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We investigate the absolute momentum flux (AMF) and vertical wind variance ρw′2‾$\left(\rho \overline{{w}^{\prime 2}}\right)$of gravity waves (GWs) along with intermittencies in the upper troposphere and lower stratosphere (UTLS) during 2017–2022 using the Middle Atmosphere Alomar Radar System at Andøya, Norway (69.30°N, 16.04°E). We categorized the AMF and ρw′2‾$\rho \overline{{w}^{\prime 2}}$into different period ranges (30 min–2 hr, 2–6 hr, 6–13 hr, 13 hr–1 day, and 30 min–1 day) to study the significance of short‐ and long‐period waves. The selection of these period bands was based on the boundary conditions of the available spectra: 30 min (Nyquist frequency), 13 hr (inertial period), and 1 day (based on our interest in maximum long‐period oscillations). Through the investigation of the AMF and ρw′2‾$\rho \overline{{w}^{\prime 2}}$, we wish to determine in detail the GW characteristics at northern polar latitudes. Furthermore, it is crucial to assess the intermittency as it considerably influences and alters the GW attributes. Our novel results indicate for both AMF and ρw′2‾$\rho \overline{{w}^{\prime 2}}$: (a) seasonal variation with minima during summer (May–September); (b) higher magnitude in the upper troposphere (<9.00 km) than the lower stratosphere; (c) short‐period components (30 min–2 hr, 2–6 hr) are more intermittent in the entire UTLS; and (d) the long‐period components (6–13 hr, 13 hr–1 day) demonstrate lower (higher) intermittency in the upper troposphere (lower stratosphere) in summer implying a plausible wave‐filtering mechanism. Atmospheric gravity waves (GWs) mostly originate in the lower atmosphere (upper troposphere and lower stratosphere [UTLS]) and play a crucial role in transporting energy and momentum to the higher layers of the atmosphere. These GWs act as the fundamental coupling mechanism between the atmospheric layers and eventually modulate global circulation. The momentum flux carried by the GWs varies with time, which is known as intermittency. It is important to not only recognize them but also distinguish the contribution of GWs with different periods and sources to parameterize them in global circulation models especially at polar latitudes due to limited observations. We examine the absolute momentum flux (AMF) and vertical wind variances ρw′2‾$\left(\rho \overline{{w}^{\prime 2}}\right)$with their intermittency (short‐term variability) for 6 years (2017–2022) over northern Norway using the winds obtained with a modern phased‐array radar. Our novel results reveal that AMF and ρw′2‾$\rho \overline{{w}^{\prime 2}}$display a seasonal variation with minimum magnitude during summer and higher magnitude in the upper troposphere than in the lower stratosphere. Furthermore, the short‐period waves are more intermittent (short‐timescale variability) in the UTLS. The long‐period waves are less intermittent in the upper troposphere and more intermittent in the lower stratosphere especially during summer suggesting an existing/prevailing wave‐filtering mechanism. Magnitude of both absolute momentum flux (AMF) and vertical wind variance is minimum during summer and maximum in the upper troposphere (<9.00 km)AMF and vertical wind variance intermittency are high for the short‐period waves (30 min–2 hr and 2–6 hr)Long‐period waves are more (less) intermittent in the lower stratosphere (upper troposphere) mainly in summer, hinting wave filtering Magnitude of both absolute momentum flux (AMF) and vertical wind variance is minimum during summer and maximum in the upper troposphere (<9.00 km) AMF and vertical wind variance intermittency are high for the short‐period waves (30 min–2 hr and 2–6 hr) Long‐period waves are more (less) intermittent in the lower stratosphere (upper troposphere) mainly in summer, hinting wave filtering

The structure, variability, and mean‐flow interactions of the quasi‐2‐day wave (Q2DW) in the mesosphere and lower thermosphere during January 2015 were studied employing meteor and medium‐frequency radar winds at eight sites from 23°S to 76°S and Microwave Limb Sounder (MLS) temperature and geopotential height measurements from 30°S to 80°S. The event had a duration of ~20–25 days, dominant periods of ~44–52 hr, temperature amplitudes as large as ~16 K, and zonal and meridional wind amplitudes as high as ~40 and 80 m/s, respectively, at middle and lower latitudes. MLS measurements enabled definition of balance winds that agreed well with radar wind amplitudes and phases at middle latitudes where amplitudes were large and quantification of the various Q2DW modes contributing to the full wave field. The Q2DW event was composed primarily of the westward zonal wavenumber 3 (W3) mode but also had measurable amplitudes in other westward modes W1, W2, and W4; eastward