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Tropical volcanism enhanced the East Asian summer monsoon during the last millennium

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Tropical volcanism enhanced the East Asian summer monsoon during the last millennium

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Extreme East Asian summer monsoon (EASM) rainfall frequently induces floods that threaten millions of people, and has been generally attributed to internal climate variability. In contrast to the hydrological weakening theory of volcanic eruptions, here we present convergent empirical and modeling evidence for significant intensification of EASM rainfall in response to strong tropical volcanic eruptions. Our multi-proxy analyses show a significantly increased EASM in the first summer after tropical eruptions from 1470 AD to the present, and the more frequent occurrence of El Niños in the first boreal winter after eruptions is necessary for the enhanced EASM. Model simulation ensembles show that a volcano-induced El Niño and the associated stronger than non-volcanic El Niño warm pool air-sea interaction intensify EASM precipitation, overwhelming volcanic-induced moisture deficiency. This work sheds light on the intertwined relationship between external forcing and internal climate variability and potential flood disasters resulting from tropical volcanic eruptions. The probability of an El Niño in the winter after large tropical volcanic eruptions increases. When this happens, summer monsoon precipitation over East Asia is enhanced, overwhelming thermodynamic precipitation reduction from volcanic cooling.
Simulated dynamic and thermodynamic responses of EASM to tropical eruptions modulated by El Niño Plotted are the composite dynamic part (ω′q¯\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{{{{\rm{\omega }}}}}}^{\prime} \bar{q}$$\end{document}) versus thermodynamic part (ω¯q′\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar{{{{{{\rm{\omega }}}}}}}{{{{{\rm{q}}}}}}^{\prime}$$\end{document}) of post-eruption EASM precipitation changes after all 78 eruptions in 11 PMIP last millennium simulations from 1470 to 1849 AD (All, dark dot), as well as those of the 21 eruptions with (EL, orange dot) and 57 eruptions without (nEL, blue triangle) El Niño responses in the first boreal winter after eruptions. CSIRO and HadCM last millennium simulations are not included due to the lack of vertical velocity. Asterisks to the right and bottom of the symbols denote the thermodynamic and dynamic anomalies significant at the 90% confidence level, respectively. The arrow indicates a change from the ensemble mean for eruptions without El Niño responses to those with El Niño responses. Results for each model are also shown by the small dots and triangles. Significant tests have not been performed for single models due to the small sample size of El Niño events. This figure was created using MATLAB 2020a (http://www.mathworks.com).
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ARTICLE
Tropical volcanism enhanced the East Asian
summer monsoon during the last millennium
Fei Liu 1, Chaochao Gao 2, Jing Chai3,4, Alan Robock 5, Bin Wang 6,7 , Jinbao Li 8, Xu Zhang 9,
Gang Huang 4& Wenjie Dong1
Extreme East Asian summer monsoon (EASM) rainfall frequently induces oods that
threaten millions of people, and has been generally attributed to internal climate variability. In
contrast to the hydrological weakening theory of volcanic eruptions, here we present con-
vergent empirical and modeling evidence for signicant intensication of EASM rainfall in
response to strong tropical volcanic eruptions. Our multi-proxy analyses show a signicantly
increased EASM in the rst summer after tropical eruptions from 1470 AD to the present,
and the more frequent occurrence of El Niños in the rst boreal winter after eruptions is
necessary for the enhanced EASM. Model simulation ensembles show that a volcano-
induced El Niño and the associated stronger than non-volcanic El Niño warm pool air-sea
interaction intensify EASM precipitation, overwhelming volcanic-induced moisture deciency.
