The formation and development of abnormally low total ozone events (LOEs), which typically last an average of 3--4 days, are analyzed to determine conditions under which these events may form. Wavelet analysis is performed on 13 years of daily total column ozone (TOZ) obtained by the Nimbus-7 Total Ozone Mapping Spectrometer each day of the record. With wavelet analysis, the contributions and relative phases of signal components may be ascertained at specific times of interest. Timescales relevant during LOEs are identified and the contributions and phase relations of all components present in the TOZ signal are compared for common patterns and features. For a location in Sweden, the formation of 96% of the 52 LOEs studied required that all components with timescales (&tgr;) less than a maximum value (&tgr;max) were present in their negative phases. This suggests that LOEs result from the simultaneous interaction of TOZ-depleting processes. The number of interacting processes varies with season and location. In addition, two criteria are seen to prevail when LOEs form in this location. First, background conditions favorable for the development of an event exist; these occur when TOZ is in a spectral state where all components with Δ&tgr;i<&tgr;<&tgr;max are in their negative phases. Second, a process takes place that can force the remaining short-timescale components (&tgr;<Δ&tgr;i) into their negative phases. Processes associated with ''initiating'' timescales (Δ&tgr;i) of ≈2--6 days are found to be a key factor in the initiation of LOEs in this location. Although the background conditions are not uncommon, the likelihood that both criteria are simultaneously met is low and the occurrence of LOEs is indeed rare (averaging 4.7 events per year from 1980--1990). ¿ American Geophysical Union 1996