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Decadal variations in the El Niño‐Southern Oscillation (ENSO)'s influence on Southeast Asian rainfall are modulated by variability in the mean state of atmospheric circulation and moisture anomalies in the Indo‐Pacific Warm Pool (IPWP). Variations in the mean state are characterized by a tripolar pattern in sea surface temperatures (SST) spanning the Indian Ocean, central IPWP, and western Pacific regions. Due to ENSO teleconnection asymmetry, Southeast Asian rainfall is more sensitive to La Niña than El Niño. During positive phases of the tripole pattern, the mean state is drier, with reduced vertical motion over mainland Southeast Asia suppressing La Niña anomalies. In negative phases, the mean state is wetter, with enhanced vertical motion amplifying La Niña anomalies. From 1980 to 2015, relative SST trends show a negative tripole pattern, strengthening the ENSO ‐ Southeast Asian rainfall relationship. However, future projections for 2050–2100 suggest a positive tripole pattern, predicting a weakening of this relationship by 2100. Plain Language Summary Year‐to‐year variations in monsoon rainfall are strongly connected to variations in ocean temperatures in the Pacific, including the warm and cool phases of the El Niño‐Southern Oscillation (ENSO). These tropical Pacific ocean temperature patterns evolve over several months and tracking their evolution is useful because they help us predict ensuing unusual rainfall conditions in regions affected by ENSO several months or seasons ahead of time. However, the ENSO‐rainfall relationship has not been steady over time. Here, we focus on understanding the cause of changes in the ENSO‐rainfall relationship in the densely populated region of mainland Southeast Asia. While previous studies emphasized the role of the Pacific Ocean, here we show how changes in the ENSO‐rainfall relationship are modulated by a combined effect of the Indian and Pacific Oceans. We also show that the ENSO‐rainfall relationship may weaken under future climate warming, meaning that rainfall in Southeast Asia may become less predictable. Key Points Decadal variations in ENSO's impact on Southeast Asian rainfall are modulated by a Tropical Tripole Pattern Positive phases of the Tropical Tripole lead to a drier mean state, suppressing La Niña‐induced rainfall enhancement in Southeast Asia Projections show a breakdown of the ENSO‐Southeast Asian rainfall relationship with a more positive Tropical Tripole Pattern by 2100

An unprecedented extreme positive Indian Ocean dipole event (pIOD) occurred in 2019, which has caused widespread disastrous impacts on countries bordering the Indian Ocean, including the East African floods and vast bushfires in Australia. Here we investigate the causes for the 2019 pIOD by analyzing multiple observational datasets and performing numerical model experiments. We find that the 2019 pIOD was triggered in May by easterly wind bursts over the tropical Indian Ocean associated with the dry phase of the boreal summer intraseasonal oscillation, and it was sustained by the local atmosphere–ocean interaction thereafter. During September–November, warm sea surface temperature anomalies (SSTA) in the central-western tropical Pacific Ocean further enhanced the Indian Ocean's easterly winds, bringing the pIOD to an extreme magnitude. The central-western tropical Pacific warm SSTA was strengthened by two consecutive Madden–Julian oscillation (MJO) events that originated from the tropical Indian Ocean. Our results highlight the important roles of cross-basin and cross-time-scale interactions in generating extreme IOD events. The lack of accurate representation of these interactions may be the root for a short lead time in predicting this extreme pIOD with a state-of-the-art climate forecast model.

Summer climate in the Northwestern Pacific (NWP) displays large year-to-year variability, affecting densely populated Southeast and East Asia by impacting precipitation, temperature, and tropical cyclones. The Pacific–Japan (PJ) teleconnection pattern provides a crucial link of high predictability from the tropics to East Asia. Using coupled climate model experiments, we show that the PJ pattern is the atmospheric manifestation of an air–sea coupled mode spanning the Indo-NWP warm pool. The PJ pattern forces the Indian Ocean (IO) via a westward propagating atmospheric Rossby wave. In response, IO sea surface temperature feeds back and reinforces the PJ pattern via a tropospheric Kelvin wave. Ocean coupling increases both the amplitude and temporal persistence of the PJ pattern. Cross-correlation of ocean–atmospheric anomalies confirms the coupled nature of this PJIO mode. The ocean–atmosphere feedback explains why the last echoes of El Niño–Southern Oscillation are found in the IO-NWP in the form of the PJIO mode. We demonstrate that the PJIO mode is indeed highly predictable; a characteristic that can enable benefits to society.

