Preliminary evidence of Madden‐Julian oscillation effects on ultrafast tropical waves in the thermosphere

Over the past two decades mounting evidence demonstrated that terrestrial weather significantly influences the dynamics and mean state of the thermosphere. While important progress has been made in understanding how this coupling occurs on hourly to daily time scales, large uncertainty still exists on this effect around intraseasonal (∼30-90 days) time scales. In this work, analyses of Thermosphere Ionosphere Mesosphere Energetics Dynamics‐Sounding of the Atmosphere using Broadband Emission Radiometry temperatures near 110 km and Gravity field and steady‐state Ocean Circulation Explorer cross‐track winds near 260 km reveal prominent intraseasonal oscillations in the equatorial (±15°) zonal mean lower and middle thermosphere. Similar intraseasonal oscillations are found in the amplitudes of the diurnal eastward propagating tide with Zonal Wavenumber 3 (DE3) and the quasi‐3‐day ultrafast Kelvin wave, two prominent ultrafast tropical waves (UFTWs) excited by deep tropical tropospheric convection. Numerical simulations from the Specified‐Dynamics Whole Atmosphere Community Climate Model eXtended demonstrate a significant connection between these UFTW and the Madden‐Julian Oscillation (MJO). Compared to the boreal winter mean state, thermospheric UFTW amplitudes are larger (+5 to +12%) during MJO Phases 2-3 and smaller (-3% to −12%) during MJO Phases 6-8. Significant variations are also found with respect to the phase of the mesospheric semiannual oscillation (MSAO) and stratospheric quasi‐biannual oscillation (SQBO), with larger (±12-16%) thermospheric amplitudes during westward MSAO/SQBO phase and smaller (±3-6%) amplitudes during eastward MSAO/SQBO phase, in accordance with theoretical interpretations. This study suggests that UFTW may play a large role in coupling tropospheric intraseasonal variability to the thermosphere, raising important questions including implications for the whole atmosphere system.