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°S, 170°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 (∼22 km) subtropical air was observed, while between 600 and 1000 K (∼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.