∼5.20 × 10 6 m −3 /SFU at 91 km). We also derived the 28‐year mean Na layer column abundance, centroid altitude, and root mean square width to be 3.92 ± 2.14 10 13 m −2 , 91.3 ± 1.0 km, and 4.62 ± 0.56 km, respectively, and deduced long‐term trend and solar cycle responses of column abundance and centroid altitude, respectively to be 7.81 ± 1.63%/decade and 16.9 ± 2.8%/100SFU, and −355 ± 35 m/decade and −1.94 ± 0.69 m/SFU. We explained conceptually how positive long‐term responses in Na density led to positive responses in column abundance and negative responses in centroid altitude.

The semiannual oscillation (SAO) in zonally averaged zonal winds develops just above the quasi-biennial oscillation (QBO) and dominates the seasonal variability in the tropical upper stratosphere and lower mesosphere. The magnitude, seasonality, and latitudinal structure of the SAO vary with the phase of the QBO. There is also an an-nual oscillation (AO) whose magnitude at the equator is smaller than those of the SAO and QBO but not negligible. This work presents the relation between the SAO, QBO, AO, and time-mean wind in the tropical upper stratosphere and lower mesosphere using winds derived from satellite geopotential height observations. The winds are generally more westerly during the easterly phase of the QBO. The SAO extends to lower altitudes during periods where the QBO is characterized by deep easterly winds. The differences in the SAO associated with the QBO are roughly con-fined to the latitudes where the QBO has appreciable amplitude, suggesting that the mechanism is controlled by verti-cal coupling. The westerly phases of the SAO and AO show downward propagation with time. This analysis suggests that forcing by dissipation of waves with westerly momentum is responsible for the westerly acceleration of both the SAO and AO. The timing and structure of the easterly phases of the SAO and AO near the stratopause are consistent with the response to meridional advection of momentum across the equator during solstices; it is not apparent that lo-cal wave processes play important roles in the easterly phases in the region of the stratopause.

Energetic particle precipitation (EPP) causes ionization of the main constituents of the Earth's atmosphere which leads to the production of nitric oxide (NO) throughout the polar mesosphere and lower thermosphere (MLT). Due to the long lifetime of NO during winter, it can also be transported deeper into the atmosphere by the mesospheric residual circulation (the indirect EEP effect). This study investigates the mesospheric indirect NO response to EEP using Whole Atmosphere Community Climate Model (WACCM) version 6. In comparison to observations from the instrument Solar Occultation For Ice Experiment (SOFIE) on the AIM (Aeronomy of Ice in the Mesosphere) satellite, a wintertime underestimation is found in the modeled mesospheric NO amount. WACCM's temperature profile is found to be vertically shifted compared to observations by SOFIE and by The Sounding of the Atmosphere using Broadband Emission Radiometry instrument on the Thermosphere Ionosphere Mesosphere Energetics Dynamics satellite (SABER). The discrepancies in NO are therefore attributed to the model's ability to simulate the dynamics responsible for the indirect EEP effect. The drivers of this transport are investigated by sensitivity runs of WACCM's gravity wave forcing. Changing the amplitude of the non‐orographic gravity waves and the Prandtl number improves the modeled vertical distribution of NO and temperature in the MLT region.

Satellite observations of middle-atmosphere temperature are used to investigate the short-term global response to planetary wave activity in the winter stratosphere. The focus is on the relation between variations in the winter and summer hemispheres. The analysis uses observations from Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics (TIMED) Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) for 2002-21 and Aura Microwave Limb Sounder (MLS) for 2004-21, and reanalysis temperatures and winds from MERRA-2 for 2002-21. We calculate temporal correlations of the Eliassen-Palm flux divergence in the winter stratosphere with global temperature. Results show a robust perturbation extending to midlatitudes of the Southern Hemisphere (SH) stratosphere during Northern Hemisphere (NH) winter. An increase in wave forcing is followed by a decrease in temperatures over the depth of the stratosphere in the SH, peaking at a lag of 3 days. Summer mesospheric temperature perturbations of the opposite sign are seen in many winters. Comparable signals in the NH summer middle-atmosphere are present during some SH winters but are weaker and less consistent than those in the SH during NH winter. A diagnostic evaluation of the patterns of correlation, the mesospheric zonal winds, and the stability criteria suggests that the temperature perturbations in the midlatitude summer mesosphere are more closely associated with the summer stratosphere directly below than with the wave activity in the winter stratosphere. This suggests that the interhemispheric coupling in the stratosphere is driving or contributing to the coupling between the winter stratosphere and the summer mesosphere that has been reported in several investigations. Significance StatementThere are many instances in which one part of the atmosphere is found to regularly respond to perturbations occurring in a distant region. In this study, we use observations to investigate one such pattern: temperature changes at high altitude (60-100 km) in the summer that follow dynamical changes near the winter pole at 40-60 km. Such analysis is useful to understand which physical processes contribute to the global connectivity and variability of the atmosphere.

