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

Abstract

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 Niñ o-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.

Total ozone trends from 1979 to 2016 derived from five merged observational datasets – the emergence into ozone recovery

“…Globally, the post-ODS peak trends vary generally between near-zero trends (NH extratropics) and positive trends of +0.7 % decade −1 (SH extratropics) with a statistical trend uncertainty of about 0.7 % decade −1 (2σ ) after 20 years of observations. We may therefore conclude that we are about to emerge into the phase of ozone recovery as is also shown by chemistry-climate and chemistry-transport models (e.g., Eyring et al, 2010;Shepherd et al, 2014;Solomon et al, 2016;Chipperfield et al, 2017). Both the regression applied to datasets (e.g., in our study) and models capture the dynamical variability well and their results are consistent.…”

“…Significant ozone depletion was also observed in the NH following the El Chichón major volcanic eruption in 1982 (e.g., Hofmann and Solomon, 1989). A positive ozone response to the El Chichón is evident in the SH middle latitudes, most likely due to the specific circulation changes induced by this volcanic event (Schnadt Poberaj et al, 2011;Aquila et al, 2013;Dhomse et al, 2015). This is also believed to have caused an initial extratropical increase in SH extratropical total ozone during the first 6 months following the Pinatubo eruption.…”

“…While Mt. Pinatubo led to enhanced ozone depletion, the Southern Hemisphere (SH) extratropical total ozone rather increased as a result of a particular dynamics condition following the El Chichón event (Schnadt Poberaj et al, 2011;Aquila et al, 2013;Dhomse et al, 2015). For El Chichón, the stratospheric aerosol optical depth (SAOD) at 550 nm from Sato et al (1993) is used as the explanatory variable, while newer data from the WACCM model are used for the period after 1990 that is dominated by the Mt.…”

Total ozone trends from 1979 to 2016 derived from five merged observational datasets – the emergence into ozone recovery

“…Hofmann and Solomon, 1989). A positive ozone response to the El Chichón is evident in the SH middle latitudes, most likely due to the specific circulation changes induced by this volcanic event (Schnadt Poberaj et al, 2011;Aquila et al, 2013). This is also believed to have caused an initial extratropical increase in SH extratropical total ozone during the first six months following the Pinatubo eruption.…”

Long-term changes in the upper stratospheric ozone at Syowa, Antarctica

“…Recent publications suggest that ozone depletion due to chemical reactions on volcanic aerosol surfaces was compensated by the QBO and EP-flux enhanced BDC transport, such that the effect of Pinatubo's eruption was not observed in the Southern Hemisphere until 1992 (Poberaj et al, 2011;Aquila et al, 2013). Note, the use of AOD proxy for analysis of Syowa ozone time series is not intended to assess chemical or dynamical effects of aerosols on stratospheric ozone.…”