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The influence of El Niño‐Southern Oscillation (ENSO) on the North Tropical Atlantic (NTA) sea surface temperature (SST) is well‐documented in boreal spring, yet it manifests greater complexity in winter. Distinct from El Niño, La Niña has a statistically significant impact on winter NTA SST, which is primarily contributed by the re‐occurring (i.e., second and third years of multi‐year) La Niña events. Despite similar atmospheric forcing, prominent SST anomalies are observed in the NTA during the re‐occurring La Niña winters but not in the first‐year La Niña winters, mainly due to different initial states associated with previous tropical Pacific forcing. Targeted pacemaker experiments well reproduce the robust impacts of re‐occurring La Niña events on the wintertime NTA SST, highlighting the crucial impact of ENSO cycle complexity in its inter‐basin linkages. Plain Language Summary: The El Niño‐Southern Oscillation (ENSO) significantly influences the sea surface temperature (SST) in the North Tropical Atlantic (NTA), particularly as ENSO events decay during boreal spring. However, during winter when ENSO reaches its peak, the NTA SST anomalies exhibit strong uncertainty. While no marked SST anomalies are observed in the NTA during El Niño winters, prominent SST anomalies can often be found in the NTA during La Niña winters. The significant La Niña‐related SST anomalies in the NTA are primarily contributed by the second and third years of multi‐year La Niña events, rather than the first‐year La Niña events. The SST difference between the first‐year and re‐occurring La Niña winters can be attributed to the SST initial state in the NTA, which is related to the preceding forcing from the tropical Pacific. By incorporating this initial state, we can more accurately reconstruct the NTA SST anomalies during La Niña winters. Targeted pacemaker experiments corroborate the significant role of re‐occurring La Niña events in influencing wintertime NTA SST. Key Points: La Niña usually has stronger impacts on winter NTA SST than El Niño, primarily contributed by re‐occurring La Niña eventsDifferent NTA SST anomalies during the first‐year and re‐occurring La Niña can be well understood by considering the NTA initial stateTargeted pacemaker experiments confirm the important effect of re‐occurring La Niña on the NTA SST [ABSTRACT FROM AUTHOR]

Atmospheric variability associated with the winter North Pacific Oscillation (NPO), preceding El Niño‐Southern Oscillation (ENSO) events by about 1 year, is driven mainly by the tropical Pacific sea surface temperature (SST) anomalies but may also affect the subsequent ENSO. To isolate the latter, we conduct an idealized hindcast experiment using a climate model MIROC6, in which the NPO‐related surface heat flux and wind stress anomalies are overridden over the North Pacific. The ENSO evolution can be overall reproduced without the NPO forcing, confirming the predominant intrinsic predictability arising from the tropical coupled dynamics. However, the extratropical atmospheric forcing associated with the NPO, when it reaches three standard deviations, can enhance the ENSO development by about 29.6%, contrasting with limited effects of the moderate NPO forcing. La Niña events exhibit much less sensitivity to the NPO forcing compared to El Niño events, possibly due to asymmetric patterns of the simulated NPO. Plain Language Summary: The winter North Pacific Oscillation (NPO), manifesting as a meridional dipole structure in sea level pressure over the North Pacific, is suggested to play a role in the subsequent El Niño‐Southern Oscillation (ENSO) development. Recent studies demonstrate that the preceding winter NPO signal mainly originates from the tropical Pacific SST forcing, making it challenging to isolate its specific impact on ENSO. In this study, we design a series of "overriding" experiments to decouple the tropical and extratropical Pacific processes based on the MIROC6 model. It is found that the ENSO evolution can be reproduced when excluding the NPO forcing, suggesting that the ENSO events is primarily driven by internal dynamics within the tropics. While the moderate NPO forcing has minor effects on the ENSO events, the exceptionally strong NPO forcing can further amplify the ENSO development. This study underscores that while NPO‐related extratropical variabilities do affect ENSO, their impact, especially under moderate conditions, should not overshadow the intrinsic predictability from the tropics. Key Points: The El Niño‐Southern Oscillation (ENSO) development is primarily driven by internal dynamics within tropicsThe NPO forcing of the extreme rather than moderate amplitude can significantly amplify the ENSO developmentDevelopment of El Niño events are more sensitive to the NPO forcing compared to La Niña events [ABSTRACT FROM AUTHOR]

