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Studies reveal the influence mechanism of volcanic eruptions and Atlantic Multi-Decadal Oscillation on Asian summer monsoon

New studies on the influence of volcanic eruptions on the Asian summer monsoon (ASM) over inter-decadal timescales reveal that over the last 110 years, volcanic eruptions have influenced ASM variations on an inter-decadal timescale via teleconnections with the Atlantic Multi-Decadal Oscillation (AMO).

To investigate the long-term variability of the ASM over the last 110 years, the high-resolution monthly observed precipitation data which is contrary to previous studies on inter-annual timescale were used to define the ASM index.

The findingentitled as “Influences of volcano eruptions on ASM over the last 110 years” were published in Scientific Reports, a journal subordinated to Nature on February 16th 2017. The lead author of this paper is Liang Ning, the associate professor of the School of Geography Science of Nanjing Normal University.

“ASM precipitation is the primary water resource for agriculture in many Asian countries that have experienced rapid economic growth in recent decades, thus implying the necessity for further investigations on both the internal variability of the ASM and the influence of external factors on the ASM.” Professor Liang Ning said.

Opposite trends in the ASM have been observed in different periods over the last six decades, indicating that ASM variability and the corresponding mechanisms should be investigated over a longer time period. In addition to its own internal variability, the ASM variability is also influenced by external forcing, including natural forcing such as solar radiation and volcanic eruptionsand anthropogenic forcing such as greenhouse gas emissions and land use/land cover change.

Regarding the influence of volcanic eruptions, previous studies have shown that large volcanic eruptions affect summer monsoon precipitation on global and regional scales. Over the Asian monsoon region, large volcanic eruptions correspond to a weakening of the East Asian Summer Monsoon (EASM) circulation and subsequent reductions in EASM rainfall although the impacts are relatively short lived while on the aspect of long-term influence, both proxy records and model simulations suggest that volcanic eruptions have played a dominant role in pacing the AMO, which has a potential influence on the EASM, since the end of the Little Ice Age.

These previous findings were further confirmed in this paper, and the mechanisms underlying the influence of the AMO on ASM precipitation were revealed at the same time.

The research on the ASM precipitation in the volcanic eruption periods was madeby using long-term high-resolution observed precipitation data and Coupled Model Intercomparison Program Phase 5 (CMIP5) model simulations.
The results suggest that: (1) during the active volcanic eruption periods (1901–1935 and 1963–1993), significantly lower ASM precipitation was observed compared with that during the inactive volcanic eruption period (1936–1962); (2)volcanic eruption is the main reason for the variability of ASMover inter-decadal timescales. By accessing the differences in the detrended winter and summer sea surface temperature (SST) between the inactive volcanic eruption period (1936–1962) and active volcanic eruption periods (1901–1935 and 1963–1993), it is found that the SSTs during the inactive volcanic eruption periods were significantly higher over the entire northern Atlantic than they were in the active volcanic eruption periods.

To further investigate the influence mechanism of volcanic eruptions and AMO on ASM, methods of linear correlation and linear regression are used to analyze the relationship between AMO and ASM precipitation and the effect of AMO on the SST, relative humidity and specific humidity.

“We found that during active volcanic eruption periods, which correspond to a negative AMO state, there is an anomalously weakened Walker circulation over the tropical Pacific that transports less moisture to the ASM region and subsequently reduces ASM precipitation.”Professor Liang Ning said.

“This new finding may help improve decadal predictions of future changes in the ASM. Besides, it is also beneficial for theup-coming resource manager and decision-makers on regulatingsuitable plans for the sustainable economic development” Professor Ning added.

Professor Jian Liu and doctor Weiyi Sun are the second and third author respectively.

This research was jointly supported by the National Natural Science Foundation of China (Grant No. 41501210), the National Key Research and Development Program of China (Grant No. 2016YFA0600401), the National Natural Science Foundation of China (Grant No. 41420104002 and Grant No. 41371209), the Jiangsu Province Natural Science Foundation (Grant No. BK20150977), and the Priority Academic Development Program of Jiangsu Higher Education Institutions (Grant No. 164320H116).