modes E1 and E2; and stationary mode S0. Of the secondary modes, W1, W2, and E2 had the larger amplitudes. Inferred MLS balance winds enabled estimates of the Eliassen‐Palm fluxes for each mode, and cumulative zonal accelerations that were found to be in reasonable agreement with radar estimates from ~35°S to 70°S at the lower altitudes at which radar winds were available. The January 2015 quasi‐2‐day wave in the Southern Hemisphere was characterized by radar wind measurements and MLS limb observationsThe dominant mode was westward zonal wavenumber 3 (W3), but W1, W2, W4, E1, E2, and S0 also had measurable amplitudes and effectsBalance winds derived from the MLS geopotential height and temperature measurements agree reasonably with meteor radar measurements

An international joint research project, entitled Interhemispheric Coupling Study by Observations and Modelling (ICSOM), is ongoing. In the late 2000s, an interesting form of interhemispheric coupling (IHC) was discovered: when warming occurs in the winter polar stratosphere, the upper mesosphere in the summer hemisphere also becomes warmer with a time lag of days. This IHC phenomenon is considered to be a coupling through processes in the middle atmosphere (i.e., stratosphere, mesosphere, and lower thermosphere). Several plausible mechanisms have been proposed so far, but they are still controversial. This is mainly because of the difficulty in observing and simulating gravity waves (GWs) at small scales, despite the important role they are known to play in middle atmosphere dynamics. In this project, by networking sparsely but globally distributed radars, mesospheric GWs have been simultaneously observed in seven boreal winters since 2015/16. We have succeeded in capturing five stratospheric sudden warming events and two polar vortex intensification events. This project also includes the development of a new data assimilation system to generate long‐term reanalysis data for the whole middle atmosphere, and simulations by a state‐of‐the‐art GW‐permitting general circulation model using the reanalysis data as initial values. By analyzing data from these observations, data assimilation, and model simulation, comprehensive studies to investigate the mechanism of IHC are planned. This paper provides an overview of ICSOM, but even initial results suggest that not only GWs but also large‐scale waves are important for the mechanism of the IHC. In the late 2000s, an interesting form of the coupling between the Northern and Southern Hemispheres was discovered: when the winter polar stratosphere warms, the upper summer mesosphere also warms several days later. An international research project called Interhemispheric Coupling Study by Observations and Modelling (ICSOM) is ongoing to examine the mechanism of this interhemispheric coupling (IHC). This IHC phenomenon is thought to be the connection in the middle atmosphere (i.e., stratosphere, mesosphere, and lower thermosphere). Several promising mechanisms have been proposed, but they remain controversial. This is because gravity waves (GWs) having small scales, which are difficult to observe and simulate, are thought to play a crucial role in the coupling. So, we have performed observations of GWs by networking radars over seven Northern Hemisphere winters, and succeeded in capturing five stratospheric warming events and two opposite events. We also developed a new data assimilation system for the entire middle atmosphere and used the global data produced by the system to simulate GWs with a high‐resolution global model. By combining these research tools, we plan to elucidate the mechanism of IHC comprehensively. This paper presents an overview of ICSOM. Initial results show that not only GWs but also large‐scale waves are important for the IHC mechanism. An international project is ongoing to elucidate the mechanism of interhemispheric coupling (IHC) in the middle atmosphereGravity waves (GWs), which are thought to play a key role in IHC, were observed by a radar network and simulated by high‐resolution global modelInitial results suggest that not only GWs but also large‐scale waves are important for the IHC mechanism An international project is ongoing to elucidate the mechanism of interhemispheric coupling (IHC) in the middle atmosphere Gravity waves (GWs), which are thought to play a key role in IHC, were observed by a radar network and simulated by high‐resolution global model Initial results suggest that not only GWs but also large‐scale waves are important for the IHC mechanism