The Quasi-Biennial Oscillation initiative (QBOi) is a model intercomparison programme that specifically targets simulation of the QBO in current global climate models. Eleven of the models or model versions that participated in a QBOi intercomparison study have upper boundaries in or above the mesosphere and therefore simulate the region where the stratopause semiannual oscillation (SAO) is the dominant mode of variability of zonal winds in the tropical upper stratosphere. Comparisons of the SAO simulations in these models are presented here. These show that the model simulations of the amplitudes and phases of the SAO in zonal-mean zonal wind near the stratopause agree well with the information derived from available observations. However, most of the models simulate time-average zonal winds that are more westward than determined from observations, in some cases by several tens of m center dot s(-1). Validation of wave activity in the models is hampered by the limited observations of tropical waves in the upper stratosphere but suggests a deficit of eastward forcing either by large-scale waves, such as Kelvin waves, or by gravity waves.

The mesospheric polar vortex (MPV) plays a critical role in coupling the atmosphere-ionosphere system, so its accurate simulation is imperative for robust predictions of the thermosphere and ionosphere. While the stratospheric polar vortex is widely understood and characterized, the mesospheric polar vortex is much less well-known and observed, a short-coming that must be addressed to improve predictability of the ionosphere. The winter MPV facilitates top-down coupling via the communication of high energy particle precipitation effects from the thermosphere down to the stratosphere, though the details of this mechanism are poorly understood. Coupling from the bottom-up involves gravity waves (GWs), planetary waves (PWs), and tidal interactions that are distinctly different and important during weak vs. strong vortex states, and yet remain poorly understood as well. Moreover, generation and modulation of GWs by the large wind shears at the vortex edge contribute to the generation of traveling atmospheric disturbances and traveling ionospheric disturbances. Unfortunately, representation of the MPV is generally not accurate in state-of-the-art general circulation models, even when compared to the limited observational data available. Models substantially underestimate eastward momentum at the top of the MPV, which limits the ability to predict upward effects in the thermosphere. The zonal wind bias responsible for this missing momentum in models has been attributed to deficiencies in the treatment of GWs and to an inaccurate representation of the high-latitude dynamics. In the coming decade, simulations of the MPV must be improved.

The mesosphere and lower thermosphere (MLT) is a dynamic layer of the earth's atmosphere. This region marks the interface at which neutral atmosphere dynamics begin to influence the upper atmosphere and ionosphere. However, our understanding of this region and our ability to accurately simulate it in global circulation models (GCMs) is limited by a lack of observations, especially in remote locations. To this end, a meteor radar was deployed from 2016 to 2020 on the remote mountainous island of South Georgia (54∘ S, 36∘ W) in the Southern Ocean. In this study we use these new measurements to characterise the fundamental dynamics of the MLT above South Georgia including large-scale winds, solar tides, planetary waves (PWs), and mesoscale gravity waves (GWs). We first present an improved method for time–height localisation of radar wind measurements and characterise the large-scale MLT winds. We then determine the amplitudes and phases of the diurnal (24 h), semidiurnal (12 h), terdiurnal (8 h), and quardiurnal (6 h) solar tides at this latitude. We find very large amplitudes up to 30 m s−1 for the quasi 2 d PW in summer and, combining our measurements with the meteor SAAMER radar in Argentina, show that the dominant modes of the quasi 5, 10, and 16 d PWs are westward 1 and 2. We investigate and compare wind variance due to both large-scale “resolved” GWs and small-scale “sub-volume” GWs in the MLT and characterise their seasonal cycles. Last, we use our radar observations and satellite temperature observations from the Microwave Limb Sounder to test a climatological simulation of the Whole Atmosphere Community Climate Model (WACCM). We find that WACCM exhibits a summertime mesopause near 80 km altitude that is around 10 K warmer and 10 km lower in altitude than observed. Above 95 km altitude, summertime meridional winds in WACCM reverse to poleward, but this not observed in radar observations in this altitude range. More significantly, we find that wintertime zonal winds between 85 to 105 km altitude are eastward up to 40 m s−1 in radar observations, but in WACCM they are westward up to 20 m s−1. We propose that this large discrepancy may be linked to the impacts of secondary GWs (2GWs) on the residual circulation, which are not included in most global models, including WACCM. These radar measurements can therefore provide vital constraints that can guide the development of GCMs as they extend upwards into this important region of the atmosphere.

