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Oceanography
Coupling is key for the tropical Indian and Atlantic oceans to boost super El Niño
Coupling is key for the Indo-Atlantic booster to super El Niño
https://orcid.org/0000-0001-9372-8401
Fan Hanjie Conceptualization Data curation Formal analysis Funding acquisition Investigation Methodology Resources Software Validation Visualization Writing - original draft Writing - review & editing 1 2 3
https://orcid.org/0000-0002-7611-0308
Wang Chunzai Conceptualization Funding acquisition Project administration Resources Supervision Validation Writing - original draft Writing - review & editing 1 *
https://orcid.org/0000-0003-1840-8429
Yang Song Conceptualization Validation Visualization Writing - original draft Writing - review & editing 2 3
https://orcid.org/0009-0008-7019-1566
Zhang Guangli Software Visualization 4
1 State Key Laboratory of Tropical Oceanography, Global Ocean and Climate Research Center, Guangdong Key Laboratory of Ocean Remote Sensing, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, China.
2 School of Atmospheric Sciences, Sun Yat-sen University, Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai, China.
3 Guangdong Province Key Laboratory for Climate Change and Natural Disaster Studies, Sun Yat-sen University, Zhuhai, China.
4 School of Marine Sciences, Sun Yat-sen University, Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai, China.
* Corresponding author. Email: cwang@scsio.ac.cn
13 9 2024
13 9 2024
10 37 eadp228114 3 2024
08 8 2024
Copyright © 2024 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).
2024
The Authors
https://creativecommons.org/licenses/by-nc/4.0/ This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license, which permits use, distribution, and reproduction in any medium, so long as the resultant use is not for commercial advantage and provided the original work is properly cited.

The influences of the tropical Indian and Atlantic oceans on the development of El Niño Southern Oscillation (ENSO), especially regarding super El Niño events, have been a subject of debate. In particular, several studies argue that these cross-basin influences may be mere statistical artifacts resulting from the high auto-correlation of ENSO. To clarify this issue, we conduct a series of perfect model hindcast experiments to untangle the individual and synergistic effects of the tropical Indian and Atlantic oceans. Our results clearly demonstrate that without these cross-basin effects, the Pacific warming would rarely reach super El Niño level. Specifically, the individual effect of the Indian Ocean efficiently enhances the development of super El Niño events. In contrast, the Atlantic’s effect is initially limited because it fails to establish the Bjerknes feedback in the Pacific. However, when coupled with the Indian Ocean, the Atlantic’s effect becomes more pronounced as it is amplified by the Bjerknes feedback established by the Indian Ocean.

The interactive coupling between the tropical Indian and Atlantic oceans promotes their boosting effects on super El Niño.

http://dx.doi.org/10.13039/501100001809 National Natural Science Foundation of China 42192564 http://dx.doi.org/10.13039/501100001809 National Natural Science Foundation of China 42088101 http://dx.doi.org/10.13039/501100001809 National Natural Science Foundation of China 42206026 National Key Research and Development Program of China 2019YFA0606701 Guangdong Province Key Laboratory for Climate Change and Natural Disaster Studies 2020B1212060025 Development fund of South China Sea Institute of Oceanology of the Chinese Academy of Sciences SCSIO202208
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pmcINTRODUCTION

The El Niño Southern Oscillation (ENSO) is a prominent periodic variation of the ocean-atmosphere system in the tropical Pacific region. Previous studies have suggested that ENSO can be influenced by anomalous climate systems in the Indian and Atlantic oceans (1–8). They demonstrate that the Indian and Atlantic oceans influence ENSO primarily by generating zonal wind anomalies over the tropical Pacific through modulating the Walker circulation (9–11) or triggering atmospheric wave response (12–14). Specifically, it has been observed that Atlantic Niña and positive Indian Ocean Dipole (IOD) events occur in the summer and autumn (referring to boreal seasons in this paper) of four super El Niño years (1972, 1982, 1997, and 2015). Building upon these observations and CMIP6 simulations, a recent study (15) proposes a mechanism termed the Indo-Atlantic Booster, which elucidates how the Indian and Atlantic oceans may synergistically enhance super El Niño events.

