Phase-locked states in Earth’s climate system were identified in a study of a set of climate indices by Swanson and Tsonis (2009)  (ST). They reported five climate shift events since 1900 based upon fea- tures in a phase-locking parameter S. The present study finds that sets of climate indices have important topological properties such as a metric diameter D that describes the magnitude of phase locking among the indices. Minima in D as a function of time are shown to be associated with climate shifts. Eighteen strong events since 1870 are identified, including the five reported by ST. Ten of these minima corre- spond to reported events such as the well documented “climate shift of the mid-1970s” and the more recent climate shift of 2001–2002. Most climate shifts tend toward radiative equilibrium.
Link to the paper is here.
Does 18 climate shifts since 1870 make sense from Douglass argument? He asks the question as to what is causing the climate shifts. He suggest one possibility as being strong short-duration events such as volcanoes, but this certainly does not answer all of the related questions.
This statement caught my eye:
That AMO lags the Pacific indices by 7 to 9 months suggests that the climate shifts originate in the Pacific. Additionally, within the Pacific indices Nino3.4 sometimes leads, suggesting that those events originate in the Tropical Pacific. These questions should be studied for each climate shift listed by varying lag times and different geographic sets of indices.
I think I have come across this idea before of a relationship between the AMO and the PDO, but I’m not sure where?