Extreme El Niño events have outsized global impacts and control the El Niño Southern Oscillation (ENSO) warm/cold phases asymmetries. Yet, a consensus regarding the relative contributions of atmospheric and oceanic nonlinearities to their genesis remains elusive. Here, we isolate the contribution of oceanic nonlinearities by conducting paired experiments forced with opposite wind stress anomalies in an oceanic general circulation model, which realistically simulates extreme El Niño events and oceanic nonlinearities thought to contribute to ENSO skewness (Tropical Instability Waves (TIWs), Nonlinear Dynamical Heating (NDH)). Our findings indicate a weak contribution of oceanic nonlinearities to extreme El Niño events in the eastern Pacific, owing to compensatory effects between lateral (NDH and TIWs) and vertical processes. These results hold across different vertical mixing schemes and modifications of the upper‐ocean heat budget mixed layer criterion. Our study reinforces previous research underscoring the pivotal role of atmospheric nonlinearities in shaping extreme El Niño events. Plain Language Summary: The El Niño‐Southern Oscillation (ENSO) is the primary driver of year‐to‐year climate variations in the tropics and beyond. Originating from air‐sea interactions in the tropical Pacific, ENSO oscillates between warm (El Niño) and cold (La Niña) phases, modulating sea surface temperature in the central and eastern equatorial Pacific. Occasionally, El Niño events intensify into "super" El Niño events, causing widespread impacts globally. Utilizing a state‐of‐the‐art oceanic model, our research challenges previous results suggesting a strong oceanic contribution to the amplitude difference between "normal" and "super" El Niño events. Instead, our findings reveal that potential oceanic influences on "super" El Niño events tend to offset each other. This is consistent with recent research highlighting the crucial role of atmospheric processes in the transformation from a "normal" to a "super" El Niño. Key Points: A state‐of‐the‐art ocean model reproduces extreme El Niño events and the corresponding nonlinear oceanic processes realisticallyContributions from oceanic nonlinearities are isolated using paired simulations forced by opposite wind stress anomaliesEffects of oceanic nonlinearities on extreme El Niño events are small, due to compensation between lateral and vertical processes [ABSTRACT FROM AUTHOR]

While the prominent influence of El Niño‐Southern Oscillation (ENSO) on the Indian Ocean Oscillation (IOD) is widely recognized, intricate relationships between them are often invoked that introduce challenges into seasonal predictions. Previous studies have shown that different flavors of El Niño exhibit distinct associations with the IOD. In this study, we demonstrate that La Niña's teleconnection to the IOD is primarily controlled by its longitudinal position. Westward‐displaced La Niña events tend to produce stronger negative convection anomalies in the central Pacific and more pronounced Walk Circulation anomalies, thereby triggering strong negative IOD events. In contrast, eastward‐displaced La Niña events are usually accompanied by feeble convection response due to the excessively cold conditions in the cold tongue, yielding insignificant IOD response. The pivotal role of La Niña's longitudinal position on the IOD's response is realistically reproduced by targeted pacemaker experiments, providing new insights into inter‐basin climate connections. Plain Language Summary: The tropical Indian Ocean usually witnesses a dipolar pattern of sea surface temperature (SST) anomalies, which is commonly referred to as the Indian Ocean Dipole (IOD). The IOD phenomenon has received much attention due to its profound global impacts, yet its seasonal prediction remains a large challenge for the climate community. The year‐to‐year variability of IOD has usually been linked to the El Niño–Southern Oscillation (ENSO), the predominant interannual climate variability in the tropical Pacific. The relationship between El Niño (i.e., warm phase of ENSO) and the intensity of IOD has been demonstrated in previous studies, which is shown to be dependent on the different El Niño flavors. In this study, we show that the longitudinal position of negative SST anomalies during La Niña events (i.e., cold phase of ENSO) in shaping their connection with IOD. Different from westward‐displaced La Niña events that can drive robust negative IOD events, eastward‐displaced La Niña events do not yield significant IOD anomalies due to the excessively cold conditions prevalent in the eastern Pacific cold tongue region. We highlight the importance of La Niña's longitudinal position in its teleconnection to other ocean basins and the associated regional climate anomalies. Key Points: The La Niña's regulation of Indian Ocean Oscillation (IOD) mainly depends on the La Niña longitudinal positionThe westward‐displaced La Niña events produce significant Walker Circulation anomalies, triggering robust negative IOD eventThe importance of La Niña longitudinal position on the IOD's response can be evidenced by the targeted pacemaker experiments [ABSTRACT FROM AUTHOR]