We have obtained horizontal phase velocity distributions of the gravity waves around 90 km from four Antarctic airglow imagers, which belong to an international airglow imager/instrument network known as ANGWIN (Antarctic Gravity Wave Instrument Network). Results from the airglow imagers at Syowa (69°S, 40°E), Halley (76°S, 27°W), Davis (69°S, 78°E), and McMurdo (78°S, 167°E) were compared, using a new statistical analysis method based on 3‐D Fourier transform (Matsuda et al., 2014) for the observation period between 7 April and 21 May 2013. Significant day‐to‐day and site‐to‐site differences were found. The averaged phase velocity spectrum during the observation period showed preferential westward direction at Syowa, McMurdo, and Halley, but no preferential direction at Davis. Gravity wave energy estimated by I′/I was ~5 times larger at Davis and Syowa than at McMurdo and Halley. We also compared the phase velocity spectrum at Syowa and Davis with the background wind field and found that the directionality only over Syowa could be explained by critical level filtering of the waves. This suggests that the eastward propagating gravity waves over Davis could have been generated above the polar night jet. Comparison of nighttime variations of the phase velocity spectra with background wind measurements suggested that the effect of critical level filtering could not explain the temporal variation of gravity wave directionality well, and other reasons such as variation of wave sources should be taken into account. Directionality was determined to be dependent on the gravity wave periods. Various variabilities of the mesospheric gravity waves are found for the first time by an airglow network observation in the AntarcticThe phase velocity spectrum that indicated wind filtering effect of gravity waves is significant over Syowa, Halley, and McMurdoOver Davis, however, no preferential propagation direction is found, which could be because of secondary wave generation

The potential energy of gravity waves (GWs) per unit mass (Ep), at altitudes of 15–70 km, has been examined from temperature profiles obtained by a Rayleigh/Raman lidar at Syowa Station (69°S, 40°E) from May 2011 to October 2013, with the exception of the summer months. The GWs with ground‐based wave periods longer than 2 h and vertical wavelengths between 1.8 and 16 km were extracted from the temperature profiles. Epwas larger in winter than in spring and fall, although in 2012, at altitudes below 30 km, Epwas larger in spring than in winter and fall. Epincreased with a mean scale height of 11.3 km. Epprofiles showed a local maximum at an altitude of 20 km and a minimum at 25 km in almost every month, which has not been reported by previous studies observed by radiosondes. The values of Epin October of 2012 were smaller at 35–60 km and larger at 20–35 km than those in October of 2011 and 2013. This difference in the Epprofile is most probably caused by different seasonal variations of zonal winds. The larger and smaller Epvalues seem to be observed both below and above the altitude at which the zonal wind speed reached 0 m s−1. This result suggests that wind filtering of gravity waves with small phase speeds is significantly important in early spring. Gravity wave potential energy at altitudes from 15 to 70 km is estimated from Rayleigh/Raman lidar observationsVertical variation of gravity wave activity in the middle atmosphere over the Antarctic is revealed quantitativelyWind filtering of gravity waves with small zonal phase speeds affects the vertical variation of gravity wave activity in early spring

Continuous observations of polar mesosphere summer echoes at heights from 81–93 km were performed using the first Mesosphere‐Stratosphere‐Troposphere/Incoherent Scatter radar in the Antarctic over the three summer periods of 2013/2014, 2014/2015, and 2015/2016. Power spectra of horizontal and vertical wind fluctuations, and momentum flux spectra in a wide‐frequency range from (8 min)−1to (20 days) −1were first estimated for the Antarctic summer mesosphere. The horizontal (vertical) wind power spectra obey a power law with an exponent of approximately −2 (−1) at frequencies higher than the inertial frequency of (13 h)−1and have isolated peaks at about 1 day and a half day. In addition, an isolated peak of a quasi‐2 day period is observed in the horizontal wind spectra but is absent from the vertical wind spectra, which is consistent with the characteristics of a normal‐mode Rossby‐gravity wave. Zonal (meridional) momentum flux spectra are mainly positive (negative), and large fluxes are observed in a relatively low‐frequency range from (1 day)−1to (1 h)−1. A case study was performed to investigate vertical profiles of momentum fluxes associated with gravity waves and time mean winds on and around 3 January 2015 when a minor stratospheric warming occurred in the Northern Hemisphere. A significant momentum flux convergence corresponding to an eastward acceleration of ~200 m s−1d−1was observed before the warming and became stronger after the warming when mean zonal wind weakened. The strong wave forcing roughly accorded with the Coriolis force of mean meridional winds. Power and momentum flux spectra in a wide‐frequency range were estimated using MST radar observations in the Antarctic mesosphereMesospheric wind spectra obey power laws at frequencies higher than the inertial frequency and have isolated peaks at 1 d and at 0.5 dVertical profiles of momentum fluxes show significant eastward forcing, consistent with the mean eastward wind shear and equatorward flow

