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

Abstract

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 (PWs), gravity waves, and atmospheric tides in driving the mean meridional circulation (MMC) in the lower thermosphere (LT) 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 LT MMC 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 PWs, 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.

Influence of Stratosphere Polar Vortex Variability on the Mesosphere, Thermosphere, and Ionosphere

“…During weak stratosphere polar vortex time periods, the residual meridional circulation in the Northern Hemisphere has southward anomalies between 10 −1 and 10 −2 hPa (65–80 km) and northward anomalies at higher altitudes (10 −3 to 10 −5 hPa, 95–120 km). The spatial structure and the magnitude of the anomalies are consistent with the anomalies that occur during SSWs (Orsolini et al., 2022). The anomalies stretch into the Southern Hemisphere with decreasing amplitude and the boundary between southward and northward anomalies increasing in altitude from the Northern to Southern Hemispheres.…”

“…The changes in wave forcing during SSWs, including from gravity waves, planetary waves, and tides, alter the mean circulation of the MLT. During SSWs, the MLT circulation is characterized by enhanced clockwise and anti‐clockwise circulation patterns in the Northern and Southern Hemispheres, respectively (Limpasuvan et al., 2016; Miyoshi et al., 2015; Orsolini et al., 2022).…”

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

“…Previous modeling studies (e.g., Miyoshi et al, 2015;Orsolini et al, 2022;Wang et al, 2022;J. Zhang et al, 2021 suggest that the climatological direction of the mesosphere-lower thermosphere (MLT) mean meridional circulation (MMC), characterized by upwelling in the middle winter latitudes, equatorward flow near ∼100-120 km, and poleward flow near ∼80-100 km, reverses following the onset of SSWs.…”

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

“…Previous modeling studies (e.g., Miyoshi et al, 2015;Zhang J. et al, 2021Zhang J. et al, , 2022Orsolini et al, 2022;Wang et al, 2022) suggest that the climatological direction of the mesosphere-lower thermosphere (MLT) mean meridional circulation (MMC), characterized by upwelling in the middle winter latitudes, equatorward flow near ∼100-120 km, and poleward flow near ∼80-100 km, reverses following the onset of SSWs. Ground-totopside model of Atmosphere and Ionosphere for Aeronomy (GAIA) model simulations from Miyoshi et al (2015) revealed that MMC reversed in the lower thermosphere.…”

“…Ground-totopside model of Atmosphere and Ionosphere for Aeronomy (GAIA) model simulations from Miyoshi et al (2015) revealed that MMC reversed in the lower thermosphere. More recently, Orsolini et al (2022) used the Whole Atmosphere Community Climate Model with thermosphere and ionosphere eXtension (WACCM-X) in the specified dynamics configuration to show that the lower thermospheric mean meridional circulation reverses for about 10 days following the onset of elevated stratopause events in the northern hemisphere largely driven by westward-propagating planetary waves. Important impacts on MLT mean circulation have also been reported observationally (e.g., Oberheide, 2022) including enhancements of both eastward and westward directed flow associated with wave dissipation.…”