The present paper examines the northern stratosphere during April 2020,
when the polar vortex split into two cyclonic vortices after a winter
with the strongest ozone depletion on record. We examine the dynamical
evolution that led to the split at middle and lower stratospheric levels
and the distribution of ozone between the main and off-spring vortices.
Finally, we look at the vertical structure of the split down to the
troposphere. The split event is analyzed with Lagrangian tools and an
Eulerian diagnostic of planetary wave activity. The findings confirm the
key role for the split played by a flow configuration with a polar
hyperbolic trajectory and associated manifolds. A trajectory analysis
illustrates how the ozone distribution between vortices was such that
ozone poor air remained in the main vortex. The off-spring vortex became
part of a deep structure from the troposphere, and connections with
tropospheric disturbances over Eurasia are suggested.