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) wer e observed over the Lauder NDSC (Network for the Detection of Stratosp heric Change) station (45 degrees S, 170 degrees E) in August 1997. Po ssible causes are examined using height-resolved O-3 measurements over Lauder, and high-resolution reverse trajectory maps of O-3 (initialis ed with Microwave Limb Sounder measurements) and of potential vorticit y. The analysis shows that O-3 poor air originated from two regions: B elow the 550 K isentrope (similar to 22 km) subtropical air was observ ed, while between 600 and 1000 K (similar to 25 - 33.5 km) the polar v ortex 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 fi lament moving over Lauder between 18 and 24 km, while simultaneously t he O-3 poor higher vortex moved away.