The Pacific Meridional Mode (PMM) has long been associated with extra‐tropical air‐sea coupling processes, which are thought to influence the development of El Niño‐Southern Oscillation (ENSO). Here we show that the PMM on seasonal to interannual timescales is closely associated with a newly proposed tropical mode known as the ENSO Combination mode (C‐mode), which arises from the nonlinear interaction between ENSO and the background annual cycle in the deep tropics. The PMM exhibits a remarkable resemblance with the C‐mode in atmospheric patterns, spectral characteristics, and local impacts. Based on a simple Hasselmann‐type model, we further demonstrate that the C‐mode‐related atmospheric anomalies can effectively drive PMM‐like sea surface temperature anomalies. As the C‐mode captures seasonally modulated ENSO characteristics, the seasonal‐to‐interannual PMM variability could naturally establish a connection with ENSO, thereby offering an alternative explanation for the observed relationship between PMM and ENSO. Plain Language Summary: Previous studies demonstrated that the Pacific Meridional Mode (PMM) is driven by the extratropical climate systems, which can impact the development of El Niño‐Southern Oscillation (ENSO) events in the tropics. In our study, we establish a close connection between the PMM on seasonal‐to‐interannual timescales and a tropical atmospheric mode arising from the nonlinear interaction between ENSO and the background annual cycle (the ENSO Combination mode, C‐mode). The C‐mode closely resembles the PMM in terms of atmospheric patterns and spectral characteristics. We further demonstrate that the C‐mode could drive a PMM‐like SST pattern in the tropical Pacific. Consequently, we suggest that the PMM on seasonal‐to‐interannual timescales has tropical origins, thereby offering an alternative explanation for the relationship between the conventional PMM index and the mature phase of ENSO. Key Points: The spatiotemporal features of Pacific Meridional Mode (PMM) on seasonal‐to‐interannual timescales exhibit a remarkable resemblance with those of the tropical combination modeC‐mode‐related atmospheric anomalies are capable of inducing PMM‐like sea surface temperature changes in the tropical PacificEl Niño‐Southern Oscillation (ENSO)'s interaction with the annual cycle manifests in PMM‐related air‐sea conditions, which explains the observed ENSO‐PMM linkage [ABSTRACT FROM AUTHOR]

Understanding the interaction between the tropical Pacific and Atlantic Oceans has challenged the climate community for decades. Typically, boreal summer Atlantic Niño events are followed by vigorous Pacific events of opposite sign around two seasons later. However, incorporating the equatorial Atlantic information to variabilities internal to the Pacific lends no significant additional predictive skill for the subsequent El Niño‐Southern Oscillation (ENSO). Here we resolve this conundrum in a physically consistent frame, in which the nascent onset of a Pacific event rapidly induces an opposite‐signed summer equatorial Atlantic event and the lead correlation of Atlantic over Pacific is a statistical artifact of ENSO's autocorrelation. This Pacific‐to‐Atlantic impact is limited to a short window around late spring due to seasonally‐amplified Atlantic atmosphere‐ocean coupling. This new frame reconciles the discrepancies between the observed and multi‐model simulated inter‐basin relationship, providing a major advance in understanding seasonally‐modulated inter‐basin climate connections as well as their predictability. Plain Language Summary: Previous studies interpreted the observed lead/lag relationship between Atlantic Niño/Niña and Pacific Niño/Niña sea surface temperature anomalies as evidence for a precursory role of the equatorial Atlantic on the development of El Niño–Southern Oscillation (ENSO) events. This study clearly demonstrates that this statistical relationship is not related to Atlantic‐to‐Pacific causality, but can rather be explained by seasonally modulated equatorial Atlantic's response to ENSO. We find that Pacific ENSO events drive equatorial Atlantic events rather than vice versa, and reconcile the apparent discrepancies between the observed and multi‐model simulated tropical Pacific/Atlantic relationship. Key Points: The lead correlation of the equatorial Atlantic over the Pacific is not related to Atlantic‐to‐Pacific causalityThe tropical Pacific/Atlantic interaction is consistent with the nascent onset of Pacific events driving the equatorial Atlantic eventsThe discrepancies between the observed and multi‐model simulated tropical Pacific/Atlantic relationship can be reconciled in this new frame [ABSTRACT FROM AUTHOR]

