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3. Total ozone amounts and trends

Figure 2 compares the mean value for the period with the mean from the Total Ozone Mapping Spectrometer (TOMS) data (Stolarski, 1993). All the main observed features are very well reproduced, including low tropical ozone in January and February, and a broad peak in Arctic ozone in April. Arctic ozone also reaches a minimum in autumn as observed. In the southern hemisphere, the Antarctic ozone hole and the mid-latitude maximum are present in October, as observed. In spite of this agreement, the model is generally about 25 DU too high except in the Arctic spring (up to 15 DU too low), the Antarctic spring (about 50 DU too low), and a more substantial 75DU too high in southern mid-latitudes during spring.

The globally averaged total ozone for the model as a function of time is compared with TOMS data in Figure 3. A clear solar signal is present in the observations whereas these processes have not been incorporated into the physics of the model. The model results are biassed high relative to the observations. The first 10 months of the model results should be ignored, as the model was still spinning up. For the remaining period of the run, the model trend was -3.3 $\pm$ 0.9 DU/decade in the annual average in agreement with the observed trend of approximately -2.0 $\pm$ 1.4 (2 $\sigma$ error bars) after removing the solar cycle (4.5 $\pm$ 1.4 DU per 100 units of F$_{10.7}$ flux). However, the observed trend is sensitive to the period chosen -- e.g. over the period 1980--1998 the observed trend was -3.1 $\pm$ 1.4 DU/decade. Also, the observed trends have been computed using data from different satellites which could have had a significant impact on the results.

Figure 2: Model total ozone climatology (DU) for the period 1980-1999 (nominally for 1990) together with a TOMS climatology. Regions not sampled by the instrument are left blank and bordered by thick lines.
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Figure 3: Modelled total ozone averaged over the latitude band 65$^{\circ }$N -- 65$^{\circ }$S in comparison with TOMS data. The solar cycle as given by the annually averaged solar flux at 10.7 cm wavelength is indicated by the dashed line.
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Figure 4: Model ozone trend (DU/decade) for the period Jan 1980 to Dec 1999 as a function of latitude and month in comparison with TOMS data.
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The total ozone trend (DU/decade) as a function of month and latitude is illustrated in Fig. 4 in comparison with TOMS data. The model results are qualitatively in agreement with observations in showing large springtime losses over the polar regions but the model Antarctic ozone hole is present well into the southern hemisphere summer. This is consistent with the known problems of climate models in simulating a polar vortex that is too strong and which is present for too long (Pawson et al., 2000, and references therein). This is further accentuated by radiative feedback in a coupled model. In middle and high latitudes of the northern hemisphere, the ozone depletion is less than observed, due in part to the absence of aerosol chemistry in this simulation (e.g. Solomon et al., 1998).

Figure 5 shows the results for the local minima in the Arctic and Antarctic spring periods, together with the maximum size of the ozone hole, as indicated by the area within the 220DU contour. Comparisons with our previous (49-level) model simulations (Austin et al., 2000a,b) are also included. In the Arctic, the model generally agrees with observations although it is biassed slightly too high whereas the 49-level model was more nearly in agreement with observations. Also, whereas the model has a negligible trend in the minimum of -1 $\pm$ 16 DU/decade the observed trend is just statistically significant at -23 $\pm$ 18 DU/decade. In the Antarctic, the 64-level model results are biassed slightly low, compared with a slight opposite bias in the 49-level results. The trend in the minimum for the 64-level model, of -51 $\pm$ 16 DU/decade agrees with the observed trend of -64 $\pm$ 12 DU/decade where 2$\sigma$ error bars are used throughout. The size of the model ozone hole, by contrast, has a different trend to that observed, being too large initially and does not increase in size as rapidly as in the observations. The cause of this error is most likely related to the dynamics of the vortex, a consequence of the use of Rayleigh friction to mimic gravity wave drag in the stratosphere and mesosphere.

Figure 5: Model simulations of Arctic and Antarctic total ozone in comparison with TOMS data.
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Figure 6: Upper panel: Model zonally and annually averaged temperature trend for the period January 1980 to Dec 1999 as a function of pressure and latitude.
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