Resonant interactions between solar activity and climate

Abstract

Solar magnetic activity exhibits chaotically modulated cycles with a mean period of 11 yr, which are responsible for slight variations in solar luminosity and modulation of the solar wind, while the earth's atmosphere and oceans support oscillations with many different frequencies. Although there are several mechanisms that might couple solar variability with climate, there is, as yet, no compelling evidence that a direct forcing is sufficiently effective to drive climatic change. In many nonlinear systems resonant coupling allows weak forcing to have a dramatic effect. An idealized model is considered, in which the solar dynamo and the climate are represented by low-order systems, each of which in isolation supports chaotic oscillations. The climate is represented by the Lorenz equations: solutions oscillate about either of two fixed points, representing warm and cold states, flipping sporadically between them. The effect of a weak nonlinear input from the dynamo to the climate that tends to push it toward the warm state is computed. This input has a significant effect when the 'typical frequencies' of each system are in resonance. The solution is now asymmetric, with the warm state preferred. The degree of asymmetry is less than might be anticipated, because resonant forcing extends the duration of oscillations about either state, and so increases the timescale for flipping. The presence of grand minima in the solar output leads to complicated intermittent behavior in the climate. Consequently, the results of frequency analysis are sensitive to the duration of time series that is used. It is clear that the resonance provides a powerful mechanism for amplifying climate forcing by solar activity.