Ozone depletion at the edge of the Arctic polar vortex 1996/1997

In winter 1996/1997 the Arctic polar stratospheric vortex was extremely long-lived. During most of its lifetime the vortex was centered at the pole, and its edge was almost permanently located over Northern Scandinavia. The ozone lidar at the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR) was operated from mid-December 1996 until mid-May 1997 when the final breakup of the vortex occurred. Comparison of the measurements with three-dimensional model calculations reveals ozone depletion of up to 40% at the 475 and 550 K level. Maximum depletion occurred by around May 5 at levels up to 550 K and by around April 20 at 675 K. Analysis of the chemical model shows that while much of the early spring ozone depletion was due to halogen chemistry, associated with chlorine activation on polar stratospheric clouds, the ongoing depletion in late April and early May was due to "summertime" NOx (=NO + NO2) chemistry. The unusual persistence of the vortex, with the isolation of high-latitude air masses until early May, permitted the occurrence of this depletion. The observations of air masses with low O-3 show a very sudden end around May 10, indicating that even in the final phase of its existence, with continuously decreasing potential vorticity, some vortex air masses were well-confined and mixing with the surroundings was small. A possible explanation of the strong confinement of vortex air is the scarcity of disturbances due to wave activity from below the vortex. It is supported by the frequent observation of intravortex layering in spring 1997.