Mesospheric gravity‐wave (GW) phase velocity spectra and total powers at two Antarctic stations, Davis and Syowa, were derived using OH airglow data from March to October in 2016. The total powers have similar seasonal variation, that is, maxima in winter at both stations. The average powers at both stations in winter were not significantly different. However, the power at Davis in September was three times smaller than that at Syowa. This lower power at Davis was attributed to GWs with omnidirectional phase velocity. These lower GW activities at Davis could be attributed to a longitudinal variation in wave filtering; a stronger wind at Davis filtered out more GWs than at Syowa. Also, to explore possible sources in the middle atmosphere, we investigated one event, in which GWs with ∼100 ms−1southeastward phase velocity appeared at Davis on 29 August. The raytracing method was applied, and its result indicated that those GWs with high southeastward phase velocity propagated from ∼45 km altitude or higher over the Southern Ocean. A large residual of the non‐linear balanced equation was found at 50 km on its ray path. GWs, very likely emitted from a tropospheric jet, were also found near the ray path at the termination altitude over the Southern Ocean and possibly appeared saturated between 45 and 50 km. Therefore, the OH imager at Davis probably captured GWs generated by a spontaneous adjustment in the upper stratosphere and/or secondary GWs produced by the breaking of the GWs that have originated from the tropospheric jet. A gravity wave (GW) is a type of atmospheric wave that transports momentum from the Earth’s surface to the edge of the atmosphere where it can drive atmospheric circulations. The temporal and spatial variation of GWs is not well understood, and their sources remain unclear. We observed GWs at the upper edge of the atmosphere over two Antarctic stations (Syowa and Davis) and calculated their phase velocity and total power spectra. The total powers have winter maxima at both stations. Although the average powers at both stations in winter were not significantly different, the power at Davis in September was three times smaller than that at Syowa. The lower GW activity at Davis could be attributed to a stronger wind at Davis, which filtered out more GWs than at Syowa. Also, to explore possible sources in the middle atmosphere, we investigated one event at Davis on 29 August 2016. We found that some GWs propagated from ∼45 km altitude or higher over the Southern Ocean. Those GWs could be emitted from the polar night jet (flow imbalance) and/or secondary GWs produced by the breaking of the GWs that have originated from the tropospheric jet. Mesospheric gravity‐wave activity at two Antarctic stations was larger in winter than in spring and fallWave activity at Davis was not significantly different with Syowa in winter but smaller in fall, which can be explained by filtering effectWe find that GWs at Davis on 29 August 2016, were very likely generated by a spontaneous adjustment and/or a tropospheric jet wave breaking Mesospheric gravity‐wave activity at two Antarctic stations was larger in winter than in spring and fall Wave activity at Davis was not significantly different with Syowa in winter but smaller in fall, which can be explained by filtering effect We find that GWs at Davis on 29 August 2016, were very likely generated by a spontaneous adjustment and/or a tropospheric jet wave breaking

We conducted two 10‐day observational campaigns in 2019 targeting turbulence in the troposphere and lower stratosphere by adopting a frequency domain radar interferometric imaging technique using Program of the Antarctic Syowa radar (PANSY radar) and radiosonde observations obtained at Syowa Station in the Antarctic. Seventy three Kelvin‐Helmholtz (K‐H) billows were detected, and two characteristic cases likely excited by the same cyclone were examined in detail. In the first case with the longest observational duration of ∼6.5 hr, the K‐H billows had a thickness of ∼800 m and a horizontal wavelength of ∼2,500 m. According to a numerical simulation, continuously existing gravity waves associated with the cyclone maintained strong vertical wind shear sufficient to cause the K‐H instability. In the second case with the deepest thickness of ∼1,600 m, the K‐H billows had a horizontal