A very deep ozone minihole in the Northern Hemisphere stratosphere at mid-latitudes during the winter of 2000

Ozone miniholes appear on total ozone maps as localized ozone minima with horizontal extentsof a few hundreds of kilometres. They are characterized by a rapid and small-scale appearanceof a columnar ozone decrease with an equally rapid recovery after a few days. They are frequentlyobserved at Northern Hemisphere mid-latitudes in winter. Evolving too rapidly to be the resultof an ozone chemical destruction, miniholes should be the result of meteorological processes.According to some authors, miniholes should be due to the northeast motions of air patcheswith low total ozone content. However, several studies attribute the formation of ozone miniholesto the uplift of air masses, which decreases the ozone columnar content by simply decreasingthe pressure thickness of the ozone layer, without changing the mixing ratio. According tothese studies, the latter mechanism explains the main reduction of ozone that occurs betweenthe tropopause and the ozone maximum during an ozone minihole event. A region of extremelow ozone values passed over Europe from 27 to 30 November 2000. The total ozone valueswere measured with the Total Ozone Mapping Spectrometer (TOMS). A radio sounding, launched on 29 November 2000 from Payerne at the place and time of the deepening of theminihole, allows us to perform a detailed analysis of its formation mechanism. It is shown thatthe uplift of isentropic surfaces plays an important role in the columnar ozone decrease andexplains the lower part of the depleted ozone profile. However, the deepening of the miniholeis explained by another mechanism: namely, at this time the minihole air column intersects thepolar vortex at high altitudes and then encounters ozone-poor air masses.