Nevertheless, the impact of the Indian and Atlantic oceans on ENSO remains a subject of ongoing debate. Several studies argue that the leading correlation between sea surface temperature (SST) modes in the Indian and Atlantic oceans, including IOD (16), Atlantic Niño/Niña (17), and the north tropical Atlantic mode (18), with subsequent ENSO events is merely a consequence of the influence of ENSO on these factors and the inherent autocorrelation of ENSO (19–22). In plain words, these studies suggest that the precursors in the Indian and Atlantic oceans for ENSO are misinterpretations of their statistical relationships. In addition, the role of the Atlantic Ocean in the development of ENSO remains uncertain, with suggestions that its influence is notably weaker compared to that of the Indian Ocean. Previous researches conclude that the Atlantic exerts a minimal impact on ENSO development by conducting comparative analyses of perfect model hindcast experiments with and without restoration of tropical Atlantic SSTs to climatological values (23, 24).

Understanding the physical mechanisms underlying super El Niño events is crucial, given their notable climate impacts worldwide (10, 25–29). Accurate comprehension of these mechanisms can enhance the prediction of such events and aid in mitigating associated climate extremes. To further elucidate the cross-basin effects of the Indian and Atlantic oceans on the development of super El Niño events, we conducted a series of perfect model hindcast experiments using a well-designed framework that isolates the individual effects of the Indian and Atlantic oceans, as well as their synergistic coupling effects. On the basis of the results from these experiments, we provide evidence supporting the notion that the coupling between the two oceans is the key to their boosting effects on super El Niño.

RESULTS

Indian and Atlantic SST anomalies during super El Niño years

Before analyzing the hindcast experiments, we provide an overview of the SST anomalies in the Indian and Atlantic oceans during super El Niño years. Specifically, we focus on the peak season (November-December-January, hereafter referred to as NDJ) and identify five super El Niño years from 1948 to 2023, namely, 1972, 1982, 1997, 2015, and 2023, where the NDJ Niño3.4 index exceeds 2 SDs (Fig. 1A). During these events, we observe several common anomalous SST modes in the Indian and Atlantic oceans, including negative Indian Ocean Basin (IOB) in the spring, Atlantic Niña in the summer, and positive IOD in the autumn (Fig. 1B), which have been proposed to promote the El Niño development in previous studies (13, 15, 30). Another potential factor influencing ENSO, the North Tropical Atlantic (NTA) mode, shows substantial variability among different super El Niño events and has a relatively small average amplitude (fig. S1).

Fig. 1. Super El Niño events and the corresponding anomalous sea surface temperature indices in the Indian and Atlantic oceans.

(A) Standardized November-December-January (NDJ) Niño3.4 index from 1948 to 2023 in observations. The red dots denote the super El Niño events when NDJ Niño3.4 index exceeds 2 SDs. (B) Standardized March-April-May (MAM) basin-wide index (BWI), June-July-August (JJA) Atl3, and September-October-November (SON) dipole mode index (DMI) during the observed super El Niño events and their average values. Error bars show the 97.5% and 2.5% confidence bounds measured by a 10,000-resampling bootstrap method. (C and D) Same as (A) and (B), except for the results from CESM simulations.

We also identify 10 super El Niño years in a 100-year fully coupled integration of the Community Earth System Model version 1 CESM1 (31), utilizing the same criterion of the NDJ Niño3.4 index surpassing 2 SDs (Fig. 1C). These model years exhibit consistent anomalous modes in the Indian and Atlantic oceans to the observations (Fig. 1D), affirming the important influence of these modes on super El Niño development. Similarly, the NTA mode does not exhibit a consistent phase in these simulated super El Niño events, further affirming its limited importance in boosting super El Niño (fig. S1). Therefore, the cross-basin effects of the Indian and Atlantic oceans mainly come from the tropical modes. However, it is important to note that the average value of the springtime basin-wide index (BWI, representing the amplitude of the IOB) in the CESM1 simulations was higher than the observed value (Fig. 1B), indicating a potential overestimation of the IOB impact by the model. Furthermore, some exceptions were indeed noted, such as a positive IOB in 2015, an Atlantic Niño in 2023, and in model year 103, suggesting that these modes may not be imperative for the occurrence of super El Niño.

