• Abstract
• References

FIGURES

• Figure 1
• Figure 2
• Figure 3
• Figure 4
• Figure 5
• Figure 6
• Figure 7

An evaluation of the potential mechanisms which are responsible for the observed correlation between the variability of the Sun and climate is very a important problem. One of the possible mechanisms is the influence of the variation of Solar UV (SUV) radiation on the temperature and ozone in the stratosphere, and the propagation of this rather weak signal downwards into the troposphere. Studies of the influence of the variation of SUV can also facilitate understanding of how the climate system responds to a weak stratospheric perturbation and thereby help to estimate the magnitude of the possible influence of stratospheric processes on the tropospheric climate/weather system. These problems have been studied intensively during the last 5 years by performing simulations with 3D General Circulation Models (GCMs) These simulations have been performed with prescribed Solar UV and ozone changes [Haigh, 1996,1999; Shindell et al. ,1999; Balachandran et al., 1999], but not with an climate-chemistry model that interactively includes all ozone-related processes.

The University of Illinois at Urbana-Champaign (UIUC) 24-layer stratosphere/troposphere GCM with interactive photochemistry described by Rozanov et al. [2000] has been used to estimate the changes of temperature, dynamics and photochemistry due to the observed increase of solar UV radiation from solar minimum to solar maximum. We have performed two 15-year steady-state model simulations with prescribed sea-surface temperature: (1) a control run with the observed average spectrum of solar radiation; and (2) an experiment with the observed increase in solar UV radiation from solar minimum to solar maximum added to the average spectrum of solar radiation.

In these simulations, the increased solar UV radiation, described by Kodera et al. [2000], influences both the solar heating rates calculated by the GCM's radiation code and the photolysis rates calculated by the GCM's photochemical code. Here we present some preliminary results of the comparison between these two runs. In the figures the regions where the changes of the different quantities are statistically significant at better than the 20% level are bounded by a red-and-black line.

The changes of the annually averaged ozone and temperature caused by the imposed SUV increase are presented in Figure 1 and Figure 2. The increased solar heating and photodissociation rates lead to an increase in the ozone mixing ratio raise of up to 3.5% and a 0.5-1K warming in the low-latitude stratosphere. These effects are statistically significant mostly in the middle and upper tropical stratosphere. The simulated ozone changes in the middle and upper stratosphere are in a good agreement with the estimations published by Miller et al. [1996]. It is worth noting that the above-mentioned changes are asymmetric; in our model, the largest changes occur in the Southern Hemisphere. Some traces of changes in ozone and temperature can also be seen in the lower stratosphere, but this signal is not pronounced in the annual-mean fields.

Figure 1. Changes (%) of the annually averaged zonal-mean ozone mixing ratio due to the SUV enhancement

Figure 2. Changes (K) of the annually averaged zonal-mean temperature due to the SUV enhancement

The geographical distributions of the changes in temperature at 50 hPa and total ozone due to the SUV enhancement are presented in Figure 3 and Figure 4, from December to March over the Northern Hemisphere. The warming in the tropical stratosphere provides the condition for the propagation of the solar signal northward. The increase of the equator-to-pole temperature contrast induces an intensification of the polar vortex during boreal winter and early spring which is most pronounced in March. As a result, some cooling of up to 4K in March occurs inside the polar vortex, while the lower stratosphere outside the vortex boundaries tends to be warmer. The most statistically significant warming of up to 4K takes place in January over eastern Siberia. The same behavior can be seen in the total ozone changes. Again, there is some ozone depletion inside the polar vortex because the latter becomes more stable and isolated. The most statistically significant ozone depletion of up to 10% occurs in March. Outside the vortex the total ozone increased mainly over Siberia and Central Asia. It should be noted that the total ozone increase of up to 4% has been simulated in low latitudes, especially in February and March.

Figure 3. Changes (K) of the monthly zonal-mean temperature at 50 hPa due to the SUV enhancement

Figure 4. Changes (%) of the monthly zonal-mean total ozone due to the SUV enhancement

The changes in surface air temperature (T_s) are presented in Figure 5. The simulated T_s changes in December consist of a statistically significant warming of up to 2K over northern Europe, Russia and the U.S., and a cooling over Alaska, Greenland and Asia. This pattern is similar to that observed following powerful volcanic eruptions, with a concomitant warming in the lower tropical stratosphere [e.g., Robock, 2000]. This means that the signature of the solar signal is rather close to that of volcanic effects. This is not surprising because the initial perturbation for the both forcings consist of the warming of the lower stratosphere in the tropics.

Figure 5. Changes (K) of the monthly zonal-mean surface air temperature due to the SUV enhancement

The geographical distributions of the changes in temperature at 50 hPa and total ozone from August to October over the Southern Hemisphere are presented in Figure 6 and Figure 7 . The signal over the Southern Hemisphere seems to be more pronounced than that in the Northern Hemisphere. During early spring, some acceleration of the zonal wind leads to an intensification of the polar vortex, a cooler environment and a significant enlargement of the ozone hole. The depletion of total ozone in October for the experiment is almost 10% larger than for the control run. The maximum effects occur in the South American sector of Antarctica.

The preliminary analysis presented here provides some useful insight into the changes in temperature and ozone induced by the increase in Solar UV radiation .

Figure 6. Changes (K) of the monthly zonal-mean temperature at 50 hPa due to the SUV enhancement

Figure 7. Changes (%) of the monthly zonal-mean total ozone due to the SUV enhancement

References

Balachandran, N. K, D. Rind, P. Lonergan, D. T. Shindell, Effects of solar cycle variability on the lower stratosphere anti the troposphere, J. Geophys. Res, 104, 1999

Haigh, J. D., The impact of solar variability on climate, Science, 272, 981, 1996.

Haigh, J. D., A GCM study of climate change in response to the 11-year solar cycle, Q. J. R. Meteorol.Soc., 125, 001-999, 1999

Kodera et al., The GRIPS initiative to study the impact of solar forcing, this issue.

Miller, A.J. et al., Comparison of the observed ozone trends and solar effects in the stratosphere through examination of ground-based Umkehr and combined solar backscattered ultraviolet (SBUV) and SBUV 2 satellite data, J. Geophys. Res., 101, 9017-9021, 1996

Robock, A., Volcanic eruptions and climate, Rev. of Geophys., 38, 191-219, 2000.

Rozanov, E. V., M. E. Schlesinger, and V. A. Zubov, The UIUC 3-D Stratosphere/Troposphere General Circulation Model with Interactive Ozone Photochemistry: 15-year Control Run Climatology, J. Geophys. Res., 2000 (submitted).

Shindell, D., D. Rind, N. Balachandran, J. Lean, and P. Lonergran, Solar cycle variability, ozone, and climate, Science, 284, 305-308, 1999.