Abstract
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.