Diverse Surface Signatures of Stratospheric Polar Vortex Anomalies.

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 weak and strong vortex winters. Over Eurasia, relatively few weak vortex winters are associated with large‐scale cold conditions, suggesting that the strength of the observed cold signature could be inflated due to insufficient sampling. For both weak and strong vortex winters, the canonical temperature pattern in Eurasia only clearly arises when North Atlantic sea surface temperatures are in phase with the NAO. Over North America, while the main driver of interannual winter temperature variability is the El Niño–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 not be fully captured by the small observational sample size. Plain Language Summary: The strength of the winds in the stratosphere over the Arctic provides useful information for seasonal forecasts of wintertime weather over Europe and North America. When we study these linkages, it is a challenge that we have few winters—only about 40—with reliable observations from the stratosphere. Here, we use data from a seasonal forecast model to generate a large collection of 3,000 possible winters, and we use these to examine different patterns of surface temperature and sea level pressure for winters with the strongest and weakest winds in the polar stratosphere. Some real‐world episodes have attracted wide attention, including recent cold winters linked to weak stratospheric winds, and there seems to be an anticipation that weak winds in the stratosphere are synonymous with extremely cold weather in many regions. However, our results indicate that these expected surface signatures are in fact not particularly common. There are also scenarios when instead the opposite surface signature emerges. We find that it is not sufficient to know the state of the stratosphere; regional sea surface temperatures can either support or counteract the stratospheric influence on winter weather in any given year. Key Points: The broad spectrum of surface signatures of stratospheric polar vortex anomalies is obscured by the small observational sample sizeA large ensemble indicates the observed magnitude of negative North Atlantic Oscillation and related surface anomalies during weak vortex winters may be inflatedOver North America the main driver of winter temperature variability is El Niño–Southern Oscillation (ENSO), but the stratosphere can modulate ENSO teleconnections [ABSTRACT FROM AUTHOR]