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Previous studies suggested that spring precipitation over the tropical western Pacific Ocean can influence the development of El Niño–Southern Oscillation (ENSO). To identify crucial precipitation patterns for post-spring ENSO evolution, a singular value decomposition (SVD) method was applied to spring precipitation and sea surface temperature (SST) anomalies, and three precipitation and ENSO types were obtained with each highlighting precipitation over the Maritime Continent (MC) or western north Pacific (WNP). High MC spring precipitation corresponds to the slow decay of a multi-year La Niña event. Low MC spring precipitation is associated with a rapid El Niño-to-La Niña transition. High WNP spring precipitation is related to positive north Pacific meridional mode and induces the El Niño initiation. Among the three ENSO types, ocean current and heat content behave differently. Based on these spring precipitation and oceanic factors, a statistical model was established aimed at predicting winter ENSO state. Compared to a full dynamical model, this model exhibits higher prediction skills in the winter ENSO phase and amplitude for the period of 1980–2022. The explained total variance of the winter Niño-3.4 index increases from 43% to 75%, while the root-mean-squared error decreases from 0.82 °C to 0.53 °C. The practical utility and limitations of this model are also discussed. [ABSTRACT FROM AUTHOR]

Previous studies suggested that spring precipitation over the tropical western Pacific Ocean can influence the development of El Niño–Southern Oscillation (ENSO). To identify crucial precipitation patterns for post-spring ENSO evolution, a singular value decomposition (SVD) method was applied to spring precipitation and sea surface temperature (SST) anomalies, and three precipitation and ENSO types were obtained with each highlighting precipitation over the Maritime Continent (MC) or western north Pacific (WNP). High MC spring precipitation corresponds to the slow decay of a multi-year La Niña event. Low MC spring precipitation is associated with a rapid El Niño-to-La Niña transition. High WNP spring precipitation is related to positive north Pacific meridional mode and induces the El Niño initiation. Among the three ENSO types, ocean current and heat content behave differently. Based on these spring precipitation and oceanic factors, a statistical model was established aimed at predicting winter ENSO state. Compared to a full dynamical model, this model exhibits higher prediction skills in the winter ENSO phase and amplitude for the period of 1980–2022. The explained total variance of the winter Niño-3.4 index increases from 43% to 75%, while the root-mean-squared error decreases from 0.82 °C to 0.53 °C. The practical utility and limitations of this model are also discussed.

The Indian summer monsoon intraseasonal oscillation (MISO) is the most influential mode of tropical atmospheric intraseasonal variability in boreal summer (May–October). This study investigates how the MISO drives the intraseasonal variability (ISV) of zonal currents (U) in the Indo‐Pacific warm pool (IPWP) region. An analysis of satellite observations confirms a close association between U ISV and the MISO, with MISO‐induced surface U anomalies reaching 16–19 cm s−1 near the equator and 4–5 cm s−1 in the Northern Hemisphere portion of the IPWP. These ISVs account for 15%–35% of the intensity changes in the major zonal currents. The spatial distribution of U ISV shows coherent enhancement and suppression of MISO convections with a lag of approximately 7 days as a delayed response to the tropical wind forcing. ISV anomalies show northward emanation following propagation of the MISO. Ocean model experiments confirm that large‐scale ISV in surface U is primarily caused by the wind forcing of the MISO. Owing to the convergence/divergence of MISO‐induced U ISV, significant warm water volume anomalies occur in the western‐to‐central equatorial Pacific. Plain Language Summary: The Indo‐Pacific warm pool holds a unique place with the largest area of warm water temperatures above 28°C, which causes vigorous atmospheric convective activities. These convective activities include the winter Madden‐Julian Oscillation (MJO) and the Summer Monsoon Intraseasonal Oscillations (MISO), which have profound effects on the upper ocean. While previous studies have emphasized more on the role of the MJO and the effects of convective activities on temperature and salinity, our work first demonstrates that the large‐scale intraseasonal variability (ISV) of zonal currents is significantly associated with the MISO, and reveals the characteristics and mechanisms of ISV in ocean surface currents. MISO‐induced ISV can significantly modulate the summer mean currents. Ocean model experiments confirm that this large‐scale ISV is mainly driven by MISO‐related wind stress forcing. MISO‐induced ISV in ocean surface circulation further leads to ISV of the warm water volume, which may potentially feedback to the MISO and play a role in the El Niño‐Southern Oscillation, typhoon and marine heatwaves. Key Points: The MISO induces strong U ISV anomalies in the IPWP, which account for 15%–35% of the intensity changes in the major zonal currentsLarge‐scale U ISV is primarily caused by the MISO wind forcingThe MISO leads to significant ISV in the warm water volume of the western‐to‐central equatorial Pacific [ABSTRACT FROM AUTHOR]

