Observations show that equatorial ionospheric vertical drifts during solar minimum differ from the climatology between late afternoon and midnight. By analyzing WACCM-X simulations, which reproduce this solar cycle dependence, we show that the interplay of the dominant migrating tides, their propagating and in-situ forced components, and their solar cycle dependence impact the F-region wind dynamo. In particular, the amplitude and phase of the propagating migrating semidiurnal tide (SW2) in the F-region plays a key role. Under solar minimum conditions, the SW2 tide propagate to and beyond the F-region in the winter hemisphere, and consequently its zonal wind amplitude in the F-region is much stronger than that under solar maximum conditions. Furthermore, its phase shift leads to a strong eastward wind perturbation near local midnight. This in turn drives a F-region dynamo with an equatorial upward drift between 18-1 hour local times.

The observed seasonal variation of zonal wind reversal in the mesosphere and lower thermosphere (MLT) is often not well captured in whole atmosphere general circulation models (GCMs) with current gravity wave parameterization schemes. In this study, we investigate the possible physical mechanisms controlling this seasonal variation. It is found that adaptation of an anisotropic parameterized gravity wave source spectrum with stronger eastward and weaker westward propagating waves can reproduce this seasonal feature. Furthermore, additional stratospheric forcing is needed to control the large winter stratospheric zonal wind and alleviate the “cold-pole” problem in the southern winter. This is accomplished by the application of an inertial gravity wave parameterization scheme. With these changes, the Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM-X) can produce zonal mean zonal wind that is in better agreement with climatology from the stratosphere to MLT, including the seasonal variation of the zonal wind reversal.

Quantification of heat and constituent transport by gravity waves in global models is challenging due to limited model resolutions. Current parameterization schemes suffer from over-simplification and often underestimate the transport rate. In this study, a new approach is explored to quantify the effective vertical eddy diffusion by using a high-resolution WACCM simulation based on scale invariance. The WACCM simulation can partially resolve the mesoscale gravity wave spectrum down to ~250 km horizontal wavelength, and the heat flux and the effective vertical eddy diffusion by these waves are calculated directly. The effective vertical diffusion by the smaller-scale, unresolved waves, is then deduced based on scale invariance, following the method outlined by Liu (2019) in quantifying gravity wave momentum flux and forcing. The effective vertical diffusion obtained is generally larger than that obtained from parameterizations, and is comparable with that derived from observations in the mesosphere and lower thermosphere region.

Solar and atmospheric variability influences the ionosphere, causing critical impacts on satellite and ground based infrastructure. Determining the dominant forcing mechanisms for ionosphere variability is important for prediction and mitigation of these threats. However, this is a challenging task due to the complexity of solar-terrestrial coupling processes. At high latitudes, diurnal and semidiurnal variations of temperature and neutral wind velocity can be forced from either below (lower atmosphere waves) or from above (geomagnetic and in-situ solar forcing). We analyse measurements from the incoherent scatter radar (ISR) facility operated by the European Incoherent Scatter Scientific Association (EISCAT). They are complemented by meteor radar data and compared to global circulation models. Experimental and model data both indicate the existence of strong semidiurnal oscillations in a two-band structure at altitudes $\lesssim110$ km and $\gtrsim130$ km, respectively. Analysis of the phase progressions suggests the upper band to be forced \textit{in situ} while the lower band corresponds to upwards propagating tides from lower atmosphere. These results show that the actual transition of tides in the altitude region between 90 and 130 km is more complex than described so far.

Whole atmosphere models that fully capture the propagation of wave dynamics from lower to upper atmosphere are believed sufficient to reproduce the type of short-term variability in the neutral upper atmosphere that produces observed variations in ionospheric parameters. However, recent studies suggest that upper atmospheric observations are needed to accurately represent short-term variability in both planetary-scale mass transport and tidal behavior crucial to representing the structure of the thermosphere and the wind-dynamo coupling in the ionosphere. To address this, we use atmospheric specifications from the prototype High-Altitude Navy Global Environmental Model (HA-NAVGEM) from the ground to 92 km to nudge the Whole Atmosphere Community Climate Model extended version (WACCM-X) coupled to the Navy Highly Integrated Thermosphere Ionosphere Demonstration System (Navy-HITIDES) ionospheric model. The HA-NAVGEM data assimilation/forecast system is run in two configurations: a reference experiment for the time period December 2012-March 2013, where satellite-based middle atmospheric observations (SABER temperature retrievals; Aura MLS temperature, ozone, and water vapor retrievals; and SSMIS microwave radiances) are included between 20-90 km; and a perturbed experiment, during the same time period, in which the middle atmospheric observations are removed. The resulting nudged simulations using WACCM-X coupled to Navy-HITIDES are used to study the impact of upper atmospheric observations in reproducing the observed short-term variability in the thermosphere-ionosphere system, both in terms of the thermospheric structure and the ionospheric response via wind-dynamo coupling. The role of solar thermal and lunar gravitational tides is discussed, as well as the impact of observations on the weather of the day in the lower thermosphere.

A new version of NCAR Whole Atmosphere Community Climate Model with thermosphere/ionosphere extension (WACCM-X) has been developed. The main feature of this version is the species-dependent spectral element (SE) dynamical core, adapted from the standard version for the Community Atmosphere Model (CAM). The SE is on a quasi-uniform cubed sphere grid, eliminating the polar singularity and thus enabling simulations at high-resolutions. Molecular viscosity and diffusion in the horizontal direction are also included. The Conservative Semi-Lagrangian Multi-Tracer Transport Scheme (CSLAM) is employed for the species transport. An efficient regridding scheme based on the Earth System Modeling Framework (ESMF) is used to map fields between the physics mesh and geomagnetic grid. Simulations have been performed at coarse (~200 km and 0.25 scale height) and high (~25 km and 0.1 scale height) resolutions. The spatial distribution of the resolved gravity waves from the high-resolution simulations compare well with available observations in the middle and upper atmosphere. The forcing by the resolved gravity waves improves the wind climatology in the mesosphere and lower thermosphere in comparison to the coarse resolution simulations with parameterized forcing. It also impacts the thermospheric circulation and compositional structures, as well as thermospheric variablity. While larger scale waves are dominant energetically at most latitudes, smaller scale waves contribute significantly to the total momentum flux, especially at mid-high latitudes. The waves in the thermosphere are shown to be strongly modulated by the large-scale wind through Doppler shift and molecular damping, and they cause large neutral atmosphere and plasma perturbations.

