The objective of this study was to examine both the climatology of the residual mean circulation, and the roles of resolved wave (RW) and unresolved wave (UW) forcings over four Mars years, based on the transformed Eulerian mean equation system using the EMARS reanalysis dataset. While RW forcing was estimated directly as Eliassen–Palm flux divergence, the forcing by UWs, including subgrid-scale gravity waves, was estimated indirectly using the zonal momentum equation. This indirect method, devised originally for study of Earth’s middle atmosphere, is applicable to latitudinal regions having angular momentum isopleths connected from the surface to the top of the atmosphere, which are usually mid- and high-latitude regions. In low latitudes of the winter hemisphere, a strong residual mean poleward flow is observed at an altitude range of 40–80 km, where the latitudinal gradient of the absolute angular momentum is small. The strong poleward flow crosses the isopleths of angular momentum in the regions of its northern and southern ends, indicating the necessity of the wave forcing. Our results suggest that the structure of the residual mean circulation at mid- and high-latitude regions is largely determined by UW forcing, particularly above the altitude of 60 km, whereas the RW contribution is also large below the altitude of 60 km.

The objective of this study was to examine both the climatology of the residual mean circulation, and the roles of resolved wave (RW) and unresolved wave (UW) forcings over four Mars years, based on the transformed Eulerian mean equation system using the EMARS reanalysis dataset. While RW forcing was estimated directly as Eliassen–Palm flux divergence, the forcing by UWs, including subgrid-scale gravity waves, was estimated indirectly using the zonal momentum equation. This indirect method, devised originally for study of Earth’s middle atmosphere, is applicable to latitudinal regions having angular momentum isopleths connected from the surface to the top of the atmosphere, which are usually mid- and high-latitude regions. In low latitudes of the winter hemisphere, a strong residual mean poleward flow is observed at an altitude range of 40–80 km, where the latitudinal gradient of the absolute angular momentum is small. The strong poleward flow crosses the isopleths of angular momentum in the regions of its northern and southern ends, indicating the necessity of the wave forcing. Our results suggest that the structure of the residual mean circulation at mid- and high-latitude regions is largely determined by UW forcing, particularly above the altitude of 60 km, whereas the RW contribution is also large below the altitude of 60 km.

Atmospheric gravity waves (GWs) play a key role in determining the thermodynamical structure of the Earth’s middle atmosphere. Despite the small spatial and temporal scales of these waves, a few high-top general circulation models (GCMs) that can resolve them explicitly have recently become available. This study compares global GW characteristics simulated in one such GCM, the Japanese Atmospheric GCM for Upper-Atmosphere Research (JAGUAR), with those derived from three-dimensional (3-D) temperatures observed by the Atmospheric Infrared Sounder (AIRS) aboard NASA’s Aqua satellite. The target period is from 15 December 2018 to 8 January 2019, including the onset of a major sudden stratospheric warming (SSW). The 3-D Stockwell transform method is used for GW spectral analysis. The amplitudes and momentum fluxes of GWs in JAGUAR are generally in good quantitative agreement with those in the AIRS observations in both magnitude and distribution. As the SSW event progressed, the GW amplitudes and eastward momentum flux increased at low latitudes in the summer hemisphere in both the model and observation datasets. Case studies demonstrate that the model is able to reproduce comparable wave events to those in the AIRS observations with some differences, especially noticeable at low latitudes in the summer hemisphere. Through a comparison between the model results with and without the AIRS observational filter applied, it is suggested that the amplitudes of GWs near the exits and entrances of eastward jet streaks are underestimated in AIRS observations.

We examined large-amplitude inertia gravity waves (GWs) over Syowa Station, Antarctica, comparing PANSY radar data and ERA5 reanalysis from October 2015 to September 2016. Focusing on large-amplitude events with a large absolute momentum flux (AMF), hodograph analysis was applied to estimate the wave parameters and found that the percentage of these waves with a downward phase velocity increased with altitude. Vertical wavelengths shortened, intrinsic periods lengthened, and horizontal wavelengths became longer with increasing altitude. Southward propagation of GWs was predominant in the stratosphere. Compared to a previous study, the wave parameters’ altitude variation remained consistent, but horizontal and vertical wavelengths were longer in this study. ERA5 underestimated AMF by about 1/5 between 5 and 12.5 km, with a larger underestimation at higher altitudes. The underestimation was related to the power spectra of horizontal and vertical winds, particularly vertical winds. The greater underestimation in the stratosphere might be due to ERA5’s vertical grid spacing and shorter vertical wavelengths of dominant GWs.

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-art GW-permitting general circulation model using 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 gravity waves but also large-scale waves are important for the mechanism of the IHC.

Since 2004, following prolonged stratospheric sudden warming (SSW) events, it has been observed that the stratopause disappeared and reformed at a higher altitude, forming an elevated stratopause (ES). The relative roles of atmospheric waves in the mechanism of ES formation are still not fully understood. We performed a hindcast of the 2018/19 SSW event using a gravity-wave (GW) permitting general circulation model that resolves the mesosphere and lower thermosphere (MLT) and analyzed dynamical phenomena throughout the entire middle atmosphere. An ES formed after the major warming on 1 January 2019. There was a marked temperature maximum in the polar upper mesosphere around 28 December 2018 prior to the disappearance of the descending stratopause associated with the SSW. This temperature structure is referred to as a mesospheric inversion layer (MIL). We show that adiabatic heating from the residual circulation driven by GW forcing (GWF) causes barotropic and/or baroclinic instability before the MIL formation, causing in situ generation of planetary waves (PWs). These PWs propagate into the MLT and exert negative (westward) forcing, which contributes to the MIL formation. Both GWF and PW forcing (PWF) above the recovered eastward jet play crucial roles in ES formation. The altitude of the recovered eastward jet, which regulates GWF and PWF height, is likely affected by the MIL structure. Simple vertical propagation from the lower atmosphere is insufficient to explain the presence of the GWs observed in this event.

