The heterogeneous chemistry that ultimately leads to strastospheric ozone loss in winter and spring, occurs on the surface of the polar stratospheric clouds (PSC), forming in the cold winter stratosphere. A measure of the coldness of the winter stratosphere that is widely used in ozone research is the PSC volume, or PSC potential.

We examine the meteorological conditions in the troposphere, which lead to a cold polar stratosphere, a strong polar night jet and a high PSC potential in the northern hemisphere, using 40 years of re-analysed data. In addition to the well-known connection between the NAO, AO and the polar night jet, we emphasize the large influence of North Pacific anomalies on the PSC potential.

The boreal winter of 2004/05 had by the largest PSC potential ever. We will briefly discuss a new technique for the estimation of ozone loss in 2004/05 based on the assimilation of satellite observations by the Microwave Limb Sounder (MLS) instrument. The technique offers great promise as it better accounts for mixing and in-homogeneities within the vortex.