The interactions between El Niño‐Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) are known to have great implications for global climate variability and seasonal climate predictions. Observational analysis suggests that the ENSO–IOD inter‐basin connection is time‐varying and related to the Atlantic Multidecadal Oscillation (AMO) with weakened ENSO–IOD relationship corresponding to AMO warm phases. A suite of Atlantic pacemaker simulations successfully reproduces the decadal fluctuations in ENSO–IOD relationship and its link to the AMO. The warm sea surface temperature (SST) anomalies associated with the AMO drive a series of Indo‐Pacific mean climate changes through tropical‐wide teleconnections, including the La Niña‐like mean SST cooling over the central Pacific and the deepening of mean thermocline depth in the eastern Indian Ocean. By modulating ocean–atmosphere feedback strength, those mean state changes decrease both ENSO amplitude and the Indian Ocean sensitivity to ENSO forcing, therefore decoupling the IOD from ENSO. Plain Language Summary: The El Niño‐Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) are two most important interannual climate signals exciting world‐wide climate and socioeconomic impacts. Their interactions are known to have great implications for global climate variability and seasonal climate predictions. However, the ENSO–IOD relationship is time‐varying and show prominent decadal fluctuations. Such changes in ENSO–IOD relationship are associated with the decadal variation in IOD teleconnections and predictability. The prediction skill of IOD is high when the ENSO–IOD relationship is strong, and vice versa. But it remains unknown what controls the decadal modulation of Indo‐Pacific inter‐basin connection. Our findings suggest that the Atlantic Multidecadal Oscillation (AMO) may modulate the ENSO–IOD inter‐basin connection. The AMO sea surface temperature anomalies drive a series of Indo‐Pacific mean climate changes through tropical‐wide teleconnections. Those mean state changes further influence both ENSO amplitude and the Indian Ocean sensitivity to ENSO forcing, thereby modulating the ENSO–IOD relationship. This study advances our understandings on inter‐basin and cross‐scale interactions in the climate system. Key Points: The inter‐basin connection between El Niño‐Southern Oscillation (ENSO) and Indian Ocean Dipole is time‐varying and controlled by the remote Atlantic Multidecadal Oscillation (AMO)The AMO sea surface temperature anomalies drive a series of Indo‐Pacific mean climate changes through tropical‐wide teleconnectionsThe AMO induced mean state changes influence both ENSO amplitude and the Indian Ocean sensitivity to ENSO forcing [ABSTRACT FROM AUTHOR]

Quasi‐decadal (QD) climate variability is detected in the tropical Pacific based on the recent 70 years of observations. This QD variability is identified in equatorial sea surface temperatures (SSTs), the pattern of which resembles the Central Pacific (CP) El Niño‐Southern Oscillation (ENSO) but extends further meridionally to the northeastern subtropical Pacific. Whereas equatorial upper‐ocean heat content and SSTs are in quadrature for ENSO, these two quantities are almost in phase on the QD timescale. Further analysis shows that nonlinear dynamical heating, primarily originating from strong El Niño events, tends to lead QD SSTs by a quarter of its dominant period (approximately 30 months) and shapes the dominant QD periodicity in observations. Our results suggest that the observed QD variability largely originates from ENSO nonlinearity and thus is fundamentally different from ENSO's oscillatory nature. Plain Language Summary: The climate variability of the tropical Pacific atmosphere‐ocean system profoundly influences the global weather and climate patterns on a wide range of temporal scales. The dynamical understanding of interannual climate variability dominated by the well‐known El Niño‐Southern Oscillation (ENSO) phenomena has established a promising way for the operational seasonal to interannual climate prediction. So far, nevertheless, there is still a lack of consensus for the understanding of tropical Pacific decadal climate variability despite the increasing desire for climatic information on longer timescales. In this article, we illustrate the evolution features and possible governing mechanisms of a prominent quasi‐decadal (QD) variability in the tropical Pacific with the utilization of an advanced spectrum approach and reliable observational data sets. The results show that this QD variability that makes up a substantial portion of tropical Pacific low‐frequency variance is largely equatorial originated from the ENSO nonlinearity, which is dynamically different from the oscillatory nature of the ENSO phenomenon. Our study has important implications for the decadal climate prediction and improving the representations of tropical Pacific decadal variability in climate models. Key Points: Observations show that CP‐ENSO‐like quasi‐decadal variability originates in the equatorial Pacific and extends to the off‐equatorial regionThe equatorial ocean heat content and sea surface temperature are in phase on quasi‐decadal timescales, whereas they are in quadrature for interannual ENSOEquatorial nonlinear dynamical heating is proposed as the dynamical origin of the quasi‐decadal variability [ABSTRACT FROM AUTHOR]

