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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]

Sea surface temperature (SST) variability in the East China Sea–Kuroshio (EK) region has important implications for the surrounding weather, climate, and marine ecology. The year-to-year variations of the EK SST are expectedly linked to El Niño–Southern Oscillation (ENSO), the predominant predictability source of seasonal-to-interannual climate variability. Surprisingly, no significant SST signal is observed in the EK region when focusing on the ENSO autumn–winter season with the persistent and pronounced SST anomalies in the tropical Pacific. We find that a remarkable seasonal reversal appears in the ENSO–EK SST connection, shifting from a negative relationship in autumn [Aug(0)–Oct(0)] to a positive relationship in winter [Dec(0)–Feb(1)]. This reversal is mainly attributed to the seasonally varying ENSO-associated western North Pacific (WNP) atmospheric circulation patterns. During ENSO autumns, the anomalous WNP anticyclone is confined south of 20°N, which is accompanied with cyclonic circulation anomalies in the EK region. The associated anomalous northerly wind tends to enhance the background northerly wind, thereby facilitating the local SST cooling mainly via the wind–evaporation–SST effect. In the subsequent winter, the ENSO-related WNP anticyclonic anomalies intensify and extend toward the EK region. Consequently, the weakened background northerly wind induced by southerly wind anomalies leads to the increase of downward latent and sensible heat flux in the EK region, fostering the local SST warming. The observed seasonal reversal of ENSO impacts can be evidenced by the tropical Pacific pacemaker experiments, emphasizing the importance of seasonally modulated ENSO teleconnection and holding implications for the local SST climate prediction. [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]

El Niño-Southern Oscillation (ENSO) sea surface temperature (SST) anomaly skewness encapsulates the nonlinear processes of strong ENSO events and affects future climate projections. Yet, its response to CO2 forcing remains not well understood. Here, we find ENSO skewness hysteresis in a large ensemble CO2 removal simulation. The positive SST skewness in the central-to-eastern tropical Pacific gradually weakens (most pronounced near the dateline) in response to increasing CO2, but weakens even further once CO2 is ramped down. Further analyses reveal that hysteresis of the Intertropical Convergence Zone migration leads to more active and farther eastward-located strong eastern Pacific El Niño events, thus decreasing central Pacific ENSO skewness by reducing the amplitude of the central Pacific positive SST anomalies and increasing the scaling effect of the eastern Pacific skewness denominator, i.e., ENSO intensity, respectively. The reduction of eastern Pacific El Niño maximum intensity, which is constrained by the SST zonal gradient of the projected background El Niño-like warming pattern, also contributes to a reduction of eastern Pacific SST skewness around the CO2 peak phase. This study highlights the divergent responses of different strong El Niño regimes in response to climate change. [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 three strongest positive Indian Ocean dipole (pIOD) events over the past four decades occurred in 1994, 1997, and 2019 and caused severe social-economic impacts in the Indian Ocean (IO) rim countries. These three strongest positive pIOD events exhibited similar oceanic and atmospheric anomaly patterns during their mature phase, though with different physical origins. The extreme amplitudes of these pIOD events were closely related to concurrent El Niño or Madden–Julian oscillation (MJO) activity. For the 1997 extreme pIOD case, the concurrent extreme El Niño in the Pacific triggered strong easterly wind anomalies over the equatorial IO and therefore favored the rapid growth of the pIOD via intense upwelling along the Sumatra-Java coast. In contrast, the long stalled MJO convective anomalies over the western IO exerted an important role in the 2019 extreme pIOD event especially during September–October by causing easterly anomalies over the equatorial IO. During that year, the Pacific moderate warming had likely a small impact since no strong associated convection was present over the western and central equatorial Pacific due to the small amplitude of the Pacific sea surface temperature anomalies. Different from the 1997 and 2019 cases, both El Niño conditions in the Pacific as well as MJO activity over the western IO contributed to the 1994 extreme pIOD event. Our results emphasize that MJO activity should be considered as one physical drivers for the extreme pIOD event in addition to the traditional El Niño-associated forcing. [ABSTRACT FROM AUTHOR]

This study demonstrates the main physical mechanism for the maintenance of the western North Pacific (WNP) anomalous anticyclone (WNPAC) during the El Niño decaying summer by analyzing the respective effects of the cold sea surface temperature (SST) anomalies in the WNP, the warm SST anomalies in the Indian Ocean (IO), and the El Niño and Southern Oscillation (ENSO) combination mode. We find that the WNPAC is usually accompanied by significant cold WNP SST anomalies in the El Niño mature winter and following spring, which almost disappear in the decaying summer and cannot explain the maintenance of the WNPAC in summer. The influence of the IO warm SST anomalies on the WNPAC exhibits conspicuous decadal differences. Before the 2000s, the IO warm SST anomalies play a role in the WNPAC maintenance through the response of the baroclinic atmospheric Kelvin wave; however, this effect cannot be evidently detected after the 2000s. This decadal difference may be related to changes in the decaying speed of ENSO events. In contrast to El Niño events before the 2000s, El Niño events after the 2000s decay more rapidly, and the associated tropical central-eastern Pacific SST features a La Niña-like condition in the El Niño decaying summer. Concomitantly, no significant warm SST anomalies appear over the tropical Indian Ocean, exerting a weak influence on the WNPAC. Relative to the cold WNP SST anomalies and warm IO SST anomalies, the ENSO combination mode, originating from the nonlinear interaction between ENSO and the annual cycle, has a relatively more stable relationship with the WNPAC during the El Niño decaying summer, which exhibits a crucial role in the maintenance of the WNPAC. Considering the persistence of the ENSO combination mode, the WNPAC and associated climate variability during the El Niño decaying summer can be skillfully predicted at least one season in advance based on the ENSO combination mode. [ABSTRACT FROM AUTHOR]

