Sudden stratospheric warmings (SSWs) are impressive fluid dynamical events in which large and rapid temperature increases in the winter polar stratosphere (∼10–50 km) are associated with a complete reversal of the climatological wintertime westerly winds. SSWs are caused by the breaking of planetary‐scale waves that propagate upwards from the troposphere. During an SSW, the polar vortex breaks down, accompanied by rapid descent and warming of air in polar latitudes, mirrored by ascent and cooling above the warming. The rapid warming and descent of the polar air column affect tropospheric weather, shifting jet streams, storm tracks, and the Northern Annular Mode, making cold air outbreaks over North America and Eurasia more likely. SSWs affect the atmosphere above the stratosphere, producing widespread effects on atmospheric chemistry, temperatures, winds, neutral (nonionized) particles and electron densities, and electric fields. These effects span both hemispheres. Given their crucial role in the whole atmosphere, SSWs are also seen as a key process to analyze in climate change studies and subseasonal to seasonal prediction. This work reviews the current knowledge on the most important aspects of SSWs, from the historical background to dynamical processes, modeling, chemistry, and impact on other atmospheric layers.

We conduct observational and modeling studies of thermospheric composition responses to weak geomagnetic activity (nongeomagnetic storms). We found that the thermospheric O and N 2 column density ratio (∑O/N 2) in part of the Northern Hemisphere measured by Global-scale Observations of the Limb and Disk (GOLD) exhibited large and long-lived depletions during weak geomagnetic activity in May and June 2019. The depletions reached 30% of quiet time values, extended equatorward to 10°N and lasted more than 10 hr. Furthermore, numerical simulation results are similar to these observations and indicate that the ∑O/N 2 depletions were pushed westward by zonal winds. The ∑O/N 2 evolution during weak geomagnetic activity suggests that the formation mechanism of the ∑O/N 2 depletions is similar to that during a geomagnetic storm. The effects of weak geomagnetic activity are often ignored but, in fact, are important for understanding thermosphere neutral composition variability and hence the state of the thermosphere-ionosphere system. Plain Language Summary The column density ratio of O and N 2 (∑O/N 2) has been used to monitor geomagnetic storm effects in the thermosphere, as well as providing valuable information about the ionosphere. This triggers an important question: Can weak geomagnetic activities cause changes in thermospheric composition too? Here, we conduct studies based on geostationary orbit observations and numerical simulations. Model outputs replicate the general morphology of this variability for the cases examined. This made it possible to understand the cause of the composition response to weak geomagnetic forcing. We found that the ∑O/N 2 depletion observed was pushed westward by the zonal wind. During weak geomagnetic activity, the ∑O/N 2 response is similar to the response during a geomagnetic storm, albeit it is weaker. In summary, our study suggests that weak geomagnetic activity can also generate strong and long-lived responses in thermosphere composition during solar minimum and that this response can be important to understanding the thermosphere and ionosphere variability during the so-called quiet times.

Observations from the recently launched Global‐Scale Observations of the Limb and Disk (GOLD) instrument on the geostationary SES‐14 communications satellite reveal a substantial response of the mean state of the thermosphere to the Sudden Stratospheric Warming (SSW) event in early January 2019. The observed O/N 2 column density depletion of more than 10% starts at the onset of the SSW, maximizes at the time of the stratospheric wind reversal, and recovers toward the end of the SSW. A connection between SSW and thermospheric composition was previously predicted by model simulations but could not be observed before. The GOLD measurements support the scenario that enhanced global‐scale wave activity during SSWs causes an enhanced wave driving of the lower thermosphere zonal mean circulation that leads to a reduction in lower thermosphere atomic oxygen, which then propagates through molecular diffusion into the upper thermosphere.

Capsule Summary Contributions and the remaining challenges of COSMIC and other RO observations to weather, climate, and space weather since 2011 and potential contributions to research and operations of COSMIC-2 are summarized.

