Abrupt Change in the Lower Thermospheric Mean Meridional Circulation during Sudden Stratospheric Warmings and its Impact on Trace Species

Abstract

<p><span>Based on the hourly output from the 2000–2014 simulations of the National Center for Atmospheric Research’s vertically extended version of the Whole Atmosphere Community Climate Model in specified dynamics configuration, we examine the roles of planetary waves, gravity waves and atmospheric tides in driving the mean meridional circulation in the lower thermosphere and its response to the sudden stratospheric warming phenomenon with an elevated stratopause in the northern hemisphere. Sandwiched between the two summer-to-winter overturning circulations in the mesosphere and the upper thermosphere, the climatological lower thermosphere mean meridional circulation is a narrow gyre that is characterized by upwelling in the middle winter latitudes, equatorward flow near 120 km, and downwelling in the middle and high summer latitudes. Following the onset of the sudden stratospheric warmings, this gyre reverses its climatological direction, resulting in a “chimney-like” feature of un-interrupted polar descent from the altitude of 150 km down to the upper mesosphere. This reversal is driven by the westward-propagating planetary waves, which exert a brief but significant westward forcing between 70 and 125 km, exceeding gravity wave and tidal forcings in that altitude range. The attendant polar descent potentially leads to a short-lived enhanced transport of nitric oxide into the mesosphere (with excess in the order of 1. parts per million), while carbon dioxide is decreased.</span></p>

Direct Observational Evidence of Altered Mesosphere Lower Thermosphere Mean Circulation From a Major Sudden Stratospheric Warming

“…Effects associated with SW1 and SW3 are found to play a smaller role, in contrast with the discussion by R. Zhang et al. (2022). Spectral analyses also reveal the presence of a ∼3‐day UFKW1 with enhanced amplitudes (∼15 m/s) near 106 km occurring during 10–20 January and 0°–10°N that closely resemble the SW2 amplifications near 93 km.…”

“…R. Zhang et al. (2022) used Ionospheric Connection Explorer (ICON) Michelson Interferometer for Global High‐resolution Thermospheric Imaging (MIGHTI) and Ion Velocity Meter (IVM) data to investigate IT coupling during the January 2021 SSW, suggesting the importance of the F‐region northward wind changes in driving anomalous ionospheric field‐aligned flows. Laskar and Pallamraju (2014) presented evidence of equatorward winds near 95 km at high latitudes using Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics Doppler Interferometer (TIMED/TIDI) wind measurements and proposed that a new cell of meridional circulation in the MLT is set up during SSWs.…”