Origins of East Asian Summer Monsoon Seasonality

Journal Article · Mon Aug 17 00:00:00 EDT 2020 · Journal of Climate

DOI:https://doi.org/10.1175/jcli-d-19-0888.1· OSTI ID:1802555

Chiang, J. C. H. ^[1]; Kong, W. ^[2]; Wu, C. H. ^[3]; Battisti, D. S. ^[4]

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1. Univ. of California, Berkeley, CA (United States); OSTI
2. Univ. of California, Berkeley, CA (United States)
3. Academia Sinica, Taipei (Taiwan)
4. Univ. of Washington, Seattle, WA (United States)

The East Asian summer monsoon is unique among summer monsoon systems in its complex seasonality, exhibiting distinct intraseasonal stages. Previous studies have alluded to the downstream influence of the westerlies flowing around the Tibetan Plateau as key to its existence. We explore this hypothesis using an atmospheric general circulation model that simulates the intraseasonal stages with fidelity. Without a Tibetan Plateau, East Asia exhibits only one primary convective stage typical of other monsoons. As the plateau is introduced, the distinct rainfall stages—spring, pre-mei-yu, mei-yu, and midsummer—emerge, and rainfall becomes more intense overall. This emergence coincides with a pronounced modulation of the westerlies around the plateau and extratropical northerlies penetrating northeastern China. The northerlies meridionally constrain the moist southerly flow originating from the tropics, leading to a band of lower-tropospheric convergence and humidity front that produces the rainband. The northward migration of the westerlies away from the northern edge of the plateau leads to a weakening of the extratropical northerlies, which, coupled with stronger monsoonal southerlies, leads to the northward migration of the rainband. When the peak westerlies migrate north of the plateau during the midsummer stage, the extratropical northerlies disappear, leaving only the monsoon low-level circulation that penetrates northeastern China; the rainband disappears, leaving isolated convective rainfall over northeastern China. In short, East Asian rainfall seasonality results from the interaction of two seasonally evolving circulations—the monsoonal southerlies that strengthen and extend northward, and the midlatitude northerlies that weaken and eventually disappear—as summer progresses.

• Research Organization: Univ. of California, Oakland, CA (United States)
• Sponsoring Organization: National Science Foundation (NSF); USDOE Office of Science (SC)
• Grant/Contract Number: SC0014078
• OSTI ID: 1802555
• Journal Information: Journal of Climate, Journal Name: Journal of Climate Journal Issue: 18 Vol. 33; ISSN 0894-8755
• Publisher: American Meteorological Society
• Country of Publication: United States
• Language: English

Related Subjects

54 ENVIRONMENTAL SCIENCES
Asia
atmosphere
climate models
meteorology & atmospheric sciences
monsoons
seasonal cycle
topographic effects