The El Niño–Southern Oscillation (ENSO) is known to influence the North Tropical Atlantic (NTA) sea surface temperature (SST) during its decaying spring, providing an essential source of NTA predictability. However, the positive ENSO–NTA SST relationship is absent during boreal winter, impeding accurate anticipation of wintertime NTA SST fluctuations. We find that this absence is mainly due to different NTA preconditions. Taking into account the previous summer NTA SST state as an initial condition, a robust relationship is established between ENSO and wintertime NTA SST variability. Furthermore, we show that the NTA initial conditions are in part traced to the ENSO preconditions. Besides, the simultaneous local atmospheric conditions play a role in shaping the initial NTA SST variabilities, especially in the developing summer of El Niño events. The understanding of the complicated ENSO–wintertime NTA SST relationship has implications for exploring trans‐basin climate interactions and associated climate predictability. Plain Language Summary: One of the most striking features of the North Tropical Atlantic (NTA) sea surface temperature (SST) variability is its pronounced dependency on the El Niño–Southern Oscillation (ENSO). The ENSO remote effects are felt in the NTA SST a few months after the mature phase. However, an absence of statistically significant linkage between wintertime NTA SST anomalies and ENSO largely limits our understanding of the wintertime climatic variations over the NTA sector. In this study, we show that a robust positive linkage exists between ENSO and NTA SST variability during winter in light of the NTA SST tendency, and the previous summer NTA SST state as an initial condition acts to explain the complicated ENSO–wintertime NTA SST relationship. We also discuss about the possible dynamical origins of the NTA initial conditions, which are attributable to the preceding ENSO conditions as well as the simultaneous local atmospheric conditions. Key Points: A robust linkage between ENSO and wintertime NTA SST variability could be established by considering the previous summer NTA SST stateThe NTA initial state is in part traced to the preceding ENSO conditions, and also partially attributable to the local disturbanceThe understanding of the ENSO–wintertime NTA SST relationship has implications for extending the seasonal predictability for the NTA SST [ABSTRACT FROM AUTHOR]

Previous studies suggest that the El Niño–Southern Oscillation (ENSO) impact on tropical Atlantic climate variability shows strong seasonal preference, manifesting itself primarily in the decaying boreal spring season. However, here we show that a robust ENSO signal in tropical Atlantic precipitation anomalies can be detected throughout ENSO's whole lifecycle. We find that the ENSO impact on the tropical Atlantic precipitation migrates meridionally, following the seasonal migration of the Atlantic Intertropical Convergence Zone (ITCZ). Taking into account the modulation of the local seasonal cycle, the full spatiotemporal characteristics of ENSO's impact on tropical Atlantic precipitation are uncovered. Furthermore, we demonstrate that climate models can simulate the modulation of ENSO‐related precipitation over the tropical Atlantic by the local seasonal cycle despite the substantial mean state model bias in the Atlantic ITCZ. This extended perspective on ENSO's impact on tropical Atlantic climate variability enhances prospects for skillful year‐round regional climate prediction. Plain Language Summary: El Niño–Southern Oscillation (ENSO) provides the primary source of predictability for the tropical Atlantic precipitation on interannual time scales. In most previous studies, a strong seasonal preference in the ENSO teleconnection to the Atlantic was identified. We here demonstrate that ENSO's impact on tropical Atlantic precipitation can be detected from ENSO developing to decaying phases, which is strongly modulated by the local seasonal cycle regarding its meridional migration. On this basis, we establish a robust year‐round statistical relationship between ENSO and tropical Atlantic precipitation by involving the local seasonal cycle. This observed modulation of the local seasonal cycle on ENSO's impact can also be simulated in the state‐of‐the‐art climate models. It is suggested that the realistic representation of the Atlantic seasonal cycle, as well as the improved ENSO prediction, could directly translate into enhanced prediction skill over the tropical Atlantic sector. Key Points: ENSO impact on the tropical Atlantic precipitation migrates meridionally along with the local seasonal cycle phaseA robust year‐round relationship can be established between ENSO and tropical Atlantic precipitation by involving the seasonal cycleThe extended perspective on ENSO's impact enhances prospects for seasonal climate prediction over the tropical Atlantic [ABSTRACT FROM AUTHOR]

