The frequency and size of ozone ''mini-hole'' events at northern midlatitudes in February

Ozone ''mini-holes'' represent regions of low total ozone (<300 D U) that persist for several days over extratropical latitudes in winter. These events are usually associated with breaking Rossby waves in the upper troposphere that allow poleward injections of ozone-poor, low potential vorticity (PV) tropospheric air into the lower stratosphere. Using daily TOMS total ozone data together with daily PV maps derived from reanalyzed NCEP data, the interannual variability of ozone mini-hole events in the northern hemisphere is investigated for one winter month (February) during the 1979-1993 period. For this purpose, the total area of anomalously low total ozone and PV between 40 degrees N and 50 degrees N and between 90 degrees W and 90 degrees E is computed for February of each year. The low-ozone and low-PV areas both peak during February of 1988, 1989, and 1990, coincident with the lowest monthly zonal mean total ozone values observed at northern midlatitudes prior to the Pinatubo eruption.