This work sheds light on the intertwined relationship between external forcing and internal
climate variability and potential ood disasters resulting from tropical volcanic eruptions.
https://doi.org/10.1038/s41467-022-31108-7 OPEN
1School of Atmospheric Sciences Sun Yat-Sen University, Key Laboratory of Tropical Atmosphere-Ocean System Ministry of Education, and Southern Marine
Science and Engineering Guangdong Laboratory, Zhuhai 519082, China. 2College of Environmental and Resource Sciences, Zhejiang University, Hangzhou
310058, China. 3Plateau Atmosphere and Environment Key Laboratory of Sichuan Province, School of Atmospheric Sciences, Chengdu University of
Information Technology, Chengdu 610225, China. 4State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics,
Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China. 5Department of Environmental Sciences, Rutgers University, New
Brunswick, NJ 08901, USA. 6Department of Atmospheric Sciences and International Pacic Research Center, University of Hawaii at Manoa, Honolulu, HI
96822, USA. 7Earth System Modeling Center and Climate Dynamics Research Center, Nanjing University of Information Science & Technology, Nanjing
210044, China. 8Department of Geography, University of Hong Kong, Hong Kong SAR, China. 9Alpine Paleoecology and Human Adaptation Group
(ALPHA), State Key Laboratory of Tibetan Plateau Earth System, Resources and Environment (TPESRE), Institute of Tibetan Plateau Research, Chinese
Academy of Sciences, Beijing, China. email: gaocc@zju.edu.cn;wangbin@hawaii.edu
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The record-breaking EASM rainfall in 2020 caused a
humanitarian disaster, affecting 45.4 million people in
China and resulting in over 142 human casualties and
more than 16 billion US dollars in economic loss1. EASM rainfall
is usually enhanced by moisture advection and convergence due
to southwesterly anomaly of an enhanced western North Pacic
subtropical high, following an El Niño peak in the previous
winter26. A recent study also proposed that El-Niño-related
tropical tropospheric warming could shift the midlatitude wes-
terlies southward, which would impinge on the Tibetan Plateau
and induce northerlies downstream of the plateau to intensify the
East Asian early summer rainband7.
Large volcanic eruptions are one of the major natural external
forcings of Earths climate variability from interannual to cen-
tennial time scales8,9. The EASM is found to be suppressed after
volcanic eruptions in model simulations1012 due to the reduction
of surface shortwave radiation and the associated slowdown of
the global hydrological cycle1316. Volcanic eruptions have been
suggested as the cause of some historical drought events1719.
However, no signicant drought was recorded in the Chinese
chronicles following the large 1815 Tambora eruption20, nor in
observations following the 1982 El Chichón and 1991 Pinatubo
eruptions21. These case studies raise a question about the per-
ception of EASM suppression by volcanic perturbations.
Previous studies of the effects of volcanic eruptions on EASM
commonly overlook the role of El Niño, although it has been
suggested by a number of proxy-reconstructed El Niño/Southern
Oscillation (ENSO) indices that the likelihood of an El Niño
increases after tropical eruptions2228. Two main mechanisms have
been proposed using state-of-the-art model simulations to explain
this connection: the ocean dynamic thermostat mechanism2931
and the land-sea thermal contrast mechanism3234.
This work investigates the response of the EASM-tropical
Pacic system to tropical volcanism to understand the interaction
and relative roles of internal climate feedback and external for-
cing. We analyze long-term multi-proxy data and multi-model
simulations and nd that a volcano-induced El Niño and the
associated warm pool air-sea interaction can intensify the EASM
precipitation, overshadowing the hydrological weakening by
volcanic eruptions.
Results
Proxy evidence of enhanced EASM rainfall following erup-
tions.Werst examine the EASM-tropical Pacic response to 22
precisely dated tropical large volcanic eruptions35 for the period
from 1470 to 1999 AD using the gridded precipitation proxy
data36 and the ensemble mean of 11 sets of ENSO paleoclimate
reconstructions (see Methods).
