Our sun does not radiate evenly. The best known example of radiation fluctuations is the famous 11-year cycle of sun spots. Nobody denies its influence on the natural climate variability, but climate models have, to-date, not been able to satisfactorily reconstruct its impact on climate activity.

Researchers from the USA and from Germany have now, for the first time, successfully simulated, in detail, the complex interaction between solar radiation, atmosphere, and the ocean. As the scientific journal Science reports in its latest issue, Gerald Meehl of the US-National Center for Atmospheric Research (NCAR) and his team have been able to calculate how the extremely small variations in radiation brings about a comparatively significant change in the System "Atmosphere-Ocean".

Katja Matthes of the GFZ German Research Centre for Geosciences, and co-author of the study, states: „Taking into consideration the complete radiation spectrum of the sun, the radiation intensity within one sun spot cycle varies by just 0.1 per cent. Complex interplay mechanisms in the stratosphere and the troposphere, however, create measurable changes in the water temperature of the Pacific and in precipitation".

Top Down - Bottom up

In order for such reinforcement to take place many small wheels have to interdigitate. The initial process runs from the top downwards: increased solar radiation leads to more ozone and higher temperatures in the stratosphere. "The ultraviolet radiation share varies much more strongly than the other shares in the spectrum, i.e. by five to eight per cent, and that forms more ozone" explains Katja Matthes. As a result, especially the tropical stratosphere becomes warmer, which in turn leads to changed atmospheric circulation. Thus, the interrelated typical precipitation patterns in the tropics are also displaced.

The second process takes place in the opposite way: the higher solar activity leads to more evaporation in the cloud free areas. With the trade winds the increased amounts of moisture are transported to the equator, where they lead to stronger precipitation, lower water temperatures in the East Pacific and reduced cloud formation, which in turn allows for increased evaporation. Katja Matthes: "It is this positive back coupling that strengthens the process". With this it is possible to explain the respective measurements and observations on the Earth's surface.

Professor Reinhard Huettl, Chairman of the Scientific Executive Board of the GFZ (Helmholtz Association of German Research Centres) adds: "The study is important for comprehending the natural climatic variability, which - on different time scales - is significantly influenced by the sun. In order to better understand the anthropogenically induced climate change and to make more reliable future climate scenarios, it is very important to understand the underlying natural climatic variability. This investigation shows again that we still have substantial research needs to understand the climate system". Together with the Alfred Wegener-Institute for Polar and Marine Research and the Senckenberg Research Institute and Natural History Museum the GFZ is, therefore, organising a conference "Climate in the System Earth" scheduled for 2./3. November 2009 in Berlin.

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Meehl, G.A., J.M. Arblaster, K. Matthes, F. Sassi, and H. van Loon (2009), Amplifying the Pacific climate system response to a small 11 year solar cycle forcing, Science, 325, 1114-1118.