wavelength of ∼4,320 m. Numerical simulation suggested that an enhanced upper‐tropospheric jet associated with a well‐developed synoptic‐scale cyclone caused the K‐H instability. Such background conditions, frequently observed in the Antarctic coastal region, are typical mechanisms for K‐H excitation. Linear stability analysis also indicated that the characteristics of the observed K‐H billows were consistent with the most unstable modes. Furthermore, statistical analysis was performed using data of all 73 observed cases. The characteristics of K‐H billows observed at Syowa Station are similar to those observed over Japan. However, the K‐H billows tend to have longer wave periods over Syowa Station than over Japan, likely due to the weaker tropospheric jet in the Antarctic. Two 10‐day observational campaigns were conducted in 2019 for tropospheric and lower stratospheric turbulence with the Program of the Antarctic Syowa radar (PANSY radar) and radiosondes at Syowa Station in the Antarctic. Seventy three Kelvin‐Helmholtz (K‐H) billows were detected and two distinct cases were studied in detail by numerical simulations: the first case has the longest observational duration of the 73 cases and was most likely caused by instability due to continuous gravity waves associated with a cyclone that imparts strong vertical wind shear. The second case is the deepest and is most likely caused by instability due to a strong upper‐tropospheric jet associated with a well‐developed synoptic‐scale cyclone. Such background conditions are frequently observed in the Antarctic coastal region, suggesting typical mechanisms for K‐H excitation in this region. Theoretical calculations also show that the observed K‐H billow characteristics were consistent with the most unstable mode. Furthermore, statistical analysis using the 73 observed cases indicates that the characteristics of K‐H billows observed at Syowa Station are similar to those over Japan except for the long wave periods, which may be due to the weaker tropospheric jet in the Antarctic. Seventy‐three Kelvin‐Helmholtz (K‐H) billows were detected in Program of the Antarctic Syowa radar observations using a frequency radar interferometric imaging techniqueGravity waves and well‐developed cyclones unique to the Antarctic coastal region cause strong vertical shearK‐H billows in the coastal Antarctic region have comparable spatial scale and strength to those in the mid‐latitudes Seventy‐three Kelvin‐Helmholtz (K‐H) billows were detected in Program of the Antarctic Syowa radar observations using a frequency radar interferometric imaging technique Gravity waves and well‐developed cyclones unique to the Antarctic coastal region cause strong vertical shear K‐H billows in the coastal Antarctic region have comparable spatial scale and strength to those in the mid‐latitudes

This study presents an analysis of OH airglow images observed from an all‐sky camera (ASC) at King Sejong Station (KSS), Antarctic for the 2012–2016 period. The two‐dimensional power spectra of short‐period gravity waves (<1 hr) as a function of phase velocities are obtained using the M‐transform method that employs the time sequence of ASC images. The amplitudes of the power spectral densities show that the mesospheric wave activity is the largest during winter (May, June, and July) and is the smallest in fall (February, March, and April). Wind‐blocking diagrams are constructed on the same two‐dimensional domain as in the two‐dimensional spectra using horizontal winds obtained from MERRA‐2 reanalysis at z= 0–80 km and from KSS meteor radar data at z= 80–90 km. Climatologically, the spectral regions of slowly propagating gravity waves (<30 m s−1) are overlaid by the wind‐blocking areas, which suggests the filtering of gravity waves with small phase speeds by winds below the upper stratosphere. Eastward propagating gravity waves in winter and intense south‐eastward waves in spring (October) are found to be unfiltered by the stratospheric winds. It is also found from the spectral analysis that these unfiltered gravity waves can originate from the upper stratosphere or the lower mesosphere, and not from the troposphere, which suggests the possibility of ASC observation of the secondary gravity waves generated near the stratopause. Spectra of gravity waves in the Antarctic mesosphere are examined through an analysis of 5‐year all‐sky OH airglow imagesEffects of wind filtering on slowly propagating gravity waves are shown through wind‐blocking diagramsEastward and southeastward propagating mesospheric gravity waves in winter and spring can be generated near the stratopause Spectra of gravity waves in the Antarctic mesosphere are examined through an analysis of 5‐year all‐sky OH airglow images Effects of wind filtering on slowly propagating gravity waves are shown through wind‐blocking diagrams Eastward and southeastward propagating mesospheric gravity waves in winter and spring can be generated near the stratopause