The composite results of East Asian rainfall reconstruction in
the rst post-eruption summer show signicant (at the 90%
condence level) positive precipitation anomalies centered
around the Yangtze River basin, northeast China, and Kyushu
of Japan, accompanied by negative, albeit not signicant,
anomalies over the Indo-China Peninsula and the Philippines
(Fig. 1a). This meridional dipole precipitation anomaly pattern
resembles the enhanced EASM-Pacic High response in the rst
post-El Niño summer, which is well observed with instrumental
observations2,4,5. Enhanced EASM precipitation in the rst post-
eruption summer is also observed in two other well-known
precipitation reconstructions over China37 and over all of Asia38
(Supplementary Fig. 1). Signicant positive EASM (25°34°N,
106°122°E) precipitation anomalies (at the 95% condence
level), mainly over the middle-lower reach of Yangtze River, are
only observed in the rst boreal summer after the eruption
(Fig. 1b).
Strengthened El Niño-EASM relationship. The ensemble mean
of 11 available ENSO reconstructions indicates a signicant post-
eruption precipitation increase following an El Niño (Fig. 2a), in
contrast to the normal EASM precipitation anomalies after an
eruption without a preceding El Niño (Fig. 2b). Signicantly
negative anomalies, on the other hand, are observed over the
Indo-China Peninsula, Taiwan, and the Philippines, demon-
strating compelling evidence for an El Niño-enhanced Pacic
High and its role in modulating the volcanic-induced EASM
response. For the non-volcanic El Niño (Fig. 2c), the EASM
precipitation increase is much weaker than that after a volcano-
induced El Niño. Our multi-proxy analysis results thus demon-
strate that explosive tropical eruptions tend to strengthen the El
Niño-EASM relationship and increase EASM rainfall in the rst
subsequent boreal summer, suggesting seasonal monsoon-ocean
feedback after large tropical eruptions.
Multi-model evidence of enhanced EASM following eruptions.
Next, we investigate the last-millennium climate simulation
results of 13 different models from the Paleoclimate Modeling
Intercomparison Project phases 3 and 4 (PMIP3 & PMIP4) to
interpret the monsoon-ocean response to tropical large volcanic
eruptions in the context of the current state-of-the-art models.
Based on the actual volcanic forcing used in each model, the
13 simulations amount to 92 eruption events for the common
pre-industrial period from 1470 to 1849 AD, and they are used to
construct a superposed epoch analysis (see Methods). The
average of these 92 eruption simulations shows a relative El Niño
signal, i.e., a reduced zonal gradient of Pacic equatorial sea
surface temperature (SST), in the rst post-eruption boreal winter
(Supplementary Fig. 2a). In the rst post-eruption summer, Indo-
Pacic oceanic cooling and Asian drying are simulated, and
maximum drying is found over the western Indo-China peninsula
(Fig. 3a). Positive precipitation anomalies are simulated over the
middle-lower reach of Yangtze River but are signicant only in a
small area. The associated EASM circulation index, dened by the
lower-tropospheric zonal wind shear vorticity of the enhanced
Pacic High (see Methods), exhibits a signicantly negative
anomaly, demonstrating an enhanced EASM circulation in the
multi-model ensemble mean results (Fig. 3a).
The responses of EASM precipitation to the direct volcanic
forcing and to an El Niño can be differentiated by comparing two
sets of composites with or without the preceding El Niño among
the 92 events (Supplementary Fig. 2). In the rst post-eruption
summer (Fig. 3b), the composite for 65 out of the 92 events
without a preceding El Niño mainly exhibits a direct cooling and
drying volcanic effect over most parts of the globe. A relative El
Niño signal is simulated and the reduced zonal SST gradient
across the Pacic weakly enhances the Pacic High, represented
by an anticyclonic anomaly mainly over the South China Sea.
