Impact of the Polar Vortex on Sub‐Seasonal O/N2 Variability in the Lower Thermosphere Using GOLD and WACCM‐X.

We provide observational evidence that the stability of the stratospheric Polar vortex (PV) is a significant driver of sub‐seasonal variability in the thermosphere during geomagnetically quiet times when the PV is anomalously strong or weak. We find strong positive correlations between the Northern Annular Mode (NAM) index and subseasonal (10–90 days) Global Observations of the Limb and Disk (GOLD) O/N2 perturbations at low to mid‐northern latitudes, with a largest value of +0.55 at ∼30.0°N when anomalously strong or weak (NAM >2.5 or < −2.1) vortex times are considered. Strong agreement for O/N2 variability and O/N2‐NAM correlations is found between GOLD observations and the Whole Atmosphere Community Climate Model with thermosphere‐ionosphere eXtension (WACCM‐X) simulations, which is then used to delineate the global distribution of O/N2‐NAM correlations. We find negative correlations between subseasonal variability in WACCM‐X O/N2 and NAM at high northern and southern latitudes (as large as −0.54 at ∼60.0°S during anomalous vortex times). These correlations suggest that PV driven upwelling at low latitudes is accompanied by corresponding downwelling at high latitudes in the lower thermosphere (∼80–120 km), which is confirmed using calculations of residual mean meridional circulation from WACCM‐X. Plain Language Summary: The stratospheric Polar vortex (PV) is a large‐scale circulation pattern that forms over the poles during the winter months, characterized by strong winds circulating in a counterclockwise direction in the Northern Hemisphere. It has previously been demonstrated that the breakup of the PV during sudden stratospheric warming (SSW) events has a large effect on the composition of the thermosphere. We demonstrate that the PV also influences thermospheric composition during non‐SSW times, including time periods in which the vortex is strong and times in which the vortex is inactive, by correlating observations of thermospheric O/N2 from the Global Observations of the Limb and Disk (GOLD) instrument with the Northern Annular Mode (NAM) index, which tracks the strength of the PV. We also find strong anti‐correlations between O/N2 and tidal amplitudes from model simulations, which suggests that the PV influences thermospheric composition via changes in mean circulation induced by the dissipation of enhanced waves. We use model simulations to confirm that the NAM‐O/N2 correlations reflect PV induced changes in global circulation patterns. We find that the PV causes enhanced upwelling at low latitudes and corresponding downwelling at high latitudes in both the northern and southern hemispheres. Key Points: Polar vortex (PV) activity is correlated with sub‐seasonal O/N2 variability during low Kp winter times at all latitudesWe observe enhanced O/N2 at high southern latitudes and reduced O/N2 at low latitudes in response to northern hemisphere PV activityPV‐O/N2 correlations are shown to reflect PV induced modulations of the residual mean meridional circulation (MMC) [ABSTRACT FROM AUTHOR]