23 September 2019

East Asian Early Summer Rainband Intensifies in a Warming Climate


We show that the early summer rainfall over East Asia—characterized by a rainband that extends from southeastern China to southern Japan—greatly intensifies in a set of future climate simulations. The cause of this intensification is attributed to changing atmospheric westerly flows that 'linger' at the latitudes of the Tibetan Plateau instead of migrating northwards, and which in turn produces convergent atmospheric flows downstream that concentrates and delivers more moisture to the rainband.


The early summer 'pre-Meiyu' rainband accounts for much of the rainfall over southeastern China, and its potential intensification in the future will have consequences for water resources and impacts from flooding. The research suggests that efforts to understand and predict this phenomenon should focus on the westerlies interacting with the Tibetan Plateau as the causal influence, rather than the low-level monsoonal circulations that is the more typical focus. 


Early summer rainfall over East Asia is projected to increase in the future as a consequence of two related physical changes resulting from global warming: more moisture in the air with warmer temperatures, and stronger monsoonal circulations. A set of state‐of‐the‐art simulations of the future climate that we analyzed suggests that the rainfall increase will be especially pronounced in the “pre‐Meiyu” stage (mid‐May to late June) when the East Asian rainband extends from southeastern China to southern Japan; at the end of the century, rainfall over southeastern China is projected to increase up to 40 percent. The cause of this intensification is different: it occurs because the westerlies “linger” at the latitudes of the Tibetan Plateau during this time instead of migrating northward, and the resulting atmospheric flow downstream brings about stronger convergent flows that concentrate and delivers more moisture to the pre‐Meiyu rainband. More work, and using other models, will be needed to demonstrate that this future pre‐Meiyu intensification is real. However, the significance of our result from a climate science viewpoint is in demonstrating that the change in the westerlies impinging on the Tibetan Plateau can have a first‐order effect on future changes to the East Asian summer rainfall climate.

John Chiang
University of California - Berkeley
Chiang, JC, J Fischer, W Kong, and MJ Herman.  2019.  "Intensification of the Pre‐Meiyu Rainband in the Late 21st Century."  Geophysical Research Letters 46(13): 7536-7545.  https://doi.org/10.1029/2019gl083383.