Studies have established that SST gradients across the tropical
Pacic strongly inuence global rainfall39. Thus, an El Niño-like
response in non-El Niño events tends to offset the cooling-
induced dry monsoon over East Asia, resulting in an insignicant
decrease in the EASM rainfall, consistent with reconstructions
(Fig. 2b). In the presence of a preceding El Niño, the model
results show increased post-eruption summer precipitation in the
EASM region, and the most signicant enhancement is located
over the middle-lower reach of the Yangtze River, driven by the
convergence and moisture advection of southwesterly wind
anomalies of the enhanced Pacic High and extratropical
northerly anomalies, albeit not signicant, downstream of the
Tibetan Plateau (Fig. 3c). Thus, the post-eruption El Niño cases
can overwhelm the no-post-eruption El Niño cases, resulting in
an overall increase in the EASM rainfall.
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This El Niño-increased EASM precipitation after volcanic
eruptions is also simulated when considering all 228 eruption
events during the last millennium covering 8501849 AD in all 13
PMIP models (Supplementary Fig. 3). Ten Community Earth
System Model (CESM) last-millennium ensemble simulations
from 1470 to 1999 AD also conrm the critical role of El Niño
after volcanic eruptions in increasing EASM precipitation
(Supplementary Fig. 4).
Volcanic-induced Pacic air-sea interaction. Eruption-induced
radiative cooling leads to an El Niño in the post-eruption winter
or an El Niño-like response in the following summer when the El
Niño is not fully developed. The responses are determined by the
coupled atmosphere-land-ocean dynamics. The decrease in SST
gradient, i.e., relative warmer eastern Pacic, can be initiated by
eruption-induced global cooling through the ocean dynamic
thermostat mechanism29,40 and land-sea thermal contrast32,33.
This decreased SST gradient gives rise to a weakened pressure
gradient and hence weaker easterly winds and Walker circulation
(Supplementary Fig. 2a), which may, in turn, reduce the SST
gradient, a mechanism known as the Bjerknes feedback33,41.
Among the 92 eruption events, the post-eruption EASM
precipitation change is signicantly correlated with the preceding
El Niño (0.19, at the 90% condence level) two seasons before,
and the correlation is higher for the EASM circulation (0.44, at
the 99% condence level) than for the precipitation. This is
because the EASM circulation index, i.e., the Pacic High, is
better captured by numerical models than precipitation42. These
signicant relations indicate that a stronger El Niño tends to
induce a larger EASM increase, consistent with the instrumental
observations43.
After a volcano-induced El Niño, strong negative precipitation
anomalies are simulated over South Asia, consistent with previous
studies demonstrating the drying effect of large volcanic eruptions
on the South Asian monsoon44,45. The enhanced Pacic High is
Fig. 1 East Asian summer monsoon (EASM) response to tropical eruptions based on reconstructions. a Superposed epoch analysis results of East Asian
precipitation anomalies (shading) in the rst boreal summer after 22 tropical eruptions from 1470 to 1999 AD. Stippling indicates precipitation anomalies
not signicant at the 90% condence level. The red curve is the Yangtze River, and the red box indicates the EASM region (25°34°N, 106°122°E).
bComposite EASM-averaged precipitation anomaly for 22 tropical eruptions. Condence limits (90, 95, and 99%) are marked by horizontal lines. Red and
blue colors mark the post-eruption and pre-eruption composites, respectively. Year 0 denotes the eruption year. This gure was created using MATLAB
2020a (http://www.mathworks.com).
Fig. 2 Role of El Niño in post-eruption EASM change based on reconstructions. Superposed epoch analysis results of East Asian precipitation anomalies
(shading) in the rst boreal summer after a9 tropical eruptions with and b13 eruptions without an El Niño in the rst winter after the eruption, and after
c148 non-volcanic El Niños during the period of 14701999 AD. An El Niño event is dened by the average of the 11 reconstructions of the ENSO index
(see Methods). Stippling indicates precipitation anomalies not signicant at the 90% condence level. The red curve is the Yangtze River, and the red
box indicates the EASM region. This gure was created using MATLAB 2020a (http://www.mathworks.com).