We present the climatology of mesopause temperatures using high‐latitude and middle‐latitude meteor radars. The daily mesopause temperatures are estimated using ambipolar diffusion coefficient data from the meteor radars at Davis Station (68.6°S, 77.9°E), in Antarctica, Svalbard (78.3°N, 16°E), Tromsø (69.6°N, 19.2°E) in the Arctic, and Mohe (53.5°N, 122.3°E) and Beijing (40.3°N, 116.2°E) in the northern middle latitudes. The seasonal variations in the meteor radar‐derived temperatures are in good agreement with the temperatures from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument onboard the TIMED satellite. Interhemispheric observations indicate that the mesopause temperatures over the southern and northern polar regions show a clear seasonal asymmetry. The seasonal variations in the Davis Station meteor radar temperatures in the southern polar mesopause are dominated by an annual oscillation (AO) with a relatively weak semiannual oscillation (SAO), which show a clear minimum during summer and a maximum during winter. The mesopause temperatures in the northern high and middle latitudes observed by the Svalbard, Tromsø, Mohe, and Beijing meteor radars mainly show an AO, with a maximum during winter and a minimum during summer. The AO in the northern polar regions is stronger than that in the southern polar regions, while the SAO in the southern polar regions is relatively strong compared to that in the northern polar regions. Interhemispheric mesopause temperatures estimated by using high‐latitude and middle‐latitude meteor radarsMesopause temperatures estimated from meteor radars agree with the SABER and MLS temperaturesInterhemispheric comparisons indicate that the mesopause temperature over the southern and northern polar regions show a seasonal asymmetry Interhemispheric mesopause temperatures estimated by using high‐latitude and middle‐latitude meteor radars Mesopause temperatures estimated from meteor radars agree with the SABER and MLS temperatures Interhemispheric comparisons indicate that the mesopause temperature over the southern and northern polar regions show a seasonal asymmetry

Free traveling Rossby wave normal modes (RNMs) are often investigated through large‐scale space‐time spectral analyses, which therefore is subject to observational availability, especially in the mesosphere. Ground‐based mesospheric observations were broadly used to identify RNMs mostly according to the periods of RNMs without resolving their horizontal scales. The current study diagnoses zonal wave numbers of RNM‐like oscillations occurring in mesospheric winds observed by two meteor radars at about 79°N. We explore four winters comprising the major stratospheric sudden warming events (SSWs) 2009, 2010, and 2013. Diagnosed are predominant oscillations at the periods of 10 and 16 days lasting mostly for three to five whole cycles. All dominant oscillations are associated with westward zonal wave number m=1, excepting one 16‐day oscillation associated with m=2. We discuss the m=1 oscillations as transient RNMs and the m=2 oscillation as a secondary wave of nonlinear interaction between an RNM and a stationary Rossby wave. All the oscillations occur around onsets of the three SSWs, suggesting associations between RNMs and SSWs. For comparison, we also explore the wind collected by a similar network at 54°N during 2012–2016. Explored is a manifestation of 5‐day wave, namely, an oscillation at 5–7 days with m=1), around the onset of SSW 2013, supporting the associations between RNMs and  SSWs. Most detectable atmospheric Rossby waves are associated with atmospheric intrinsic properties, including the free traveling Rossby wave normal modes and forced resonant stationary Rossby waves. The former is less understood than the latter because they are relatively weak and often distorted by the background atmosphere. In the current work, we customize a compact wave identifying technique to estimate the horizontal wavelength of Rossby wave‐like predominant 10‐ and 16‐day oscillations detected by two arctic radars during stratospheric sudden warming events. Results illustrate that five of the six most oscillations are westward traveling with zonal wave number 1. We explain the oscillations as free transient Rossby wave normal modes. Two radars at polar latitude are combined to diagnose zonal wave number mof Rossby wave‐like oscillations occurring around four SSW onsetsTransient 10‐ and 16‐day oscillations are mostly associated with m=1, and therefore should be Rossby wave normal modesA 16‐day oscillation is associated with m=2, resulting potentially from nonlinear interactions of the 16‐day wave with a stationary Rossby wave