PDO and AMO Modulation of the ENSO–Asian Summer Monsoon Teleconnection During the Last Millennium

Observations show that the teleconnection between the El Niño-Southern Oscillation (ENSO) and the Asian summer monsoon (ASM) is non-stationary. However, the underlying mechanisms are poorly understood due to inadequate availability of reliable, long-term observations. This study uses two state-of-the-art data assimilation-based reconstructions of last millennium climate to examine changes in the ENSO–ASM teleconnection; we investigate how modes of (multi-)decadal climate variability (namely, the Pacific Decadal Oscillation, PDO, and the Atlantic Multidecadal Oscillation, AMO) modulate the ENSO–ASM relationship. Our analyses reveal that the PDO exerts a more pronounced impact on ASM variability than the AMO. By comparing different linear regression models, we find that including the PDO in addition to ENSO cycles can improve prediction of the ASM, especially for the Indian summer monsoon. In particular, dry (wet) anomalies caused by El Niño (La Niña) over India become enhanced during the positive (negative) PDO phases due to a compounding effect. However, composite differences in the ENSO–ASM relationship between positive and negative phases of the PDO and AMO are not statistically significant. A significant influence of the PDO/AMO on the ENSO–ASM relationship occurred only over a limited period within the last millennium. By leveraging the long-term paleoclimate reconstructions, we document and interrogate the non-stationary nature of the PDO and AMO in modulating the ENSO–ASM relationship.