Observations with high vertical resolution have revealed that power spectra of horizontal wind and temperature fluctuations versus vertical wavenumber m have a universal shape with a steep slope in a high m range, approximately proportional to m^(-3). Several theoretical models explaining this spectral slope were proposed under an assumption of gravity wave (GW) saturation. However, little evidence has been obtained to show that these universal spectra are fully composed of GWs. To confirm the validity of this assumption, two kinds of m spectra are calculated using outputs from a GW-permitting high-top general circulation model. One is the spectra for GWs designated by fluctuations having total horizontal wavenumbers of 21–639. The other is the spectra of fluctuations unfiltered except extracting a linear trend in the vertical that were often analyzed in the observational studies. Comparison between the two shows that GWs dominate the observed spectra only in a higher m part of the steep slope, whereas disturbances other than GWs significantly contribute to its lower m part. Moreover, geographical distributions of the characteristic wavenumbers, slopes, and spectral densities of GW spectra are examined for several divided height regions of the whole middle atmosphere. It is shown that strong vertical shear below the zonal wind jets as well as the wave saturation is responsible for the formation of the steep slopes of GW spectra.

Gravity waves play an essential role in driving and maintaining global circulation. To understand their contribution in the atmosphere, the accurate reproduction of their distribution is important. Thus, a deep learning approach for the estimation of gravity wave momentum fluxes was proposed, and its performance at 100 hPa was tested using data from low resolution zonal and meridional winds, temperature, and specific humidity at 300, 700, and 850 hPa in the Hokkaido region (Japan). To this end, a deep convolutional neural network was trained on 29-year reanalysis datasets (JRA-55 and DSJRA-55), and the final 5-year data were reserved for evaluation. The results showed that compared to ground truth data, the fine-scale momentum flux distribution of the gravity waves could be estimated at a low computational cost. Particularly, in winter, when gravity waves are stronger, the median RMSE of the maximum momentum flux in the target area was 0.06–0.13 mPa.

In September 2019, a minor but strong sudden stratospheric warming (SSW) event occurred in the Southern Hemisphere. We examine the dynamical characteristics of the gravity waves (GWs) and Rossby waves (RWs), especially quasi-6-day waves (Q6DWs), during this event based on Program of the Antarctic Syowa (PANSY) radar observations and high-resolution Japanese Atmospheric General circulation model for Upper Atmosphere Research (JAGUAR) simulations. For the GWs, strongly negative vertical fluxes of zonal momentum in the stratosphere were observed around the edge of the polar vortex during the SSW event. In the mesosphere, strongly positive momentum fluxes were observed in the Eastern Hemisphere, where westward winds were dominant associated with the SSW event. For the RWs, two types of Q6DWs appeared during the SSW event: one with eastward phase velocity (Q6DW-E) and one with westward phase velocity (Q6DW-W). These waves had a baroclinic structure in vertical, differing from normal-mode 5-day Rossby waves. It is shown that Q6DW-E, which was observed prior to the SSW onset, was an unstable wave owing to the baroclinic instability in the high-latitude mesosphere. Conversely, Q6DW-W was observed after the onset and had characteristics of an upward-propagating internal RW. It is considered to be generated by barotropic/baroclinic instability in the upper stratosphere. This instability was likely caused by forcings resulting from the in situ generated Q6DW-E and RWs originating from the mid- and high-latitude troposphere, as well as the GW forcings, which were positive in the mesosphere and negative in the stratosphere associated with the SSW event.

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 (PANSY) radar and radiosonde observations obtained at Syowa Station in the Antarctic. Overall, 73 cases of Kelvin-Helmholtz (K-H) billows were detected, and 2 characteristic cases were examined in detail. In the first case with the longest duration of ~6.5 h, the K-H billows had thickness of ~800 m and horizontal wavelength of ~2500 m. According to a numerical simulation of the environmental conditions, continuously existing orographic gravity waves maintained strong vertical shear of the horizontal winds sufficient to cause the K-H instability. In the second case with the deepest thickness of ~1600m, the K-H billows had duration of ~1.5 h and horizontal wavelength of ~4320m. 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, statical 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 weaker vertical shear and longer wave period of the K-H billows over Syowa Station reflect that the tropospheric jet over the Antarctica is not as strong as that over Japan.

Atmospheric gravity waves (GWs) during the February 2018 sudden stratospheric warming (SSW) were simulated using the T639L340 whole neutral atmosphere general circulation model. Their characteristic morphology around the drastically evolving polar vortex was revealed by three-dimensional (3D) visualization and ray-tracing analyses. The 3D morphology of simulated GWs was described for the three key days that represent the pre-SSW, the mature stage for the vortex splitting, and the late SSW. The combination of strong winds along the polar vortex edge and underneath the tropospheric winds with similar wind directions consisted of the deep waveguide for the upward-propagating GWs, forming GW hot spots in the middle atmosphere. The GW hot spots associated with the development of the SSW were limited to North America and Greenland, and they included the typical upward-propagating orographic GWs with relatively long vertical wavelengths. Different types of characteristic GW signatures were also recognized around the Canadian sub-vortex (CV). The GWs having short vertical wavelengths formed near the surface and obliquely propagated over long distances along the CV winds. The non-orographic GWs with short vertical wavelengths formed in the middle stratosphere through the spontaneous adjustment of flow imbalance around the CV. Those GWs cyclonically ascended into the mesosphere along CV winds.