Systematic Changes of Northern Hemisphere Ozone and Their Relationship to Random Interannual Changes

Abstract

Northern Hemisphere ozone underwent a monotonic decline during the 1980s and 1990s. Systematic changes associated with that trend are shown to have a close relationship to random changes of ozone. These two components of interannual variability share a common structure. In it, ozone changes at high latitude are compensated at low latitude by changes of opposite sign. The out-of-phase relationship between ozone changes at high and low latitudes is consistent with a change of the residual mean circulation of the stratosphere, and so is the seasonality of systematic changes. Compensating trends at high and low latitudes amplify simultaneously?during winter, when the polar-night vortex is disturbed by planetary waves that force residual motion. Analogous relationships are obeyed by Northern Hemisphere temperature. The strong resemblance between systematic and random changes of Northern Hemisphere ozone implies that a major portion of its decline during the 1980s and 1990s involved a systematic weakening of the residual circulation. Anomalous forcing of the residual circulation is strongly correlated to random changes of ozone, which in turn have the same structure as systematic changes. The magnitude and structure of the ozone trend are broadly consistent with the climate sensitivity of ozone with respect to a change of the residual circulation. Derived from random changes over a large population of winters, the climate sensitivity implies an ozone trend quite similar to the observed trend, but with about two-thirds of its magnitude. When account is taken of both the anomalous residual circulation and anomalous photochemistry, the climate sensitivity of ozone reproduces the major structure as well as the magnitude of the observed trend.