The East Asian Summer Monsoon (EASM) is a crucial climate system influencing rainfall patterns across East Asia. Record-breaking EASM rainfall in 2020 resulted in devastating floods, impacting millions and causing significant economic losses. Typically, EASM intensification is linked to moisture advection and convergence due to the enhanced western North Pacific subtropical high, often following an El Niño event. A recent study also suggested that El Niño-related tropical tropospheric warming shifts midlatitude westerlies southward, impacting the Tibetan Plateau and intensifying the East Asian early summer rainband. Volcanic eruptions are significant natural climate forcings, influencing variability across various timescales. Previous modeling studies suggested EASM suppression after volcanic eruptions due to reduced surface shortwave radiation and a slowed global hydrological cycle, associating them with historical droughts. However, the absence of significant droughts following major eruptions like Tambora (1815), El Chichón (1982), and Pinatubo (1991) raises questions about the extent of volcanic influence on EASM suppression. Previous research often overlooked El Niño's role, despite evidence suggesting increased El Niño likelihood after tropical eruptions. Two main mechanisms – the ocean dynamic thermostat and the land-sea thermal contrast – have been proposed to explain this link. This study investigates the EASM-tropical Pacific response to tropical volcanism to understand the interaction between internal climate feedback and external forcing, using multi-proxy data and multi-model simulations to determine whether volcano-induced El Niño events can intensify EASM precipitation despite the overall cooling effects of volcanic eruptions.

The existing literature presents a somewhat contradictory picture regarding the impact of volcanic eruptions on the EASM. While some studies using climate models have suggested a suppression of the EASM following volcanic eruptions due to the reduction in solar radiation and a subsequent weakening of the hydrological cycle, observational evidence and historical records don't always support this conclusion. The lack of widespread drought following major eruptions like Tambora in 1815 challenges the notion of consistent EASM suppression. Furthermore, the role of El Niño in modulating the volcanic impact on the EASM has been largely unexplored or given less prominence. While some proxy-reconstructed El Niño indices have hinted at a potential increase in El Niño events after volcanic eruptions, the mechanism behind this interaction has not been fully elucidated. Studies using state-of-the-art climate models have proposed mechanisms such as the ocean dynamic thermostat and the land-sea thermal contrast to explain the El Niño-volcano link, but comprehensive investigation incorporating both model simulations and paleoclimate data is still lacking.

This research employed a multifaceted approach combining paleoclimate reconstructions and climate model simulations. For the paleoclimate analysis, the study used gridded precipitation proxy data covering the Asian continent from 1470 to 1999 AD. This dataset, primarily based on historical records, tree-ring data, and ice cores, provides a long-term perspective on EASM rainfall variability. The analysis incorporated eleven different ENSO reconstructions to account for uncertainties inherent in proxy data. The researchers focused on 22 precisely dated large tropical volcanic eruptions during this period. A superposed epoch analysis was conducted to examine the EASM response to these eruptions, differentiating events with and without preceding El Niño conditions. The study further examined the results against two other precipitation reconstructions, one focusing specifically on China and another encompassing all of Asia. To complement the paleoclimate data, the study analyzed climate simulation results from thirteen different models from the Paleoclimate Modeling Intercomparison Project phases 3 and 4 (PMIP3 & PMIP4). These simulations cover the last millennium (with some focused on the pre-industrial period) and provide a large ensemble of eruption events, allowing for robust statistical analysis. To distinguish between the direct effects of volcanic forcing and the indirect effects mediated by El Niño, the researchers compared model simulations with and without preceding El Niño conditions. Ten Community Earth System Model (CESM) last-millennium simulations were additionally examined to further corroborate the findings. The study employed various climate indices including the Niño3.4 index (to define El Niño events), and an EASM circulation index derived from 850 hPa zonal wind differences to quantify the strength of the Pacific High and its influence on EASM rainfall. The study also performed a decomposition of the precipitation change into its dynamic and thermodynamic components, attributing variations to changes in circulation and moisture availability respectively. Statistical methods such as bootstrapped resampling were used to assess the significance of the observed patterns.

