Late Holocene Asian summer monsoon dynamics from small but complex networks of paleoclimate data
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Internal variability of the Asian monsoon system and the relationship amongst its sub-systems, the Indian and East Asian Summer Monsoon, are not sufficiently understood to predict its responses to a future warming climate. Past environmental variability is recorded in Palaeoclimate proxy data. In the Asian monsoon domain many records are available, e.g. from stalagmites, tree-rings or sediment cores. They have to be interpreted in the context of each other, but visual comparison is insufficient. Heterogeneous growth rates lead to uneven temporal sampling. Therefore, computing correlation values is difficult because standard methods require co-eval observation times, and sampling-dependent bias effects may occur. Climate networks are tools to extract system dynamics from observed time series, and to investigate Earth system dynamics in a spatio-temporal context. We establish paleoclimate networks to compare paleoclimate records within a spatially extended domain. Our approach is based on adapted linear and nonlinear association measures that are more efficient than interpolation-based measures in the presence of inter-sampling time variability. Based on this new method we investigate Asian Summer Monsoon dynamics for the late Holocene, focusing on the Medieval Warm Period (MWP), the Little Ice Age (LIA), and the recent period of warming in East Asia. We find a strong Indian Summer Monsoon (ISM) influence on the East Asian Summer Monsoon during the MWP. During the cold LIA, the ISM circulation was weaker and did not extend as far east. The most recent period of warming yields network results that could indicate a currently ongoing transition phase towards a stronger ISM penetration into China. We find that we could not have come to these conclusions using visual comparison of the data and conclude that paleoclimate networks have great potential to study the variability of climate subsystems in space and time.
KeywordsAsian summer monsoon Complex networks Irregular sampling Little ice age Medieval warm period
This research was financially supported by the the German Federal Ministry of Education and Research (BMBF project PROGRESS, 03IS2191B), the German Science Foundation (DFG research group FOR 1380 “Himalaya: Modern and Past Climates (HIMPAC))”, the DFG graduate school GRK 1364 “Shaping Earth’s Surface in a Variable Environment”) and the Schweizer National Fond (SNF Sinergia grant CRSI22 132646/1). The authors would like to thank M. Yadava, R. Ramesh and H. Borgaonkar for providing data from India as well as G. Helle and M. Freund for helpful discussions about tree-ring data. Software to analyze irregularly sampled time series using the methods in this paper can be found on http://tocsy.pik-potsdam.de.
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