The morphology and evolution of the stratospheric ozone (O3) distribution at high latitudes in the Northern Hemisphere (NH) are examined for the late summer and fall seasons of 1999. This time period sets the O3 initial condition for the SOLVE/THESEO field mission performed during winter 1999/2000. In situ and satellite data are used along with a three-dimensional model of chemistry and transport (CTM) to determine the key processes that control the distribution of O3 in the lower-to-middle stratosphere. O3 in the vortex at the beginning of the winter season in late November is found to be nearly constant from 500 to above 800 K with a value at 3 ppmv ¿ ~10%. Values outside the vortex above 550 K are up to a factor of 2 higher and increase significantly with potential temperature. The seasonal time series of data from POAM shows that the relatively low O3 mixing ratios, which characterize the vortex in late November, are already present at high latitudes at the end of summer in mid-September before the vortex circulation sets up. Analysis of the CTM output shows that the minimum O3 and increase in variance in the middle stratosphere in late summer are the result of (1) stirring of polar concentric O3 gradients by nascent wave-driven transport and (2) an acceleration of net photochemical loss with decreasing solar illumination. The segregation of low O3 mixing ratios into the vortex as the circulation strengthens through the fall suggests a possible feedback role between O3 chemistry and the vortex formation dynamics that may need to be better understood in order to make confident predictions of the recovery of NH O3 at high latitudes.