major atmospheric flow phenomena. An SSW event severely disrupts the
stratospheric polar night jet, as the polar vortex is either displaced
equatorwards and sheared into a comma shape (a displacement SSW) or
torn into two individual vortices (a splitting SSW). Similarly,
blocking events severely disrupt the extra-tropical circumpolar
tropopause-level jet, which is either displaced poleward of or splits
around the block's core of anomalously low potential vorticity at
Here we focus on the dynamical linkage between to two phenomena by
examining their multiple co-occurrence over a climatological time
period, and analyzing specific events. Specifically, we find clear
evidence that blocking events are precursors to SSW events.
Spatial frequency composites of atmospheric blocking are constructed
for the ten-day period preceding splitting and displacement SSW
events, as objectively identified in the ERA-40 data set in an earlier
study. Distinct differences in the location and amplitude of the
blocking frequency distributions are found. Displacement events are
preferentially preceded by blocking in the Atlantic basin, whereas
splitting events are preceded by blocking events occurring in the
Pacific basin or both basins concomitantly. These differences in the
blocking distribution are mirrored in significantly different
planetary wave patterns prior to the warming events.
Analysis of individual events supports the idea of tropospheric
blocking as a key precursor to SSW events, and sheds further light on
the relationship between the two phenomena.