The Interdecadal Pacific Oscillation (IPO) and Atlantic Multidecadal Variability (AMV) substantially affect global climate system. Model studies suggested a fast interaction between the IPO and AMV through atmospheric teleconnections, but observations exhibit a weak IPO–AMV contemporaneous correlation. To address this paradox, we apply linear inverse model (LIM) in observations to decode the interaction. We reveal that a cancel effect of the interaction lowers the observed IPO–AMV contemporaneous correlation. When only retaining the one‐way modulation (the IPO forces the AMV or the reversed one) in the observational LIM, their correlation peaks nearly simultaneously, consistent with the fast IPO–AMV interaction in model experiments. We further demonstrate that the fast interaction is associated with both tropical and extratropical processes. Our study reconciles the discrepancy between observations and models on the IPO–AMV interaction. Plain Language Summary Many decadal‐to‐multidecadal climate and biological variations could be attributed to two climate modes, the Interdecadal Pacific Oscillation (IPO) and Atlantic Multidecadal Variability (AMV). In model simulations, the two modes interact via fast atmospheric teleconnections, implying a high contemporaneous correlation between them. However, in observations, the IPO–AMV contemporaneous correlation is weak. Here, by using observations and model simulations, we show that the weak contemporaneous correlation results from the competition between the IPO‐forced positive correlation and the AMV‐forced negative correlation. Our study confirms a fast interaction between the IPO and AMV in observations, which has broad implications for improving climate simulation and decadal climate prediction. Key Points The Interdecadal Pacific Oscillation–Atlantic Multidecadal Variability interaction lowers their contemporaneous correlation Both observational linear inverse model and general circulation model experiments support the cancel effect of the interaction The commonly used correlation analysis in climate science may lead to misunderstanding in a coupled system

The tropical Pacific Ocean is a key regulator of Earth's climate, with teleconnections that influence remote locations all around the world. Here we use partially coupled climate model experiments to show that tropical Pacific cooling related to an abrupt Atlantic Meridional Overturning Circulation (AMOC) slowdown can strengthen the AMOC by ∼25%. This tropical‐extratropical teleconnection occurs initially via atmospheric Rossby waves propagating from the tropical Pacific to the North Atlantic which alter surface climate conditions locally. These changes facilitate ocean heat loss from the subpolar gyre, favoring enhanced oceanic convection. The AMOC strengthening is subsequently enhanced by anomalous northward salt advection in the Atlantic, with a potential contribution from oceanic wave adjustment triggered by increased Southern Ocean westerly winds. These results highlight the influence of the tropical Pacific on the AMOC on multidecadal timescales and suggest that cold phases of tropical Pacific decadal variability could drive temporary strengthening of the AMOC. Plain Language Summary Changes in the tropical Pacific sea surface temperature can exert significant remote weather and climate impacts via physical mechanisms known as teleconnections. In this study, we report unexplored teleconnections to the Atlantic basin which act in a timescale of decades. In particular we found that a mean cooling in the tropical Pacific could act to accelerate a large‐scale ocean circulation in the Atlantic basin known as the Atlantic Meridional Overturning Circulation (AMOC). This occurs via atmospheric and oceanic waves which propagate to the North Atlantic and alter local conditions, favoring the acceleration of the AMOC. Key Points Tropical Pacific cooling can drive a strengthening of the Atlantic Meridional Overturning Circulation (AMOC) The Pacific–North Atlantic teleconnection occurs via both atmospheric and oceanic planetary waveguides Pacific–AMOC teleconnections can be induced on a multidecadal time‐scale

Jiang et al. (2023), https://doi.org/10.1029/2023gl103777 argue that the apparent impact of the equatorial Atlantic on El Niño-Southern Oscillation (ENSO) is a statistical artifact, and that the 6-month lead correlation reported in previous studies stems from early developing ENSO events driving the equatorial Atlantic zonal mode (AZM) in boreal summer and maturing in winter. Closer examination, however, reveals that most AZM events develop too early to be driven by developing ENSO, and that the influence of decaying ENSO events has to be considered too. Thus, while early developing ENSO events may play a role, they do not fully explain observed AZM behavior. Our aim is not to argue for or against an AZM influence on ENSO, but rather to show that Jiang et al.’s analysis is insufficient to resolve this issue. More analysis will be needed for a deeper understanding of Atlantic-Pacific interaction.

The impacts of tropical interbasin interaction (TBI) on the characteristics and predictability of sea surface temperature (SST) in the tropics are assessed with a linear inverse modelling (LIM) framework that uses SST and sea surface height anomalies in the tropical Pacific (PO), Atlantic (AO), and Indian Ocean (IO). The TBI pathways are shown to be successfully isolated in stochastically-forced simulations that modify off-diagonal elements of the linear operators. The removal of TBI leads to a substantial increase in the amplitude of El Niño-Southern Oscillation (ENSO) and related variability. Partial decoupling experiments that eliminate specific coupling components reveal that PO-IO interaction is the dominant contributor, whereas PO-AO and AO-IO interactions play a minor role. A series of retrospective forecast experiments with different operators shows that decoupling leads to a substantial decrease in ENSO prediction skill especially at longer lead times. The relative contributions of individual pathways to forecast skill are generally consistent with the results from the stochastically-forced experiments. Qualitatively similar results are obtained from an additional set of forecast experiments that partially apply initial conditions over specific basins, but several important differences were also found due to differences in the representations of each TBI pathway. Finally, the cause of contrasting SST anomalies over the AO after the extreme 1982/83 and 1997/98 El Niño events is explored using LIM forecast experiments to demonstrate the strength and flexibility of our LIM-based approach.

