Volcanically forced Madden–Julian oscillation triggers the immediate onset of El Niño

Domain: earth_science
Published: 2025-02-06
Editorial outcome: minor_revision

Abstract
A process-level understanding of the volcanically forced climate response is an urgent challenge due to its similarities to the potential effects of geoengineering techniques. Although the influence of volcanic forcing on El Niño events has been studied extensively, the mechanisms driving the volcanically-induced immediate onset of El Niño remain uncertain, with many climate models producing a delayed El Niño response compared to observations. In this study, using large ensemble simulations that allow us to isolate the impacts of volcanic forcing on the El Niño response, we demonstrate a mechanism that highlights the central triggering role of the Madden–Julian oscillation (MJO), which has been overlooked in existing literature. Because the land areas surrounding the Indo-Pacific warm pool dry more quickly after a volcanic eruption, the background moisture distribution becomes more favorable for the MJO to propagate eastward from the Indian Ocean into the Pacific. This increases the likelihood of ensemble members having stronger MJO activity in the western Pacific by about 35% compared to non-volcanic years, which subsequently increases the frequency of westerly wind bursts by about 76%, ultimately enhancing the probability of the onset of an El Niño by about 98% following major volcanic eruptions.

Introduction
On 15th January 2022, the explosive eruption of Hunga Tonga-Hunga Ha’apai injected the highest amount of water vapor into the stratosphere, rivaling the explosiveness of the Pinatubo eruption in 1991 1. It caused an estimated US$90.4 million in damages due to the effects of the resulting tsunami and ash fall 2. Volcanic eruptions (VEs), such as the Pinatubo eruption, would impact not only economies but also regional and global climates, as well as their seasonal predictability 3. Volcanic eruptions are a dominant form of external forcing that significantly affects the climate by injecting dust particles and sulfur dioxide into the atmosphere 4. As sulfur moves into the stratosphere, it combines with water and forms sulfate aerosols that reflect incoming shortwave solar radiation. Because it takes a few years for these sulfate aerosols to settle out of the atmosphere, they cool the Earth’s surface worldwide during this period. For example, the Mt. Pinatubo eruption in June 1991 cooled the global surface by a few tenths of a degree Celsius over about three years and led to a substantial decrease in global streamflow and land precipitation, as less shortwave radiation reached the surface, thereby reducing evaporation and stabilizing the atmosphere 5.

In addition to its significant impact on the mean climate, VEs are also known to affect natural modes of climate variability, such as the El Niño-Southern Oscillation (ENSO), the dominant mode of tropical interannual variability. However, no consensus has yet been reached on the immediate ENSO response to volcanic forcing, complicating the current understanding of ENSO dynamics 6–17. Using targeted experiments with global climate models (GCMs), most studies have reported El Niño-like responses to volcanic forcing 10, though La Niña-like responses and no significant responses have been reported as well 11,12. Anomalous westerly winds over the western Pacific have often been proposed as an important driver of the El Niño events that develop after a VE in simulations. That is, the volcanically forced radiative cooling and drying occur faster over the land areas than the ocean, especially over the Maritime Continent (MC) and surrounding area, and the increased land-ocean temperature gradient weakens the equatorial Walker circulation and enhances westerly anomalies in the western Pacific, inducing El Niño 13–17 through the Bjerknes feedback 18. The cooling and reduction of precipitation over northern Africa are also thought to be a major source of an altered Walker circulation after volcanic eruption 16,17. However, this mechanism has not been supported by a wider variety of models and, more importantly, produces delayed El Niño responses with a peak during the year after eruptions, which fails to explain the observed immediate onset of El Niño. While the volcanically forced changes in tropical westerly wind is known to be crucial for El Niño onset, the proposed mechanisms are still debated.

It has been widely accepted that the westerly anomalies associated with weaker Walker circulation consist of a series of short-lived, intense, and low-level westerly wind events that are referred to as westerly wind bursts (WWBs) 19. WWBs are often associated with large clusters of deep convective clouds 20 and are recognized as precursors to El Niño events because they excite downwelling oceanic Kelvin waves 21. In particular, when ENSO is in a neutral phase, El Niño can be strongly influenced by WWBs 19. For example, an increase in WWB activity was observed prior to the 1982/83 and 1991/92 El Niño events 21,22, which occurred in conjunction with two major tropical volcanic eruptions, El Chichón (1982) and Mt. Pinatubo (1991).

The extent to which WWBs associated with deep convective clusters contribute to the immediate onset of an El Niño event within a few months after a volcanic eruption remains unexplored, as previous studies 17,23 have relied only on monthly or seasonally averaged wind data, in which the high-frequency signals of WWBs were averaged out.

In the present study, we use daily data to determine the possible influence of volcanic-forced high-frequency WWBs on El Niño through their connection with the Madden–Julian oscillation (MJO). The MJO is the dominant mode of tropical intraseasonal variability and is characterized by an eastward-propagating, large-scale envelope of deep convective clusters that is strongly coupled with circulation anomalies 24. After beginning its lifecycle over the Indian Ocean, the MJO reaches the western Pacific if it survives the barrier effect of the Maritime Continent. In the western Pacific, studies have found that the strongest, longest-lasting, basin-scale WWBs were mostly associated with active MJO events 19,25–28. These MJO-driven WWBs in the western Pacific then increase sea surface temperatures (SST) in the eastern Pacific by a few tenths of a degree Celsius 28. Therefore, if MJO activity is enhanced over the western Pacific, the likelihood of MJO-driven WWB occurrence increases, eventually facilitating the subsequent development of an El Niño event. This hypothesis is tested in this study by proposing a plausible mechanism using reanalysis data and climate model sensitivity experiments. It is important to note that the role of volcanically forced MJO on ENSO has been overlooked in existing literature.

To test the hypothesis by isolating the volcanic forcing from other external forcings, single-forcing large ensemble experiments (SE) are utilized (see Methods for details). High model fidelity in simulating the observed climate is essential to accurately quantify the impact of forced signals on climate variability. However, many GCMs struggle with systematic biases. For example, while El Niño immediately peaks in the first winter following the major tropical volcanic eruption in reconstructions and observations 10, most GCMs simulate the El Niño peak during the second or third winter 13,15,17,29, limiting confidence in the proposed mechanisms. The present study seeks to partly resolve the issue by utilizing the Community Earth System Model Version 2.0 (CESM2) which is one of the few contemporary climate models that can realistically simulate both the ENSO and MJO 30–34. In particular, as demonstrated later, the timing of the El Niño onset after volcanic forcing (i.e., the first winter following an eruption) is captured well by the CESM2, unlike most other GCMs. This accurate simulation of response timing is a necessary condition for better understanding the underlying mechanism.

Results
This study focuses on two recent VEs: El Chichón in April 1982 and Mt. Pinatubo in June 1991. Fig. 1 presents the observed temporal changes in the monthly indices of Niño 3.4, MC moisture, MJO activity, and WWB frequency after the two VEs. The observations reveal that both VEs lead to the drying of MC moisture, which reached a minimum 6–7 months after the eruption (Fig. 1A). As documented in previous studies, El Niño intensified immediately after the two VE events and reached a peak after about 7–9 months. However, this immediate onset is not simulated in most GCMs. Before the El Niño maximum phase, in conjunction with the enhanced MJO activity over the western Pacific (Fig. 1B), the WWB frequency increases in the western Pacific before dropping sharply after about 7 months, with MJO activity also suppressed (Fig. 1C). These results suggest that an increase in MJO activity over the western Pacific is associated with WWB activity, and it may have contributed to the rapid development of an El Niño event immediately after the recent large VEs.

The spatial pattern of moisture (defined as column-integrated water vapor) 3–7 months after the VEs exhibits significant negative anomalies near the MC and positive moisture anomalies in the central to eastern tropical Pacific (Fig. 2A and B), which is consistent with previous studies 15. Regarding the African drying mechanism 16,17, the observed moisture pattern shows a less consistent and weaker response than the MC drying. During the same period, MJO activity is suppressed in the vicinity of the MC and significantly enhanced in the western to central Pacific, with the maximum near the dateline (Fig. 2C and D). The similar changes in moisture and MJO activity anomalies before the peak phase of El Niño suggest that there could be a plausible link between the moisture and MJO after recent VEs.

