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.

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.