Why Does the Ozone Hole Vary in Size?
|Fig. 1: Propagation of weak (blue) and strong (red) planetary waves.|
The ozone hole is a severe depletion of Earth's protective ozone layer which takes place each spring over Antarctica. It is caused by chemical reactions that take place primarily on the surface of polar stratospheric clouds, ice particles or liquid droplets which form at high altitudes in extreme cold. Over the long term, the size of the ozone hole is governed by emissions of chlorine and bromine containing compounds, for example chlorofluorocarbons (CFCs), generated mostly by human activities. Year-to-year variability, however, is determined mostly by temperature variations in the upper atmosphere. In colder years, more ice particles will freeze, allowing more chemical destruction of the ozone layer.
An understanding of the variability of the ozone hole allows us to better quantify the impact of changing emissions of ozone depleting chemicals, to predict the future of the ozone hole, and to detect its recovery. Our investigations into the interannual variability of the ozone hole, using the GISS global climate/middle atmosphere model, indicate that it is strongly dependent upon the amount of planetary wave energy in the troposphere (the area from Earth's surface to approximately 17 km altitude).
Planetary waves are large-scale waves in Earth's atmosphere that are created by the uneven distribution of continents and oceans over Earth's surface. These variations lead to differences in heating rates and air motions over water versus land and over mountain ranges, creating waves which travel through the atmosphere similarly to ocean waves. The amount of wave energy that moves up from the troposphere into the lower stratosphere (roughly 17 to 30 km altitude) significantly affects the temperature, and therefore the ozone depletion, at these altitudes, where the bulk of the ozone layer is located.
The schematic diagram in Figure 1 illustrates the propagation of tropospheric wave energy in the Southern Hemisphere polar region. The thin blue lines represent weak wave energy; the thick red lines strong wave energy. The tropopause (thin horizontal black line) marks the boundary between the troposphere and the stratosphere. In July, energy tends to propagate equatorward in both cases. However, in years with strong waves, some of the energy penetrates upwards, warming the lower stratosphere. The percentage of the total overhead ozone column destroyed is shown in Figure 2 for the years in our experiment with strongest and weakest wave energies. The year with weak wave energy (top) clearly shows greater ozone loss, which extends over a wider area, reaching farther past the edge of Antarctica and closer to the tip of South America. In the year with strong wave energy, by contrast, the lower stratospheric warming in July and August leads to reduced ozone losses throughout the Southern Hemisphere spring, when the sunlight that powers ozone depletion chemistry returns to the polar regions. Note the lack of ozone depletion right near the South Pole in August, while that region remains in 24 hour darkness.
|Fig. 2: Modeled percent ozone loss in years with weak (top) and strong (bottom) wave activity.|
In addition, the equatorward propagation of energy set up in July in the strong wave years is so powerful that energy continues to propagate towards the equator through October, as shown in the first diagram. In years with weak waves, on the other hand, energy goes up into the stratosphere, causing warmings in late spring, especially at higher altitudes.
Both the variations in ozone loss and the high altitude warmings which occur in the model have been observed in nature. However, our model shows considerably less variability than that observed, suggesting that other important sources of variability exist which have not been included in the model. Further experiments will explore the influence of periodic forcings such as the quasi biennial oscillation (QBO) in equatorial winds and the cyclical variation of solar radiation.
Shindell, D.T., S. Wong, and D. Rind 1997. Interannual variability of the Antarctic ozone hole in a GCM. Part 1: The influence of tropospheric wave variability. J. Atmos. Sci. 54, 2308-2319.