By fully utilizing all ensemble members in the CESM2-SE and CESM2-LE, we assess the degree to which changes in MC moisture and MJO activity are volcanically forced responses. Supplementary Fig. 1 presents the probability distribution of normalized monthly MC moisture and MJO activity indices for the 3–7 month window after two VE years and nonVE years. About 35% of the ensembles simulate anomalously strong MC drying (defined as less than –1.0σ of the index) in VE years that is about twice as high as that for the nonVE years (17.8%) (Supplementary Fig. 1A). During the same period, the likelihood of MJO activity being anomalously strong (larger than +1.0σ) in VE years increases by about 36% compared with the corresponding values for the nonVE years (Supplementary Fig. 1B). Furthermore, using the +1.0σ threshold for the 3–7 month window, the westerly wind stress (TAUx) index over the western Pacific increases by about 62% (Supplementary Fig. 1C) and WWBs frequency increases by 76% during VE years. Eventually, the chance of being strong El Niño (defined as greater than +1.0σ of Niño 3.4 index) increases by about 98% after volcanic eruptions (Supplementary Fig. 1D). To further verify the robustness of the results, the same analysis is performed with 100 ensemble members of CESM2-LE, and although the signals are weaker, the responses are generally similar to CESM2-SE (Supplementary Fig. 2). These results indicate that the observed MC drying and enhanced western Pacific MJO activity are robust responses to large volcanic forcing and suggest a possible link with WWBs and ENSO.

While comparing the full ensembles between VE and nonVE years offers an opportunity to understand forced responses in a probabilistic manner (Supplementary Figs. 1 and 2), averaging over all ensemble members may underestimate the magnitude of internal variability, as different ensemble members are in various climate states at the time of a volcanic eruption. To further clarify the mechanisms responsible for the volcanically forced MJO and ENSO, we cluster ensemble members based on their ENSO phase similar to that observed in the pre-eruption month (month 0). Sea surface height (SSH) anomaly, which is a proxy for ocean heat variation in the Pacific, is used for ensemble clustering (see “Methods”). We examine the response of the MC moisture, MJO activity, TAUx, WWB frequency, and ENSO to two major VE forcings in CESM2-SE using ensemble members that have similar ENSO states (neutral state) to that observed at month 0 and compare the composites of ensemble members to the corresponding nonVE clusters (Fig. 3). ENSO started with an initially neutral state, and about three months after the VE, an El Niño event develops and becomes significantly different from the nonVE clusters (Fig. 3A and Supplementary Fig. 3). Although somewhat weaker than the observed as the signal from each ensemble member is averaged out, the peak in the El Niño event appears about 6-7 months after the VE which roughly corresponds with observations (Fig. 1). This demonstrates that volcanic forcing can significantly influence El Niño development without a pre-existing El Niño signal at the time of the eruption. More importantly, while most GCMs simulate the peak of the El Niño event in the second or third winter 13,15,17,29, the CESM2-SE successfully reproduces the observed El Niño timing, thus providing higher confidence for the estimated forced response of the ENSO and related physical mechanism.

In conjunction with the pre-eruption neutral ENSO phase, the moisture anomaly over the MC is nearly zero at month 0 for both the VE and nonVE clusters (Fig. 3A). Supplementary Fig. 3 shows the composite difference between VE and nonVE clusters for the September-October-November (SON) averaged field. The MC moisture becomes significantly drier than the nonVE, again similar to the observed response (Fig. 3A and Supplementary Fig. 3A). Note that the moisture patterns (Supplementary Fig. 3A) are similar in two VE events. Synchronized changes of MJO activity (Fig. 3A and Supplementary Fig. 3B), enhanced westerly wind stress (TAUx), increased WWB frequency, and El Niño signal after the VE are also visible (Fig. 3A, B and Supplementary Fig. 3C, D), exhibiting significant differences from the nonVE cluster. For example, the WWB frequency during VE is about twice as high as in the nonVE cluster as the MJO activity enhances in 3 months after the VE. The MJO activity, TAUx, and WWB frequency escalate as the MC drying signal becomes robustly consistent with the observed signal, suggesting that volcanically forced MC drying enhances the MJO activity and westerly wind stress, which provides a favorable condition for El Niño onset. Once El Niño reaches its peak at 6 months, the MJO activity and related WWB continue to intensify, possibly through the MJO-ENSO coupled processes 33.

What causes the MJO to become active in the western Pacific after a volcanic eruption?

To understand the processes responsible for the enhanced MJO activity over the western Pacific, we adopt the moisture mode hypothesis 32–37. Recent theoretical, observational, and modeling studies have collectively demonstrated that the background moisture distribution in the Indo-Pacific warm pool is closely linked to MJO activity. In particular, the increased horizontal advection of background mean moisture to the east, caused by MJO circulation, and the decreased moisture advection to the west of the MJO convective phase together create favorable conditions for the MJO to propagate eastward across the MC 32–37.

Supplementary Fig. 3A shows that, after the volcanic eruption, sharp drying not only occurs over the MC as observed but also to the north of the maximum moisture region in the Indo-Pacific warm pool. The decrease in moisture poleward of the climatological maximum steepens the poleward meridional moisture gradient in both hemispheres. At the same time, the relative wetting in the equatorial central Pacific decreases the zonal moisture gradient (Fig. 2 and Supplementary Fig. 3A). The volcanically-driven stronger meridional moisture gradient over the Indo-Pacific warm pool and weaker zonal moisture gradient near the dateline together favor the eastward propagation of the MJO. To demonstrate this, Fig. 4 compares the lag composites of the MJO-filtered outgoing longwave radiation (OLR) and zonal wind at 850 hPa (U850) in the tropical belt. In the composite of the VE clusters, MJO convection and circulation couplet over the Indian Ocean propagates eastward and enters the western Pacific, thus contributing to the enhanced MJO activity over the western Pacific (Fig. 4A). In the nonVE cluster, however, the MJO convection and circulation signals move slowly eastward and are confined to a narrower longitudinal band with weaker amplitude than in the VE cluster (Fig. 4B). This demonstrates that the enhanced MJO activity in the western Pacific after the VE is partly attributable to a more coherent eastward propagating signal from the Indian Ocean into the western Pacific.

To understand the key processes associated with the propagation of the MJO, we conducted a moisture budget analysis (Fig. 4C, see “Methods” for details). While moisture to the east of the MJO convection center is enhanced in both the VE and nonVE clusters, the VE cluster shows about 71% greater moisture tendency compared to nonVE (Fig. 4C). The increased moisture tendency explains the more coherent eastward propagation of the MJO in the VE cluster. Both zonal and meridional moisture advection terms are larger in the VE than in the nonVE clusters (Fig. 4C). The zonal moisture advection term is about 5.5 times larger, and meridional moisture advection term is about 1.5 times larger in VE than nonVE cluster. By decomposing the moisture horizontal advection terms, the zonal and meridional advection of the background mean moisture due to the anomalous MJO wind is found to be the most dominant process (Fig. 4C and Supplementary Fig. 4), which is consistent with previous studies 32–37. Note that the influence of the upper-tropospheric background wind may play a significant role in shaping the dynamics of MJO propagation 38.

In summary, because the area poleward of maximum background moisture over the Indo-Pacific warm pool becomes drier and the area east of the maximum moisture becomes wetter after a VE, these changes together provide favorable conditions for the MJO to propagate further eastward. The enhanced MJO activity over the western Pacific and associated WWBs eventually support the immediate onset of El Niño.