Weakened Pacific warming by decoupling the Indian and Atlantic oceans

The perfect model hindcast experiments are conducted from 1 January of the developing year (year0) to 31 March of the subsequent year (year 1), using 10 ensemble members of slightly random-perturbed initial conditions. Initially, fully coupled hindcast experiment (referred to as “CTRL” hereafter) is conducted for each of the 10 super El Niño years to serve as a baseline for sensitive experiments and test the predictability of these events. However, for two events (years 123 and 142), the CTRL fails to reproduce the extreme tropical Pacific warming observed in the 100-year continuous integration (referred to as “truth” hereafter) (fig. S2). The low predictability of these two events might be associated with the relatively weaker warm water volume (referred to as “WWV” hereafter) and charging preconditions (table S1). Therefore, the occurrences of these two super El Niño events in the truth might be associated with other mechanisms. Specifically, the CTRL exhibited only half the strength of autumn IOD compared to the truth for year 123, while the summer Atlantic Niña was much weaker in the CTRL than in the truth for year 142 (fig. S3). The weaker cross-basin forcing from the Indian and Atlantic oceans could be one of the reasons why the Pacific warming in the CTRL is weaker than the truth.

Subsequently, three partially decoupled hindcast experiments are conducted for each of the eight super El Niño years with high predictability. In these decoupled experiments, SSTs in either the tropical Indian Ocean or the Atlantic Ocean, or both, are prescribed to daily climatology respectively (referred to as “dIO,” “dAtl,” and “dInA,” respectively hereafter, fig. S4). The decoupled regions are confined to the pantropical oceans to highlight the main boosters to super El Niño, namely, the negative IOB, positive IOD, and Atlantic Niña. In the dInA, the multi-member-mean Niño3.4 index decreases to varying degrees for all the eight events, with only year 196 exceeding 2 SDs (fig. S5). A super El Niño event could still develop in year 196 without cross-basin forcing (fig. S5G), possibly due to the extraordinary WWV and charging preconditions (table S1) resulting from the strongest and double-year La Niña in the preceding years (Fig. 1C). However, the multi-member-mean Niño3.4 index was slightly weaker in each decoupled experiment compared to the CTRL for year 196 (figs. S6G and S7G), suggesting that at the very least, the Indian and Atlantic oceans would not diminish the Pacific warming during the development of super El Niño events. All the decoupled experiments, except for the dAtl of year 191 (fig. S7F), simulate a decrease in the Niño3.4 index compared to the CTRL (figs. S5 to S7). Moreover, the inter-member uncertainties of the three partially decoupled experiments (figs. S6 and S7) are remarkably larger than those of the CTRL (fig. S5), highlighting the importance of cross-basin effects from the Indian and Atlantic oceans in accurately predicting super El Niño events. Specifically, decoupling the Atlantic induces a smaller decrease in the mean values of the Niño3.4 index compared to decoupling the Indian Ocean, but leads to a comparable increase in the inter-member spreads (figs. S6 and S7).

Figure 2 provides a more general illustration of the responses to decoupling the Indian and Atlantic oceans by compositing all members of the super El Niño events, except for year 196 when the Pacific warming is strongly determined by its high WWV and charging preconditions. The composite super El Niño condition in the CTRL is clearly accompanied by springtime IOB cooling, summertime Atlantic Niña, and autumn-time positive IOD (Fig. 2A). With the coupling of the other two tropical oceans, the composite Niño3.4 index significantly exceeds 2 SDs with a very small spread of uncertainty (Fig. 2B). On the other hand, when the cross-basin effects from the Indian and Atlantic oceans are absent in the dInA, the westerly wind anomalies and SST warming in the Pacific are much weaker (Fig. 2C). Consequently, the Niño3.4 index decreases markedly, even with the upper bound of Niño3.4 index being lower than 2 SDs (the threshold for categorizing a super El Niño event) while the spread increases (Fig. 2D). This substantial decline in the Niño3.4 index indicates that the probability of super El Niño occurrence would be greatly reduced in the absence of cross-basin forcing. By only decoupling the Indian Ocean, the Pacific warming in the dIO is also much weaker compared to the CTRL, but still slightly stronger than in the dInA (Fig. 2, E and F). In the dAtl, when only decoupling the Atlantic, the Pacific warming shows a smaller decrease in the composite mean compared to the dIO (Fig. 2G), but the Niño3.4 index displays a larger spread of uncertainty than in the dIO (Fig. 2H). This feature suggests that the Atlantic forcing plays a nonnegligible role in ensuring the occurrence of super El Niño, although its influence on the amplitude of Pacific warming is relatively weak. It is noteworthy that in the dIO, the Atlantic Niña also becomes weaker as the El Niño weakens (Fig. 2E), indicating a two-way interaction between ENSO and Atlantic Niño/Niña (15, 32).

