A climatology of ozone mini-holes over the northern hemisphere

A comprehensive climatology of ozone mini-hole occurrence over mid-latitudes of the northern hemisphere is constructed, based on daily measurements of total column ozone from the Nimbus-7 TOMS (total ozone mapping spectrometer) satellite, using version-7 data, over a 14-year period. An ozone mini-hole is a synoptic-scale area of strongly reduced total column ozone amount, which undergoes a cycle of growth and decay, in direct association with tropospheric weather systems. A computer-based analytical method for searching for local minima in the daily TOMS ozone fields is employed to build up a catalogue of mini-hole events, tracking location and intensity. Geographical and seasonal variations in the resulting data are summarised and possible trends over the data period are assessed. Ozone mini-hole activity exhibits a strong annual cycle, peaking in the late winter. Mini-holes are shown to be generally more frequent over the N. Atlantic/European sector than the N. Pacific/N. American sector. An alternative method of defining a threshold is also discussed in which ozone minima are viewed as anomalies from local climatological mean ozone levels. Mini-hole activity is seen to exhibit a close spatial correlation to each of the storm-track regions of the hemisphere.