Is 2020 super Meiyu a result of changing annual cycle of East Asian monsoon?

2020 was exceedingly difficult for humans. As the world was experienced surge waves of COVID19, East Asia was also facing a one in a century, record-breaking flood,  as the result of a super 47-day Meiyu/Baiu stage of East Asian summer monsoon. As East Asian monsoons (EAM) follow a yearly cyclical pattern, we wonder which stage(s) were collateral damages of the extended Meiyu. Was it an early termination of the anomalous dry Spring, or was it a delayed northward propagation of the rain belt, i.e. late Mid-summer? The hypothesis stems from our recent finding (Dai et al., 2020) that the duration of the Spring stage is informative for the onset of Meiyu, while the duration of Meiyu is negatively correlated with that of Mid-summer, i.e., the longer the Meiyu, the shorter the Mid-summer. To verify this, we first positioned the 2020 pre-Meiyu, Meiyu, Mid-summer stages in the 40-year climatology annual cycle (Dai et al., 2020). Although neither the onset nor the termination was beyond the 40-year variance, Meiyu indeed hastened to arrive but postponed its departure. Rain belt stalled over the Yangtze river basin and southern Japan since mid-June; until the end of July, a planetary-scale anomalous high pressure band was in place encompassing the Arabian sea and north Pacific. It hindered the South Asian monsoonal flow to the South China Sea, curbing the northward propagation of the rain belt with assistance by both southeast-ward shift of South Asian High and lower level high pressure system persistent over the northern China. With these observations, we put forward a framework of ocean-atmosphere coupled mechanisms that traces back to the summer in 2019, and reveal the climate teleconnection and circulation systems that pave the road to the 2020 super Meiyu. With this study, we address the question of whether the 2020 super Meiyu was a “black swan” or a manifestation of ongoing systematic changes of the EAM annual cycle?

 

 

 

Reference

Dai, L., Cheng, T. F., & Lu, M. (2020). Define East Asian monsoon annual cycle via a self‐organizing map‐based approach. Geophysical Research Letters, 47. e2020GL089542. https://doi.org/10.1029/2020GL089542