MLS observations of Arctic ozone loss in 1996-97

The Microwave Limb Sounder (MLS) observed ozone (O-3) loss in the Arctic vortex beginning in January 1997 at 585 K (similar to 25 hPa) and in February 1997 at 465 K (similar to 50 hPa). Minimum vortex-averaged O-3 mixing ratios observed in 1997 were higher than those in 1996, which were the lowest ever recorded by MLS. The vertical extent of O-3 loss and maximum local O-3 decreases were larger, but the decrease filled the vortex less completely, in 1997 than in 1996. Unusually low high-latitude column O-3 above 100 hPa in April 1997 resulted mainly from dynamical effects of the unusually persistent lower stratospheric vortex and winter-like temperature patterns. Column O-3 above 100 hPa averaged in comparable regions of the vortex showed a stronger decreasing trend in 1996-97 than in 1995-96, consistent with the larger vertical extent and maximum local values of lower stratospheric O-3 loss. Chemical O-3 loss resulted in an similar to 10% observed decrease in column O-3 between late January and early April 1997.