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tied to the SST cooling to its southeast (Fig. 3c), explained by the
wind-evaporation SST feedback after an El Niño4. Easterly wind
anomalies over the North Indian Ocean, which were also
suggested to increase the Pacic High5, are only signicant over
the Arabian Sea. Volcanic eruptions act to enhance this post-El
Niño western North Pacic SST cooling and intensify the Pacic
High (Fig. 3d), as found by previous simulations26,46. The
resulted EASM enhancement is much larger than a non-volcanic
El Niño would induce, conrming the proxy-nding of volcanic
strengthened El Niño-EASM relationship.
The change of monsoon precipitation can be divided into
dynamic and thermodynamic causes47, which are related to the
changes in circulation and moisture availability, respectively (see
Methods). After a volcanic eruption, dynamic-related precipita-
tion is increased while the thermodynamic-related precipitation is
decreased, demonstrating the volcanic-induced cooling and
drying (Fig. 4). The decreased thermodynamic-induced precipita-
tion counteracts the increased dynamic-induced precipitation,
resulting in a weak change in total precipitation. The presence of
an El Niño tends to enhance the dynamic-related precipitation,
while keeping the thermodynamic-related precipitation almost
intact (Fig. 4). Results from these last-millennium experiments
conrm the critical role of a preceding El Niño in enhancing the
EASM, against the well-known cooling and hydrological weak-
ening effect of volcanic eruptions1316.
Discussion
The fraction of El Niños occurring after volcanic eruptions in
reconstructions is 41% for the study period from 1470 to 1999
AD (Fig. 2), or 44% if we expand the period to cover the last
millennium from 901 to 1999 AD, which are both larger than the
expected percentage (27%) for El Niño events without a volcanic
effect. Despite the small sample sizes, i.e., 22 or 39 eruptions in
the two periods, respectively, the results consistently suggest that
tropical eruptions do increase the likelihood of an El Niño. This
explains why the composite EASM rainfall after all eruptions is
increased in reconstructions. In the models, this count fraction is
only 29% for PMIPs (Fig. 3), resulting in a feeble EASM rainfall
increase in the all-eruption case. This less frequent occurrence of
El Niño, against a robust equatorial Pacic westerly response to
tropical volcanic eruptions, was argued to be caused by weak
coupling between central Pacic precipitation and westerly
anomalies in the PMIP3 and CESM models48.
Although our ensemble analysis shows that a tropical eruption
can increase the likelihood of an El Niño, individual paleoclimate
reconstructions exhibit divergent responses22,28,49,50. We use the
ensemble mean to maximize the signals, but the results might be
affected by the proxy overlap among the 11 available ENSO
indices. The divergent ENSO26,27 and hydrology47,5154 respon-
ses, plus the time-dependent shifts of the Intertropical Con-
vergence Zone (ITCZ)55 to volcanic eruptions with asymmetric
hemispheric distributions further complicate the volcano-
EASM relationship quantication. Future study expanding to
Northern or Southern Hemispheric eruptions should correct for
SST variations associated with such ITCZ shifts.
To conclude, this work nds an east-west asymmetric
monsoonal-ocean response to the zonally symmetric volcanic
forcing, highlighting the delayed oceanic responses to volcanic
eruptions in exciting EASM precipitation changes. A precipita-
tion increase due to wind convergence induced by the enhanced
western North Pacic high overwrites the precipitation reduction
due to the thermodynamically induced moisture deciency. The
tropical Pacic plays a critical role in modulating the climate
system response to external volcanic forcing, i.e., increasing the
likelihood of an El Niño and enhanced post-El Niño warm pool
air-sea interaction. Similar cases are found in the Nuclear Niño34
or the northern Eurasia winter warming responses to strato-
spheric soot and sulfate aerosols injections during volcanic
scenarios56. Results obtained from this work demonstrate the
important role and complexity of the coupled atmosphere-ocean
dynamics in Pacic, and shed light on the potential impact of
volcanic eruptions on EASM hydroclimate.
