A series of experiments is conducted in which a variable solar irradiance is imposed for a range of frequencies and amplitudes in a simplified coupled General Circulation Model. For realistic amplitudes solar forcing dominates over internal variability in global mean surface air temperature (GM-SAT) beyond decadal timescales. Its impact increases with period up to 50 years. Evidence is found for interactions between climate variations with different timescales. A weak 22-yr solar irradiance variation excites a significant spectral peak with a 70-yr period in GM-SAT. On the regional-scale the internal variability dominates at all timescales. Patterns of internal variability and their associated variance are robust for a variable solar forcing. The temporal spectra, however, are sensitive to such forcing. Some preferred decadal timescales of the internal modes of the coupled system disappear when the solar forcing varies. ¿ 1999 American Geophysical Union