Fig. 2. Weakening of equatorial Pacific warming due to the decoupling of Indian and Atlantic oceans.

(A) Composite equatorial (5°S to 5°N) sea surface temperature (shading, °C, with stippling indicating significance at the 95% confidence level) and surface zonal wind (contours, m/s, shown only for the values exceeding the 95% confidence level) anomalies from January (0) to March (1) in the CTRL. (B) Monthly standardized Niño3.4 index from January (0) to March (1) in the CTRL. The solid line displays the composite values, whereas the shading denotes the confidence interval between the 97.5% and 2.5% bounds measured by a 10,000-resampling bootstrap method. (C and D) Similar to (A) and (B), except for the results from the dInA. In (D), the black line and shading show the composite values and confidence interval of the dInA, while the red line is the composite values of the CTRL. (E and F) Similar to (A) and (B), except for the results from the dIO. (G and H) Similar to (A) and (B), except for the results from the dAtl. In (F)/(H), the green/blue solid line and shading show the composite values and confidence interval of the dIO/dAtl, while the red and black dashed lines are the composite values of the CTRL and dInA, respectively.

Individual and synergistic effects from the Indian and Atlantic oceans

Next, we aim to distinguish the individual and synergistic effects from the Indian and Atlantic oceans on the development of super El Niño. The individual effects are achieved by the differences between the dIO/dAtl and dInA, whereas the synergistic effects are obtained by the differences between the CTRL and the dIO/dAtl (see Materials and Methods).

When decoupling the Indian and Atlantic oceans simultaneously (dInA), only the internal processes within the Pacific such as recharge process (33) and westerly wind bursts (34) contribute to the promotion of El Niño development. However, when decoupling only one ocean, i.e., dAtl or dIO, the active ocean (the Indian Ocean or the Atlantic Ocean) still plays a role in enhancing El Niño growth, in addition to processes within the Pacific. Therefore, the differences between the dAtl or dIO and the dInA (former minus latter) display the individual effect of the Indian Ocean or the Atlantic Ocean without their coupling. On the other hand, the differences between the CTRL and the dIO or dAtl (former minus latter) demonstrate the effect of the Indian Ocean in the combination with the Atlantic, or vice versa. The specific method of isolating these cross-basin effects is outlined in table S2. If the synergistic effects differ from the individual effects, it suggests that the coupling of the Indian and Atlantic oceans contributes to their boost effects on super El Niño events.

First, differences can be observed in the mean values between the synergistic and individual effects, with an amplitude of approximately 0.3 SDs during the peak seasons (Fig. 3). These differences are not particularly large, especially for the Indian Ocean (Fig. 3, A and D). However, for the Atlantic, the individual effect without coupling is nearly neutral (Fig. 3B). In this case, the synergistic effect of the Atlantic due to coupling shows a substantial enhancement compared to its individual effect (Fig. 3E). Therefore, previous studies may have underestimated the true impact of the Atlantic (22, 24) by failing to account for the synergistic interaction between the Atlantic and Indian Oceans. The more substantial differences are observed in uncertain spreads. Specifically, the spreads in the individual effects are larger than 0.5 SDs for both the Indian and Atlantic oceans (Fig. 3, A and B). Moreover, when these two individual effects are combined offline, the spread reaches a maximum of almost 1 SD (Fig. 3C). Conversely, spreads in synergistic effects are much smaller, indicating a more definitive boosting effect due to the coupling of the Indian and Atlantic oceans. With a small spread, the synergistic effect of the Indian Ocean significantly exceeds 0.5 SDs at its peak (Fig. 3D). Similarly, the Atlantic exhibits a significantly positive synergistic effect during OND(0) (Fig. 3E), whereas the individual effect remains consistently weak and uncertain throughout the developing year (Fig. 3B). The combined effect of the Indian and Atlantic oceans shows an exceptionally small spread (Fig. 3F), even smaller than that of their individual effects (Fig. 3, A to C).

Fig. 3. Individual and synergistic boost effects of the Indian and Atlantic oceans on super El Niño.

