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

Observations show that the teleconnection between the El Nino-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 Nino (La Nina) 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. Sea surface temperatures in the tropical Pacific oscillate between warmer and cooler conditions every 2-7 years. These oscillations are called "The El Nino-Southern Oscillation (ENSO)" and have been shown to affect weather and climate in remote locations. For example, ENSO has been shown to alter rainfall of the Asian summer monsoon (ASM). However, the connection between ENSO and the ASM is not dependable, making accurate ASM prediction a challenge, especially in a changing climate. Here, we use a new technique that combines geological archives of past climates like ice cores, corals, or lake sediments with climate models to examine alterations in the ENSO-ASM relationship over the past thousand years. In particular, we focus on how other oscillatory ocean temperature patterns, the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO), might affect the ENSO-ASM relationship. In the context of the last millennium, we find the differences in the ENSO-ASM relationship between all positive and negative PDO/AMO phases are not statistically significant, as the PDO/AMO modulation on the ENSO-ASM relationship evolves over long time scales. Nevertheless, the PDO itself strongly regulates the Indian summer monsoon, and the consideration of PDO in addition to ENSO enhances monsoon prediction. This information is useful for anticipating decadal-scale changes in the ASM in our changing climate. We investigate how the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO) modulate the relationship between El Nino-Southern Oscillation (ENSO) and the Asian summer monsoon (ASM) using paleo-data assimilation productsWe find that the PDO impacts ASM variability more than the AMO, and its consideration yields improved Indian summer monsoon predictionsNotably, the influence of the PDO and AMO on the ENSO-ASM relationship is highly non-stationary across the last millennium