Secular variations in CO₂ concentration and geomagnetic field can affect the dynamics of the upper atmosphere. We examine how these two factors influence the dynamics of the upper atmosphere during the Holocene, using two sets of ~ 12 000-year control runs by the coupled thermosphere–ionosphere model. The main results show that (a) increased CO₂ enhances the thermospheric circulation, but non-linearly; and (b) geomagnetic variation induced a significant hemispheric asymmetrical effect.

On timescales longer than the solar cycle, secular changes in CO₂ concentration and geomagnetic field play a key role in influencing the thermosphere. We performed four sets of ~12000-year control runs with the coupled thermosphere–ionosphere model to examine the effects of the geomagnetic field, CO₂, and solar activity on thermospheric density and temperature, deepening our understanding of long-term changes in the thermosphere and making projections for future thermospheric changes.

Periodic changes in all hydrodynamic parameters are constantly observed in the atmosphere. The amplitude of these fluctuations increases with height due to a decrease in the atmospheric density. In the upper layers of the atmosphere, waves are the dominant form of motion. We use a model of the general circulation of the atmosphere to study the contribution to the formation of the dynamic and temperature regimes of the middle and upper atmosphere made by different global-scale atmospheric waves.

We make high-resolution simulations of poorly understood decays of nonlinear atmospheric acoustic–gravity waves (AGWs) after deactivations of the wave forcing. The standard deviations of AGW perturbations, after fast dispersions of traveling modes, experience slower exponential decreases. AGW decay times are estimated for the first time and are 20–100 h in the stratosphere and mesosphere. This requires slow, quasi-standing and secondary modes in parameterizations of AGW impacts to be considered.

The MLT DW1 amplitude is suppressed during El Niño winters in both satellite observation and SD-WACCM simulations. The suppressed Hough mode (1, 1) in the tropopause region propagates vertically to the MLT region, leading to decreased DW1 amplitude. The latitudinal zonal wind shear anomalies during El Niño winters would narrow the waveguide and prevent the vertical propagation of DW1. The gravity wave drag excited by ENSO-induced anomalous convection could also modulate the MLT DW1 amplitude.

In order to have confidence in atmospheric predictions, it is important to know how well different numerical model simulations of the Earth’s atmosphere agree with one another. This work compares four different data assimilation models that extend to or beyond the mesosphere. Results shown here demonstrate that while the models are in close agreement below ~50 km, large differences arise at higher altitudes in the mesosphere and lower thermosphere that will need to be reconciled in the future.

Metal layers occur in the MLT region (80–120 km) from the ablation of cosmic dust. The latest lidar observations show these metals can reach a height approaching 200 km, which is challenging to explain. We have developed the first global simulation incorporating the full life cycle of metal atoms and ions. The model results compare well with lidar and satellite observations of the seasonal and diurnal variation of the metals and demonstrate the importance of ion mass and ion-neutral coupling.

The winter winds over Antarctica form a stable vortex. However, in 2019 the vortex was disrupted and the temperature in the polar stratosphere rose by 50°C. This event, called a sudden stratospheric warming, is a rare event in the Southern Hemisphere, with the only known major event having taken place in 2002. The 2019 event helps us unravel its causes, which are largely unknown. We have discovered a unique behaviour of the equatorial winds in 2002 and 2019 that may signal an impending SH SSW.

Our results suggest that lightning strokes would probably influence the ionosphere and thus give rise to the occurrence of a sporadic sodium layer (Na_S), with the overturning of the electric field playing an important role. Model simulation results show that the calculated first-order rate coefficient could explain the efficient recombination of Na⁺→Na in this Na_S case study. A conjunction between the lower and upper atmospheres could be established by these inter-connected phenomena.

Ground-based observations of the hydroxyl (OH) airglow have indicated that the rotational energy levels may not be in thermal equilibrium with the surrounding gas. Here we use simulations of the OH airglow to show that temperature changes across the extended airglow layer, either climatological or those temporarily caused by atmospheric waves, can mimic this effect for thermalized OH. Thus, these must be considered in order to quantify the non-thermal nature of the OH airglow.

The first long-term simulation using the high-top non-hydrostatic general circulation model (NICAM) was executed to analyze mesospheric gravity waves. A new finding in this paper is that the spectrum of the vertical fluxes of the zonal momentum has an isolated peak at frequencies slightly lower than f at latitudes from 30 to 75° S at a height of 70 km. This study discusses the physical mechanism for an explanation of the existence of the isolated spectrum peak in the mesosphere.

The terdiurnal solar tide is an atmospheric wave, owing to the daily variation of solar heating with a period of 8 h. Here, we present model simulations of this tide and investigate the relative importance of possible forcing mechanisms because they are still under debate. These are, besides direct solar heating, nonlinear interactions between other tides and gravity wave–tide interactions. As a result, solar heating is most important and nonlinear effects partly counteract this forcing.

Local time variations of polar mesospheric clouds (PMCs) in the Northern Hemisphere are studied using a combination of a global circulation model and a microphysical model. We investigate the brightness, altitude, and occurrence of the clouds and find a good agreement between model and observations. The variations are caused by tidal structures in background parameters. The temperature varies by about 2 K and water vapor by about 3 ppmv at the altitude of ice particle sublimation near 81.5 km.

The influences of sudden stratospheric warming in the Northern Hemisphere on quasi-2-day waves are studied with both observations and simulations. We found the energy of W3 is transferred to W2 through the nonlinear interaction with SPW1 and the instability at winter mesopause could provide additional amplification for W3. The summer easterly is enhanced during SSW, which is more favorable for the propagation of quasi-2-day waves.