Multi‐Step Vertical Coupling During the January 2017 Sudden Stratospheric Warming.

This study analyzes a simulation of the Arctic winter 2016–2017 with focus on multi‐step vertical coupling (MSVC) by primary, secondary, and higher‐order gravity waves (GWs). We employ the HIgh Altitude Mechanistic general Circulation Model with nudging of the large scales to MERRA‐2 reanalysis. Simulation results confirm the well‐known effects from primary GWs in the winter middle atmosphere regarding strong westward GW drag and a warm winter polar stratopause during the strong‐vortex period in late December 2016, as well as weak eastward GW drag and mesospheric cooling during the sudden stratospheric warming (SSW) in late January and early February 2017. Since the amplitudes of the primary GWs that dissipate in the middle atmosphere are weaker for a reversed or weakened polar vortex, the theory for secondary GW generation predicts reduced MSVC in this case. This is confirmed by strongly reduced secondary and higher‐order GW amplitudes during the SSW and the weak‐vortex period in February. The wintertime higher‐order GWs show partial concentric ring structures above their sources in the lower thermosphere. We find that mostly those higher‐order GWs propagate to higher altitudes that have horizontal propagation directions against the tidal winds. The simulated GWs at 300 km height and observed perturbations of total electron content over Europe and North America during selected days of low geomagnetic activity show very good agreement regarding (a) the wave characteristics and (b) the reduction of amplitudes during the SSW and early February as compared to late December. Plain Language Summary: We analyze the Arctic winter 2016–2017 using a whole atmosphere model with resolved gravity waves (GWs) and nudging of the large scales to MERRA‐2 reanalysis. Model results confirm the well‐known effects from primary GWs in the winter middle atmosphere regarding strong westward GW drag and a warm winter polar stratopause during the strong‐vortex period in late December 2016, as well as weak eastward GW drag and mesospheric cooling during the sudden stratospheric warming (SSW), which started in late January 2017 and was followed by a weak vortex in February. Since secondary and higher‐order GWs are the predominant GWs in the winter mesopause region and thermosphere, the vertical coupling from the lower to the upper atmosphere by GWs is termed multi‐step vertical coupling (MSVC). We find that MSVC is strongly reduced during and after the SSW. The simulated GWs at 300 km height and observed total electron content (TEC) perturbations over Europe and North America during days of low geomagnetic activity show very good agreement regarding the wave characteristics. Furthermore, we find a clear reduction in the TEC perturbation amplitudes during and after the SSW. This agreement between model and observations strongly suggests that quiet‐time TEC perturbations in the winter thermosphere are caused by GWs from MSVC. Key Points: Wintertime thermospheric gravity waves (GWs) simulated by a whole atmosphere model are strongly reduced during the January 2017 sudden stratospheric warming (SSW) and the subsequent weak‐vortex periodObserved quiet‐time total electron content (TEC) perturbations show reduced amplitudes during the SSW and weak‐vortex period as wellPropagation directions, wavelengths, and periods of the simulated thermospheric GWs agree with observed quiet‐time TEC perturbations [ABSTRACT FROM AUTHOR]