Shifting summer rains

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Science  02 Nov 2018:
Vol. 362, Issue 6414, pp. 518-520
DOI: 10.1126/science.aav5280

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Most of China's water supply depends on rainfall from the East Asian summer monsoon (EASM), a seasonal progression of rains that begins along the southern coast in spring, then sweeps north, reaching northeastern China in midsummer (see the photo). Projections of the EASM's response to future climate change are complicated by its complex interaction with the mid-latitude jet stream, which appears to govern the monsoon's northward march each spring and summer (1). To investigate the monsoon's sensitivity and dynamics, many scientists have turned to examining its past changes recorded in natural archives. Although past climates are not a direct analog of the 21st-century climate, they offer vital tests of the ability to describe monsoon behavior through theories and numerical models. On page 580 of this issue, Zhang et al. (2) provide new insight into the impact of past EASM and jet stream variations on rainfall patterns over China. Through examination of trace elements in Chinese stalagmites (a proxy for local precipitation amount) and climate modeling experiments, they show that cooling episodes in the North Atlantic shifted the summer jet stream south, delaying the onset of monsoon rains in northeastern China and increasing rainfall in central China. The finding demonstrates that local rainfall in the EASM regions can vary in opposition to monsoon strength, and it highlights the importance of future high-latitude warming in determining precipitation patterns in China.