Tidal and Ultra-Fast Kelvin Wave Influences on Thermospheric Intra-Seasonal Variability as Revealed by ICON/TIE-GCM, Swarm-C, and ICON

Recent observational studies suggest that strong coupling between the troposphere and the thermosphere occurs at intra-seasonal (IS, ~30-90 days) timescales. While it is well known that the lower and middle latitude (+/- 40 deg.) thermosphere is dominated by modes of IS variability not related to external drivers, theoretical understanding of the fundamental processes at play is still lacking. This is largely due to the sparsity of satellite observation in the ‘thermospheric gap’ region between about 100 and 300 km altitude, which represents a significant obstacle in the quest for whole atmosphere predictability.

The Madden-Julian Oscillation (MJO) is the dominant source of IS variability in tropical tropospheric convection and circulation and can influence the generation and propagation characteristics of atmospheric waves, such as solar tides, gravity waves (GWs), and ultra-fast Kelvin waves (UFKWs). Rising evidence suggests that a subgroup of the MJO-modulated wave spectrum may be the leading driver of IS variability in the thermosphere.

In this work, we present results of wave diagnostics applied to Swarm-C and ICON data throughout 2020-2022 that reveal prominent IS variability in tidal and UFKW1 amplitudes not presented in external drivers (e.g., F10.7, Kp) but concurrent with similar OLR variations. Furthermore, we show output from an Ionospheric Connection Explorer (ICON)-adapted version of the Thermosphere Ionosphere Electrodynamics General Circulation Model (TIE-GCM). Tidal structures at the lower boundary of the model near 97 km are obtained from ICON-MIGHTI observations using Hough Mode Extensions (HMEs), wherein ICON-MIGHTI winds and temperature from 94 km to 102 km in the latitude range of 10°S-40°N are used to fit the HMEs in 35-day running windows needed to cover 24-hours of local time. Three ICON/TIEGCM cases are examined: (a) with HMEs, (b) without HMEs (GSWM tidal LB specification), and (c) with HMEs but constant solar-geomagnetic forcing. Important modeling-based conclusions are made about the contributions of the upward-propagating tidal spectrum on the thermospheric IS variability.