Methods
Modelexperimentsanddata
We use the historical simulations of the Community Earth System Model Version 2.0 single-forcing large ensemble (CESM2-SE) experiments 46. The CESM2 reasonably simulates both the basic state and modes of tropical variabilities, such as the ENSO and MJO 30–34,47. The CESM2-SE offers a unique opportunity to quantify the relative contributions of anthropogenic and natural forcings to the evolution of the Earth system. To investigate the influence of volcanic forcing, the “EE2” experiment is analyzed, which consists of 15 members that run from 1850 to 2050 where forcings from greenhouse gases, anthropogenic aerosol, and biomass-burning aerosol are held fixed at 1850s value and forcings from “everything else” in CESM2 (EE2) are evolving. The EE2 forcing experiment includes forcing from solar insolation, stratospheric volcanic aerosol, stratospheric and tropospheric ozone, and land-use and land-cover change. Among these, the volcanic forcing has been shown to have the largest impact on the year-to-year variation of surface temperature during the historical period 46. Therefore, to disentangle the impact of volcanic forcing on MJO and ENSO, 15 ensemble members from the EE2 experiment from 1980 to 2014 are analyzed. Supplementary Fig. 7 shows the monthly mean stratospheric aerosol optical depth (AOD) at 550 nm from the Whole Atmosphere Community Climate Model Version 6 (WACCM6) that produced the volcanic aerosol forcing 46. The AOD exceeds 0.06 during two volcanic eruptions, El Chichón in 1982 and Mt. Pinatubo in 1991 (Supplementary Fig. 7)

To verify the results obtained from the CESM2-SE, CESM2 large-ensemble (CESM2-LE) simulations are compared 30. Although volcanic forcing cannot be isolated from other external forcings, CESM2-LE offers a unique opportunity for analyzing the forced responses as it consists of 100 ensemble members. We utilize all 100 ensemble members during the same period as CESM2-SE, from 1980 to 2014. To test the hypothesis drawn from CESM2-SE and CESM2-LE, the hypothesis is further examined with HadGEM3‐GC3.1-LL single forcing large ensemble experiment (hereafter referred to as HadGEM3). The HadGEM3‐GC3 reasonably simulates the MJO 39. The HadGEM3 historical volcanic-forcing simulation 48, driven by volcanic forcing only, is analyzed from 1980 to 2014 using a similar approach applied in the CESM2 analysis. Fifty ensemble members are analyzed. Note that some output variables are not available for download in HadGEM3, such as daily westerly wind stress.

Daily and monthly mean zonal wind and specific humidity are obtained from the European Center for Medium‐Range Weather Forecasting (ECMWF) Interim Reanalysis 49. The monthly mean SST comes from the National Oceanic and Atmospheric Administration (NOAA) Optimum Interpolation SSTV2 50. The OLR data is from the NOAA Advanced Very High-Resolution Radiometer 51. The monthly sea surface height (SSH) is obtained from the ECMWF Ocean Reanalysis System 4 52. For brevity, the reanalysis and satellite data sets are referred to as observations. All observations and simulations are interpolated onto a 2.5° longitude and 2.5° latitude grid. Monthly and daily anomalies are calculated by subtracting the climatology of the 1980–2014 period for both the observations and simulations. The possible impact of anthropogenic forcing is removed through linear detrending.

Volcanic eruption events and the analysis window
The Northern Hemisphere and tropical eruptions tend to favor El Niño-like conditions, while Southern Hemisphere eruptions enhance the probability of La Niña-like events 16,29. In addition, only VEs that are strong enough to inject aerosols into the stratosphere and that pass a certain threshold (i.e., absolute radiative forcing greater than 3.3 W/m2) can increase the probability of an El Niño event occurring 53. Since the satellite era, only two eruptions in the tropics have surpassed this threshold: El Chichón in Mexico (93˚W, 17˚N) in April 1982 and Mt. Pinatubo in the Philippines (120˚E, 15˚N) in June 1991. The plumes from El Chichón reached about 31 km at their highest point and released ~7.5 Tg of SO2, while Mt. Pinatubo reached 40 km and injected ~20 Tg of SO2, both exceeding volcanic explosivity index category 5 1. Notably, both eruptions occurred at tropical latitudes. We define the month of eruption (month 0) as April 1982 and June 1991, respectively. Therefore, the period three to seven months after the VE, which is the window of analysis in the present study, corresponds to July to November 1982 for El Chichón and September 1991 to January 1992 for Mt. Pinatubo. The non-VE years are defined as the years from 1980 to 2014, excluding 1982 and 1991, unless otherwise noted.

Indices
Seven indices are defined in this study: MJO activity index, Maritime Continent (MC) moisture index, northern African (NAFR) moisture index, westerly wind stress (TAUx) index, WWB frequency index, recharge index, and Niño 3.4 index. For our purposes, an intraseasonal filter is applied to retain only eastward propagating signals of wavenumbers 0–9 with periods between 30 and 96 days 54. This is hereafter referred to as the “MJO-filter”. The MJO activity is defined as the monthly standard deviation of the MJO-filtered daily OLR. The MJO activity index is then defined as the monthly MJO activity anomalies averaged over 140˚E–200˚E and 15˚S–15˚N (indicated by the blue boxes in Fig. 2C and D). The MC moisture index is defined as the monthly moisture anomalies averaged over 90˚E–150˚E and 15˚S–15˚N (indicated by the brown boxes in Fig. 2A and B). The NAFR moisture index is defined as the monthly moisture anomalies averaged over 20˚W–50˚E and 10˚S–30˚N.

The westerly wind stress index (hereafter TAUx) is defined as the monthly mean zonal wind stress anomalies averaged over 130˚E–200˚E and 5˚S–5˚N (indicated by the box in Supplementary Fig. 3C). The WWB frequency index is defined as the number of days when daily surface zonal wind stress anomaly averaged over tropical western Pacific (130˚E-200˚E, 5˚S-5˚N) exceeds 1.0σ for three consecutive days, indicating a sustained surface westerly. The daily frequency is then counted over a month to define the monthly WWB frequency index. Note that the definition of WWBs is loosely analogous to that used by Santoso et al 55. but not as strict as other definitions 56.

The recharge index is defined as the monthly SSH anomaly averaged over the western tropical Pacific (120°E–205°E, 5°S–5°N) 31. Based on the delayed oscillator and recharge–discharge oscillator paradigm for the ENSO 57, the equatorial Pacific Ocean heat content is regarded as a key precursor to the ENSO. Equatorial ocean heat accumulates (or recharges) about three to six months before the onset of El Niño and then discharges during and after El Niño. In particular, the recharge in the western Pacific is accompanied by a higher sea level associated with a deeper thermocline 58. Thus SSH is often used as a proxy for ocean heat variation. The recharge–discharge process for ENSO is well-simulated in the CESM2 31. The Niño 3.4 index is calculated using the relative SST (RSST) 59 averaged over the Niño 3.4 region (190˚E–240˚E and 5˚S–5˚N, the box in Supplementary Fig. 3D). RSST is defined as the SST signal after removing the tropical (20°S–20°N) mean SST anomalies because it better highlights the intrinsic ENSO signal after a volcanic eruption 15,17.

Ensemble clustering for the VE and nonVE groups and composite analysis
We subsample the ensemble members by considering each ensemble’s recharge phase in the pre-eruption month (month 0) into account and select those ensembles that are within a similar range as the observed recharge phase. Given that the two eruptions occurred in April (El Chichón) and June (Mt. Pinatubo), and by considering the seasonal phase locking of the ENSO, we use the April-May-June (AMJ) averaged normalized recharge index as the threshold for ensemble clustering. The observed AMJ recharge index is 0.9σ and 0.2σ for El Chichón and Mt. Pinatubo, respectively, indicating neutral ENSO conditions before both volcanic eruptions. Therefore, ensemble members are selected if the normalized AMJ recharge index ranges between 0.2σ and 0.9σ, coinciding with the observed recharge phase. About 53% of ensembles are selected as VE clusters during VE years and 25% as nonVE clusters during nonVE years. Note that, due to the phase locking of ENSO, the composites of the mean state (Supplementary Fig. 3) are calculated for the September-October-November (SON) average.

For the daily lag composite analysis (Fig. 4), a reference time series is first defined as the area-averaged MJO-filtered OLR anomalies over the Indian Ocean (80°E–110°E and 15°S–15°N). The lag composite is calculated when the reference time series from each clustered ensemble is below –1.0σ and has the minimum values between −1 and +1 day. Both monthly and daily composite analyses are conducted over the 3–7-month window for both the VE and nonVE clusters starting from AMJ.