The properties and heat budget of marine heat waves (MHWs) on the northern South China Sea (SCS) continental shelf are investigated. MHWs with warming amplitudes above 1.58C occur mainly along the coast, and their temperature anomaly decreases toward the open sea. MHWs with 18-1.58C warming and duration, 20 days dominate the northern SCS continental shelf. A heat budget analysis indicates that the main heat source is the sea surface net heat flux. Oceanic processes are dominated by the advection of mean temperature by the anomalous horizontal velocity (advha). The net contribution of advha always cools the upper layer of the ocean, resulting in the decay of MHWs. Active crossslope water exchanges exist at the east and west sides of the northern SCS continental shelf edge, which makes the dominant contributions to the advha. In the MHW developing phase, the west (east) side makes a positive (negative) contribution to the advha. In the decay phase, both sides make a negative contribution to the advha, resulting in the rapid decay of MHWs. Although the contribution of advha to the heat budget varies along the northern SCS continental shelf edge, its net effect always cools the MHWs over the shelf. These results provide new insight into the characteristics and formation mechanism of MHWs on the northern SCS continental shelf; in particular, they clarify the respective contributions of air-sea flux and oceanic processes to MHWs. [ABSTRACT FROM AUTHOR]

This study demonstrates that the generalization that strong anomalous equatorial Pacific westerly (easterly) winds during El Niño (La Niña) events display strong adjusted warm water volume (WWV) discharges (recharges) is often incorrect. Using ocean model simulations, we categorize the oceanic adjusted responses to strong anomalous equatorial winds into two categories: (i) transitioning (consistent with the above generalization) and (ii) neutral adjusted responses (with negligible WWV recharge and discharge). During the 1980–2016 period only 47% of strong anomalous equatorial winds are followed by transitioning adjusted responses, while the remaining are followed by neutral adjusted responses. Moreover, 55% (only 30%) of the strongest winds lead to transitioning adjusted responses during the pre-2000 (post-2000) period in agreement with the previously reported post-2000 decline of WWV lead time to El Niño–Southern Oscillation (ENSO) events. The prominent neutral adjusted WWV response is shown to be largely excited by anomalous wind stress forcing with a weaker curl (on average consistent with a higher ratio of off-equatorial to equatorial wind events) and weaker Rossby wave projection than the transitioning adjusted response. We also identify a prominent ENSO phase asymmetry where strong anomalous equatorial westerly winds (i.e., El Niño events) are roughly 1.6 times more likely to strongly discharge WWV than strong anomalous equatorial easterly winds (i.e., La Niña events) are to strongly recharge WWV. This ENSO phase asymmetry may be added to the list of mechanisms proposed to explain why El Niño events have a stronger tendency to be followed by La Niña events than vice versa. [ABSTRACT FROM AUTHOR]