The key findings of this study are based on a combination of proxy data analysis and climate model simulations: 1. **Proxy Evidence of Enhanced EASM Rainfall:** The analysis of multiple precipitation reconstructions revealed significantly increased EASM rainfall in the first summer following tropical volcanic eruptions between 1470 AD and 1999 AD. This increase was centered around the Yangtze River basin, northeast China, and Kyushu, Japan, forming a meridional dipole pattern similar to the EASM-Pacific High response after El Niño events. This enhancement was statistically significant (90% and 95% confidence levels). 2. **Strengthened El Niño-EASM Relationship:** The study revealed a significant increase in post-eruption precipitation only when a preceding El Niño event was present. The EASM rainfall increase after volcano-induced El Niño was considerably stronger than that observed after non-volcanic El Niño events. This suggests that tropical eruptions strengthen the El Niño-EASM relationship. 3. **Multi-Model Support:** Analysis of 13 PMIP simulations confirmed the increased EASM rainfall following tropical eruptions. However, this enhancement was only significant when a preceding El Niño was simulated. The models show that volcano-induced El Niño conditions create an overall weakening of the Walker Circulation and a shift in the SST gradient across the Pacific. This leads to an enhanced Pacific High and subsequently intensifies the EASM circulation, outweighing the direct cooling and drying effects of the eruption. 4. **Volcanic-Induced Pacific Air-Sea Interaction:** The model simulations demonstrated that eruption-induced radiative cooling can lead to El Niño development or an El Niño-like response in the following summer. This process is influenced by coupled atmosphere-land-ocean dynamics and the Bjerknes feedback. A stronger El Niño showed a stronger correlation with increased EASM rainfall. 5. **Dynamic vs. Thermodynamic Effects:** The decomposition of precipitation changes revealed an increase in dynamically-driven precipitation (due to changes in circulation) and a decrease in thermodynamically driven precipitation (due to reduced moisture availability) after volcanic eruptions. El Niño tended to amplify the dynamic component while leaving the thermodynamic component largely unaffected. 6. **Increased Likelihood of El Niño Following Eruptions:** The analysis indicated an increased likelihood of El Niño events following tropical volcanic eruptions, both in the proxy data (41-44%) and model simulations (29%). This increased El Niño frequency contributes significantly to the observed EASM enhancement after volcanic eruptions.

This study's findings challenge the prevailing notion of volcanic-induced EASM suppression, demonstrating a complex interaction between volcanic forcing, El Niño events, and EASM dynamics. The results highlight that the influence of tropical volcanic eruptions on the EASM is not simply a matter of direct radiative cooling and hydrological weakening. Instead, the indirect effects mediated through the triggering of El Niño events and the subsequent enhancement of the Pacific High play a crucial role. The observed increase in EASM rainfall after eruptions, especially in the presence of El Niño, is explained by a strengthening of the dynamic component of precipitation, largely overcoming the negative effects on thermodynamic factors. This implies that the combined effect of volcanic-induced cooling and the El Niño-driven changes in atmospheric circulation outweighs the direct cooling and drying induced by the eruptions themselves. The study emphasizes the importance of considering the intricate interaction between external forcings (volcanic eruptions) and internal climate variability (ENSO) when assessing regional climate impacts. Future studies could focus on expanding the analysis to include eruptions in other regions and refining the understanding of the interactions among various factors (ITCZ shifts, divergent ENSO responses, and variable hydrological responses) influencing the EASM.

This research demonstrates that contrary to previous assumptions, tropical volcanic eruptions can enhance EASM rainfall, primarily through the increased likelihood of El Niño events. The volcano-induced El Niño events stimulate a stronger than normal warm pool air-sea interaction, leading to an intensified Pacific High and enhanced EASM precipitation. These findings underscore the complex interplay between external forcings and internal climate variability and have implications for understanding the potential for future flood disasters linked to volcanic activity and EASM.

The study acknowledges several limitations. The relatively small number of volcanic events in the paleoclimate record might limit the statistical power of the analysis, and uncertainties associated with proxy reconstructions could influence the results. While the ensemble mean of multiple ENSO reconstructions was used to mitigate uncertainties in individual proxy records, discrepancies in individual proxy responses to volcanic eruptions remain, highlighting the complexities of reconstructing past climate variability. Furthermore, the study focused primarily on tropical eruptions; future research should examine the effects of extra-tropical volcanic eruptions on the EASM. Finally, while the model simulations provide valuable insights, the representation of certain processes in climate models may not be perfect, requiring further refinements for improved understanding.