Very low ozone episodes due to polar vortex displacement

The large-scale ozone distribution over the northern hemisphere undergoes s trong fluctuations each winter on timescales of up to a few weeks. This is closely linked to changes in the stratospheric polar vortex, whose shape, i ntensity and location vary with time. Elliptical diagnostic parameters prov ide an empirical description of the daily character of the polar vortex. Th ese parameters are used as an objective measure to define two characteristi c wintertime vortex displacements, towards northern Europe and Canada, resp ectively. The large-scale structures in bath the stratosphere and troposphe re and the 3D ozone structures are determined for both vortex displacement scenarios. A linear ozone transport model shows that the contribution of ho rizontal ozone advection dominates locally in the middle stratosphere. Neve rtheless, the largest contribution is due to vertical advection around the ozone layer maximum. The findings are in agreement with an EOF analysis whi ch reveals significant general modes of ozone variability linked to polar v ortex displacement and to phase-shifted large-scale tropospheric waves. Whe n baroclinic waves travel through the regions of vortex-related ozone reduc tion, the combined effect is to produce transient synoptic-scale areas of e xceptionally low ozone; namely dynamically induced strong ozone mini-holes.