It is relatively easy to discover overlaid (additive) signals in natural phenomena using wavelet decomposition, Fourier Transforms, CEEMD, etc, because addition in the time domain is the same as addition in the frequency domain.
However, I cannot find a clear reference on how to determine if amplitude modulation is occurring in a natural phenomena. This type of relationship between known or unknown sources results in convolution in the frequency domain, and I can't figure out how to process a time series of data to elicit possible sources of such a signal.
Let's take for example a commonly available series - monthly sunspots. There's clear peaks at 10 and 12 year intervals that can be seen by taking an FFT, resulting in the commonly known 11 year cycle average, which makes one suspect amplitude modulation somewhere in the natural processes of the sun. How would one determine a suspected carrier frequency or prove that some sort of amplitude modulation is taking place?