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\begin{document}

\title{Potential links between tropospheric and stratospheric circulation
extremes during early 2020}



\author[1]{Philip Rupp}%
\author[2]{Sheena Loeffel}%
\author[2]{Hella Garny}%
\author[3]{Xiaoyang Chen}%
\author[3]{Joaquim G Pinto}%
\author[1]{Thomas Birner}%
\affil[1]{Ludwig-Maximilians-University Munich}%
\affil[2]{Deutsches Zentrum für Luft- und Raumfahrt}%
\affil[3]{Karlsruhe Institute of Technology}%


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  \date{May 22, 2024}


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\begin{abstract}
February-March 2020 was marked by highly anomalous large-scale
circulations in the Northern extratropical troposphere and stratosphere.
The Atlantic jet reached extreme strength, linked to some of the
strongest and most persistent positive values of the Arctic Oscillation
index on record, which provided conditions for extreme windstorms
hitting Europe. Likewise, the stratospheric polar vortex reached extreme
strength that persisted for an unusually long period. Past research
indicated that such circulation extremes occurring throughout the
troposphere-stratosphere system are dynamically coupled, although the
nature of this coupling is still not fully understood and generally
difficult to quantify.\\
We employ sets of numerical ensemble simulations to provide statistical
characterizations of the mutual coupling of the early 2020 circulation
extremes. We find an overall robust coupling between tropospheric and
stratospheric anomalies: ensemble members with polar vortex exceeding a
certain strength tend to exhibit a stronger tropospheric jet and vice
versa. Moreover, members exhibiting a breakdown of the stratospheric
circulation (associated with a sudden stratospheric warming) tend to
lack periods of persistently enhanced tropospheric circulation. A
crucial component in determining the stratospheric vortex strength
appears to be the vortex geometry, corresponding to states that promote
either reflection or dissipation of planetary waves. Despite these lines
of evidence for vertical coupling, our simulations underline the role of
internal variability within each atmospheric layer. The circulation
extremes during early 2020 may be viewed as resulting from a fortuitous
alignment of dynamical evolutions within the troposphere and
stratosphere, aided by each layer's modification of the other layer's
boundary condition.%
\end{abstract}%



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