Analysis of record-low ozone values during the 1997 winter over Lauder, New Zealand

Record-low ozone (O-3) column densities (with a minimum of 222 DU) were observed over the Lauder NDSC (Network for the Detection of Stratospheric Change) station (45 degrees S, 170 degrees E) in August 1997. Possible causes are examined using height-resolved O-3 measurements over Lauder, and high-resolution reverse trajectory maps of O-3 (initialised with Microwave Limb Sounder measurements) and of potential vorticity. The analysis shows that O-3 poor air originated from two regions: Below the 550 K isentrope (similar to 22 km) subtropical air was observed, while between 600 and 1000 K (similar to 25 - 33.5 km) the polar vortex tilted over Lauder for several days. A rapid recovery of the O-3 column density was observed later, due to an O-3 rich polar vortex filament moving over Lauder between 18 and 24 km, while simultaneously the O-3 poor higher vortex moved away.