Missing Stratospheric Ozone Decrease at Southern Hemisphere Middle Latitudes after Mt. Pinatubo: A Dynamical Perspective

Although large total ozone decreases occurred in the Northern Hemisphere extratropics in the years after the volcanic eruption of Mt. Pinatubo that are generally attributed to the eruption, comparable decreases did not emerge in the Southern Hemisphere. To study this missing decrease, a multiple linear regression was applied to the Chemical and Dynamical Influences on Decadal Ozone Change (CANDIDOZ) Assimilated Three-Dimensional Ozone (CATO) dataset including the solar cycle, the quasi-biennial oscillation (QBO), the effect of volcanic eruptions, the lower stratospheric (LS) Eliassen–Palm (EP) flux to describe the Brewer–Dobson circulation, and stratospheric chlorine increase as explanatory variables. Volcanically induced chemical ozone depletion was overcompensated by the QBO and by a pronounced EP flux anomaly. Using NCEP–NCAR reanalysis data, it is found that the anomalous EP flux was caused by several significant stratospheric wave events (SWEs) from September–November 1991 through 1992 that, together with aerosol heating, led to a significantly enhanced Brewer–Dobson circulation and more ozone transport from the tropics to the extratropics. The onset of the volcanic ozone loss was shifted into 1992 and the strength of the signal was reduced. Most SWEs can be traced back to the troposphere and a significant fraction was associated with atmospheric blocking patterns preceding the SWEs. In 1991/92, the southern annular mode was in a negative phase and El Niño–Southern Oscillation in a warm phase. It is suggested that this constellation favored a flow preconditioning toward quasi-stationary features including blocking, which was significantly enhanced in 1991/92. During June–August 1992, blocking occurred preferably over the southeastern Pacific, pointing to a major ENSO influence on LS wave activity.