(A) The response of the monthly standardized Niño3.4 index to the individual effect of the Indian Ocean (dAtl minus dInA) from January (0) to March (1). The solid line displays the composite values, whereas the shading denotes the confidence interval between the 97.5% and 2.5% bounds measured by a 10,000-resampling bootstrap method. (B) Same as (A), except for the individual effect of the Atlantic (dIO minus dInA). (C) Same as (A), except for the offline summation of the individual effects of the two oceans [(A) plus (B)]. (D) Same as (A), except for the synergistic effect of the Indian Ocean with the coupling of the Atlantic (CTRL minus dIO). (E) Same as (A), except for the synergistic effect of the Atlantic with the coupling of the Indian Ocean (CTRL minus dAtl). (F) Same as (A), except for the online joint effect of the two oceans (CTRL minus dInA). The schematics above and below the time series of Niño3.4 index show how to calculate the individual and synergistic boost effects.

The stronger and more determined cross-basin effect resulting from the coupling of the Indian and Atlantic oceans can be understood from two perspectives. First, the synergistic effect of the Atlantic Ocean is bolstered by the stage set by the Indian Ocean’s impact. Because the Indian Ocean has a greater and earlier impact on Pacific SST warming (Figs. 3A and 4D), the warm pool in the dAtl extends further eastward, with an approximate 5° to 15° eastward shift in its eastern edge, compared to that in the dInA (Fig. 4, A and B). This eastward shift of the warm pool creates a warmer state and stronger convection in the central Pacific, favoring stronger zonal wind responses to remote forcing from the Atlantic (Fig. 4, A and B). These amplified zonal wind responses subsequently generate larger zonal current anomalies in the central Pacific, ultimately leading to greater SST warming responses through enhanced zonal advection (Fig. 4, A to C). Second, while the Indian Ocean effect is stronger and efficiently establishes the Bjerknes feedback, it may introduce more uncertainties in the development of the El Niño event. Notably, the inter-member spread of the dAtl experiments is larger than that of the dIO (Fig. 2, F and H), reflecting the additional noise introduced by the Indian Ocean’s individual effect. However, in the CTRL when the effect of the Atlantic is active, the El Niño events simulated by the model not only have larger amplitudes but also exhibit substantially reduced uncertainties (Figs. 2B and 4E). Therefore, despite its smaller amplitude, the forcing of the Atlantic ensures the total cross-basin effects during the development of super El Niño events. Ultimately, the joint effect of the coupled Indian and Atlantic oceans provides an additional basin-wide warming of more than 1 SD, which is crucial for tropical Pacific warming to reach the level of a “super El Niño” event.

Fig. 4. The Atlantic as a booster with coupling of the Indian Ocean.

(A) The responses of Niño4-averaged surface zonal winds (red solid line, m/s), equatorial (5°S to 5°N) sea surface temperature (shading, °C, with stippling indicating significance at the 95% confidence level), and oceanic zonal currents (contours, cm/s) to the individual effect of the Atlantic (dIO minus dInA) from January (0) to March (1). Blue solid line atop the shading denotes the warm pool edge (29°C isotherm) in the dInA. For reference, the response of Niño4-averaged surface zonal winds to the synergistic effect of the Atlantic (CTRL minus dAtl) and the warm pool edge in the dAtl are displayed by dashed gray and blue lines. (B) Same as (A), except for the synergistic effect of the Atlantic with the coupling of the Indian Ocean. (C) Similar to the shading and contours in (A), but showing differences between the individual and synergistic effects of the Atlantic [(B) minus (A)]. (D) Similar to (C), except for the individual effect of the Indian Ocean (dAtl minus dInA), with shading displayed only for the values exceeding the 95% confidence level. (E) Same as (D), except for the online joint effect of the two oceans (CTRL minus dInA).

DISCUSSION

Here, we provide experimental evidence that supports the crucial role of the coupling between the Indian and Atlantic oceans in enhancing the intensity of super El Niño events. Through a series of well-designed perfect model hindcast experiments, we are able to isolate and examine the individual and synergistic effects of these two oceans. By decoupling the Indian and Atlantic oceans, we observe a significant weakening in seven of eight super El Niño events in the CESM1 model, suggesting that the cross-basin effects from these two oceans are instrumental in amplifying the Pacific warming to the extreme levels observed in the CTRL experiments.