The mechanisms and predictability of extreme El Niño events, as in 1982, 1997, and 2015, remain unclear. Here we investigate the relative roles of the ocean initial heat recharge and westerly wind bursts (WWBs) that occur during different seasons in the generation of extreme El Niño events in a climate model (CESM1.2). Several sets of 100‐member ensemble simulations, along with CMIP5 and atmospheric reanalysis data, are used. We show that while the probability of extreme El Niño increases progressively with ocean heat recharge, tropical conditions by year‐end are least predictable (the largest ensemble spread) for moderately‐to‐strongly recharged ocean initial states, rather than neutrally charged. Even after strong early‐year WWBs and recharged ocean the occurrence of extreme El Niño has limited predictability. While the early‐year WWBs are still important for strong El Niño, it is to a lesser extent than suggested previously, as mid‐year WWBs play a critical role in El Niño progression. Plain Language Summary: El Niño is a climate phenomenon that originates in the tropical Pacific and affects weather patterns worldwide, causing dramatic changes in local temperatures and rainfall. Such impacts are especially pronounced during extreme El Niño events, whose prediction would have broad socio‐economic benefits. In this study, we use model simulations and observations to examine the relative roles of episodic westerly wind bursts and initial upper‐ocean heat content in the tropical Pacific—two potential precursors of El Niño—in the development of extreme events. We show that with increased ocean heat content in January, the likelihood of extreme El Niño by year‐end increases, but so does the forecast spread for year‐end tropical conditions. This is because westerly wind bursts that occur mid‐year become more critical for El Niño development than early‐year bursts. Thus, our study implies that even after strong early‐year westerly wind bursts with high positive initial ocean heat content the predictability of an extreme El Niño remains limited unless we can find a way to anticipate mid‐year westerly wind bursts. Key Points: Chances of extreme El Niño increase progressively with the initial ocean heat recharge, but the prediction uncertainty increases as wellEarly‐year westerly wind bursts may have weaker impacts on the occurrence of extreme El Niño than previously suggestedMid‐year westerly wind bursts are a major factor controlling the strength of El Niño and hence critical for extreme El Niño events [ABSTRACT FROM AUTHOR]

The relationship between the equatorial Pacific warm water volume (WWV) and El Niño–Southern Oscillation (ENSO) sea surface temperature (SST) has varied considerably on decadal timescales. These changes are strongly related to the occurrence frequency of central Pacific (CP) ENSO events. While both eastern Pacific (EP) and CP ENSO events show clear signatures of WWV recharge/discharge, their phase‐lag relationships between WWV and Niño3.4 SST are different. The WWV usually leads the Niño3.4 SST by two to three seasons during EP ENSO, while the lead time is reduced to one season during CP ENSO. The different phase‐lag relationships can be explained by distinct periodicities of the two ENSO types. Hence, ENSO regime changes associated with decadal predominance of either EP or CP ENSO events can give rise to decadal variations in the statistical WWV‐ENSO SST relationship. We emphasize the importance of identifying these different ENSO types and potentially different ENSO regimes to assess ENSO predictability. Key Points: Relationship between WWV and Niño3.4 SST varies on decadal timescales, corresponding to the occurrence frequency of central Pacific ENSOBoth eastern and central Pacific ENSO events show recharge/discharge signatures but with different WWV/Niño3.4 phase‐lag relationshipsThis difference can be explained by the existence of two different ENSO types characterized by distinct intrinsic periodicities [ABSTRACT FROM AUTHOR]

Several previous studies have suggested that a cool western Indian Ocean may induce convection over Indonesia, leading to surface convergence and easterlies over the western equatorial Pacific Ocean. These easterlies then increase the Pacific Warm Water Volume favouring El Niño in the next year. We investigate this mechanism of Indian-Pacific interaction using output from two simulations (at 0. 1 ∘ and 1 ∘ ocean model resolution) with the Community Earth System Model (CESM). No conclusive evidence for the suggested interaction mechanism is found in CESM. Like many other coupled models, CESM has an overly strong sea surface temperature variability in the eastern Indian Ocean due to an exaggerated sensitivity of the sea surface temperature to thermocline variations. Due to this bias the effect of the western Indian Ocean signals on the Pacific, as found from observational analysis, is hidden. Our analysis shows that this bias can be traced to errors in the time-mean vertical temperature profile in the Indian Ocean. This bias needs to be reduced to allow a better investigation of the subtle interactions between the Indian and Pacific Oceans. [ABSTRACT FROM AUTHOR]

