Planetary waves and the global ozone distribution.
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Date
1994
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Abstract
Planetary waves are known to play an important role in determining the annual variability in the severity of Antarctic ozone depletion. In this thesis, data obtained from the TOMS experiment on board the Nimbus-7 satellite are used to create global maps of total column ozone for each day from 1979 to 1992. Planetary wave morphology within these distributions is examined using spatial spectral analysis. The wave powers obtained are compared with a measure of the depth and area of the Antarctic ozone hole. A theoretical overview is given of middle atmosphere dynamics and how it influences the global distribution of ozone. Planetary waves play a predominant role in this dynamical distribution as they are responsible for the eddy transport of heat, momentum and long-lived tracers. A discussion of the equations of motion governing planetary wave propagation and their physical interpretation is given. The use of derived meteorological parameters, such as potential vorticity, in the study of planetary waves is developed. The theoretical tools, together with the equations of motion, are used to examine the excitation mechanisms for planetary waves. This theoretical analysis is also used to show that the strength of the westerly jet within the polar vortex determines which planetary wave modes can be expected in the geopotential height fields. In general only wavenumber 1 and 2 propagate during the Southern Hemisphere winter and the strength of these waves strongly modulates the severity of the Antarctic ozone depletion. Furthermore, the timing of the breakdown of the polar vortex is connected with the temporal variation of planetary wave power. A review of the current research in the field of planetary waves and the global distribution of ozone is given. To quantify planetary wave activity, spatial spectral analysis (Fourier Transforms) of midlatitude zonal profiles of total column ozone are used to calculate wave powers in the ozone distribution. Powers at wavenumber 1 to 6 are calculated for each day from 1979 to 1992 for both the Northern and Southern Hemispheres. The powers are seen to decrease with wavenumber throughout the year and the amplitude of wave modes 3 and higher is suppressed during the winter in accordance with the theoretical discussion. Southern Hemisphere wave powers are compared with a daily depleted mass of ozone over the Antarctic from 1979 to 1992. It is found that during years of high (low) planetary wave activity Antarctic ozone depletion is very weak (severe). Finally, the planetary wave morphology observed in the Southern Hemisphere total column ozone distribution is compared with geopotential height waves reported in the literature. Comparison of Northern Hemisphere wave powers and Arctic ozone depletion shows little or no correlation. To examine whether planetary waves may effect the interannual variability in Antarctic vortex temperatures, winter averaged 100 hPa upper air temperatures from SANAE (70°18' S, 2°21' W) are compared with derived planetary wave powers. It is found that during years of high (low) planetary wave activity the SANAE 100 hPa temperatures are above (below) the mean. Analysis of daily total column ozone and upper air temperatures at SANAE, during the winters of 1987 and 1988, shows that wave 1 forcing can significantly influence the day-to-day variation in these quantities. A statistical model of total ozone variation over the entire globe, from 1979 to 1992, has been developed. This model incorporates a long-term linear trend, an annual variation, a quasi-biennial oscillation, a solar cycle variation and a semiannual variation. Since monthly average total column ozone data are used in this model, short term planetary wave influences are masked, and the variations in global ozone are determined primarily by the 5 signals discussed above. However, analysis of the model coefficients indicates that planetary wave activity may significantly contribute to zonal asymmetry in global total ozone trends. Differences between model results and measured data are compared with planetary wave activity. The timing of the breakdown of the Antarctic circumpolar vortex causes large interannual differences in monthly average total column ozone for the months of October and November in the Southern Hemisphere. This analysis is made more specific for South Africa by examining the variation in monthly average total column ozone over the five South African cities of Pretoria, Bloemfontein, Durban, Port Elizabeth and Cape Town for the years 1979 to 1992. A model of surface erythemal irradiance as a function of total column ozone, time of the year and other meteorological parameters is developed. Total ozone data for the 5 cities, used within the context of the statistical model and the DV irradiance model, allows the prediction of future DV levels over South Africa to the year 2000. Planetary wave motion owes its existence to the conservation of potential vor-ticity. However, to make use of this conservation principle as a diagnostic for planetary wave propagation, it is necessary to know the conditions under which it is conserved. A formalism developed by Plumb and Ko (1992) has been used to suggest a technique whereby the 'lifetime' of potential vorticity may be determined. Use was made of data from the NCAR CCM2 model to test this hypothesis. The concentrations of long-lived tracers (CH4 and NzO) extracted from the CCM2 also show the effects of planetary wave breaking on the meridional distribution of these species. This study has important implications for airborne polar chemistry campaigns.
Description
Thesis (Ph.D.)-University of Natal, 1994.
Keywords
Theses--Physics., Atmospheric ozone., Ozone layer depletion., Rossby waves., Geophysics.