Fig. 3 Simulated EASM-ocean responses to tropical eruptions. Composite SST anomalies (shading over ocean), precipitation anomalies (shading over
land), and 850 hPa wind anomalies (vectors) in the rst boreal summer after aall 92 simulated tropical eruptions, b65 eruptions without, and c27
eruptions with an El Niño response in the rst boreal winter after the eruption, as well as dcomposites for 1215 non-volcanic El Niños in 13 PMIP last
millennium simulations from 1470 to 1849 AD. Stippling and gray vectors indicate precipitation and temperature anomalies and wind anomalies not
signicant at the 90% condence level, respectively. The red curve is the Yangtze River. The red rectangles denote the locations where the EASM
circulation index is dened: the 850 hPa zonal wind averaged in the southern box minus that in the northern box (see Methods). A negative value of this
index represents enhanced Pacic high and associated East Asian subtropical rainfall. Maps created with The NCAR Command Language (Version 6.6.2)
[Software]. (2019). Boulder, Colorado: UCAR/NCAR/CISL/TDD. https://doi.org/10.5065/D6WD3XH5.
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Methods
Proxy data. The proxy data we analyzed mainly include a long-term reconstruc-
tion of Asian summer precipitation36 and 11 reconstructed ENSO indices (Sup-
plementary Table 1). The Feng et al.36 data are a gridded (0.5° × 0.5°)
reconstruction of annual MaySeptember precipitation from 1470 to 1999 AD over
the whole Asian continent and are mainly based on 500-year historical doc-
umentary records, tree-ring data, ice-core records, and a few long-term instru-
mental data series. A regional empirical orthogonal function method is utilized to
increase the signal-to-noise ratio. Two other gridded reconstructions of pre-
cipitation based on point-by-point regression methods are also analyzed in parallel,
serving as conrmation of the results: a 530-yr multi-proxy MaySeptember pre-
cipitation reconstruction over China (0.5° × 0.5°) based on the tree-ring chron-
ologies and drought/ood indices from 1470 to 2000 AD37, and an annual June-
August rainfall reconstruction over the Asian land region (2° × 2°) generated by
merging 453 tree-ring width chronologies and 71 historical documentary records
from 1470 to 201338.
To test the potential role of a preceding El Niño on volcanic-induced EASM
rainfall change, the ensemble mean of 11 available ENSO indices is reconstructed
and applied, following the method used before26 except adding one new proxy49.
All of the 11 ENSO indices rst go through a 9-yr Lanczos high-pass lter57 to
isolate the ENSO signal, and then are normalized according to their own standard
deviations. The ensemble mean is utilized to remove proxy uncertainty. Since these
11 indices cover different periods, with the longest extending from 900 to 2002 AD
and the shortest from 1706 to 1997 AD, the ensemble mean for each year from
1470 to 1999 AD is calculated as the average of available indices. The ensemble
mean with any individual index removed is still highly correlated (above the 99%
condence level), with a minimum correlation coefcient of 0.95, to the ensemble
mean of all 11 indices. The resulting ensemble mean has a high correlation with the
instrumental DecemberFebruary Niño3.4 index of 0.83 (at the 99% condence
level) for the period of 18711999, based on the averaged (5°N5°S, 120°170°W)
SST anomaly from Hadley Centre Ice and SST version 1 (HadISST1)58.
All 22 tropical eruptions from the Sigl et al.35 volcanic reconstructions during
the period of 14701999 AD, when Asian precipitation proxies are available, are
used for the composite analysis. We dene the peak aerosol loading year as the
eruption year 0 (Supplementary Table 2).
