Role of Asian Westerly Jet Core's Zonal Migration in Holocene East Asian Summer Monsoon Precipitation

Paleoclimatic records reveal spatially asynchronous changes of East Asian summer monsoon (EASM) precipitation during the Holocene, but the underlying mechanisms are not fully understood. This study explores the role of the zonal shift of the Asian westerly jet (AWJ) core on the spatial-temporal variability of EASM precipitation based on Holocene climate simulations using the KCM and CESM forced by orbital variations. We analyzed the composite differences of summer precipitation and associated atmospheric circulations between the early (middle) and middle (late) Holocene. Simulations suggest a dipole pattern of summer precipitation with increasing (decreasing) precipitation in central (southern) China from the early to late Holocene, which was shaped by the eastward shift of the AWJ core. The eastward shift of the AWJ core and the accompanied eastward movement of the upper-level convergence over the AWJ's right exit region weakened the low-level downward motions over central China. Meanwhile, the low-level upward motions over southern China were weakened by the concurrent eastward (westward) migration of the South Asian High (western Pacific subtropical High) during the Holocene. Consequently, summer precipitation increased (decreased) in central (southern) China from the early to late Holocene. The eastward shift of the AWJ core was primarily driven by amplified meridional insolation gradient at middle latitudes via strengthening upper-level westerly intensity during the Holocene. In addition, land cover difference between East Asia and central Asia caused a greater enhancement of westerlies intensity in East Asia, further modulating the eastward movement of the AWJ core during the Holocene.

Automated summary

This study examines the impact of the zonal shift of the Asian westerly jet (AWJ) core on the spatial-temporal variability of East Asian summer monsoon (EASM) precipitation. Through Holocene climate simulations, it is found that the eastward shift of the AWJ core is responsible for the changes in summer precipitation in central and southern China. This shift affects the atmospheric circulations and low-level motions, resulting in the observed changes in precipitation. Additionally, differences in land cover between East Asia and central Asia further modulate the movement of the AWJ core.