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During strong East Asian summer monsoons, the Yangtzee River Valley gets less rain than normal – in present times and possibly also in the past, according to recently published research. This satellite image shows the region upstream of the Three Gorges Dam in November, 2006. Credit: Jesse Allen, NASA.

The tripole pattern of East China precipitation over the past 425,000 years

Rainfall patterns in China over the past half a million years may have been more like the present than previously thought. Gaowen Dai writes about his new study.

Written by Gaowen Dai, guest researcher / PhD candidate at the Bjerknes Centre and NORCE

The classical paleoclimatological view holds that monsoonal China was overall wetter when the East Asian summer monsoon was strong during glacial and interglacial cycles. However, this view is at odds with the understanding of modern climate, which demonstrates an analogous “tripole precipitation pattern” in monsoonal China.

Over the past several decades, when the East Asian summer monsoon is strong, more precipitation falls over the Yellow River Valley (North China) and the Pearl River Valley (South China) while less precipitation appears in the Yangtze River Valley (Central-East China). During weak summer monsoons, the pattern is the opposite – less precipitation in the valleys in the north and the south, and more in the central, Yangtze River Valley. 

Tripole precipitation pattern
The changes in ensemble mean precipitation (unit: mm, shading colors) due to intensified summer monsoon. (a) Precipitation anomalies from May to September. (b) Anomalies in annual mean precipitation. Only changes that are significant at the 95% confidence level are shown. For both (a and b), the black vectors indicate the anomaly summer (JJA) wind (m/s) at 850 hPa due to intensified summer monsoon. The red rectangle shows the Central-East China region. From Dai et al., 2021

Using NorESM-L climate models, we showed that such analogous “tripole precipitation pattern” in monsoonal China also may have appeared during the past 425,000 years. To some extent this result can be compared to the paleoclimate records available from China.

Our results agree with the present understanding of modern East Asian climate, and furthermore confirm that the boreal summer insolation is the dominant forcing for the intensity of the East Asian summer monsoon, and that the response of subtropical high pressures is fundamental in modulating the precipitation pattern in monsoonal China on orbital timescales.

This temporal and spatial variability of precipitation shows the potential high complexities in hydroclimatic conditions in monsoonal China throughout the past glacial-interglacial cycles. Nevertheless, the tripole precipitation pattern in monsoonal China during glacial-interglacial cycles will be further validated by new well-dated precipitation proxies that can explicitly indicate precipitation from Central-East China.

Reference

Dai, G., Zhang, Z., Otterå, O. H., Langebroek, P. M., Yan, Q., and Zhang, R., 2021. A Modeling Study of the Tripole Pattern of East China Precipitation Over the Past 425 ka. Journal of Geophysical Research: Atmospheres 126.