On the interpretation of midlatitude ozone changes

Abstract

The attribution of the Antarctic ozone hole to anthropogenic emissions of halogen species is well established. The same chemistry is operative in the Arctic, although is strongly modulated by year-to-year dynamical variability. In midlatitudes, stratospheric ozone has also been observed to have decreased over the past 20 or so years, but the attribution of these midlatitude ozone changes is far less clear than is that of the polar late-winter/spring depletion. Because of concurrent changes in Northern Hemisphere dynamical quantities such as stratospheric planetary-wave drag, tropopause height, and the Northern Annular Mode (NAM) index, some argue that it is the dynamical changes that have caused the ozone changes. (The dynamical changes are not sufficiently well known in the Southern Hemisphere to be discussed at this point.)

This talk (prefaced by a brief primer on stratospheric dynamics and ozone for the non-specialists!) will describe some of the fundamental issues involved, and the available evidence, for understanding the observed midlatitude ozone changes, including the key question of how to separate natural variability from anthropogenic influences. The stratospheric fingerprint of the Mount Pinatubo volcanic eruption will also be considered. Because natural variability and the dynamical response to volcanic eruptions, greenhouse gas changes, and ozone losses ALL seem to project onto the NAM, understanding the stratosphere-troposphere coupling apparent in the NAM appears to be a crucial key to the puzzle.