Very low ozone episodes due to polar vortex displacement

The large-scale ozone distribution over the northern hemisphere undergoes strong fluctuations each winter on timescales of up to a few weeks. This is closely linked to changes in the stratospheric polar vortex, whose shape, intensity and location vary with time. Elliptical diagnostic parameters provide an empirical description of the daily character of the polar vortex. These parameters are used as an objective measure to define two characteristic wintertime vortex displacements, towards northern Europe and Canada, respectively. The large-scale structures in bath the stratosphere and troposphere and the 3D ozone structures are determined for both vortex displacement scenarios. A linear ozone transport model shows that the contribution of horizontal ozone advection dominates locally in the middle stratosphere. Nevertheless, the largest contribution is due to vertical advection around the ozone layer maximum. The findings are in agreement with an EOF analysis which reveals significant general modes of ozone variability linked to polar vortex displacement and to phase-shifted large-scale tropospheric waves. When baroclinic waves travel through the regions of vortex-related ozone reduction, the combined effect is to produce transient synoptic-scale areas of exceptionally low ozone; namely dynamically induced strong ozone mini-holes.