Scientists Find the Beginning Signals of East Asian Summer Monsoon
The East Asian summer monsoon (EASM), covering the subtropics and midlatitudes, affects about one-third of the world’s population. It is crucial for the rainy season in East Asia which generally begins with the summer monsoon onset and ends with its withdrawal. As the advancement or postponement of the rainy season can severely influence the agriculture planting and human production. it is of great significance to find the precursor signals of the onset of the monsoon rainy season.
"EASM shows substantial variations from year to year in term of the onset, duration, and total precipitation. Yet the controlling factors are not clear." said Prof. DUAN Anmin at the Institute of Atmospheric Physics of the Chinese Academy of Sciences.
Prof. DUAN and his team analyzed multisource observational data and reanalysis data sets during 1961-2016 and used a numerical simulation to examine the controlling factors during different stages of EASM. They found that the positive surface sensible heating and condensation heating anomalies over the Tibetan Plateau lead the onset, duration, and total precipitation of the rainy season in South China (the first stage of the EASM) by about 2 and 1 months, respectively. Moreover, a significant positive condensation heating anomaly over the Tibetan Plateau can be detected about 1 month before the onset of the rainy season in Southwest China (the second stage of the EASM). By contrast, the relationship between heating of the Tibetan Plateau and the onset of the rainy season is relatively weak during the remaining stages of the EASM, including the Meiyu and North China rainy seasons.
"We conclude that the persistent above-normal diabatic heating of the Tibetan Plateau intensifies the low‐level southwesterly circulation to the southeast and facilitates the convergence of moisture and ascending motion, leading to an earlier onset of the rainy season." introduced Prof. DUAN.
Ratio of the monthly precipitation in (a) April, (b) May, (c) June, (d) July, (e) August, and (f) September to the average precipitation from April to September during the time period 1960–2015. The three green curves from south to north denote the Yangtze, Huaihe, and Yellow Rivers, respectively, and the red curve denotes region of the Tibetan Plateau with an altitude >2,500 m. The black frames in part (a) denote the rough outline of the South China (SC), Southwest China (SWC), Meiyu, and North China (NC) regions. (Image by HU Die)
The team further did sensitive experiments with a linear baroclinic mode, which confirmed the role of heating of the Tibetan Plateau in generating circulation conditions favorable for the onset of the rainy season in both South and Southwest China.
This study provides further proof that the diabatic heating anomaly of the Tibetan Plateau acts as an independent precursor of the early stage onset of the EASM in addition to the influence from the atmospheric and oceanic modes.
The related results recently have been published in Journal of Geophysical Research: Atmospheres.
This work was jointly supported by the National Natural Science Foundation of China (grants 41725018 and 91937302).
Duan, A., Hu, D., Hu, W., & Zhang, P. (2020). Precursor effect of the Tibetan Plateau heating anomaly on the seasonal march of the East Asian summer monsoon precipitation. Journal of Geophysical Research: Atmospheres, 125, e2020JD032948. https://doi.org/10.1029/2020JD032948
Contact: Duan Anmin, firstname.lastname@example.org