Effects Of Volcanic Activity Over The Past Millennium On East Asian Summer Monsoon Precipitation In Reconstructions And Simulations

Prediction of East Asian summer monsoon(EASM)change is of great importance for our daily life due to its effect on about one third of the world's population.Climate variation of the EASM is caused by both internal variability and external forcing.However,our understanding of the EASM's sensitivity to external climate forcing remains uncertain.Here,we focus on volcanic forcing since volcanism is known to be a leading natural external forcing of climate change on the interannual timescale.Besides,global warming is one of the most serious challenges for human beings during 21^st century.At present,the use of global emission reduction measures to slow down the global warming trend is still facing great challenges.Therefore,the scientific community has proposed a solar radiation intervention geoengineering method with the goal of reducing the solar radiation reaching the atmosphere and the ground.Stratospheric aerosol injection and surface albedo increase are considered as a backup approach to mitigate global warming.The most important mechanism by which a volcano perturbs climate is the injection of large amounts of SO₂ gas into the stratosphere.The resulted aerosols as stratospheric geoengineering's natural analogues could provide important reference for us to understand the impact of stratospheric geoengineering on EASM.Using observations,multi-source reconstruction data,Paleoclimate Modeling Inter-comparison Project Phase 3/4(PMIP3/PMIP4)and Community Earth System Model(CESM)models in the past millennium as well as the superposed epoch analysis,diagnostic analysis,numerical simulation and other methods,the temporal and spatial characteristics of EASM precipitation influenced by volcanic eruptions are investigated.The main conclusions are summarized as follows:(1)After an explosive tropical volcano eruption,it will not only have a direct climate effect on EASM precipitation,but will also be modulated by internal mode.The Tambora eruption of April 1815 is one of the most explosive tropical volcanic eruptions,which resulted in the so-called“year without summer”in many locations around the world in 1816.However,based on the long-term reconstruction of precipitation,the EASM precipitation is found to be enhanced by the equatorial eruptions.This response can be convincingly confirmed by all tree ring and Chinese documentary proxies used in this study.We show that the suppressed EASM is a forced response to external forcing,whereas the first EOF mode of model control run represents internal variability with increased EASM.The increased type responds to both external forcing and internal variability,while the decreased type responds mainly to external forcing.(2)Using three state of the art paleoclimate reconstructions of EASM precipitation,we show a robust evidence for a significantly response of increased EASM in the first summer after explosive tropical volcanism.14 model simulation results demonstrate the key role of El Ni?o in the first boreal winter in the identified EASM intensification,which is related to cold SST anomaly over western North Pacific caused by a decaying El Ni?o.Finally,multi-source reconstruction datasets show increased likelihood of El Ni?o after tropical eruptions,supporting this El Ni?o-enhanced EASM relationship.(3)Through examining simulations over the last millennium by 11 different models,we show that a tropical volcano eruption can robustly excite a western-to-central equatorial Pacific(WCEP)westerly anomaly at 850 h Pa in eight out of the 11 models;such a westerly anomaly is favorable for El Ni?o development.Under the volcanic forcing,there are significant extratropical continent surface cooling and tropical drying with negative precipitation anomalies over the South-South East Asia(SSEA),West African monsoon,and Intertropical Convergence Zone(ITCZ)regions.This common precipitation suppression response occurs in most of the models.Sensitivity experiments show that a WCEP westerly anomaly can be excited by the tropical land cooling,especially the SSEA cooling induced precipitation suppression rather than by the extratropical land surface cooling.Theoretical results show that a WCEP westerly anomaly is excited due to a Gill response to reduced precipitation over the SSEA and West African monsoon regions;and the SSEA contributes more than the West African monsoon does.The ITCZ weakening,however,excites an easterly wind anomaly.(4)Using the observation and tree based long-term reconstructed proxy data,we discover that the EASM precipitation is reduced significantly by the Northern Hemisphere(NH)and tropical volcanic forcing and is enhanced by the remote volcanic forcing occurring in the other hemisphere.Through examining simulations over the last millennium by 11 different models,we show that NH and tropical volcano eruption can excite EASM precipitation reduction.The enhanced EASM precipitation induced by Southern Hemisphere volcanic eruption,however,cannot be simulate.The models with more reasonable meridional propagation of volcanic eruptions tend to simulate an asymmetrical response of EASM precipitation to hemispheric eruption more easily.In addition,the NH volcanic eruptions are mainly through weaking the off-equatorial monsoon circulation in weakening the EASM,while the tropical one tends to both reduce moisture heavily and cause strong divergence anomalies.