Assessing the Occurrence of Elevated Stratopause Events

The northern hemisphere polar winter exhibits significant dynamical variability that affects temperatures, winds, and atmospheric chemistry in the middle and upper atmosphere. Extreme events known as elevated stratopause (ES) events can occur during the winter after some sudden stratospheric warmings (SSWs). Atmospheric circulation changes during such events have been shown to enhance the transport of NO_x (NO + NO₂) generated via energetic particle precipitation (EPP) from the mesosphere and thermosphere into the stratosphere. ES events occur after the polar vortex has been disrupted to the point that it breaks down completely. During this time, the stratopause becomes indistinct for a number of days, after which it reforms at higher altitudes than before the warming, around 20 km higher than the seasonal climatological mean height. The ES is caused by adiabatic heating from increased descent near 80 km, which is induced by a strengthening of the upper branch of the residual circulation. This results in enhanced descent of EPP-NO_x from the mesosphere into the stratosphere, where it can then participate in catalytic ozone depletion. Many methodologies have been utilized to investigate ES events in observations and models, however identified years in the modern literature disagree significantly as a result of the use of different indicators. Here we investigate various low-altitude indicators in reanalyses (MERRA, MERRA-2, and ERA-40) as well as satellite observations (MLS and SABER) in order to resolve discrepancies in documented events, and assess the occurrence of ES events in the historical data record. A close assessment of MERRA and MERRA-2 is also undertaken during polar winter, and differences in the reanalyses that are important for the atmospheric coupling modulating the flux of EPP-NO_x are discussed.