El Niño‐Southern Oscillation (ENSO), the primary source of year‐to‐year climate variability on Earth, has profound impacts on the Indian Ocean dipole (IOD), another important climate pattern. Much attention has been paid to potentially increased ENSO predictability by utilizing IOD conditions, inferred from a statistically significant correlation with ENSO at a long lead time. However, the intrinsic dynamics for the causality of this IOD‐ENSO relationship remain largely elusive. Here, we demonstrate that the observed nonstationary IOD‐ENSO lead‐lag relationship is mainly ENSO‐driven and therefore adds no additional information for ENSO predictability. The nonstationarity of their correlation is a manifestation of ENSO cycle complexity. Multi‐climate model and theoretical results further demonstrate that ENSO pacing tightly controls the statistical IOD‐ENSO relationship via changes in ENSO periodicity and regularity. This highlights that ENSO dominates the inter‐basin Pacific‐Indian Ocean interactions, shedding light on the key predictors for interannual pantropical climate variability. Plain Language Summary: Mounting attention has been paid to finding potential predictability sources outside the tropical Pacific to extend the prediction horizon of the El Niño‐Southern Oscillation (ENSO). This study shows that the observed inter‐basin lead‐lag relationship in the Indo‐Pacific, that was previously invoked to identify the Indian Ocean dipole as an independent predictor of ENSO, is in fact a manifestation of ENSO cycle complexity. We demonstrate that ENSO pacing tightly controls the inter‐basin Pacific‐Indian Ocean interannual relationship via changes in fundamental ENSO properties. The causal relationship identified here regarding inter‐basin climate interactions is of crucial relevance for future studies investigating pantropical climate interactions. Key Points: The observed Indian Ocean dipole (IOD)‐El Niño‐Southern Oscillation (ENSO) lead‐lag relationship is primarily ENSO‐driven, which adds no additional information to ENSO predictabilityThe observed nonstationarity of inter‐basin Pacific‐Indian Ocean interannual connectivity is a manifestation of ENSO cycle complexityENSO pacing regulates the IOD‐ENSO relationship via changes in fundamental ENSO properties [ABSTRACT FROM AUTHOR]

During early boreal summer 2020, the western North Pacific (WNP) was characterized by record‐low tropical cyclone (TC) activity, reaching the lowest historical level since 1979. Only one short‐lived TC was observed in June and the whole of July underwent an unprecedented quiescent TC period. Concurrent Indian Ocean warming provided unfavorable conditions for cyclogenesis over the WNP, which partly (∼40%) explains the extremely low TC number from a statistical standpoint. Accompanied with the Indian Ocean warming, an unprecedentedly long‐lasting Madden‐Julian Oscillation (MJO) event stalled over the Indian Ocean sector, persisting almost through the entire June and July. This simultaneous quasi‐stationary MJO activity further caused unfavorable background conditions for cyclogenesis in the WNP. The Indian Ocean warming combined with the anomalous MJO conditions together largely (>80%) explain the record‐low TC number over the WNP during early summer 2020. Plain Language Summary: Tropical Cyclone (TC) is one of the most intense storm systems with considerable socioeconomic impacts. Near one‐third of global TC forms in the western North Pacific (WNP), the largest breeding ground for TCs on the planet. Climatologically, the active TC season starts from boreal early summer when background environments are favorable for TC formation. During early summer 2020, however, the WNP remains in an almost quiet state and the TC frequency sank to the lowest level on record since 1979. The physical processes responsible for it remain unclear. Accompanied with this extremely low TC frequency, the Indo‐Pacific Oceans are characterized by strong Indian Ocean basin warming and ongoing La Niña development in the equatorial eastern Pacific. The large‐scale tropical SST conditions provide unfavorable environment background for cyclogenesis and statistically explain about 50% of the extremely low TC frequency. Simultaneously, an unprecedentedly long‐stalled Madden‐Julian Oscillation (MJO) event was observed in the Indian Ocean. The MJO further worsens the background conditions for cyclogenesis and additionally explains above 40% of the record‐low TC frequency. We here demonstrated that the record‐low TC frequency in early summer 2020 is due to the combined effects of large‐scale tropical SST conditions and the extraordinary MJO event. Key Points: The western North Pacific (WNP) tropical cyclone activity was a record low in early summer 2020 with only one short‐lived tropical stormThe Indian Ocean basin experienced a remarkable warming, unfavorable for cyclogenesis over the western North PacificThe simultaneous quasi‐stationary MJO activity caused unfavorable background condition, causing the extremely inactive cyclogenesis [ABSTRACT FROM AUTHOR]

