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Diverse surface signatures of stratospheric polar vortex anomalies
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  • Erik W. Kolstad,
  • Simon Haydn Lee,
  • Amy Hawes Butler,
  • Daniela I.V. Domeisen,
  • Christoph Ole Wilhelm Wulff
Erik W. Kolstad
NORCE Norwegian Research Center

Corresponding Author:[email protected]

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Simon Haydn Lee
Columbia University
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Amy Hawes Butler
NOAA Chemical Sciences Laboratory
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Daniela I.V. Domeisen
ETH Zurich
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Christoph Ole Wilhelm Wulff
NORCE Norwegian Research Centre AS
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The Arctic stratospheric polar vortex is an important driver of winter weather and climate variability and predictability in North America and Eurasia, with a downward influence that on average projects onto the North Atlantic Oscillation (NAO). While tropospheric circulation anomalies accompanying anomalous vortex states display substantial case-by-case variability, understanding the full diversity of the surface signatures requires larger sample sizes than those available from reanalyses. Here, we first show that a large ensemble of seasonal hindcasts realistically reproduces the observed average surface signatures for weak and strong vortex winters and produces sufficient spread for single ensemble members to be considered as alternative realizations. We then use the ensemble to analyze the diversity of surface signatures during the 25% weakest and strongest vortex winters. Over Eurasia, only one of three weak vortex clusters yields continent-wide cold conditions, suggesting that the observed Eurasian cold signature could be artificially strong due to insufficient sampling. For both weak and strong vortex cases, the canonical temperature pattern in Eurasia only clearly arises when North Atlantic sea surface temperatures exhibit the tripolar structure in-phase with the NAO. Over North America, while the main driver of interannual winter temperature variability is the El Nino;Southern Oscillation (ENSO), the stratosphere can modulate ENSO teleconnections, affecting temperature and circulation anomalies over North America and downstream. These findings confirm that anomalous vortex states are associated with a broad spectrum of surface climate anomalies on the seasonal scale, which may be obscured by the small observational sample size.