To further assess the relative contribution of MC drying and NAFR drying on the onset of El Niño, we subsample the ensemble members by taking each ensemble’s MC index and NAFR index in the pre-eruption month (month 0) into account and select those ensembles that are less than −1.0σ, indicating strong drying over each continent right before the volcanic eruptions.

Moisture budget analysis
To explain the key processes involved in MJO propagation (Fig. 4C), the following column-integrated moisture budget equation is utilized:

<∂q/∂t> = -<u∂q/∂x> - <v∂q/∂y> - <ω∂q/∂p> - P' + E' (1)

where q, u, v, ω, P, and E are the specific humidity, zonal wind, meridional wind, vertical wind, precipitation, and evaporation, respectively. The angled bracket indicates a mass-weighted vertical integral and the prime denotes MJO-filtered anomalies. We focus on the tendency term and horizontal (zonal and meridional) advection terms because the horizontal advection processes are key to MJO eastward propagation 36,37. The first three terms (the moisture tendency term and zonal and meridional advection terms) are calculated using the daily CESM2-SE output, whereas the sum of the other terms, which is often referred to as column processes 60, is obtained as the residual. Note that the role of column processes on the MJO is non-negligible and warrants further analysis. Changes in moisture advection could be attributed to changes in the background mean moisture distribution, anomalous MJO circulation or the nonlinear interaction between the two. To further understand the contribution of the mean state and the MJO to the key processes in the dominant term, the moisture advection terms are decomposed into different timescale interactions as follows:

-u∂q/∂x - v∂q/∂y = -u'∂q/∂x - v'∂q/∂y - u∂q'/∂x - v∂q'/∂y - u'∂q'/∂x - v'∂q'/∂y + h.o.t (2)

where the overbar represents the background mean (September-October-November average after volcanic eruptions). Moisture budget terms are averaged from a lag of −2 to +2 days over the western Pacific (100˚E–160˚E, 20˚S–15˚N, as indicated by the boxes in Supplementary Fig. 4).