There are strong day-to-day variations in ionosphere parameters such as the maximum electron density of the F2 layer (N m F 2 ) and the total electron content (TEC). Changes in solar radiation, geomagnetic disturbances, and lower atmosphere forcing can all contribute to this day-to-day variability. During what is generally considered to be geomagnetically quiet time (for example, magnetic activity index Kp < 3), this dayto-day variability of the ionosphere can reach 20%-200% (

Simulations with the Community Earth System Model, version 2, using the Whole Atmosphere Community Climate Model version 6 [CESM2(WACCM6)] configuration, show evidence of dynamical coupling from the high latitudes of the winter middle atmosphere to the tropics and the middle and high latitudes of the summer hemisphere. Analysis of monthly and daily output covering 195 simulation years indicates that the response in the summer middle and high latitudes has a weak overall magnitude of a few kelvins or less in temperature but has a repeatable pattern whose structure and phase agree with observational studies. Lag correlation indicates that perturbations in wave activity in the winter stratosphere, as quantified by Eliassen–Palm (EP) flux divergence, are accompanied by perturbations in the transformed Eulerian-mean meridional wind extending into the summer hemisphere. There is not an appreciable correlation with momentum forcing in the summer hemisphere by either resolved waves or parameterized gravity waves. The rapid circulation response and the lack of a wave response in the summer hemisphere suggest that the interhemispheric coupling that is simulated in WACCM6 in both the stratosphere and the mesosphere owes its existence to a circulation that develops to restore balance to the zonally averaged state of the atmosphere. This is an alternative explanation for the coupling from the winter stratosphere to the summer mesosphere; previous studies have assumed a necessary role for wave activity in the summer hemisphere.

This study employs a troposphere to lower thermosphere assimilation model data set generated by the Whole Atmosphere Community Climate Model with data assimilation provided by the Data Assimilation Research Test Bed (WACCM + DART) to explore the sources, sinks, and propagation characteristics of the quasi 6‐day wave (Q6DW) in the year 2007. WACCM + DART reproduces the burst‐like Q6DW and compares well with Sounding of the Atmosphere using Broadband Emission Radiometry and Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics Doppler Interferometer observations. The most prominent Q6DW took place in later February and mid‐October, while the Q6DW was absent during solstice conditions in 2007. The occurrence of a large Q6DW in the equinoctial mesosphere and lower thermosphere is highly dependent on wave amplification and overreflection processes associated with barotropic/baroclinic instabilities and wave critical layers defined by the zonal mean zonal winds. During solstices, the winter hemisphere waveguide is negative and prevents the vertical wave propagation from the source region into the mesosphere and lower thermosphere. Meanwhile, the critical layer for the Q6DW encloses the unstable region in the summer hemisphere and thus blocks the energy conversion from the mean flow to the wave. The resulting circulation pattern due to the Q6DW momentum deposition is upward and poleward in both hemispheres and thus weakens the residual mean circulation in the summer hemisphere but strengthens it in the winter hemisphere. Also, the Q6DW impact on the residual mean circulation points to broader implications for the mean state of the upper atmosphere, for example, the thermospheric O/N2 ratio due to upward constituent transport and related changes in the ionospheric plasma.

We use the CESM2-Whole Atmosphere Community Climate Model, to study the importance of ozone in the vertical coupling between lower and upper atmosphere during sudden stratospheric warmings (SSWs). During SSWs, the build up of stratospheric ozone concentrations at tropical latitudes and its increased asymmetrical distribution carries the potential to affect the generation of migrating and nonmigrating semidiurnal solar tides. Much of the upper atmospheric variability associated with SSWs is known to be driven by large changes in the vertically propagating semidiurnal migrating (SW2) and nonmigrating (SW1 and SW3) solar tides. In this study, we investigate the effect of stratospheric ozone variability during SSWs on these solar tides. For this purpose, a case study of the 2009 SSW event is carried out using the WACCM with two distinct simulation setups. In the first setup, the ozone concentrations are interactively calculated in the model and resemble the ozone observations during the 2009 SSW event, while in the second setup, the ozone concentrations are specified using zonal mean values. We constrain both of the simulations to the Modern-Era Retrospective Analysis for Research and Applications-2 reanalysis so that the background atmosphere through which the solar tides propagate are almost identical in each case. Following the onset of the SSW, we find that in the vicinity of the peak enhancements of SW1, SW2, and SW3 in the mesosphere-lower thermosphere (MLT), the amplitudes of these semidiurnal solar tides are approximately about 15-50% larger for the simulation with interactive ozone as compared with the one with prescribed ozone, indicating that the stratospheric ozone variability plays an important role in driving semidiurnal solar tidal changes during SSWs. Key Points: • The stratospheric ozone variability effect on SW2 amplitudes in the vicinity of the peak SW2 enhancements can be up to 15-25% at MLT heights • Results suggest that enhanced QSPW1 in the NH before the onset of 2009 SSW may lead to large variability of SW1 and SW3 in the MLT of SH • Before the SSW onset, the SW1 and SW3 variability in the SH seems to be related to the nonlinear interaction between SW2 and QSPW1