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\begin{document}

\title{Formation of a mesospheric inversion layer and the subsequent elevated
stratopause associated with the major stratospheric sudden warming in
2018/19}



\author[1,1,1]{Haruka Okui}%
\author[2,2,2]{Kaoru Sato}%
\author[1,1,1]{Dai Koshin}%
\author[3,3,3]{Shingo Watanabe}%
\affil[1]{The University of Tokyo}%
\affil[2]{University of Tokyo}%
\affil[3]{Japan Agency for Marine-Earth Science and Technology}%


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  \date{May 17, 2024}


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\begin{abstract}
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 1 January 2019. There was a marked
temperature maximum in the polar upper mesosphere around 28 December
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.%
\end{abstract}%



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