Tropical Pacific Modulation of the Asian Summer Monsoon Over the Last Millennium in Paleoclimate Data Assimilation Reconstructions.

Large uncertainties exist in climate model projections of the Asian summer monsoon (ASM). The El Niño‐Southern Oscillation (ENSO) is an important modulator of the ASM, but the ENSO‐ASM teleconnection is not stationary. Furthermore, teleconnections between ENSO and the East Asian versus South Asian subcomponents of the ASM exhibit distinct characteristics. Therefore, understanding the variability of the ENSO‐ASM teleconnection is critical for anticipating future variations in ASM intensity. To this end, we here use paleoclimate records to extend temporal coverage beyond the instrumental era by millennia. Recently, data assimilation techniques have been applied for the last millennium, which facilitates physically consistent, globally gridded climate reconstructions informed by paleoclimate observations. We use these novel data assimilation products to investigate variations in the ENSO‐ASM relationship over the last 1,000 years. We find that correlations between ENSO and ASM indices are mostly negative in the last millennium, suggesting that strong ASM years are often associated with La Niña events. During periods of weak correlations between ENSO and the East Asian summer monsoon, we observe an El Niño‐like sea surface temperature (SST) pattern in the Pacific. Additionally, SST patterns associated with periods of weak correlations between ENSO and South Asian summer monsoon rainfall are not consistent among data assimilation products. This underscores the importance of developing more precipitation‐sensitive paleoclimate proxies in the Indian subcontinental realm over the last millennium. Our study serves as a baseline for future appraisals of paleoclimate assimilation products and an example of informing our understanding of decadal‐scale ENSO‐ASM teleconnection variability using paleoclimate data sets. Plain Language Summary: A large fraction of floods and droughts in Asia are associated with extreme Asian summer monsoon (ASM) events, but unfortunately, the prediction of the ASM is still highly uncertain. Scientists often use El Niño—warm sea surface temperature (SST) events in the eastern Pacific Ocean that occur before the ASM—as a basis to predict the ASM, but it does not work perfectly all the time. In order to understand why El Niño fails to predict the ASM sometimes, we looked at climate data spanning the last 1,000 years. The data was based on paleoclimate reconstructions using archives including tree rings, corals, speleothems, and ice cores. Scientists recently synthesized all these data together with climate model simulations. We find that more intense summer monsoon rainfall in East and South Asia likely corresponds to colder SSTs in the Eastern Pacific Ocean. Also, when El Niño fails to predict the monsoon in East Asia, the background SST in the eastern Pacific is warmer than normal. Our results could help better predict ASM rainfall in the future and mitigate impacts of fire and flooding caused by extreme monsoon events. Key Points: The El Niño‐Southern Oscillation (ENSO) teleconnections to the Asian summer monsoon have evident decadal‐to‐centennial variability in the last 1,000 yearsDuring most of the last 1,000 years, El Niño tends to be associated with weak Asian summer monsoon rainfallWeak ENSO‐East Asian summer monsoon teleconnection is associated with sea surface temperature anomalies resembling El Niño events [ABSTRACT FROM AUTHOR]