Possible evidence of the resonant influence of solar forcing on the climate system

Abstract An assumption of the existence of natural climatic oscillations driven by solar activity enables an explanation of phase differences between variations in solar activity and rainfall level in Fortaleza, Brazil. Decadal and interdecadal variations in rainfall level can be reproduced using a forced oscillation equation with a driving force term that describes the variation in the sunspot number and with the assumption of the existence of 31.7-year interdecadal and 12.96-year decadal natural climatic oscillations. This equation satisfactorily reproduces the periodicity with a length of approximately 22 yr in the interdecadal rainfall variation before and up to the middle of the past century as well as the subsequent phase inversion, period and amplitude increase in the variation that followed the corresponding increases in the interdecadal sunspot number variation. The equation accurately reproduces the irregular phase shifts between decadal variations in rainfall level and in sunspot number over the entire 160 yr of recorded observations.