The influence of the tropical Atlantic on El Niño-Southern Oscillation (ENSO) is examined using dedicated climate model experiments with sea-surface temperature (SST) restoring. Partial SST restoring to climatology in the tropical Atlantic leads to slower decay of ENSO events and to a shift of the power spectrum to longer periods. Perfect model hindcast experiments with and without restoring tropical Atlantic SST to climatology indicate that both the northern tropical and equatorial Atlantic have a very small influence on ENSO development. During decaying ENSO events, on the other hand, northern tropical Atlantic SST anomalies strongly accelerate the decay. Key to the Atlantic influence on ENSO decay are Atlantic SST anomalies just north of the equator (~ 5°N). These lead to local convection anomalies that change the Walker circulation so as to accelerate ENSO decay. Importantly, anomalous events in either the northern tropical or equatorial Atlantic fail to develop in the hindcast ensemble mean, when tropical Pacific SSTs are restored to climatology. This indicates that anomalous tropical Atlantic events in boreal spring and summer are strongly dependent on preceding ENSO events in boreal winter. Thus, the role of the tropical Atlantic is to mediate a negative feedback of ENSO on itself. Despite this passive role of the tropical Atlantic in the Pacific-Atlantic interaction, accurate simulation of the Atlantic feedback should play some role in ENSO prediction. Further model experiments will be required to evaluate model dependence of these findings and to quantify the impact of the Atlantic on ENSO prediction skill.

Sea surface temperature (SST) anomalies over the North Tropical Atlantic (NTA) during the early boreal spring can trigger El Niño‐Southern Oscillation (ENSO) events in the following boreal winter. However, the future changes in the impact of the NTA on ENSO remain controversial. Here, we show distinct changes in the strength of the NTA−ENSO relationship due to global warming by comparing models from the Coupled Model Intercomparison Project (CMIP) 5 and CMIP6. The impact of the NTA on ENSO under greenhouse warming is notably enhanced in CMIP6 compared to CMIP5. A wetter mean state over the subtropical eastern North Pacific and increased oceanic sensitivity over the equatorial central Pacific are key factors that enhance the impact of the NTA SST on ENSO. Therefore, differences in the mean state under greenhouse warming between the CMIP5 and CMIP6 models can modulate the strength of the NTA−ENSO relationship. Plain Language Summary The North Tropical Atlantic (NTA) region can affect the development of El Niño events, significantly impacting global weather systems. Our study compares two generations of climate models, CMIP5 and CMIP6, to understand how the influence of the NTA sea surface temperature (SST) on El Niño might change due to global warming. We found that the influence of the NTA SST on El Niño is stronger in the CMIP6 models than in the CMIP5 models. This stronger impact is due to changes in the climatological background states, including a wetter mean state over the subtropical eastern North Pacific and increased oceanic sensitivity over the equatorial central Pacific. Understanding these changes is crucial for improving predictions of future climate variabilities and their potential impacts on weather, ecosystems, and food production worldwide. Key Points The relationship between the North Tropical Atlantic (NTA) and El Niño‐Southern Oscillation (ENSO) is significantly enhanced by global warming in CMIP6 compared to CMIP5 Mean state change under greenhouse warming is responsible for the strengthening of the NTA‐ENSO relationship

Previous studies have demonstrated that the tropical Indian Ocean basin mode (IOB) favors the phase transition of ENSO; however, they have not differentiated the two types of ENSO. This study reports that the boreal winter-spring IOB appears more likely to induce the central-Pacific (CP) ENSO than the eastern-Pacific ENSO one year later based on observation data and CMIP5 simulations. We find a strong asymmetry in the forcing of the IOB on the CP ENSO. The impact of IOB warming on CP La Niña events is more significant than the impact of IOB cooling on CP El Niño events. IOB warming during late winter to early spring produces easterly wind anomalies over the western equatorial Pacific and an anomalous descending motion of the Walker circulation near the dateline, inducing cooling in the central Pacific during the subsequent summer to winter. This suggests that interbasin interactions with the tropical Indian Ocean, in addition to the northern tropical Atlantic Ocean, are capable of generating CP La Niña events. Most CMIP5 models generally capture the negative IOB-CP ENSO lagged correlation. However, the cooling center and sinking branch of the anomalous Walker circulation in the Pacific Ocean forced by the IOB warming are weaker and shifted eastward in the multimodel ensemble than in the observations. Additionally, the observed asymmetry of the IOB impact on the CP ENSO is not captured by the CMIP5 models. This study indicates that improving the simulation of the IOB as a contributor to ENSO diversity remains a challenge.