Dynamical component of seasonal and year-to-year changes in Antarctic and global ozone

Abstract

Using a zonally averaged model in which all transport fields are fixed by input temperature data, we study dynamical aspects of the Antarctic ozone hole problem as a response to the observed year-to-year temperature change in August, September, October, and November. For the period studied, from 1979 to 1985, October column ozone density averaged over longitude is observed to decrease by 125 DU at 85°S. Our model produces 100 DU of decline from temperature change alone. At 60°S the model ozone decreases from 375 DU in 1979 to 325 DU in 1985, while the observed zonal mean column ozone declined from 410 DU in 1979 to 325 DU in 1985. The quasi-biennial signal in the year-to-year variations in column ozone is well-produced by the model with low (high) ozone correlating with westerly (easterly) phase of the tropical quasi-biennial oscillation (QBO). It is estimated that our small underprediction of 25 DU of polar ozone decline for the period 1979–1985 is uncertain by a factor of 2 and that if the effect on temperature cooling by reduced ozone heating in later years is taken into account, our model underprediction may increase to 50 DU. The underprediction of the October mean for 1985 by the model appears to be caused by the underprediction of the seasonal decline in September in the present model without heterogeneous chemistry. However, the seasonal declines during the earlier years are well simulated. With dynamics accounting for between 60 to 80% of the Antarctic October mean, zonal mean ozone decline for the period 1979–1985, we infer, subject to uncertainties in the model and data inputs, that dynamics should play a more important role in the phenomenon than merely providing a special condition for heterogeneous chemical destruction of ozone. However, chemical depletion of ozone may have become more important after 1984. Because heterogeneous chemistry is not incorporated in the present model, the model may have overestimated the magnitude of upwelling after 1984. It is possible that the reduced springtime radiative heating due to chemically reduced ozone amount may have suppressed vertical motion in September and early October in the later years.