Last-millennium simulations. All of the 10 last-millennium climate simulations in
the PMIP3 and three simulations in the PMIP4 are analyzed to understand the
underlying mechanisms of observed EASM response to large volcanic eruptions
(Supplementary Table 3). Additional outputs from CESMs 10 last-millennium all-
forcing simulations are also investigated, due to CESMs good performance in
simulating ENSO seasonality, amplitude, frequency, and teleconnection59. Since
different volcanic forcing reconstructions are used in the last-millennium simula-
tions from PMIP models and CESM (Supplementary Table 3), the accuracy of the
eruption date and strength in each last-millennium simulation is determined
according to the volcanic forcing used, and year 0 denotes the year with maximum
annual-mean stratospheric sulfate aerosol injection for each eruption, consistent
with the using of peak aerosol loading year in reconstruction analysis35. A tropical
eruption is dened when it has an aerosol density or aerosol optical depth in both
hemispheres, following the ice core-based forcing reconstructions60,61. Thus, tro-
pical eruptions have their maximum aerosol density or optical depth in the tropics
(20°S20°N)47. Since the PMIP simulations only cover the pre-industrial period
before 1850, we have 92 eruption events in the 13 PMIP model simulations for a
period of 14701849 AD and 228 events for the whole last millennium of 8501849
AD, listed in Supplementary Table 4. In the 10 full forcing ensembles of CESM we
have 90 eruption events for the period of 14701999 AD when the Asian pre-
cipitation reconstructions are available.
-15 -10 -5 0 5 10
10-6
-5
0
5
10
15
10-5
CCSM4
FGOALS-s2
MRI
BCC
MPI
EC-Earth3
MIROC-ES2L
MRI-ESM2
EL
CCSM4
IPSL
FGOALS-s2
MRI
BCC
MPI
GISS
MIROC
EC-Earth3
MIROC-ES2L
MRI-ESM2
All volcano cases
Volcano with El Nino
Volcano without El Nino
~
~
ALL
nEL
Fig. 4 Simulated dynamic and thermodynamic responses of EASM to tropical eruptions modulated by El Niño. Plotted are the composite dynamic part
(ω0
q) versus thermodynamic part (
ωq0) of post-eruption EASM precipitation changes after all 78 eruptions in 11 PMIP last millennium simulations from
1470 to 1849 AD (All, dark dot), as well as those of the 21 eruptions with (EL, orange dot) and 57 eruptions without (nEL, blue triangle) El Niño responses
in the rst boreal winter after eruptions. CSIRO and HadCM last millennium simulations are not included due to the lack of vertical velocity. Asterisks to the
right and bottom of the symbols denote the thermodynamic and dynamic anomalies signicant at the 90% condence level, respectively. The arrow
indicates a change from the ensemble mean for eruptions without El Niño responses to those with El Niño responses. Results for each model are also
shown by the small dots and triangles. Signicant tests have not been performed for single models due to the small sample size of El Niño events. This
gure was created using MATLAB 2020a (http://www.mathworks.com).
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Composite and signicance. A superposed epoch analysis22 is used to evaluate
the inuence of explosive tropical volcanoes on global cooling and related EASM-
ocean interaction. To isolate the climate responses to an eruption from the back-
ground noise, we obtain the anomaly by removing the climatology of the ve years
preceding the eruption. We apply the superposed epoch analysis to the recon-
structed EASM precipitation and Niño3.4 indices based on the 22 reconstructed
tropical eruptions, and internal variability is expected to be ltered out. A super-
posed epoch analysis is also performed on multi-model simulations from PMIP3
and PMIP4 and on multi-ensemble simulations from CESM. The signicance of
the superposed epoch analysis results for the 11-year window of ve years before
and six years after each eruption is calculated by the bootstrapped resampling
method with 10,000 random draws from the full 500-year (i.e., 14701990 AD)
study period22.
