This work estimates global‐mean Kzz using Sounding of the Atmosphere using Broadband Emission Radiometry/Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics monthly global‐mean CO2 profiles and a one‐dimensional transport model. It is then specified as a lower boundary into the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIE‐GCM). Results first show that global‐mean CO2 in the mesosphere and lower thermosphere region has annual and semiannual oscillations (AO and SAO) with maxima during solstice seasons along with a primary maximum in boreal summer. Our calculated AO and SAO in global‐mean CO2 are then modeled by AO and SAO in global‐mean Kzz. It is then shown that our estimated global‐mean Kzz is lower in magnitude than the suggested global‐mean Kzz from Qian et al. (2009) that can model the observed AO and SAO in the ionosphere/thermosphere (IT) region. However, our estimated global‐mean Kzz is similar in magnitude with recent suggestions of global‐mean Kzz in models with explicit gravity wave parameterization. Our work therefore concludes that global‐mean Kzz from global‐mean CO2 profiles cannot model the observed AO and SAO in the IT region because our estimated global‐mean Kzz may only be representing eddy diffusion due to gravity wave breaking. The difference between our estimated global‐mean Kzz and the global‐mean Kzz from Qian et al. (2009) thus represents diffusion and mixing from other nongravity wave sources not directly accounted for in the TIE‐GCM lower boundary conditions. These other sources may well be the more dominant lower atmospheric forcing behind the AO and SAO in the IT region.

Quantifying the eddy diffusion coefficient profile in the mesosphere and lower thermosphere (MLT) is critical to the constituent density distributions in the upper mesosphere and thermosphere. Previous work by Swenson et al. (2018, https://doi.org/10.1016/j.jastp.2018.05.014) estimated the global mean eddy diffusion (kzz) values in the upper mesosphere using atomic oxygen (O), derived from Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) hydroxyl (OH). In this study, vertical eddy diffusive transport velocities of O were determined from continuity of mass in the mesopause region (80–97 km), primarily via the HOx chemistry. Global average constituent climatology from previously deduced SABER ozone (O3) and atomic hydrogen (H) was applied. Furthermore, we extended the global mean eddy transport velocities to new heights (105 km) in the MLT using the newly available global mean Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) data. The combined method of determining O3 loss and O density climatology from SCIAMACHY, as well as an improved global mean background atmosphere from SABER, provides new information for eddy diffusion determination in the MLT. Three prominent results to emerge from this study include (i) global mean kzz profiles between 80 and 105 km derived from MLT constituent climatologies, SABER, and SCIAMACHY global mean O density profiles averaged for approximately one solar cycle, (ii) determination of O eddy diffusion velocities in the MLT consistent between two satellite measurements and the thermosphere‐ionosphere‐mesosphere‐electrodynamics general circulation model, and (iii) resolution of historically large differences between deduced kzz determined from O versus CO2 by analysis of SABER and SCIAMACHY measurements.

Global Navigation Satellite System radio occultation signals often show extremely strong levels of scintillation when passing through the ionospheric E region. This is related to sporadic E (Es)-dense layers of metallic ions that can form in the E region, influencing terrestrial and satellite radio propagation. In our report on the 2007-2014 variation of E region S4 amplitude fluctuation indices measured by the FORMOSAT-3/Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC) satellite constellation, we find that the spatial and temporal variation of the maximum S4 index in the E region is proportionate to the occurrence rate of extreme scintillation and by extension, sporadic E. We also find that the monthly median extreme S4 amplitude fluctuation index in the E region midlatitudes shows a dependence on variation of the El Niño-Southern Oscillation (ENSO) in the troposphere that has not been previously reported. The ENSO-related variation of the E region median extreme S4 indices varies closely with the tropopause height, with both parameters lagging the Oceanic Niño Index by roughly 1 to 2 months, while also displaying a similar spectrum of periodicities. This similarity is especially strong in the southern midlatitudes. These results indicate that ENSO signatures can be transmitted to Es formation mechanisms, potentially through modulation of vertically propagating atmospheric tides that alter lower thermospheric wind shears. The end result is the modulation of the interannual variation of extreme Es values by ENSO.Plain Language Summary Global Navigation Satellite System signals often become extremely unstable when passing through altitudes between approximately 90 and 110 km. This is related to dense layers of metallic ions deposited by meteors burning up in the atmosphere, known as sporadic E (Es). In our observations of the fluctuation of Global Navigation Satellite System signals passing through this region measured by the FORMOSAT-3/Constellation Observing System for Meteorology Ionosphere and Climate satellite constellation, we find that the level of signal fluctuation shows a dependence on variation of the El Niño-Southern Oscillation (ENSO) in the troposphere that has not been previously reported. A similar ENSO-related variation is also observed in the height of the tropopause between 10 and 20 km, showing that ENSO can affect atmospheric phenomena that are capable of propagating upward to altitudes where Es occurs. This result illustrates a vertical connection in the atmosphere that has not been previously observed.