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.

We used observations and model simulation to examine the atmospheric pulses that dominate the far field in the hours after the January 2022 Tonga eruption. We analyzed radiance observations taken from the Himawari-8 geostationary satellite and showed that both a Lamb wave front with the expected horizontal phase speed ~315 m-s-1 and a distinct front with phase speed ~245 m-s-1 can be detected. The slower phase speed is consistent with that expected for the global internal resonant mode that had been proposed by Pekeris in 1937 and in other idealized theoretical studies over the past century, but which had never been detected in the atmosphere. A simulation of the eruption aftermath was performed with a high resolution atmospheric general circulation model. A hot anomaly over the volcano location was introduced instantaneously to the model fields and the model was integrated for another 12 hours. This produced a simulated wave pulse that, in the far field, agreed reasonably well with barograph observations of the Lamb wave. The model results also showed the presence of the slower pulse and that this disturbance had a vertical structure with a 180o phase shift in the stratosphere, in agreement with the theoretical prediction for the internal mode. An implication of this result is that the continuously ringing Lamb wave global normal modes that have been seen in analysis of long observational records ought to have lower frequency internal Pekeris mode counterparts, a prediction that we confirm though analysis of 57 years of hourly global reanalysis data.

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.

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.