Influence of Equatorial Spring Insolation on Abrupt Asian Summer Monsoon Decline at Orbital Scale

The dominant influence of precession-induced changes in summer insolation on orbital-scale variability of the Asian summer monsoon (ASM) during the Holocene has been widely proposed; however, it remains unclear why the decline of the ASM started several thousand years after the peak summer insolation. Through comparisons of climate simulations and proxy records, our study reveals that the abrupt decline in the ASM coincided with an increase in spring insolation at the equator. The reduced spring insolation resulted in a cooler tropical Indian Ocean, which weakened and shifted northward the westerly jet due to decreased meridional thermal gradient. The South Asian high moved northward in conjunction with the westerly jet, causing anomalous upwards over northern South Asia, the Tibetan Plateau, as well as southwestern and northern China. The associated anomalous cyclone over the Tibetan Plateau enhanced the monsoonal moisture transport, subsequently intensifying the ASM circulation and precipitation. The ASM was enhanced by the decrease in spring insolation and was weakened by the opposite. The abrupt decline of the ASM was associated with an increase in spring insolation superimposed on a decrease in summer insolation. Consequently, orbital-scale ASM variability is dominated by the precession not only via insolation changes in summer but also changes in spring.