Confidence interval
The confidence interval for the results shown in Fig. 3 and Supplementary Fig. 3 is calculated using the bootstrap method. First, the anomaly composite is constructed by selecting random years from the entire period (1980–2014) with the same number of VE (2 years) and nonVE groups (33 years) with initially neutral ENSO conditions in AMJ without overlapping years. The anomalies are then calculated between the sampled VE and nonVE groups, respectively. The difference between the VE and nonVE group composites is then calculated. This process is repeated 1000 times with replacement to obtain a probability distribution function of the differences. The 2.5th and 97.5th percentile values are then used to define the 95% confidence interval.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review of “Volcanically forced Madden–Julian oscillation triggers the immediate onset of El Niño” by Kim et al.\nThe authors investigate the mechanisms underlying the responses of El Niño to volcanic eruptions, focusing on the role of\nthe MJO and its associated westerly wind busts. They showed that, following the volcanic eruptions, the background\nmoisture over the Maritime Continent dries faster, thus favoring the eastward propagation of the MJO into the western\nPacific. The increased MJO activity further enhances the westerly wind bursts and thus triggers El Niño. The results might\nadd useful information about the mechanism of MJO-ENSO interaction and their responses to volcanic eruptions – which is\nimportant for geoengineering studies. Most of the conclusions are supported by evidence from the large ensemble\nsimulations, but others might need additional thought. My concerns are listed below.\nLines 108-110: I was not able to follow. I guess Fig. S1 shows the phase composite of the MJO, and so, by construction,\nMJO phases 6-8 are in the western Pacific while phases 2-4 are in the Indian Ocean. It is unclear how we can see the\nincrease in the frequency of MJO phases 6-8. Or do the authors mean this is compared to the climatological mean or non-VE\ncases?\nLine 128: Do the authors mean Figs. S2 and S3?\nLine 132: Pre-eruption month is month (-1)? Since the figure captions state that the eruption month is month (0)?\nLine 162: Does the drying pattern (Fig. S4) depend on the eruptions’ location?\nLines 195-196: Since the authors repeatedly state that the MC dries faster than other areas after the VEs, it is worth\ndiscussing why this happens.\nLine 316: In order to determine which budget terms contribute most to the propagation, previous studies (e.g., Andersen and\nKuang, 2012; Bui and Maloney, 2020) projected each budget quantity onto the tendency term, so I wonder if the authors also\nconsider such kind of projection here.\nLine 327: This is surprising, but I guess CESM2-LE provides the surface latent heat flux, which is directly related to\nevaporation?\nFig. 4:\n- Consider showing the ensemble spread.\n- Since the authors analyzed the large ensemble simulations, did the author also examine the across-ensemble (or\nensemble-wise) to address the natural variability?\n- It might be worth adding a bit of discussion on the contribution of synoptic eddies (v’dq’) in regulating the intraseasonal\nmoisture budget\nSupplementary: Line 31: Panel (B)?\nAndersen, J. A., & Kuang, Z. (2012). Moist static energy budget of MJO-like disturbances in the atmosphere of a zonally\nsymmetric aquaplanet. Journal of Climate, 25(8), 2782–2804.\nBui, H. X., & Maloney, E. D. (2020). Changes to the Madden-Julian Oscillation in coupled and uncoupled aquaplanet\nsimulations with 4xCO2. Journal of Advances in Modeling Earth Systems, 12, e2020MS002179.\nHien Bui", "reviewer_2": "The manuscript entitled “Volcanically forced Madden–Julian oscillation triggers the immediate onset of El Niño” by Hyemi\nKim et al. focuses on the physical processes of how strong tropical volcanic eruptions trigger El Niño events. Based on the\nanalysis of observational data and CESM2 ensemble simulations, a new mechanism is proposed where volcanic eruptions\nenhance the Madden-Julian Oscillation (MJO), subsequently rapidly triggering El Niño. This study is interesting and\ninnovative. However, there are significant issues in the analysis process. As of now, it seems that the paper does not meet\nthe publication criteria of the journal Nature Communications.\nIn this study, the authors have innovatively emphasized the crucial role of the Madden-Julian Oscillation (MJO). Following\nstrong volcanic eruptions, the notable enhancement of MJO over the western Pacific is the most important and innovative\naspect of this study. However, the current definition of the MJO activity index in this paper raises significant concerns about\nwhether it genuinely reflects the characteristics of MJO activity. The MJO index definition (lines 272-277) in this study reflects\nsubseasonal to seasonal-scale oscillations in the western Pacific region, but it cannot be conclusively established\nas a signal of MJO activity. If it cannot be confirmed that this indeed represents the MJO, the novelty of this work would be\nundermined.\nAdditionally, the majority of CMIP6 models currently exhibit limited skill in simulating the MJO, with most producing weak\ncharacteristics of eastward propagation from the Indian Ocean, even the CESM2 model encounters this issue. Therefore,\nusing model results to prove the volcanic enhancement of eastward MJO propagation, particularly in the western Pacific\noscillation, cannot be conclusively demonstrated based on existing analysis. Consequently, both the applicability of the MJO\nindex and the validation of the MJO's significant role through model data raise critical concerns. As a result, it becomes\ndifficult to agree with the viewpoints proposed by the authors.\nSpecific comments:\nLines 71-72, most coupled models exhibit poor simulation of eastward-propagating MJO, especially the process where MJO\ncrosses the Maritime Continent to reach the western Pacific. In other words, in the models, it is very difficult for the MJO to\ncross the Maritime Continent.\nLines 93 to 112, in the observational analysis, due to the applicability issues of the MJO index mentioned in the major\ncomment, the current analysis cannot provide insights into the activity characteristics of the MJO following the volcanic\neruptions in 1991 and 1982. As a result, it's not possible to establish the lead-lag relationship between MJO and ENSO, MC\nindicators. Did the MJO coincidentally propagate to the western Pacific in the third month following both volcanic eruptions?\nIf this is the case, I suggest the authors present time-longitude propagation maps of MJO's eastward propagation from the\nIndian Ocean and its subsequent propagation to the western Pacific in the third month after the volcanic eruptions. This\nwould more comprehensively demonstrate the significance of MJO in triggering El Niño during these two observational\nprocesses.\nLines 106-112, if MJO is suppressed over the Indian Ocean, would it become more challenging for MJO to propagate\neastward across the Maritime Continent and reach the western Pacific? Furthermore, based on some previous studies,\nwhen the Maritime Continent becomes drier, it becomes challenging for the MJO to traverse from the eastern Indian Ocean\nthrough the MC and reach the western Pacific.\nLines 121-126, how can it be proven that the intensified MJO over the western Pacific rapidly triggered the WWBs rather\nthan other seasonal-scale oscillations?\nLines 165-166, when the western side of the MJO becomes drier and the eastern side becomes wetter, it will favor the\neastward propagation of the MJO. However, when the MJO is over the eastern Indian Ocean, if the MC becomes drier, it will\nmake it more difficult for the MJO to traverse through the MC, rather than promoting MJO propagation across the MC.\nSome other comments:\nThe MJO represents internal variability in climate system simulations, and it is challenging to comprehend the phenomenon\ndescribed in this paper, where considering volcanic forcing signal leads to a strengthening of MJO variability (as shown in\nFigures 4a and 4b). Typically, external forcings have a limited ability to alter the spatial characteristics of internal climate\nsystem variability; they mainly affect the evolution period. Additionally, regarding Figures 4a and 4b, the region around\n120E, which is near the Maritime Continent, displays a discontinuity in OLR. This also indicates the difficulty for the MJO to\nacross the MC and reach the western Pacific. Oscillations in the western Pacific might not be related to MJO, such as the\nwestward signals near 180E.\nThere are certain issues concerning the selection of volcanic and non-volcanic samples. According to the description in this\nstudy, only simulation data from the years 1982 and 1991 are categorized as volcanic samples. However, for instance, the\natmospheric concentration of volcanic aerosols was very high in 1992 and 1993, especially in 1992 when the volcanic\naerosol content surpassed even that of 1991. This year's impact on global climate was more significant. Therefore, when\nselecting volcanic and non-volcanic samples, it would be more accurate to consider the concentration of atmospheric sulfate\naerosols as a criterion, given its direct relation to volcanic influence."}, "author_response": "Response to the reviewers’ comments\n[Response to the reviewers’ comments received on September-02, 2023]\n“Volcanically forced Madden–Julian oscillation triggers the immediate onset of El Niño”\nHyemi Kim, Seung-Ki Min, Daehyun Kim, and Daniele Visioni\nWe wish to express our gratitude to the reviewers for their time and effort spent in\nreviewing this manuscript. The revision underwent substantial changes to address the reviewer’s\ncomments. We would appreciate it if the reviewer could review our revision again, as we believe\nthe methodologies have changed substantially and the conclusions have become more robust. The\nmain points addressed are:\n(1) Analyzing the simulation output from sensitivity experiments (CESM2 and HadGEM3\nsingle-forcing and large-ensemble experiments) to isolate the volcanic eruption impacts\non MJO-ENSO interaction.\n(2) Improving the methodology (MJO-filter, ensemble clustering)\n(3) Addressing the relative importance of northern Africa drying on El Niño onset which is\nknown as an important mechanism.