Abstract
The term “polar vortex” remained largely a technical term until early
January 2014 when the United States media used it to describe an
historical cold air outbreak in eastern North America. Since then,
“polar vortex” has been used more frequently by the media and public,
often conflating circulation features and temperatures near the surface
with only partially related features at the tropopause and in the
stratosphere. The polar vortex in its most common scientific usage
refers to a hemispheric-scale stratospheric circulation over the Arctic
that is present during the Northern Hermisphere cold season. Reversal of
the zonal-mean zonal winds circumnavigating the stratospheric polar
vortex (SPV), termed major sudden stratospheric warmings (SSWs) can be
linked to mid-latitude cold air outbreaks. However, this mechanism does
not explain the cold US winter of 2013/14. This study revisits the
winter of 2013/14 to understand how SPV variability may still have
played a role in the severe winter weather. Observations indicate that
anomalously strong vertical wave propagation occurred throughout the
winter and disrupted, but did not fully break, the SPV. Instead,
vertically propagating waves were reflected back downward, building a
blocking high near Alaska and downstream troughing across central North
America, a classic signature for extreme cold air outbreaks across
central and eastern North America. Thus, the association of the term
“polar vortex” with winter 2013/14, while not justified by the most
common usage of the term, serves as a case study of the wave-reflection
mechanism of stratospheric polar vortex influence on mid-latitude
weather.