We explore the decadal variability of teleconnection from tropical Pacific to the Indian summer monsoon rainfall (ISMR) using various observational and Reanalysis datasets for the period 1958-2008. In confirmation with the earlier findings, we find that the interannual correlations between the various SST indices of ENSO and ISMR have continued to weaken. Interestingly, we find that even the robust lead correlations of the tropical pacific warm-water-volume with ISMR have weakened since late 1970s. Our analysis suggests that there is a relative intensification of the cross-equatorial flow from the southern hemisphere into the equatorial Indian Ocean associated with ISMR due to strenghtening of Mascarene High. Further, a shift in the surface wind circulation associated with monsoon over the northern pacific since late 1970s has resulted in a strenghtened cyclonic seasonal circulation south-east of Japan. These changed circulation features are a shift from the known circulation-signatures that efficiently teleconnect El Niño forcing to South Asia. These recent changes effectively weakened the teleconnection of the El Niño to ISMR. [ABSTRACT FROM AUTHOR]

The western equatorial Pacific oceanic heat content (warm water volume in the west or WWVw) is the best El Niño–Southern Oscillation (ENSO) predictor beyond 1‐year lead. Using observations and selected Coupled Model Intercomparison Project Phase 5 simulations, we show that a discharged WWVw in boreal fall is a better predictor of La Niña than a recharged WWVw for El Niño 13 months later, both in terms of occurrence and amplitude. These results are robust when considering the heat content across the entire equatorial Pacific (WWV) at shorter lead times, including all Coupled Model Intercomparison Project Phase 5 models or excluding Niño‐Niña and Niña‐Niño phase transitions. Suggested mechanisms for this asymmetry include (1) the negatively skewed WWVw distribution with stronger discharges related to stronger wind stress anomalies during El Niño and (2) the stronger positive Bjerknes feedback loop during El Niño. The possible role of stronger subseasonal wind variations during El Niño is also discussed. Plain language summary: El Niño and La Niña have strong societal impacts at the global scale, especially large‐amplitude El Niño events like in 1982–1983, 1997–1998, and 2015–2016. It is hence important to identify early warning signals for the occurrence of El Niño/La Niña. The equatorial Pacific Ocean heat content is a well‐known predictor of El Niño/La Niña several seasons ahead. In this study, we show that negative heat content anomalies lead more systematically to La Niña events than positive heat content to El Niño events. We suggest that the enhanced predictability of La Niña relative to El Niño is due to larger negative heat content anomalies ahead of La Niña events and a more unstable (and hence less predictable) ocean‐atmosphere system during El Niño. Key Points: The western equatorial Pacific heat content is the best El Niño‐Southern Oscillation oceanic predictor beyond 1‐year leadThis relation is asymmetrical: La Niña amplitude and occurrence is more predictable than that of El NiñoWeak western Pacific heat content recharges, stronger air‐sea coupling, and atmospheric stochasticity contribute to this asymmetry [ABSTRACT FROM AUTHOR]

Abstract: A wind forced ocean model is used to decompose the equatorial Pacific warm water volume (WWV) between 1980 and 2016 into two components: the (i) adjusted wind response, which is found by letting the model evolve unforced for three months, and (ii) instantaneous wind response, which are the instantaneous WWV changes due to Ekman transports. Our results suggest that roughly half of WWV variability is only as predictable as the winds that drive the instantaneous change. Separate examinations of pre‐2000 and post‐2000 periods reveal (i) nearly equal importance of instantaneous and adjusted responses for the pre‐2000 period and (ii) dominance of the instantaneous response during the post‐2000 period, which is most apparent during the recharged phase. This increasing instantaneous contribution prominence explains the post‐2000 reduction in WWV/El Niño‐Southern Oscillation sea surface temperature lead times (from six to nine months pre‐2000 down to three months post‐2000) and is consistent with the reduction in post‐2000 El Niño‐Southern Oscillation prediction skill. [ABSTRACT FROM AUTHOR]