\nThe original comments from the reviewers are in black, and the responses are in blue.\nReviewer #1\nThe authors investigate the mechanisms underlying the responses of El Niño to volcanic\neruptions, focusing on the role of the MJO and its associated westerly wind bursts. They showed\nthat, following the volcanic eruptions, the background moisture over the Maritime Continent dries\nfaster, thus favoring the eastward propagation of the MJO into the western Pacific. The increased\nMJO activity further enhances the westerly wind bursts and thus triggers El Niño. The results\nmight add useful information about the mechanism of MJO-ENSO interaction and their responses\nto volcanic eruptions – which is important for geoengineering studies. Most of the conclusions are\nsupported by evidence from the large ensemble simulations, but others might need additional\nthought. My concerns are listed below.\n- Lines 108-110: I was not able to follow. I guess Fig. S1 shows the phase composite of the MJO,\nand so, by construction, MJO phases 6-8 are in the western Pacific while phases 2-4 are in the\nIndian Ocean. It is unclear how we can see the increase in the frequency of MJO phases 6-8. Or\ndo the authors mean this is compared to the climatological mean or non-VE cases?\n Thanks for pointing out. This no longer applies to the revised version, so we have removed the\ncorresponding sentence.\n- Line 128: Do the authors mean Figs. S2 and S3?\n The figure order is now changed in the revision.\n- Line 132: Pre-eruption month is month (-1)? Since the figure captions state that the eruption\nmonth is month (0)?\n Thank you very much for catching this. It is now clarified that pre-eruption month is month-0\nand figure captions are corrected.\n- Line 162: Does the drying pattern (Fig. S4) depend on the eruptions’ location?\n As shown in the figure below (Figure A), the overall spatial patterns of moisture anomaly are\nsimilar (drying in the poleward direction from the maximum moisture center). This is now\nmentioned in the revision as (L173) “Note that the moisture patterns (Fig. S3A) are similar in two\nVE events (not shown).”\nFigure A. Spatial pattern of variables in response to volcanic eruptions with initially neutral ENSO\nconditions. Spatial patterns of ensemble cluster composites of moisture for (A) El Chichón and (B)\nPinatubo for the September-October-November after each eruption. Contour represents the total\nfield of nonVE cluster and shading is the difference between VE and nonVE cluster. Black boxes\nindicate the area calculated for the MC moisture index.\n- Lines 195-196: Since the authors repeatedly state that the MC dries faster than other areas after\nthe VEs, it is worth discussing why this happens.\n Thank you for your suggestion. Now we noticed that, in the single-forcing experiment, not only\nthe Maritime Continent (MC) but also the land area surrounding the Indo-Pacific warm pool dries\nfaster as well. This is now discussed in the revised manuscript as follows:\n(L57) “That is, the volcanically forced radiative cooling and drying occur faster over the land areas\nthan the ocean, especially over the Maritime Continent (MC) and surrounding area, and the\nincreased land-ocean temperature gradient weakens the equatorial Walker circulation and\nenhances westerly anomalies in the western Pacific, inducing El Niño (13-17) through the Bjerknes\nfeedback (18).”\n(L193) “Supplementary Figure S3A shows that, after volcanic eruption, sharp drying not only\noccurs over the MC as observed, but also to the north of the maximum moisture region in the Indo-\nPacific warm pool.”\n- Line 316: In order to determine which budget terms contribute most to the propagation, previous\nstudies (e.g., Andersen and Kuang, 2012; Bui and Maloney, 2020) projected each budget quantity\nonto the tendency term, so I wonder if the authors also consider such kind of projection here.\nAndersen, J. A., & Kuang, Z. (2012). Moist static energy budget of MJO-like disturbances in the atmosphere of a\nzonally symmetric aquaplanet. Journal of Climate, 25(8), 2782–2804.\nBui, H. X., & Maloney, E. D. (2020). Changes to the Madden-Julian Oscillation in coupled and uncoupled aquaplanet\nsimulations with 4xCO2. Journal of Advances in Modeling Earth Systems, 12, e2020MS002179.\n Thank you for your suggestion and providing references. Although the projection method is\nnovel, we think that our composite analysis may be more intuitive, thus easier to interpret with\nsimple explanation. We have now included additional budget terms to quantify their relative\ncontributions on the tendency term.\n- Line 327: This is surprising, but I guess CESM2-LE provides the surface latent heat flux, which\nis directly related to evaporation?\n The experiment is now changed to CESM2 Single Forcing experiment and in the budget term,\nthe column process term is deleted as it is just the residual of all terms. We have revised the\nsentence as (L420) “Note that the role of column processes on the MJO is non-negligible (not\nshown) and warrants further analysis”\n- Figure 4: Consider showing the ensemble spread.\n Following the reviewer’s suggestion, the ensemble spread is now added to Figure 4 (C).\nFigure B (Fig. 4. C in the revision) Decomposed terms of horizontal moisture advection averaged\neast of the MC (100˚E–160˚E, 20˚S–15˚N, indicated by the box in Fig. S4) and over –2 to 2 days\nduring the VE (dark blue) and nonVE (light blue) events. Units are 10-6 kg m-2 s-1. The orange\nwhiskers represent the spread of ensemble members, outlining ±1.0 standard deviation for each\nterm.\n- Since the authors analyzed the large ensemble simulations, did the author also examine the\nacross-ensemble (or ensemble-wise) to address the natural variability?\n We now have changed the experiment to CESM2 single forcing (CESM2-SE) experiment to\nisolate the role of the volcanic forcing on ENSO-MJO interaction. CESM2-SE consists of 15\nensemble members which is not enough to examine across-ensembles to address natural variability.\nNevertheless, we have provided the spread of ensemble members in Figure 4(C), which shows\noverall consistent responses, i.e. stronger horizontal moisture advection due to the enhanced MJO\nactivity, across ensemble members. We have noted this in the revised text (L209-223, Figure 4)\n- It might be worth adding a bit of discussion on the contribution of synoptic eddies (v’dq’)\nin regulating the intraseasonal moisture budget\n Following the reviewer’s suggestion, we now have added budget terms (including synoptic\neddies) (Figure B above). As can be seen, the contribution of synoptic eddies is negligible, thus\nwe focus on the main contributing terms only in the discussion.\nSupplementary: Line 31: Panel (B)?\n Thank you for pointing this out. It is now fixed.\nReviewer #2\nThe manuscript entitled “Volcanically forced Madden–Julian oscillation triggers the immediate\nonset of El Niño” by Hyemi Kim et al. focuses on the physical processes of how strong tropical\nvolcanic eruptions trigger El Niño events. Based on the analysis of observational data and CESM2\nensemble simulations, a new mechanism is proposed where volcanic eruptions enhance the\nMadden-Julian Oscillation (MJO), subsequently rapidly triggering El Niño. This study is\ninteresting and innovative. However, there are significant issues in the analysis process. As of now,\nit seems that the paper does not meet the publication criteria of the journal Nature Communications.\nIn this study, the authors have innovatively emphasized the crucial role of the MJO. Following\nstrong volcanic eruptions, the notable enhancement of MJO over the western Pacific is the most\nimportant and innovative aspect of this study. However, the current definition of the MJO activity\nindex in this paper raises significant concerns about whether it genuinely reflects the characteristics\nof MJO activity. The MJO index definition (lines 272-277) in this study reflects subseasonal to\nseasonal-scale oscillations in the western Pacific region, but it cannot be conclusively established\nas a signal of MJO activity. If it cannot be confirmed that this indeed represents the MJO, the\nnovelty of this work would be undermined.\n Thank you for pointing this out. Following the reviewer's suggestion, we have now changed\nthe definition of the MJO and the MJO index that better reflect the characteristics of the MJO.\nNow, we believe the results are more robust. Please see our detailed comments. We appreciate the\nreviewer’s comment.\nAdditionally, the majority of CMIP6 models currently exhibit limited skill in simulating the\nMJO, with most producing weak characteristics of eastward propagation from the Indian Ocean,\neven the CESM2 model encounters this issue. Therefore, using model results to prove the volcanic\nenhancement of eastward MJO propagation, particularly in the western Pacific oscillation, cannot\nbe conclusively demonstrated based on existing analysis. Consequently, both the applicability of\nthe MJO index and the validation of the MJO's significant role through model data raise critical\nconcerns. As a result, it becomes difficult to agree with the viewpoints proposed by the authors.\n Thank you for pointing this out. Our detailed response about the MJO simulation fidelity in\nthe CESM2 and HadGEM3 is addressed below.\nSpecific comments\n- Lines 71-72, most coupled models exhibit poor simulation of eastward-propagating MJO,\nespecially the process where MJO crosses the Maritime Continent to reach the western Pacific. In\nother words, in the models, it is very difficult for the MJO to cross the Maritime Continent.\n Figure C shows the MJO life-cycle composite for CESM2 large-ensemble simulation compared\nwith NOAA OLR. Although the MJO simulation is poor in many GCMs, the CESM2 and\nHadGEM3-GC3.1-LL which are used in this study, are one of the high MJO fidelity models\nregarding their MJO propagation ability (Back et al. 2024). Those models are used for MJO\nprocess studies (Ahn et al. 2020), mean-state and MJO interaction (Kang et al. 2020), and MJO\ndiversity studies (Back et al. 2024). Therefore, we argue that MJO simulation skills of CESM2\nand HadGEM3-GC3.1-LL are reasonable to utilize for process studies. We have added this point\nto the revised text.\n(L101) \"The present study seeks to partly resolve the issue by utilizing the Community Earth\nSystem Model Version 2.0 (CESM2) which is one of the few contemporary climate models that\ncan realistically simulate both the ENSO and MJO (30-35).