Every summer, the East Asian monsoon brings rainfall to China, Korea, Japan, and surrounding regions. Over one third of the global population resides in the paths of these warm storms. Because of their societal impact, the rains have been the subject of much research investigating their dynamics.
In a new study of East Asian monsoon rains, Chiang et al. investigate the causes behind year-to-year changes in the geographic patterns of rainfall that occur over the course of each summer. They used wind and rainfall data to test a hypothesis proposed by Chiang and other colleagues in 2015, which had previously been supported only by model simulations.
The 2015 hypothesis proposed that changes in rainfall patterns are caused by shifts in the timing and duration of four well-known stages of varying rainfall patterns that occur over the course of every summer monsoon; these stages are labeled by the authors as spring (April through mid-May), pre-Meiyu (mid-May through mid-June), Meiyu (mid-June to mid-July), and midsummer (mid-July through mid- to late August; also known as the post-Meiyu period). In turn, shifts in the timing and duration of these stages may be caused by northward drift of westerly winds relative to the Tibetan Plateau.
To test this idea, the researchers probed daily wind and rainfall data gathered from 1951 to 2007. They analyzed the rainfall data using a computational tool called a self-organizing map, which automatically groups the complex data, revealing the rainfall patterns associated with each stage. They used this technique to investigate the annual timing and duration of the stages, including Meiyu, or “plum rain,” a particularly rainy period over central eastern China that occurs in the early summer.
The analysis revealed a link between the timing of Meiyu and typical geographic rainfall patterns in July and August. Specifically, during years in which July and August saw less rain in central eastern China and more rain in northeastern and southeastern China, Meiyu ended early, and the next stage, midsummer, lasted longer.
Further analysis revealed a link between Meiyu timing changes and shifting wind patterns. Over the course of a typical summer, westerly winds that flow across the Tibetan Plateau and into eastern China gradually shift north of the plateau. During years when Meiyu ended early, these westerly winds migrated northward earlier than usual.
Overall, the data support the 2015 hypothesis, but more research is needed to confirm the role of the westerly winds and determine how they might control the timing of Meiyu, midsummer, and other stages. These findings could ultimately help improve seasonal rainfall forecasts and aid efforts to predict how climate change might affect East Asian rainfall in the future. (Geophysical Research Letters, https://doi.org/10.1002/2017GL072739, 2017)
—Sarah Stanley, Freelance Writer
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