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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. Plain Language Summary: 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. Key Points: 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 [ABSTRACT FROM AUTHOR]

Simulations of the solar thermal migrating semidiurnal (SW2) tide in the mesosphere‐lower‐thermosphere (MLT) are compared against meteor wind observations from a longitudinal chain of high‐latitude Super Dual Auroral Radar Network radars. The simulations span the year 2015 and are performed using a mechanistic primitive equation model. The model employs a whole‐atmosphere tide forcing based on temperature tendency fields from the Specified Dynamics Whole Atmosphere Community Climate Model with Thermosphere and Ionosphere Extension, and a background atmospheric specification based on zonal wind and temperature data from the Navy Global Environmental Model‐High Altitude meteorological analysis system. Results show that the model accurately reproduces the observed seasonal variability of the SW2 tide in both the amplitude and phase. Numerical experiments are performed to investigate how the tidal forcing, dissipation terms, and seasonal variations in the background atmosphere shape the seasonal variations of the simulated SW2 tide. Notable results are that the background atmosphere most strongly impacts the SW2 tide forced in the troposphere, and that the specification of a narrow surface friction profile enhances the net SW2 amplitude in the MLT between April and October. Eddy diffusion is found to damp the simulated tide predominantly around summer solstice and in December. Key Points: Simulations of the migrating semidiurnal (SW2) tide are validated against observations from a high‐latitude array of Super Dual Auroral Radar Network meteor radarsNumerical experiments investigate the impact of the background atmosphere, tidal dissipation, and tidal forcing on the simulation resultsThe simulated SW2 is largely shaped by the background atmosphere, while being sensitive to eddy diffusion and surface friction [ABSTRACT FROM AUTHOR]

This work presents a comprehensive evaluation of tides in the mesosphere and lower thermosphere (MLT) of the Whole Atmosphere Community Climate Model with thermosphereionosphere eXtension (WACCMX) and data assimilation provided by the Data Assimilation Research Testbed (DART) ensemble Kalman filter. A total of 26 diurnal and semidiurnal tidal components are calculated using the latest hourly data assimilation cycling during January 12–March 15, 2009 time period. When averaged over this entire yaw cycle, most tidal components display a latitudinal and vertical structures similar to that observed by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER). However, WACCMX + DART amplitudes of some diurnal tidal components are a factor of ∼1.2–2 times smaller and the peak altitudes and latitudes are shifted compared to SABER. The amplitudes of semidiurnal tides are likewise underestimated in the model, as well as being shifted in altitude and latitude, and some wavenumbers are not captured. Daily tidal variations are calculated in the model and compared with SABER and Super Dual Auroral Radar Network (SuperDARN) radar observations. Both the model and the radars show an amplification of the migrating semidiurnal tide during the week following the peak SSW. Enhancements in nonmigrating diurnal tides are also shown around the SSW peak, which could be produced by the nonlinear interaction between migrating tides and planetary waves. This study finds that, despite using data assimilation to constrain model dynamics, more work is needed in order to accurately simulate the full tidal spectrum and responses to SSWs in the MLT region. [ABSTRACT FROM AUTHOR]

Abstract: Observations from satellites and a ground‐based station are combined to construct a global data set for investigating the tertiary ozone maximum in the winter mesosphere for the period August 2004 to June 2017. These give a comprehensive picture of this ozone maximum in latitude, pressure, and time. The location of the tertiary ozone maximum shifts in latitude and pressure with the evolving season; the ozone peak occurs at lower latitude and higher pressure around the winter solstice. Highest average nighttime ozone concentrations and greatest degree of interannual variability are seen in late winter in the Northern Hemisphere (NH). The hemispheric differences and interannual variability in nighttime ozone are related to variations of temperature, H2O, and OH associated with dynamical activity. Elevated stratopause events in the NH winter are associated with transport of air that is depleted in H2O and enhanced in OH; photochemistry then leads to downward displacement of the altitude of maximum ozone and enhancement in the ozone amount. Transport by planetary waves in the NH extends the region of high ozone further from the pole and leads to longitudinal variations. The analysis shows that while the tertiary ozone maximum responds to a particular radiative situation as shown in previous studies, it is also the result of very dry air found in the winter polar mesosphere. [ABSTRACT FROM AUTHOR]