\"\n(L299) \"The CESM2 reasonably simulates both the basic state and modes of tropical variabilities,\nsuch as the ENSO and MJO (30-35)\"\n(L322) \"The HadGEM3‐GC3 reasonably simulates the MJO (35).\"\nFigure C. Observed and simulated MJO. MJO lifecycle composite of MJO-filtered OLR anomalies\nfor (left) NOAA OLR and (right) CESM2-LE during Northern Hemisphere extended winter (Oct-\nMarch) from 1980 to 2014.\nBack, S., D. Kim, and S. Son, 2024: MJO Diversity in CMIP6 Models. J. Climate, 37, 4835–\n4850, https://doi.org/10.1175/JCLI-D-23-0656.1.\nAhn, M. et al. MJO Propagation Across the Maritime Continent: Are CMIP6 Models Better Than\nCMIP5 Models? Geophys Res Lett 47, (2020).\nKang, D. et al. The Role of the Mean State on MJO Simulation in CESM2 Ensemble Simulation.\nGeophys Res Lett 47, (2020).\n- Lines 93 to 112, in the observational analysis, due to the applicability issues of the MJO index\nmentioned in the major comment, the current analysis cannot provide insights into the activity\ncharacteristics of the MJO following the volcanic eruptions in 1991 and 1982. As a result, it's not\npossible to establish the lead-lag relationship between MJO and ENSO, MC indicators.\n Thank you for pointing this out. Following the reviewer's suggestion, we define the MJO using\nmore robust definition throughout the entire paper. (L354) \"For our purposes, an intraseasonal\nfilter is applied to retain only eastward propagating signals of wavenumbers 0–9 with periods\nbetween 30 and 96 days (Kiladis et al. 2005). This is hereafter referred to as the “MJO-filter\".\nKiladis, G. N., K. H. Straub, and P. T. Haertel: Zonal and Vertical Structure of the Madden–Julian\nOscillation. J. Atmos. Sci., 62, 2790–2809 (2005)\nDid the MJO coincidentally propagate to the western Pacific in the third month following both\nvolcanic eruptions? If this is the case, I suggest the authors present time-longitude propagation\nmaps of MJO's eastward propagation from the Indian Ocean and its subsequent propagation to the\nwestern Pacific in the third month after the volcanic eruptions. This would more comprehensively\ndemonstrate the significance of MJO in triggering El Niño during these two observational\nprocesses.\n Thank you for pointing this out. Figure D shows the MJO-filtered anomaly over the tropics for\nEl Chichón (left) and Pinatubo (right) eruptions. The brown boxes are the 3-7 months periods after\neruptions. It is shown that the MJO activity becomes more active in the western Pacific (west of\n120E) after two eruptions with subsequent eastward propagation to the western Pacific. This\nfeature tends to become stronger after the third month, particularly for Pinatubo, consistent with\nthe enhanced MJO activity (Figure 1B). This is clearer in the single forcing experiments as\ndescribed in the main text (Figure 4, S3, S4)\nFigure D. MJO-filtered anomaly averaged over the tropical belt [10S-10N] after (A) El Chicon\nand (B) Pinatubo eruptions. Boxes indicate approximate 3-7 months period after each eruption.\n- Lines 106-112, if MJO is suppressed over the Indian Ocean, would it become more challenging\nfor MJO to propagate eastward across the Maritime Continent and reach the western Pacific?\nFurthermore, based on some previous studies, when the Maritime Continent becomes drier, it\nbecomes challenging for the MJO to traverse from the eastern Indian Ocean through the MC and\nreach the western Pacific.\n Thank you for pointing this out. When the new MJO definition is used following the reviewer’s\nsuggestion, the MJO suppressed anomaly in the western Pacific is less clear. Additionally, we have\nprovided clear discussion on how the drying over the Maritime Continent surrounding land area\nenhances the MJO propagation. As there has substantial changes, we would appreciate the\nreviewers view on our revised manuscript.\n- Lines 121-126, how can it be proven that the intensified MJO over the western Pacific rapidly\ntriggered the WWBs rather than other seasonal-scale oscillations?\n More detailed evidences from previous studies are added now as (L84), \"After beginning its\nlife cycle over the Indian Ocean, the MJO reaches the western Pacific if it survives the barrier\neffect of the Maritime Continent. In the western Pacific, studies have found that the strongest,\nlongest-lasting, basin-scale WWBs were mostly associated with active MJO events (19, 25-28).\nThese MJO-driven WWBs in the western Pacific then increase sea surface temperatures (SST) in\nthe eastern Pacific by a few tenths of a degree Celsius (28).\"\n19. A. Seiki, Y. N. Takayabu, Westerly Wind Bursts and Their Relationship with Intraseasonal Variations and ENSO.\nPart I: Statistics. Monthly Weather Review 135, 3325-3345 (2007).\n25. M. Puy, J. Vialard, M. Lengaigne, E. Guilyardi, Modulation of equatorial Pacific westerly/easterly wind events\nby the Madden–Julian oscillation and convectively-coupled Rossby waves. Climate Dynamics 46, 2155-2178\n(2016).\n26. H. H. Hendon, M. C. Wheeler, C. Zhang, Seasonal Dependence of the MJO–ENSO Relationship. Journal of\nClimate 20, 531-543 (2007).\n27. W. S. Kessler, M. J. McPhaden, K. M. Weickmann, Forcing of intraseasonal Kelvin waves in the equatorial Pacific.\nJournal of Geophysical Research: Oceans 100, 10613-10631 (1995).\n28. A. M. Chiodi, D. E. Harrison, G. A. Vecchi, Subseasonal Atmospheric Variability and El Niño Waveguide\nWarming: Observed Effects of the Madden–Julian Oscillation and Westerly Wind Events. Journal of Climate\n27, 3619-3642 (2014).\n Figure E below (modified from Figure 3 in the main text) shows the change of MJO activity\nand WWB frequency composite during the volcanic eruption (VE) and nonVE clusters. In both\nclusters, WWB frequency varies along with the MJO activity.\nFig. 3. Responses of the variables to VEs with initially neutral ENSO conditions. Averaged\nmonthly time series of (A) MJO activity (blue line, right y-axis) and WWB frequency indices (gray\nline, left y-axis) from the CESM2-SE for VE (solid line) and nonVE (dashed line) clusters. Month\n0 indicates the pre-eruption month, and dashed vertical lines indicate 3 and 7 months after the\neruption. Dots indicate months when the difference between the VE and nonVE clusters is\nstatistically significant at the 95% confidence level (closed dot) based on 1,000-sample\nbootstrapping.\n- Lines 165-166, when the western side of the MJO becomes drier and the eastern side becomes\nwetter, it will favor the eastward propagation of the MJO. However, when the MJO is over the\neastern Indian Ocean, if the MC becomes drier, it will make it more difficult for the MJO to\ntraverse through the MC, rather than promoting MJO propagation across the MC.\n Thank you for pointing this out. To address the reviewer’s comment, the zonal advection term\nis added and discussed in details in the revised manuscript. Part of the discussion is as:\n(L216) “By decomposing the moisture horizontal advection terms, the zonal and meridional\nadvection of the background mean moisture due to the anomalous MJO wind is found to be the\nmost dominant process (Fig. 4C and Fig. S4), which is consistent with previous studies (36, 37).\nIn summary, because the area poleward of maximum moisture over the Indo-Pacific warm pool\nbecomes drier and the area east of the maximum moisture becomes wetter after a VE, these\nchanges together provide favorable conditions for the MJO to propagate further eastward.”\n- The MJO represents internal variability in climate system simulations, and it is challenging to\ncomprehend the phenomenon described in this paper, where considering volcanic forcing signal\nleads to a strengthening of MJO variability (as shown in Figures 4a and 4b). Typically, external\nforcings have a limited ability to alter the spatial characteristics of internal climate system\nvariability; they mainly affect the evolution period. Additionally, regarding Figures 4a and 4b, the\nregion around 120E, which is near the Maritime Continent, displays a discontinuity in OLR. This\nalso indicates the difficulty for the MJO to across the MC and reach the western Pacific.\nOscillations in the western Pacific might not be related to MJO, such as the westward signals near\n180E.\n The volcanic forcing influences on the mean state such as SST and moisture distribution as\nshown in this paper. Natural variability, such as ENSO and MJO, can substantially be changed due\nto the change in mean state. As discussed in the main text, the MJO can be altered by the\nbackground state, especially the moisture distributions (moisture-mode theory). This is now\nheavily discussed in the revision (L185-225).\n The new Figure 4 below shows more robust eastward MJO propagation as the MJO-filter is\nnow changed following the reviewer’s suggestion.\nFig. 4. Longitude-time evolution of the MJO and moisture budget terms. (A and B) The lag\ncomposites of MJO-filtered OLR (W m-2, shading) and U850 (m s-1, contour interval 0.5, starting\nat 0.3) averaged over the tropics (10˚S–10˚N) for the 3–7 months after the VE and nonVE events.\nGray solid lines indicate a lag of 0 days and 120˚E.\n- There are certain issues concerning the selection of volcanic and non-volcanic samples.\nAccording to the description in this study, only simulation data from the years 1982 and 1991 are\ncategorized as volcanic samples. However, for instance, the atmospheric concentration of volcanic\naerosols was very high in 1992 and 1993, especially in 1992 when the volcanic aerosol content\nsurpassed even that of 1991. This year's impact on global climate was more significant. Therefore,\nwhen selecting volcanic and non-volcanic samples, it would be more accurate to consider the\nconcentration of atmospheric sulfate aerosols as a criterion, given its direct relation to volcanic\ninfluence.\n In general, the SO₂ from volcanic eruption all converted in sulfate aerosols in a month.\nPinatubo erupted in June, and the peak was before the end of 1991. By the end of 1992, there is\n50% less sulfate. This was same for El Chichon eruption. Figure E shows the stratospheric AOD\nafter 1991 Pinatubo eruption for observation and models (Quaglia et al. 2023, Figure 4).