[1] Statistical trend analyses have been performed for monthly zonal average total ozone data from both TOMS and SBUV satellite sources and ground-based instruments over the period 1978-2002 for detection of a ''turnaround'' in the previous downward trend behavior and hence evidence for the beginning of an ozone recovery. Since other climatic and geophysical changes can impact ozone behavior and can influence the detection of turnaround and recovery, we also focus on accounting for ozone variations that may be ascribed to various physical and chemical influences. Thus we include in the statistical trend modeling and analysis the effects of various dynamical and circulation variations in the atmosphere, including those associated with the quasibiennial oscillation (QBO), Arctic Oscillation (AO) and Antarctic Oscillation (AAO), and Eliassen-Palm (EP) flux influences, as well as influences of solar cycle. A notable result of the analysis is that for latitude zones of 40°and above in both hemispheres, large positive and significant estimates of a change in trend (since 1996) are obtained (on the order of 1.5 to 3 DU per year). The dynamic index series, AO/AAO and EP flux, are found to have a substantial influence on total ozone for these higher latitudes, and significant influences of lesser magnitude are also found for lower latitudes. The feature of positive significant change in trend in total ozone over recent years, however, is obtained both without and with the dynamical index terms included in the statistical models.
[1] Because of control measures from international agreements, it is predicted that previous decreases in ozone should lessen and eventually turn around as a recovery of ozone. It is important to statistically determine from available ozone data that a change in the rate of downward trend in ozone occurs and that there is an overall ''turnaround'' in the downward trend. For this purpose, characteristics of a statistical trend model that allows for a change in trend (either a flattening or a turnaround) at some specified date are investigated. This model permits the use of data from before as well as after the change date to be used in the trend analysis and affords the opportunity for detection of a change in the downward ozone trend as an early signal in addition to detection of positive trend (recovery) after the change. Total Ozone Mapping Spectrometer data and ground station total ozone data from the northern midlatitudes are used to estimate the number of years of data required to detect a change in trend and to detect a positive trend recovery, under two different assumptions about future ozone trends. Results show that for midlatitudes (30°-60°) a positive change in trend of reasonably assumed magnitude can be detected within $7-8 years from the change date, whereas detection of a positive trend recovery can require roughly 15-20 years for southern midlatitude zonal average data and 20 -25 years for northern midlatitude zonal average data. In addition, preliminary trend analysis of recent ground station total ozone data is performed to illustrate estimation using the proposed trend model, with some informative results.
Abstract. International agreements for the limitation of ozone-depleting substances have already resulted in decreases in concentrations of some of these chemicals in the troposphere. Full compliance and understanding of all factors contributing to ozone depletion are still uncertain; however, reasonable expectations are for a gradual recovery of the ozone layer over the next 50 years. Because of the complexity of the processes involved in ozone depletion, it is crucial to detect not just a decrease in ozone-depleting substances but also a recovery in the ozone layer. The recovery is likely to be detected in some areas sooner than others because of natural variability in ozone concentrations. On the basis of both the magnitude and autocorrelation of the noise from Nimbus 7 Total Ozone Mapping Spectrometer ozone measurements, estimates of the time required to detect a fixed trend in ozone at various locations around the world are presented. Predictions from the Goddard Space Flight Center (GSFC) two-dimensional chemical model are used to estimate the time required to detect predicted trends in different areas of the world. The analysis is based on our current understanding of ozone chemistry, full compliance with the Montreal Protocol and its amendments, and no intervening factors, such as major volcanic eruptions or enhanced stratospheric cooling. The results indicate that recovery of total column ozone is likely to be detected earliest in the Southern Hemisphere near New Zealand, southern Africa, and southern South America and that the range of time expected to detect recovery for most regions of the world is between 15 and 45 years. Should the recovery be slower than predicted by the GSFC model, owing, for instance, to the effect of greenhouse gas emissions, or should measurement sites be perturbed, even longer times would be needed for detection.
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