Fundamental understanding of the climate responses to solar variability is obscured by the large and complex climate variability. This long-standing issue is addressed here by examining climate responses under an extreme quiet sun (EQS) scenario, obtained by making the sun void of all magnetic fields. It is used to drive a coupled climate model with whole atmosphere and ocean components. The simulations reveal robust responses, and elucidate aspects of the responses to changes of troposphere/surface forcing and stratospheric forcing that are similar and those that are different. Planetary waves (PWs) play a key role in both regional climate and the mean circulation changes. Intermediate scale stationary waves and regional climate respond to solar forcing changes in the troposphere and stratosphere in a similar way, due to similar subtropical wind changes in the upper troposphere. The patterns of these changes are similar to those found in a warming climate, but with opposite signs. Responses of the largest scale PW during NH and SH winters differ, leading to hemispheric differences in the interplay between dynamical and radiative processes. The analysis exposes remarkable general similarities between climate responses in EQS simulations and those under nominal solar minimum conditions, even though the latter may not always appear to be statistically significant.

The extent to which terrestrial weather below 30 km in altitude can influence the dynamics and mean state of the thermosphere (ca., 100-500 km) is a fascinating discovery of the last two decades or so. Waves that are excited by deep convection in the tropical troposphere and propagate vertically into the thermosphere are responsible for much of this influence. Tropospheric convection associated with the Madden JulianOscillation(MJO), the dominant mode of intra-seasonal variability in tropical convection and circulation, has been known to modulate the intensity of upward propagating gravity waves and Kelvin waves. An MJO impact on tides was already proposed over two decades ago, but only recent gains in satellite observational capabilities allows one to quantify their effect from observations. Previous work by Gasperini et al. [2017a] demonstrate that a 90-day oscillation in tropospheric convection during 2009-2010 is imprinted on both thermospheric mean winds and the eastward propagating wavenumber 3 diurnal (DE3) tidal amplitudes observed by the GOCE and CHAMP satellites and modeled with the TIME-GCM. In a follow-on modeling-based study, Gasperini at al. [2020] present statistical evidence for a strong connection (+/-12%) between the phase of the tropospheric MJO and the amplitudes of the thermospheric DE3 and 3-day ultra-fast Kelvin wave (UFKW), two of the most prominent and well-established waves from the tropical wave spectrum that preferentially propagate into the thermosphere. These recent studies demonstrate that strong coupling between the troposphere and the thermosphere occurs on intra-seasonal timescales, raising important questions that have implications for the whole atmosphere system. In this work, we present evidence for a strong quasi-60 day oscillation in GOLD column-integrated O/N2 and Swarm-C total mass density during 2018-2019. A similar and concurrent oscillation is observed in the DE3 and SE2 tidal amplitudes derived from SD/WACCM-X. Spectral analysis of OLR reveals a similar oscillation in tropical tropospheric convection that is shown to be eastward propagating with s=-1 and is thus consistent with an MJO event.

Fundamental understanding of the climate responses to solar variability is obscured by the large and complex climate variability. This long-standing issue is addressed here by examining climate responses under an extreme solar minimum (ESM) scenario, obtained by making the sun void of all magnetic fields. It is used to drive a whole atmosphere climate model with coupled ocean. The simulations reveal robust responses in the coupled climate system, and elucidate similarities and differences of responses to bottom-up and top-down forcing. Planetary waves (PWs) play a key role in both regional climate and the mean circulation changes. Responses of the largest scale PW during NH and SH winters differ, leading to hemispheric differences in the interplay between dynamical and radiative processes. The analysis exposes remarkable general similarities between climate responses in ESM simulations and those under nominal solar minimum conditions, even though the latter may not appear to be statistically significant.

This work uses the Specified Dynamics-Whole Atmosphere Community Climate Model with Ionosphere/Thermosphere eXtension (SD-WACCM-X) to determine and explain the seasonality of the migrating semidiurnal tide (SW2) components of tropical upper mesosphere and lower thermosphere (UMLT) temperature, zonal-wind and meridional-wind. This work also quantifies aliasing due to SW2 in satellite-based tidal estimates. Results show that during equinox seasons, the vertical profile of tropical UMLT temperature SW2 and zonal wind SW2’s amplitudes have a double peak structure while they, along with meridional wind SW2, have a single peak structure in their amplitudes in June solstice. Hough mode reconstruction reveals that a linear combination of 5 SW2 Hough modes cannot fully reproduced these features. Tendency analysis reveals that for temperature, the adiabatic term, non-linear advection term and linear advection term are important. For the winds, the classical terms, non-linear advection term, linear advection term and gravity wave drag are important. Results of our alias analysis then indicate that SW2 can induce an ~60% alias in zonal-mean and DW1 components calculated from sampling like that of the Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite and the Aura satellite. This work concludes that in-situ generation by wave-wave interaction and/or by gravity waves play significant roles in the seasonality of tropical UMLT temperature SW2, zonal wind SW2 and meridional wind SW2. The alias analysis further adds that one cannot simply assume SW2 in the tropical UMLT is negligible.