Interstadial diversity of East Asian summer monsoon linked to changes of the Northern Westerlies.

During the last glacial period, the iconic Greenland ice-core records provide evidence of interstadial warmings with various durations ranging from a century to millennia. However, whether differences in interstadial duration are mirrored by distinct hydroclimate responses in the tropics remains unclear. Here we present four speleothem δO records from the Indian summer monsoon (ISM) and East Asian summer monsoon (EASM) regions, spanning both short and long interstadials during the last glacial period. Greenland and ISM records show broadly similar isotopic responses across events, however, the EASM records exhibit markedly different δO depletions between short and long interstadials. Using an isotope-enabled climate model, we attribute these differences to a further northward shift of the Northern Westerlies during short interstadials, driven by intensified high-latitude warming. This shift promoted the northwestward expansion of Western Pacific Subtropical High and hence the delivery of isotopically enriched near-sourced vapor to eastern China, dampening δO depletion during stadial-to-interstadial transitions. Our findings highlight a previously unrecognized sensitivity of EASM precipitation δO to nuanced meridional shifts in the Northern Westerlies in contrast to the uniform responses of the ISM during interstadials.