The Stratosphere-troposphere Processes And their Role in Climate (SPARC) Quasi-Biennial Oscillation initiative (QBOi) seeks to improve confidence in general circulation and earth system model (GCM and ESM) simulations of the QBO, a prominent feature of middle atmosphere tropical variability first identified nearly sixty years ago. Although only five out of 47 models contributing to the Coupled Model Intercomparison Project Phase 5 (CMIP5) had spontaneous QBOs, simulated QBOs are anticipated to be more common among CMIP6 models as more atmospheric GCMs are able to reproduce the phenomenon, both by ensuring adequate vertical resolution in the stratosphere and by parametrizing accelerations due to subgrid nonorographic gravity waves (NOGWs). The complexity of CMIP6 models and their forcing scenarios, however, is an obstacle to using the CMIP6 multimodel ensemble for analysis of modelling uncertainties that are specific to the QBO and its impacts. The QBOi multimodel ensemble represents an alternative approach in which modelling uncertainties related to the QBO are assessed by performing coordinated experiments with atmospheric GCMs that have simplified external forcings and boundary conditions, designed to characterize QBO representation and its response to idealised future climate scenarios. Results are presented from an analysis of QBOs in thirteen atmospheric GCMs forced with both observed and annually repeating sea surface temperatures (SSTs). Mean QBO periods in most of these models are close to, though shorter than, the period of 28 months observed in ERA-Interim. Amplitudes are within ±20% of the observed QBO amplitude at 10hPa, but typically about half of that observed at lower altitudes (50 and 70hPa). For almost all models the oscillation's amplitude profile shows an overall upward shift compared to reanalysis and its meridional extent is too narrow. Asymmetry in the duration of eastward and westward phases is reasonably well captured though not all models replicate the observed slowing as the westward shear descends. Westward phases are generally too weak, and most models have an eastward time mean wind bias throughout the depth of the QBO. Intercycle period variability is realistic and in some models is enhanced in the experiment with observed SSTs compared to the experiment with repeated annual cycle SSTs. Mean periods are also sensitive to this difference between SSTs but only when parametrized NOGW sources are coupled to tropospheric parameters and not prescribed with a fixed value. But, overall, modelled QBOs are very similar whether or not the prescribed SSTs vary interannually. A portrait of the overall ensemble performance is provided by a normalised grading of QBO metrics. To simulate a QBO all but one model used parametrized NOGWs, which provided the majority of the total wave forcing at altitudes above 70hPa in most models. Thus the representation of NOGWs either explicitly or through parametrization is still a major uncertainty underlying QBO simulation in these present-day experiments.

The paper presents climatology of the poorly known, particularly at high latitudes, short-period (8- and 6-h) tides observed by meteor radars at high latitudinal stations Tromsø (70°N, 19°E) and Svalbard (78°N, 16°E) based on 16 years (2003–2018) and 18 years (2001–2018) of measurements, respectively. The main focus of this study is to clarify the seasonal variability and vertical structure of the two tides observed at both sites. It is found that at the two not very distant high-latitude stations Tromsø and Svalbard both tides have not only different seasonal variability but also some distinction in the altitude structure as well. In general the short-period tides at both sites are vertically upward propagating waves but with seasonally depending vertical wavelength. The two tides at Tromsø and the 8-h one at Svalbard reveal some inter-annual variability with a period of quasi-2 years. An assessment of the solar heating as the main mechanism for the generation of the 8- and 6-h tides simulated by the model WACCM6 is presented as well. The results could be used as benchmarks for model simulations and for understanding better the forcing mechanisms of these short-period tides at high latitudes.