ENSO history recorded in Agathis australis (kauri) tree rings. Part B: 423 years of ENSO robustness

Abstract

Part A of this study identified the potential of kauri tree rings for reconstructing the history of past ENSO activity. Plausible indices of multidecadal to centennial-scale ENSO activity (phase dominance and robustness) that could be derived from kauri regional master chronologies are hypothesized here, and they are tested against the instrumental SOI record, using partitioning and graphical analyses. The subset of the indices passing this first screening are then tested for sensitivity to the complex evolving sample depth and site-composition mix that characterizes regional tree-ring chronologies. Corrections for evolving sample depth are developed where possible. The two indices of ENSO activity remaining after these two screening phases (evolving chronology time-series variance and decadal-scale spectral signature) are used to infer 423 years of ENSO robustness and the results are compared to two multiproxy ENSO reconstructions. Results suggest that ENSO robustness (1) peaked in the 20th century, (2) is characterized by persistent 55–80 year cyclicity, (3) was low in the mid-20th century only relative to robust ENSO activity at the beginning and end of that century, (4) reached a pre-20th century peak in the mid-18th century, and (5) was weakest near the beginning of the 19th century.