International Journal of Bifurcation and Chaos, Vol. 21, No. 12 (2011) 3549–3556 c  World Scientific Publishing Company DOI: 10.1142/S0218127411030714 CLIMATE MODE COVARIABILITY AND CLIMATE SHIFTS ANASTASIOS A. TSONIS ∗ and KYLE L. SWANSON † Department of Mathematical Sciences, Atmospheric Sciences Group, University of Wisconsin-Milwaukee, Milwaukee, WI 53217, USA ∗ aatsonis@uwm.edu † kswanson@uwm.edu Received September 3, 2010; Revised January 12, 2011 It has previously been shown from the collective behavior of a network of observed climate indices that this network synchronized several times in the period 1900–2000. Further, it has been found that in those cases where the synchronous state was followed by a steady increase in the coupling strength between the indices, the synchronous state was destroyed, after which a new climate state emerged. These shifts are associated with significant changes in global temperature trend and in El Ni˜ no/Southern Oscillation variability. Subsequently, the evidence for such type of behavior has been found to occur in three climate simulations using state- of-the-art models as well as in the observed data in the 21st century. This was the first time that this mechanism, which appears consistent with the theory of synchronized chaos, has been discovered in a physical system of the size and complexity of the climate system. Here we extend this approach to consider proxy data for climate modes going back several centuries. While noise in the proxy data in some cases masks the mechanism, we find significant coherence between both synchronization and coupling and global temperature. These results provide further support that the above mechanism for climate shifts is a robust feature of the climate system. Keywords : Climate variability; synchronized chaos; paleoclimate. 1. Introduction One of the most important and mysterious events in recent climate history is the climate shift in the mid- 1970s [Graham, 1994]. In the northern hemisphere 500-hPa atmospheric flow the shift manifested itself as a collapse of a persistent wave-3 anomaly pattern and the emergence of a strong wave-2 pattern. The shift was accompanied by sea-surface temperature (SST) cooling in the central Pacific and warming off the coast of western North America [Miller et al., 1994]. The shift brought sweeping long-range changes in the climate of northern hemisphere. Incidentally, after “the dust settled”, a new long era of frequent El Ni˜ no events superimposed on a sharp global temperature increase began. While several possible triggers for the shift have been suggested and investigated [Graham, 1994; Miller et al., 1994; Graham et al., 1994], the actual phys- ical mechanism that led to this shift is not known. Understanding the dynamics of such phenomena is essential for our ability to make useful predic- tion of climate change. A major obstacle to this understanding is the extreme complexity of the climate system, which makes it difficult to disen- tangle causal connections leading to the observed climate behavior. Here, we extend the analysis which has revealed an important new mechanism in climate dynamics and explained several aspects 3549