Individually, the Indian Ocean has the capacity to induce substantial warming in the tropical Pacific by effectively establishing a positive feedback loop between zonal wind and SST (known as the Bjerknes feedback). On the other hand, the individual effect of the Atlantic is limited due to its weak amplitude and inability to establish the Bjerknes feedback in the Pacific. However, when coupled with the Indian Ocean, the influence of the Atlantic becomes stronger and more determined, attributed to the amplification by the Indian Ocean–established Bjerknes feedback. Overall, the synergistic effect of the Indian and Atlantic oceans can lead to an additional warming in the tropical Pacific of more than 1 SD, significantly boosting the development of super El Niño events with minimal uncertainty. It is important to note that the cross-basin effect of the Atlantic has shown an intensification over the past few decades (10, 35). This intensification could potentially be linked to changes in the mean state of a warmer western Pacific and an expanded warm pool. However, according to future projections by climate models, it is expected that the eastern Pacific will warm at a faster rate, while the deep convection favoring pool will be confined to the western Pacific (36). This confinement of the deep convection favoring pool in the western Pacific might dampen the cross-basin effect of the Atlantic (37).

While the cross-basin boosting effects are evident in most of the super El Niño events analyzed, one event (year 196) appears to be less influenced by the Indian and Atlantic oceans. Before this particular super El Niño event, a historically strong La Niña event occurred and persisted for 2 years. The exceptional charging precondition resulting from this preceding La Niña event may have played an important role in the subsequent growth of the super El Niño event. It is worth noting that in both the truth and CTRL experiments of year 196, there are substantial summertime Atlantic Niña and autumn-time positive IOD, confirming the influence of early-onset ENSO in the Pacific on the other two oceans (13).

Our findings partly align with the arguments put forth by the researchers who argued that the summertime Atlantic Niña and autumn-time positive IOD during developing years of El Niño events are only responses to Pacific SST anomalies and do not contribute significantly to the growth of Pacific warming (19, 22). We concur with their perspective that the Pacific Ocean plays a pivotal role in driving SST variations in the Indian and Atlantic oceans. However, our results demonstrate the significant contributions of the Indian and Atlantic oceans in boosting ENSO, or at least in the case of super El Niño events. In particular, it is important to acknowledge the potential effect of the Atlantic Ocean, as it can further increase the likelihood of super El Niño events by providing extra fuel, in addition to the effect of the Indian Ocean.

In addition, it is important to note that we do not claim that the Indian and Atlantic oceans uniformly exert a positive influence on every super El Niño event. In reality, because of internal variability, the SST anomalies in these two oceans display varying amplitudes across different super El Niño events, and may even create unfavorable conditions that hinder the development of El Niño. Addressing the reasons for such uncertainty is essential to enhance our comprehension of inter-basin interactions. However, this aspect is beyond the scope of the current study and will be addressed in our future research endeavors. Last, while the CESM model used in the present study yielded promising results, conducting multi-model coordinated experiments is necessary to enhance the reliability of the crucial role of coupling in amplifying the effects of the Indian and Atlantic oceans on super El Niño events.

MATERIALS AND METHODS

Observational data

The observed monthly SST data used in this study are from the National Oceanic and Atmospheric Administration (NOAA) Extended Reconstructed SST version 5 (ERSST5) (38) with a resolution of 2° × 2°, covering a period from 1948 to 2023.

Definition of indices

Four SST-based indices are used in this study, including Niño3.4 index to measure ENSO, BWI to represent IOB, dipole mode index (DMI) to show IOD, and Atl3 index to display Atlantic Niño/Niña. The Niño3.4 index is the average of SST anomalies in the region of 120° to 170°W, 5°S to 5°N. The BWI is measured by the area-averaged SST anomalies in the tropical Indian Ocean (40° to 110°E, 20°S to 20°N) (39). The DMI is calculated as the difference in SST anomalies between the IOD western pole region (50° to 70°E, 10°S to 10°N) and eastern pole region (90° to 110°E, 10°S to 0°) (16). In observations, the definition of the Atl3 index is the mean SST anomalies in 0° to 20°W, 3°S to 3°N (6). In the CESM1, the longitude range to define the Atl3 index shifts 5° westward considering the system bias of the model (15). In addition, the NTA index is the mean SST anomalies in 20° to 60°W, 10° to 25°N (6).

