Formation of a Mesospheric Inversion Layer and the Subsequent Elevated Stratopause Associated With the Major Stratospheric Sudden Warming in 2018/19

Since 2004, following prolonged stratospheric sudden warming (SSW) events, it has been observed that the stratopause disappeared and reformed at a higher altitude, forming an elevated stratopause (ES). The relative roles of atmospheric waves in the mechanism of ES formation are still not fully understood. We performed a hindcast of the 2018/19 SSW event using a gravity‐wave (GW) permitting general circulation model that resolves the mesosphere and lower thermosphere (MLT) and analyzed dynamical phenomena throughout the entire middle atmosphere. An ES formed after the major warming on January 1, 2019. There was a marked temperature maximum in the polar upper mesosphere around December 28, 2018 prior to the disappearance of the descending stratopause associated with the SSW. This temperature structure is referred to as a mesospheric inversion layer (MIL). We show that adiabatic heating from the residual circulation driven by GW forcing (GWF) causes barotropic and/or baroclinic instability before the MIL formation, causing in situ generation of planetary waves (PWs). These PWs propagate into the MLT and exert negative (westward) forcing, which contributes to the MIL formation. Both GWF and PW forcing (PWF) above the recovered eastward jet play crucial roles in ES formation. The altitude of the recovered eastward jet, which regulates GWF and PWF height, is likely affected by the MIL structure. Simple vertical propagation from the lower atmosphere is insufficient to explain the presence of the GWs observed in this event. Plain Language Summary A stratospheric sudden warming (SSW), a rapid and strong warming in the winter polar stratosphere, occurred in January 2019. Following this event, the stratopause disappeared and reformed at a much higher altitude. This phenomenon is called an elevated stratopause (ES), whose formation mechanism is not fully understood. In this study, we conducted hindcast simulations of this SSW event using a high‐resolution high‐top general circulation model. Prior to the SSW onset, a marked temperature maximum characterized as a mesospheric inversion layer (MIL) appeared in the polar upper mesosphere. We examined the formation mechanisms of the ES and MIL quantitatively and three‐dimensionally. The results shows that wave forcing of planetary‐scale waves (PWs) caused the ES formation. The MIL is likely caused by both PWs and gravity waves (GWs) which are small‐scale waves. In addition, it is inferred that these PWs responsible for the