The buildup of the warm water in the equatorial Pacific prior to an El Niño event is considered a necessary precondition for event development, while the event initiation is thought to be triggered by bursts of westerly wind. However, in contrast to the view that warm water slowly builds up years before an El Niño event, the volume of warm water in the equatorial Pacific doubled in the first few months of 2014 reaching values that were consistent with the warm water buildup prior to the extreme 1997/1998 El Niño. It is notable that this dramatic warm water buildup coincided with a series of westerly wind bursts in the western tropical Pacific. This study uses linear wave theory to determine the effect of equatorial and off-equatorial westerly wind events on the Warm Water Volume (WWV) of the Pacific. It is found that westerly wind events have a significant impact on equatorial WWV with all events initially acting to increase WWV, which highlights why WWEs are so effective at exciting ENSO. In fact, our results suggest that the single westerly wind burst, which peaked in the first few days of March in 2014, was largely responsible for the coincident dramatic observed increase in WWV. How long the equatorial region remains charged, however, depends on the latitude of the westerly wind event. For instance, a single equatorially symmetric westerly wind event ultimately acts to discharge WWV via the reflection of upwelling Rossby waves, which makes it difficult to more gradually build WWV given multiple WWEs. In contrast, when the wind events occur off the equator, the subsequent discharge is significantly damped and in some cases the equatorial region can hold the heat charge for the duration of the simulations (~6 months). As such, off-equatorial WWEs can not only charge equatorial region WWV in the short term, but are also a mechanism to more gradually build equatorial region WWV in the longer term. Given that these off-equatorial WWEs have a relatively small projection onto the equatorial Kelvin wave, we argue these events can be considered as a mechanism to modulate the background state in which ENSO operates. [ABSTRACT FROM AUTHOR]

One of the leading theories to explain the oscillatory nature of the El Niño Southern Oscillation is the recharge–discharge oscillator paradigm, which roots on warm waters exchanged between the equatorial and off-equatorial regions. This study tests the relevance of this theory to account for the Eastern and recently mediated Central Pacific El Niño events. The recharge–discharge of the equatorial Pacific, measured here as changes in Warm (>20°C) Water Volume (WWV), is analysed using monthly 1993–2010 sea level anomaly (a proxy for WWV) obtained from altimetry, and a validated 1958–2007 DRAKKAR simulation. An Agglomerative Hierarchical Clustering (AHC) technique performed on the observed and modelled WWV tendency shows the existence of five distinct clusters, which characterise the Eastern Pacific (EP) and Central Pacific (CP) El Niño, La Niña, after EP El Niño and neutral conditions. The AHC results, complemented with an analysis of lagged-regression analysis, and 3-month averages of typical EP and CP El Niño events, indicate that the equatorial band WWV discharge during CP is not as pronounced as during EP El Niño. To understand the differences, we analysed the balance of horizontal mass transports accounting for changes in WWV tendency. The analysis indicates an overall poleward transport during EP El Niño, which is not the case during CP El Niño. Instead, a compensating effect with a poleward (equatorward) transport occurring in the western (eastern) Pacific is evident, in line with changes in the zonal thermocline slopes occurring in the western (eastern) half of the basin. The WWV changes are discussed with respect to the conceptual phases of the recharge–discharge oscillator paradigm. [ABSTRACT FROM AUTHOR]

Variations of the volume of warm water above the thermocline in the equatorial Pacific are a good predictor of ENSO (El Niño/Southern Oscillation) and are thought to be critical for its preconditioning and development. In this study, the Warm Water Volume (WWV) interannual variability is analysed using forced general circulation model experiments and an original method for diagnosing processes responsible for WWV variations. The meridional recharge/discharge to higher latitudes drives 60% of the ENSO-related equatorial WWV variations, while diabatic processes in the eastern equatorial Pacific account for the remaining 40%. Interior meridional transport is partially compensated by western boundary transports, especially in the southern hemisphere. Diabatic equatorial WWV formation (depletions) during La Niña (El Niño) are explained by enhanced (reduced) diathermal transport through enhanced (reduced) vertical mixing and penetrating solar forcing at the 20°C isotherm depth. The respective contribution of diabatic and adiabatic processes during build-ups/depletions strongly varies from event-to-event. The WWV build-up during neutral ENSO phases (e.g. 1980-1982) is almost entirely controlled by meridional recharge, providing a text-book example for the recharge/discharge oscillator's theory. On the other hand, diabatic processes are particularly active during the strongest La Niña events (1984, 1988, 1999), contributing to more than 70% of the WWV build-up, with heating by penetrative solar fluxes explaining as much as 30% of the total build-up due to a very shallow thermocline in the eastern Pacific. This study does not invalidate the recharge/discharge oscillator theory but rather emphasizes the importance of equatorial diabatic processes and western boundary transports in controlling WWV changes. [ABSTRACT FROM AUTHOR]