One of the most significant events in the evolution
of the ozone layer over southern mid-latitudes since the
late 1970s was the large decrease observed in 1985. This
event remains unexplained and a detailed investigation of
the mechanisms responsible for the event has not previously
been undertaken. In this study, the 1985 Southern Hemisphere
mid-latitude total column ozone anomaly is analyzed
in detail based on observed daily total column ozone fields,
stratospheric dynamical fields, and calculated diagnostics of
stratospheric mixing. The 1985 anomaly appears to result
from a combination of (i) an anomaly in the meridional circulation
resulting from the westerly phase of the equatorial
quasi-biennial oscillation (QBO), (ii) weaker transport
of ozone from its tropical mid-stratosphere source across the
sub-tropical barrier to mid-latitudes related to the particular
phasing of the QBO with respect to the annual cycle,
and (iii) a solar cycle induced reduction in ozone. Similar
QBO and solar cycle influences prevailed in 1997 and 2006
when again total column ozone was found to be suppressed
over southern mid-latitudes. The results based on observations
are compared and contrasted with analyses of ozone
and dynamical fields from the ECHAM4.L39(DLR)/CHEM
coupled chemistry-climate model (hereafter referred to as
E39C). Equatorial winds in the E39C model are nudged towards
observed winds between 10o S and 10o N and the ability
of this model to produce an ozone anomaly in 1985, similar
to that observed, confirms the role of the QBO in effecting
the anomaly