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This image shows a time series of solar activity (bottom) and the North Atlantic Oscillation in two model simulations, without (blue) and with (yellow) solar forcing.
Credit: Graphics, GEOMAR.
Are climate predictions over periods of several years reliable if weather forecast are still only possible for short periods of several days? Nevertheless there are options to predict the development of key parameters on such long time scales. A new study led by scientists at GEOMAR Helmholtz Centre for Ocean Research Kiel shows how the well-known 11-year cycle of solar activity affects the long-time development of dominant large-scale pressure systems in the Northern Hemisphere. For their investigations the scientists used a coupled ocean-atmosphere model. In addition, this model includes an interactive chemistry module which can for instance cope with the effect of ultraviolet radiation (UV) in the upper atmosphere. This additional component seemed to be key to transmit the variations in the solar radiation which might have only a small direct impact on Earth's surface, through a complex mechanism from the stratosphere (10-50 km altitude) to the lower atmosphere. "We have carried out several experiments," says Dr. Rémi Thiéblemont from GEOMAR, lead author of the study. "We conducted model experiments over a period of 145 years, with and without the influence of solar activity ," Thiéblemont continued. The sun's influence could clearly be identified in the so-called North Atlantic Oscillation, which is roughly speaking the pressure difference between the Azores high and the Iceland low. The ratio between these two pressure systems often determines the weather in Europe over longer time periods, such as whether the winter months turn warm and stormy or cold and snowy. The researchers found a time lag between variations in solar irradiance and atmospheric pressure patterns of about one to two years, which can be explained by an interaction between the atmosphere and the ocean. By comparing the two experiments with or without solar activity, they were able to prove for the first time that the sun irradiance serves as a phase-lock for the North Atlantic Oscillation. With this context, an increase of the predictability of the decadal NAO phase can be expected. |
