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.

Horizontal gravity wave (GW) refraction was observed around the Andes and Drake Pas- sage during the SouthTRAC campaign. GWs interact with the background wind through refraction and dissipation. This interaction helps to drive mid-atmospheric circulations and slows down the polar vortex by taking GW momentum flux from one location to an- other. The SouthTRAC campaign was composed to gain improved understanding of the propagation and dissipation of GWs. This study uses observational data from this cam- paign collected by the German research aircraft on 12 September 2019. During the cam- paign a minor sudden stratospheric warming in the Southern Hemisphere occurred, which heavily influenced GW propagation and refraction and thus also the location and amount of GW momentum flux deposition. Observations include, amongst others, measurements from below the aircraft by GLORIA (Gimballed Limb Observer for Radiance Imaging of the Atmosphere), and above the aircraft by ALIMA (Airborne Lidar for the Middle Atmosphere). Refraction is identified in two different GW packets as low as ≈4 km and as high as 58 km. One GW packet of orographic origin and one of non-orographic ori- gin is used to investigate refraction. Observations are supplemented by the Gravity-wave Regional Or Global Ray Tracer (GROGRAT), a simplified mountain wave model, ERA5 data and high-resolution (3 km) WRF data. Contrary to some previous studies we find that refraction makes a noteworthy contribution in the amount and the location of GW momentum flux deposition. This case study highlights the importance of refraction and provides compelling arguments that models should account for this.

Atmospheric gravity waves (GWs) are generated globally in the lower atmosphere by various weather phenomena during all seasons. They propagate upward, carry a significant amount of energy and momentum to higher altitudes, and significantly influence the general circulation of the middle and upper atmosphere. We use a three-dimensional first-principle general circulation model (GCM) with an implemented nonlinear whole atmosphere GW parameterization to study the global climatology of wave activity and produced effects at altitudes up to the upper thermosphere. The numerical experiments were guided by the GW momentum fluxes and temperature variances as measured in 2010 by the SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) instrument onboard NASA’s TIMED (Thermosphere Ionosphere Mesosphere Energetics Dynamics) satellite. This includes the latitudinal dependence and magnitude of GW activity in the lower stratosphere for the boreal summer season. The modeling results were compared to the SABER and Upper Atmosphere Research Satellite (UARS) data in the mesosphere and lower thermosphere. Simulations suggest that, in order to reproduce the observed circulation and wave activity in the middle atmosphere, smaller than the measured GW fluxes have to be used at the source level in the lower atmosphere. This is because observations contain a broad spectrum of GWs, while parameterizations capture only a portion relevant to the middle and upper atmosphere. Accounting for the latitudinal variations of the source appreciably improves simulations.

The Quasi-Biennial Oscillation (QBO) is a major mode of climate variability with periodically descending westerly and easterly winds in the tropical stratosphere, modulating transport and distributions of key greenhouse gases such as water vapor and ozone. In 2016 and 2020, anomalous QBO easterlies disrupted the QBO’s 28–month period previously observed. Here, we quantify the impact of these QBO disruptions on lower stratospheric circulation, and water vapour and ozone using reanalyses and satellite observations, respectively. Both constituents decrease globally from early spring to late autumn during 2016, while they only weakly decrease during 2020. These dissimilarities result from differences in upwelling and cold-point tropopause temperatures caused by anomalous planetary and gravity wave forcing. Our results highlight the need for a better understanding of the causes of QBO disruptions, their interplay with other modes of climate variability, and their impacts on water vapor and ozone in the face of a changing climate.