We also adopt two indices to denote the subsurface preconditions in the Pacific. The first one is the warm water volume (WWV) index defined as the sum of depth above the 20°C isotherm within 5°N to 5°S, 120°E to 80°W (40). The other is the sea surface height gradient (SSHG) index obtained by the relative magnitude of the equatorial (0°, 120°E to 80°W) sea surface height anomalies with respect to the off-equatorial (5°N to 5°S, 120°E to 80°W) ones (41).

Perfect model hindcast experiments

To examine the cross-basin effects from the Indian and Atlantic oceans on the development of super El Niño events, the Community Earth System Model version 1 (CESM1; 31) from the National Center for Atmospheric Research (NCAR) is used for our model experiments. This model has demonstrated excellent capability in accurately reproducing the mean climate state and has been extensively applied in ENSO studies (42–44).

The ocean component of the coupled system is the Parallel Ocean Program, version 2, which has a horizontal resolution of approximately 1° and 60 vertical levels with a maximum depth at 5375 m. The atmospheric component is the Community Atmospheric Model, version 4.0, with a horizontal resolution of 2.5° longitude × 1.9° latitude approximately and 26 vertical levels. All the experiments in this study are conducted with the external forcing fixed at the level of year 2000.

Four sets of perfect model hindcast experiments are performed for the super El Niño events in a 100-year fully coupled integration after model spin-up. The first set, referred to as the “CTRL,” serves as the control experiment where global SSTs are fully coupled. The remaining three sets are partially decoupled experiments, including two sets of single-ocean decoupled experiments where either the Indian Ocean (dIO) or the Atlantic (dAtl) is decoupled individually, and a set of two-ocean decoupled experiment where both the Indian and Atlantic oceans are decoupled (dInA). The experimental framework and decoupled regions for the Indian Ocean and the Atlantic can be seen in fig. S4. All the hindcast experiments are conducted from 1 January of year 0 to 31 March of year 1, with 10 ensemble members for each of the super El Niño events. In the CTRL experiment, two events (years 123 and 142) fail to simulate the extreme warming in the Pacific, indicating their low predictability (fig. S3). Therefore, the decoupled experiments are only performed for the remaining eight predictable super El Niño events.

Isolation of individual and synergistic effects

By comparing the differences between the four sets of perfect model hindcast experiments, we can isolate the individual and synergistic effects of the Indian and Atlantic oceans (table S2).

1. In the dInA, both the effects of the Indian and Atlantic oceans are suppressed, while in the dAtl/dIO, the Indian Ocean/Atlantic Ocean is coupled. Therefore, by calculating the differences between the dInA and dAtl/dIO (latter minus former), we can determine the individual impact of the Indian Ocean and the Atlantic Ocean.

2. With active Indian Ocean/Atlantic Ocean in the dAtl/dIO, we can assess the synergistic effect of the Atlantic Ocean/Indian Ocean by subtracting the dAtl/dIO from the CTRL.

Last, the differences between the CTRL and dInA (former minus latter) illustrate the joint effect of the two oceans.

This isolation method allows us to distinguish how the Indian and Atlantic oceans independently and jointly influence the development and prediction of super El Niño events.

Acknowledgments

Funding: This work was supported by National Natural Science Foundation of China 42192564 (C.W.), National Natural Science Foundation of China 42088101 (S.Y.), National Natural Science Foundation of China 42206026 (H.F.), National Key Research and Development Program of China 2019YFA0606701 (C.W.), Guangdong Province Key Laboratory for Climate Change and Natural Disaster Studies 2020B1212060025 (S.Y.), and Development Fund of South China Sea Institute of Oceanology of the Chinese Academy of Sciences SCSIO202208 (C.W.).

Author contributions: Conceptualization: C.W., H.F., and S.Y. Formal analysis: H.F. Investigation: H.F. Methodology: H.F. Software: G.Z. Visualization: H.F., S.Y., and G.Z. Funding acquisition: H.F. and C.W. Validation: S.Y. and C.W. Writing—original draft: H.F., C.W., and S.Y. Writing—review and editing: H.F., C.W., and S.Y.

Competing interests: The authors declare that they have no competing interests.

Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. The NOAA ERSST5 SST data used in this study are publicly available from their website at https://ncei.noaa.gov/products/extended-reconstructed-sst. The data of model simulations are publicly available via Dryad: https://doi.org/10.5061/dryad.c59zw3rh1.

Supplementary Materials

This PDF file includes:

Figs. S1 to S7

Tables S1 and S2
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