The East Asian summer monsoon (EASM) stands as the most northerly extending monsoon system, defined by its remarkable seasonal march. It features a stepwise northward migration of rainfall stages, starting with the pre-Meiyu season in southern China in mid-May, progressing into the Meiyu season (known as Baiu in Japan and Changma in Korea) from mid-June to mid-July, and ultimately extending to northern China by mid-July. While the future behavior of this seasonal march remains uncertain, the mid-Pliocene warm period (~3.3 to 3 million years ago) was the last time that atmospheric CO2 levels (~400 ppm) were similar to today’s. There were extensive continental greening and fewer ice sheets. The terrestrial geography and ocean basin configuration were largely unchanged, many floral and faunal species were still extant. Thus, the mid-Pliocene is regarded as a potential analog for the future. 

A view of northeastern East Asia after the mid-summer rains. (Image by HE Linqiang)

However, what the mid-Pliocene epoch can teach us about future seasonal march of East Asian summer monsoon rainband remains unknown. A study recently published in the Journal of Climate reveals that the seasonal march of the mid-Pliocene EASM was about 10 days earlier than today, mainly driven by extratropical vegetation greening and tropical shelf exposure.

Led by Prof. ZHOU Tianjun from the Institute of Atmospheric Physics, Chinese Academy of Sciences, the study synthesizes multi-source model simulations to disentangle the influences of atmospheric CO₂, vegetation, and geography on the EASM.  "The seasonal march of the mid-Pliocene East Asian summer monsoon could provide implications for the future change from a paleoclimate perspective." ZHOU said. 

The results show that extratropical vegetation greening and tropical shelf exposure modulate the zonal and meridional atmospheric teleconnection, respectively, leading to anticyclonic anomalies over East Asia. This further results in a northward shift of the western Pacific subtropical high, ultimately advancing the monsoon rainband.

The greening of extratropical vegetation, as a long-term indirect response to sustained CO₂ forcing, facilitates an earlier seasonal march. In contrast, the direct effect of CO₂ tends to delay this seasonal march. “These findings suggest that the future change of the EASM’s seasonal march will result from a tug-of-war between the direct and indirect impacts of CO₂,” explained Dr. HE Linqiang, the lead author of the study. He has completed his PhD at the University of the Chinese Academy of Sciences and is currently a postdoctoral researcher at Columbia University.