\nQuaglia et al. (2023) \" Interactive stratospheric aerosol models' response to different amounts and\naltitudes of SO2 injection during the 1991 Pinatubo eruption\" Atmos. Chem. Phys., 23, 921–948,\n2023, https://doi.org/10.5194/acp-23-921-2023"}, {"round": 2, "reviewer_comments": {"reviewer_3": "From 118-121, it stated that \"These results suggest that an increase in MJO activity over the western Pacific is associated\nwith WWB activity, and it may have contributed to the rapid development of an El Niño event immediately after the recent\nlarge VEs\".\nWhat VE's parameter that can be adjusted or making it called \"Large VE\"? Is it the amount of aerosol blow to air? or the high\nof the eruption? or even dimension of mountain that erupted? It have to be cleared, since small mountain that erupted, can\nalso produced high amount of aerosol, or even high eruption.\nFrom 249-251, it stated that \"We conclude that the realistic response of the MJO associated with MC drying in CESM2 is\nlikely one of the reasons for the immediate onset of El Niño after an eruption\"\nOnset El Niño can be develop immediate after an eruption of VE, that is assure. But how big the VE? is it VE location near the\nEquator? What time the VE? All of these parameters can not be put aside. What if the longevity of MJO Phases also\nbeen driven by the VE, perhaps in a very short time after VE in near equator?", "reviewer_4": "Fascinating paper and previous reviewer exchange. I found the previous reviews thorough, their most important criticisms\nsimilar to my own, and I found the authors' responses to these major concerns compelling. There is a lot of uncertainty with\nregard to the association between volcanic events and El Niño development, with such a small sample size in observations,\nrendering it possible that coincidence could explain some of observed association. The authors suggest that drying of the\nMaritime Continent region favors MJO propagation across the Maritime Continent, which favors west Pacific westerly wind\nevents, which then can favor El Niño development. It's unclear that models sufficiently represent the associated phenomena,\nor the full MJO/ENSO relationship. Yet their results paint a picture that I think it clear enough, with sufficient caveats, to\nmerit publication. The claims appear to fit their evidence posed, along with the argument supported by the moisture mode\nhypothesis. Potential flaws pointed out by reviewers, such as the subseasonal index used, actually rendered the results\nstronger when an MJO index was used instead of their original west Pacific subseasonal index. The revised manuscript is\nthus improved. The results should be reproducible based on the descriptions given.\nThe authors' general conclusion that drying near the Maritime Continent after a volcanic eruption favors MJO crossing is\nsupported by their arguments to the extent that the simple moisture mode explains MJO propagation, and in that context I\nthink their explanation is sufficiently supported. The drying base state of the Maritime Continent is also consistent with my\nown recent finding that when the upper troposphere over the region is most zonally diffluent, implying that the Walker\ncirculation over the Indian Ocean is stronger than average, the MJO tends to fail to cross the Maritime Continent, and some\nof this effect is explained by dry dynamics (advection of diffluent background zonal wind by the MJO zonal wind collapses\nthe MJO upper tropospheric zonal wind because it implies advection of background wind of the opposite sign to the MJO\nwind). The Maritime Continent is most moist during periods when the Walker circulation is strongest over the Indian Ocean.\nThus the upper tropospheric dry dynamics and the authors' lower tropospheric moist dynamics appear to work in the same\ndirection and support their conclusion.\nSigned, Paul Roundy"}, "author_response": "[Response to the reviewers’ comments received on January-06, 2025]\n“Volcanically forced Madden–Julian oscillation triggers the immediate onset of El Niño”\nHyemi Kim, Seung-Ki Min, Daehyun Kim, and Daniele Visioni\nWe wish to express our gratitude to the reviewers for their time and effort spent in reviewing this\nmanuscript. The original comments from the reviewers are in black, and the responses are in blue.\nReviewer #3\n(1) From 118-121, it stated that \"These results suggest that an increase in MJO activity over the\nwestern Pacific is associated with WWB activity, and it may have contributed to the rapid\ndevelopment of an El Niño event immediately after the recent large VEs\".\n(2) What VE's parameter that can be adjusted or making it called \"Large VE\"? Is it the amount of\naerosol blow to air? or the high of the eruption? or even dimension of mountain that erupted?\nIt have to be cleared, since small mountain that erupted, can also produced high amount of aerosol,\nor even high eruption.\n(3) From 249-251, it stated that \"We conclude that the realistic response of the MJO associated\nwith MC drying in CESM2 is likely one of the reasons for the immediate onset of El Niño after an\neruption\". Onset El Niño can be develop immediate after an eruption of VE, that is assure. But\nhow big the VE? is it VE location near the Equator? What time the VE? All of these parameters\ncannot be put aside. What if the longevity of MJO Phases also been driven by the VE, perhaps in a\nvery short time after VE in near equator?\n (Response to 1-3) The reviewer's multiple questions regarding the threshold for defining “large\nVE” are addressed as follows:\n(L338) \"..... In addition, only VEs that are strong enough to inject aerosols into the stratosphere\nand that pass a certain threshold (i.e., absolute radiative forcing greater than 3.3 W/m2) can increase\nthe probability of an El Niño event occurring (52). Since the satellite era, only two eruptions in the\ntropics have surpassed this threshold: El Chichón in Mexico (93˚W, 17˚N) in April 1982 and Mt.\nPinatubo in the Philippines (120˚E, 15˚N) in June 1991. The plumes from El Chichón reached\nabout 31 km at their highest point and released ~7.5 Tg of SO2, while Mt. Pinatubo reached 40\nkm and injected ~20 Tg of SO2, both exceeding volcanic explosivity index category 5. Notably,\nboth eruptions occurred at tropical latitudes.\"\n(4) What if the longevity of MJO phases also been driven by the VE, perhaps in a very short time\nafter VE in near equator?\n Although, no study exists regarding this topic, volcanic eruptions can significantly impact the\natmospheric and oceanic systems through a variety of mechanisms, potentially affecting the MJO\nlongevity, as discussed by the reviewer.\nTo check the MJO characteristics change after VE in observations, Figure A shows the MJO-\nfiltered anomaly over the tropics after the El Chichón (left) and Pinatubo (right) eruptions. The\nbrown boxes are the 3-7 months periods after eruptions. It is shown that the MJO activity becomes\nmore active in the western Pacific (west of 120E) after two eruptions with subsequent eastward\npropagation to the western Pacific. Based on this metrics, no clear signal is detected regarding the\nMJO longevity. However, changes in MJO longevity may be more effectively detected using more\nadvanced metrics, which can be explored in future studies.\nFigure A. MJO-filtered anomaly averaged over the tropical belt [10S-10N] after (A) El Chicon\nand (B) Pinatubo eruptions. Boxes indicate approximate 3-7 months period after each eruption.\nReviewer #4\nFascinating paper and previous reviewer exchange. I found the previous reviews thorough,\ntheir most important criticisms similar to my own, and I found the authors' responses to these major\nconcerns compelling. There is a lot of uncertainty with regard to the association between volcanic\nevents and El Niño development, with such a small sample size in observations, rendering it\npossible that coincidence could explain some of observed association. The authors suggest that\ndrying of the Maritime Continent region favors MJO propagation across the Maritime Continent,\nwhich favors west Pacific westerly wind events, which then can favor El Niño development. It's\nunclear that models sufficiently represent the associated phenomena, or the full MJO/ENSO\nrelationship. Yet their results paint a picture that I think it clear enough, with sufficient caveats, to\nmerit publication. The claims appear to fit their evidence posed, along with the argument supported\nby the moisture mode hypothesis. Potential flaws pointed out by reviewers, such as the subseasonal\nindex used, actually rendered the results stronger when an MJO index was used instead of their\noriginal west Pacific subseasonal index. The revised manuscript is thus improved. The results\nshould be reproducible based on the descriptions given.\nThe authors' general conclusion that drying near the Maritime Continent after a volcanic eruption\nfavors MJO crossing is supported by their arguments to the extent that the simple moisture mode\nexplains MJO propagation, and in that context, I think their explanation is sufficiently supported.\nThe drying base state of the Maritime Continent is also consistent with my own recent finding that\nwhen the upper troposphere over the region is most zonally diffluent, implying that the Walker\ncirculation over the Indian Ocean is stronger than average, the MJO tends to fail to cross the\nMaritime Continent, and some of this effect is explained by dry dynamics (advection of diffluent\nbackground zonal wind by the MJO zonal wind collapses the MJO upper tropospheric zonal wind\nbecause it implies advection of background wind of the opposite sign to the MJO wind). The\nMaritime Continent is most moist during periods when the Walker circulation is strongest over the\nIndian Ocean. Thus the upper tropospheric dry dynamics and the authors' lower tropospheric moist\ndynamics appear to work in the same direction and support their conclusion.\nSigned, Paul Roundy\n We appreciate your positive and thorough review of the original and the revised manuscript. A\nrecent study (Roundy, 2024) is now added as it discusses the plausible impact of the upper\ntropospheric circulation on MJO propagation.\n(L219) “Note that the influence of the upper-tropospheric background wind may play a significant\nrole in shaping the dynamics of MJO propagation (38).”\nRoundy, P. E., 2024: Role of Dry Dynamics in the Maritime Continent Barrier Effect in the\nMadden–Julian Oscillation. J. Atmos. Sci., 81, 2041–2050, https://doi.org/10.1175/JAS-D-24-\n0072.1.\n3"}]

Decision letter