Climate indices and variability. The Niño3.4 index is dened as the area-averaged
SST anomaly over the area (5°N5°S, 120°170°W). To use the same standard for
selecting an El Niño event list among reconstructions with different scales, an El
Niño event is dened when the boreal winter Niño3.4 index is >0.5 standard
deviations in both reconstructions and simulations. A non-volcanic El Niño is
dened as when it occurs in a normal year without an eruption. Using different
thresholds to select El Niño events does not change our results qualitatively. Boreal
summer is dened as MaySeptember, and boreal winter, as DecemberFebruary,
consistent with those used for reconstructions. The EASM region is dened as the
region over (25°34°N, 106°122°E) in both reconstructions and simulations.
Based on the simulated western North Pacic subtropical high, the EASM circu-
lation index is dened as the 850-hPa zonal wind difference between the northern
(22°32°N, 130°160°E) and southern (9°19°N, 120°150°E) regions, a little dif-
ferent from the original index between northern (22.5°32.5°N, 110°140°E) and
southern (5°15°N, 90°130°E) regions from instrumental observations62.
The monsoon precipitation index Pcan be represented by moisture
convergence47,63:
P0¼ð
qωq0
ωþq0ω ð1Þ
where qdenotes surface specic humidity and ωis the pressure velocity at 500 hPa.
The overbar denotes the mean state before the eruption, while the prime, the
anomaly after the eruption.
qω0and q0
ωdenote the dynamic and thermodynamic-
related precipitation changes, respectively. q0ω0represents the nonlinear feedback
and is usually much smaller than the dynamic or thermodynamic counterparts.
Data availability
All data used in this study were obtained from publicly available sources. The PMIP
model outputs are distributed by the Earth System Grid Federation (ESGF) at https://
esgf-node.llnl.gov/projects/esgf-llnl/ (see Supplementary Table 3 for model details and
references). Output from the CESM Last Millennium Ensemble (CESM-LME) can be
downloaded at https://www.earthsystemgrid.org/dataset/ucar.cgd.ccsm4.cesmLME.html.
Proxy reconstruction datasets are available at the National Centers of Environmental
Information via https://www.ncdc.noaa.gov/data-access/paleoclimatology-data/datasets/
climate-reconstruction.
Code availability
The code used to create Figs. 14in this study is available on https://zenodo.org/record/
6503745. Other analytical scripts are available from the authors upon request. All of the
gures are created by the authors using MATLAB 2020a (http://www.mathworks.com/)
or NCAR Command Language (NCL; http://www.ncl.ucar.edu/). The atlas imbedded in
the gures are the by-default atlas from MATLAB or NCL.
Received: 21 November 2021; Accepted: 20 May 2022;
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Acknowledgements
F.L. is supported by the National Natural Science Foundation of China (Grant
41975107). C.G. is supported by the National Natural Science Foundation of China
(Grant 41875092). A.R. is supported by the U.S. National Science Foundation grant AGS-
2017113. B.W. acknowledges support from U.S. National Science Foundation grant
2025057. G.H. is supported by the National Natural Science Foundation of China (Grant
41721004, 41831175).
Author contributions
F.L., C.G., and B.W. conceptualized and led the work. F.L., C.G., and J.C. contributed to
data analysis including validation and interpretation of the results. F.L., C.G., A.R., and
B.W. wrote the manuscript, J.L., X.Z., G.H., and W.D. reviewed and edited the
manuscript.
Competing interests
The authors declare no competing interests.
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... However, Lin et al. (2018) proposed that extreme precipitation is more sensitive to aerosol forcing than it is to greenhouse-gas forcing. Natural forcing, such as a strong tropical volcanic eruption, can cause a significant enhancement in East Asian Summer Monsoon (EASM) rainfall (Liu et al., 2022a). ...
... Volcanic eruptions and solar irradiance are the main natural external forcings. Related studies (Liu et al., 2022a) show that EASM rainfall is significantly enhanced during strong tropical volcanic eruptions. Therefore, it is necessary to conduct an attribution analysis of natural Observation (a, b, HadEX), CMIP6 multimodel simulations with ALL (c, d), GHG (e, f), AER (g, h) and NAT (i, j) forcing. ...
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