This project examines the variations in lower stratospheric ozone and temperature that accompany the 11-year cycle in sunspots and solar (mostly ultraviolet) radiation received by the earth, in which higher ozone values are associated with greater solar radiation. The research tests the hypothesis that the stratospheric response to 11-year solar forcing is influenced by tropical air-sea interactions, as increased insolation leads to enhanced sea surface evaporation, which in turn produces a precipitation pattern that generates an atmospheric flow response extending into the stratosphere. The hypothesis will be tested through simulations of the Whole Atmosphere Community Climate Model (WACCM), in which the upper levels of the atmosphere are resolved and the photochemical reactions that determine stratospheric ozone concentration are represented. In this model, the influence of air-sea interactions can be determined through simulations in which ocean surface temperatures are either fixed or allowed to interact with the overlying atmosphere. Statistical analysis of both simulations and corresponding observations will also be performed to further examine the relevance of the proposed mechanism for real-world stratospheric variability.
The work will have broader impacts through the support and training of a graduate student, which will develop the scientific workforce in this scientific area. In addition, a better understanding of the impact of solar variability on climate may prove useful for decision makers concerned with the practical implications of climate variability and climate change.
This one-year project was an effort to compare observational estimates of the atmospheric effects of the 11-year solar cycle with those that are simulated by advanced climate models. The main purposes were (a) to test the reality of the observed effects by determining whether they can be simulated in a realistic model; and (b) to evaluate whether solar ultraviolet forcing of the upper atmosphere (mainly the stratosphere) is involved in producing 11-year signals in the lower stratosphere and at the Earth's surface. We began by comparing the observationally estimated solar cycle response of the stratosphere (ozone and temperature) to simulations using a climate model developed at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado. Results were mixed. On the one hand, the simulations were able to produce part of the observed vertical structure of the ozone and temperature responses. On the other hand, the model responses in the lower stratosphere and at the Earth's surface were much weaker than those derived from observations. We then began to analyze model results from a more advanced version of the NCAR model and from three simulations previously performed by a group at the Free University of Berlin in Germany. The latter group used a simpler climate model - an atmosphere-ocean general circulation model. In this model, the solar-induced changes in the stratosphere (mainly changes in ozone and radiative heating) were prescribed rather than being calculated self-consistently. However, their approach had an advantage in that each simulation assumed a different solar cycle change in ozone. This allowed a more direct test of whether the details of the stratospheric response were important for producing a realistic response in the troposphere near the surface. Because of the short project duration, we have so far been able to carry out a complete comparison only for the surface climate response during northern hemisphere winter using the simulations done by the German group. We find that the model results are indeed sensitive to the assumed ozone variation.The simulation that used the most realistic ozone variation (one specified from satellite observations) yielded a surface climate signal in sea level pressure and sea surface temperature that agreed best with observations, especially in the Pacific region. Although these results are provisional and need to be verified using other model simulations, they indicate an important role of the stratosphere in producing the observationally estimated surface climate response to the solar cycle during northern winter. Future work is needed to better document and understand this ``top-down'' mechanism for solar forcing of surface climate. A better understanding of this natural component of climate variability may lead to improvements in climate models in general that will be useful for other applications.