During June–July 2020, the record‐breaking flooding in the recent four decades struck the plum rain belt over China, Japan, and Korea. Concurrent with this persistent heavy rainfall, pronounced Indian Ocean basin warming (IOBW) was observed, following the previous El Niño event in the transition to La Niña‐like mean state this summer. This tropical Indo‐Pacific large‐scale thermal condition provided favorable conditions for rainfall surpluses over the plum rain belt via the western North Pacific anticyclone. Superimposed on the tropical Indo‐Pacific large‐scale thermal condition, an extraordinary long‐lasting and quasistationary Madden‐Julian Oscillation (MJO) active phase persisted in the Indian Ocean throughout June–July 2020, lasting for 59 days. The MJO‐associated teleconnection was mainly responsible for the extreme rainfall over the plum rain belt, which was facilitated by the conducive large‐scale Indo‐Pacific oceanic‐atmospheric condition. Plain Language Summary: The East Asian rainy season (commonly called the plum rain, also known as Meiyu in China, Baiu in Japan, and Jangma in Korea), is caused by rainfall along a persistent stationary front. The plum rain season usually starts in June and persists into the middle of July. During June–July 2020, the region of the plum rain belt over China, Japan, and Korea, was ravaged by the strongest flooding in decades with catastrophic losses. The physical mechanisms driving this extreme flooding remains unclear. Concurrent with this persistent rainfall surpluses, the Indo‐Pacific oceans featured a developing La Niña‐like state in the eastern tropical Pacific and pronounced sea surface temperature warming in the Indian Ocean basin. Although these Indo‐Pacific thermal conditions can provide a favorable large‐scale background for the rainfall surpluses over East Asia, they cannot explain the extreme rainfall amount over the plum rain belt during 2020. We show that an exceptionally long‐lasting and quasistationary Madden‐Julian Oscillation activity over the Indian Ocean was the key reason for this extreme event. Key Points: The rainfall amount over the East Asian plum‐rain belt during summer 2020 was the largest in the recent four decadesThe intense flooding coincided with an exceptionally persistent Madden‐Julian Oscillation (MJO) active phase in the Indian Ocean throughout June and JulyThis extraordinary MJO activity was facilitated by the conducive pantropic background oceanic‐atmospheric conditions [ABSTRACT FROM AUTHOR]

Australia suffered a long‐lasting extensive drought in 2019 with catastrophic wildfires creating about $4.4 billion damages, the worst record in the recent four decades. Concurrent with this extreme drought, the tropical Indo‐Pacific oceans exhibited an extraordinary combination of sea surface temperature (SST) anomalies, characterized by a Central‐Pacific (CP) El Niño event with westernmost location and the third strongest positive Indian Ocean Dipole (IOD) event. We here show evidence that this unique combination was responsible for the pan‐Australian drought as a westward located CP El Niño event and a strong positive IOD event can exacerbate precipitation reduction in northeastern and southern Australia, respectively. These continent‐wide extreme droughts could become more frequent over Australia in a warming world, considering projected increases in both CP El Niño and extreme positive IOD events superimposed on secular warming and drying trends. Plain Language Summary: Australia is the driest inhabited continent in the world, 70% of which is either arid or semiarid land. During 2019, Australia was ravaged by the worst drought in decades with devastating wildfires destroying around 3,000 homes and affecting about one billion animals. The physical mechanisms driving this extreme drought remains unclear. Concurrent with this long‐lasting continent‐wide drought, the Indo‐Pacific oceans showed pronounced sea surface temperature anomalies associated with a record westernmost‐located Central Pacific (CP) El Niño event and the third strongest Indian Ocean Dipole (IOD) event co‐occurring. Our observational diagnostics and numerical model experiments show that these two compounding events caused atmospheric circulation anomalies that were responsible for the extreme precipitation deficiencies over Australia. With global warming, both CP El Niño and extreme positive IOD events are projected to become more frequent, which hints to increased risk of the severe Australian droughts in the future. Key Points: Australia suffered a long‐lasting extensive drought in 2019, the worst record in the recent four decadesThis extreme drought can be well explained by combined effects of the westernmost‐located CP El Niño and third strongest positive Indian Ocean Dipole (IOD) eventsSevere Australian droughts could occur more frequently in a warming world with projected increases in both CP El Niño and extreme IOD events [ABSTRACT FROM AUTHOR]

The temperature of the subsurface water entrained into the surface mixed layer plays a key role in controlling the sea surface temperature (SST) and its interannual variability in the equatorial Pacific. In this paper, we combine a hyperbolic tangent function bounded by the warm pool SST and centered at the thermocline depth with a variable sharpness parameter to describe the time‐space evolutions of the subsurface temperature. Under simple approximations of the sharpness parameter, this concise expression becomes remarkably efficient in capturing the observed and climate‐model simulated subsurface temperature variability in terms of anomalies of the thermocline depth and SST of the El Niño‐Southern Oscillation (ENSO) phenomenon. The formulations for the subsurface temperature and thermocline sharpness developed in this work should be useful tools for evaluating and understanding the role of the thermocline feedback in ENSO behaviors in both theoretical and comprehensive climate models. Plain Language Summary: While significant advances have been made in our ability to understand, simulate, and predict the El Niño‐Southern Oscillation (ENSO), its remarkable complexity is still not well understood. The subsurface ocean temperature in the equatorial Pacific is known to play a key role in controlling sea surface temperature and its interannual variations. An accurate description of its relationship with thermocline depth variability is of essential importance to understanding ENSO's complex dynamics in mechanistic studies. In this paper, we develop a concise expression which captures the observed relationship between anomalies of the subsurface temperature and thermocline depth remarkably well. This expression should be useful for ENSO mechanistic model studies and diagnostic investigations into climate models' performances in their ENSO simulations. Key Points: The equatorial Pacific subsurface temperature is expressed as a hyperbolic tangent function of thermocline depth and sharpnessEffective parameterizations for the subsurface temperature are derived by simple approximations of the thermocline sharpnessThese parameterizations can be useful tools for evaluating the impacts of thermocline feedback on ENSO [ABSTRACT FROM AUTHOR]

