Decrease of total ozone at low latitudes in the southern hemisphere by a combination of linear and nonlinear processes

Detailed analysis of the behaviour of the edge of the Antarctic polar vorte x shows that it has to be considered as a region of finite latitudinal exte nt rather than as an infinitely narrow boundary. A new definition of the ex ternal boundary of the vortex edge allows the study of events where the edg e becomes strongly dilated towards lower, even subtropical, latitudes. Such events have been observed by Argentinian subtropical ultra-violet measurem ent stations and can also be found in the Total Ozone Mapping Spectrometer total-ozone observations. The dilation of the vortex edge is due to a linea r deformation of isentropic surfaces, but eventually leads to irreversible nonlinear mixing at low latitudes in the surf zone, where the horizontal gr adients in Ertel potential vorticity are small. The dilation of the vortex edge in the studied events is demonstrated to be due to the action of planetary waves. Planetary waves not only affect the low-latitude ozone by causing vortex-edge dilation events, but may also by themselves lower the ozone column locally, as they adiabatically decrease t he isentropic density in the ozone layer in some regions (and increase it i n other regions).