The unstable East Asian Summer Monsoon - ENSO relationship over the past 700 years

The El Niño-Southern Oscillation (ENSO) is an important modulator of the East Asian Summer Monsoon (EASM), but their teleconnection has been unstable during the instrumental era. Due to the short duration of instrumental records, we utilized paleoclimate records to investigate the EASM-ENSO teleconnection over the past 700 years. Building upon an established paleoclimate data assimilation method — the Last Millennium Reanalysis framework — that effectively fuses paleoclimate proxies and model simulations, we reconstructed seasonal sea surface temperature (SST), 500 hPa geopotential height, 850 hPa wind, and summer precipitation fields in the tropical and mid-latitude regions. Our findings confirm the instability of the EASM-ENSO teleconnection, with transitions between positive and negative correlation periods linked to tropical SST patterns and the Western North Pacific Anticyclone (WNPA). We demonstrate that cooling in the Western North Pacific, rather than SST warming in the central and eastern Pacific, dominates the development of the WNPA, facilitating a positive EASM-ENSO teleconnection. Additionally, the tropical Indian Ocean SST shows no significant differences between positive and negative correlation periods, underscoring the critical influence of Western North Pacific SST. Our results demonstrate how the EASM and ENSO are interconnected and how their teleconnections changed during the past 700 years, providing insights for projecting future EASM changes and the EASM-ENSO teleconnection.