East Asian historiographical traditions tell of flourishing civilization based on successful control of floods, and dynasty collapse due to failed adaption to drought. Located between the largest continent (Eurasia) and ocean (Pacific), the change of East Asian summer monsoon causes East Asian flood and drought during its extreme wet and dry phases driven by natural climate variations. But volcanic eruptions complicate the picture. Tropical volcanic eruptions tend to slowdown the hydrological cycle by reducing sunlight at the surface. At the same time, they increase the likelihood of having an El Niño, as seen in paleoclimate analyses and model simulations. It was not clear, however, how the East Asian summer monsoon would respond following tropical eruptions, due to the combined impacts of direct volcanic drying and the impacts on atmospheric circulation from the resulting El Niño.

In this context, Fei Liu from Sun Yat-sen University, Chaochao Gao from Zhejiang University and Bin Wang from the University of Hawaii at Manoa led a study exploring the robust impacts of explosive tropical volcanism on East Asian summer monsoon throughout the last millennium (Liu et al., 2022).

This study employed comprehensive multi-proxy and multi-numerical-model analyses. They include long-term reconstructions of Asian summer precipitation based on Chinese documents and tree rings, a multi-ice-core reconstruction of explosive volcanism from which 22 large tropical eruptions with precise dates were extracted, 11 sets of reconstructed El Niño indices, and last-millennium climate simulations of 14 different climate models participating in Climate Model Intercomparison Projects 5 and 6.

The merged results showed significant intensification of East Asian summer monsoon rainfall in response to strong tropical volcanic eruptions, mainly around the Yangtze River basin, northeast China, and Kyushu of Japan. The more frequent occurrence of El Niños in the first post-eruption winter is necessary for this enhancement, and tropical eruptions increase the likelihood of El Niño from an average without volcanic eruptions of 27% to 41%. Tropical eruptions also cool the western Pacific after an El Niño to enhance the Pacific subtropical high pressure, which brings more moisture to East Asia.

In contrast to the hydrological weakening theory of volcanic eruptions, this work demonstrates that the volcanic intensified El Niño influence could overwrite the direct aerosol drying effect on East Asian summer monsoons. The results shed light on the intertwined relationship between external forcing and internal climate variability resulting from stratospheric soot and sulfate aerosols injections during volcanic, nuclear scenarios, or stratospheric aerosol geoengineering (also known as “solar radiation modification”). In a future warming world, more moisture in the atmosphere is likely to strengthen such connections (Yang et al., 2022) and potential flood disasters.   

References:

Liu, Fei, Chaochao Gao, Jing Chai, Alan Robock, Bin Wang, Jinbao Li, Xu Zhang, Gang Huang and Wenjie Dong. Tropical volcanism enhanced the East Asian summer monsoon during the last millennium. Nature Communications, 13, 3429 (2022). https://doi.org/10.1038/s41467-022-31108-7

Yang, Linshan, Yujuan Gao, Chaochao Gao, and Fei Liu. Climate responses to Tambora-size volcanic eruption and the impact of warming climate. Geophysical Research Letters, 49, e2021GL097477 (2022). https://doi.org/10.1029/2021GL097477