Tropical instability waves (TIWs), the dominant form of eddy variability in the tropics, have a peak period at about 5 weeks and are strongly modulated by both the seasonal cycle and El Niño–Southern Oscillation (ENSO). In this study, we first demonstrated that TIW‐induced nonlinear dynamical heating (NDH) is basically proportional to the TIW amplitude depicted by a complex index for TIW. We further delineated that this NDH, capturing the seasonally modulated nonlinear feedback of TIW activity onto ENSO, is well approximated by a theoretical formulation derived analytically from a simple linear stochastic model for the TIW index. The results of this study may be useful for the climate community to evaluate and understand the TIW‐ENSO multiscale interaction. Plain Language Summary: Tropical instability waves (TIWs) are westward propagating high‐frequency waves having a main period about 5 weeks. Their activity is strongly modulated by the cold tongue annual cycle and El Niño–Southern Oscillation (ENSO). At the same time, TIW activity as a whole systematically transports heat meridionally from warm to cold regions, and thus when they are modulated by ENSO, they can have a nonlinear rectification effect on ENSO in return. We find that the TIW‐induced rectification effect on ENSO can be related to the amplitude of a simple index that captures the main propagative wavy feature of TIW. This feedback effect prevents the growth of La Niña (El Niño) events by promoting a warming (cooling) through meridional convergence of TIW heat transport. Finally, we introduce a theoretical formulation for TIW‐induced effect by adopting a simple linear stochastic model for TIW focused on the complex index for TIW. This validated formulation shall be useful, for instance, for evaluating and understanding the climate model's ability in simulating the TIW‐ENSO multiscale interaction. Key Points: Nonlinear dynamical heating (NDH) due to tropical instability waves (TIWs) is largely proportional to the amplitude of a simple TIW indexTIW feedback onto El Niño–Southern Oscillation through TIW‐induced NDH is nonlinear and strongly seasonal dependentA theoretical derived simple expression for this feedback is in agreement with the deduced result from the reanalysis data [ABSTRACT FROM AUTHOR]

Previous studies argued that the Pacific Meridional Mode (PMM) impacts tropical cyclone (TC) genesis variability over the southeastern part of the western North Pacific (SE‐WNP). Here, we find that the statistical relationship between PMM and SE‐WNP TC genesis frequency is dominated by their co‐variability on decadal timescales. The decadal component of the PMM exhibits very similar temporal and spatial features to quasi‐decadal tropical Pacific sea surface temperature (SST) variability. The latter can affect SE‐WNP TC activity via changes in both zonal vertical wind shear and low‐level vorticity. In contrast, the interannual component of the PMM exhibits no statistically significant correlation with SE‐WNP TC genesis. Furthermore, observations show that both interannual and decadal variability of SE‐WNP TC activity are well correlated with the commonly used Niño3.4 El Niño‐Southern Oscillation index. Thus, equatorial Pacific SST variability is the dominant source of SE‐WNP TC activity predictability on different timescales. Key Points: PMM and WNP TC variability show a strong relationship on quasi‐decadal timescales but less so on interannual timescalesPMM and central Pacific ENSO‐like SST variability are indistinguishable on quasi‐decadal timescalesEquatorial Pacific SST variability impacts WNP TCs on both interannual and decadal timescales [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]

Abstract: We here propose a new statistical method to interpret nonstationary running correlations by decomposing them into a stationary part and a first‐order Taylor expansion approximation for the nonstationary part. Then, this method is applied to investigate the nonstationary behavior of the El Niño–Southern Oscillation (ENSO)‐East Asian winter monsoon (EAWM) relationship, which exhibits prominent multidecadal variations. It is demonstrated that the first‐order approximation of the nonstationary part can be expressed to a large extent by the impact of the nonlinear interaction between the Atlantic Multidecadal Oscillation (AMO) and ENSO (AMO*Niño3.4) on the EAWM. Therefore, the nonstationarity in the ENSO‐EAWM relationship comes predominantly from the impact of an AMO modulation on the ENSO‐EAWM teleconnection via this key nonlinear interaction. This general method can be applied to investigate nonstationary relationships that are often observed between various different climate phenomena. [ABSTRACT FROM AUTHOR]