Previous studies have demonstrated that the Atlantic multidecadal oscillation (AMO) could affect El Niño–Southern Oscillation (ENSO) through thermocline adjustment, with a stronger ENSO sea surface temperature (SST) amplitude during a negative AMO phase than during a positive phase. In this study, we find that the ENSO atmospheric anomaly amplitudes in the tropical Pacific during different AMO phases are not necessarily consistent with these ENSO SST changes. For El Niño episodes, the low-level wind and precipitation anomalies over the tropical Pacific in the boreal winter are more pronounced during the negative AMO phase than during the positive phase, corresponding well to the stronger SST anomalies. However, La Niña events during the negative AMO phase are accompanied by weaker atmospheric anomalies in the tropical Pacific, although their SST anomalies are stronger than those during the positive phase. We suggest that this mismatch between La Niña SST and atmospheric anomalies can be largely attributed to AMO decadal modulation. A positive AMO favors intensified trade winds and weakened precipitation in the central tropical Pacific by modifying Walker circulation. Therefore, when La Niña coincides with a positive AMO, the low-level easterly and negative precipitation anomalies are superimposed, which gives rise to stronger atmospheric perturbations. In contrast, under a negative AMO background, the atmospheric anomalies induced by La Niña anomalous SST are partly counteracted by the AMO remote decadal modulation, thereby resulting in weaker anomaly amplitudes. Here, we highlight that AMO decadal forcing needs to be considered when investigating ENSO atmospheric variabilities and related regional climate impacts. [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]

Although the 1997/98 and 2015/16 El Niño events are considered to be the strongest on record, their subsequent La Niña events exhibited contrasted evolutions. In this study, we demonstrate that the extremely strong period of tropical instability waves (TIWs) at the beginning of boreal summer of 2016 played an important role in hindering the subsequent La Niña's development by transporting extra off-equatorial heat into the Pacific cold tongue. By comparing the TIWs' contribution based on an oceanic mixed layer heat budget analysis for the 1998 and 2016 episodes, we establish that TIW-induced nonlinear dynamical heating (NDH) is a significant contributor to the El Niño–Southern Oscillation (ENSO) phase transition in 2016. TIW-induced NDH contributed to around 0.4°C warming per month during the early boreal summer (May–June) following the 2015/16 El Niño's peak, which is found to be an essential inhibiting factor that prevented the subsequent La Niña's growth. A time-mean eddy kinetic energy analysis reveals that anomalous TIWs during 2016 mainly gained their energy from the baroclinic instability conversion due to a strong SST warming in the northeastern off-equatorial Pacific that promoted an increased meridional SST gradient. This highlights the importance of accurately reproducing TIW activity in ENSO simulation and the benefit of off-equatorial SST anomalies in the eastern Pacific as an independent precursor for ENSO predictions. [ABSTRACT FROM AUTHOR]

The El Niño-Southern Oscillation (ENSO), the primary driver of year-to-year global climate variability, is known to influence the North Tropical Atlantic (NTA) sea surface temperature (SST), especially during boreal spring season. Focusing on statistical lead-lag relationships, previous studies have proposed that interannual NTA SST variability can also feed back on ENSO in a predictable manner. However, these studies did not properly account for ENSO's autocorrelation and the fact that the SST in the Atlantic and Pacific, as well as their interaction are seasonally modulated. This can lead to misinterpretations of causality and the spurious identification of Atlantic precursors for ENSO. Revisiting this issue under consideration of seasonality, time-varying ENSO frequency, and greenhouse warming, we demonstrate that the cross-correlation characteristics between NTA SST and ENSO, are consistent with a one-way Pacific to Atlantic forcing, even though the interpretation of lead-lag relationships may suggest otherwise. It has been suggested that sea surface temperatures in the North Tropical Atlantic exert strong influence on the evolution of the El Nino Southern Oscillation (ENSO). Here, the authors argue that observed statistics are fully consistent with ENSO driving climate variations in the Atlantic and not vice versa. [ABSTRACT FROM AUTHOR]

Variabilities in precipitation frequency carry profound implications for many aspects of socioeconomic activities. Based on the observations from 613 meteorological stations in China, the dominant mode of wintertime precipitation days and its possible causes are investigated in this article. It is revealed that the leading mode accounts for 35.0% of the total variance and mainly displays quasi‐uniform anomalies of precipitation days to the south of 40°N with the monopolar centre over southeastern China. The associated principal component exhibits remarkable interannual variability with a peak periodicity at 2–3 years. We suggest that this leading mode is largely contributed by the variabilities of El Niño‐Southern Oscillation (ENSO) in the tropical Pacific and Eurasian (EU) teleconnection over the mid‐high latitudes. While ENSO mostly affects the variability of precipitation frequency in South China, the EU teleconnection is largely responsible for the changes of precipitation days in North China. On one hand, when El Niño events occur, the anomalous anticyclone over the western North Pacific propagates abundant moisture towards South China, leading to more frequent precipitation occurrence there. On the other hand, during the winters with a negative EU pattern, North China tends to be rainy due to an evident moisture transport and a profound regional anomalous ascending flow which are embedded in the large‐scale eastward‐propagating wave‐train. The responses are roughly reversed in sign when these two factors are in their opposite phases. Based on the impacts from ENSO and the EU teleconnection, a binary statistical regression model is established and over 20% variance of the precipitation days can be realistically captured in eastern China. Our results could be beneficial for wintertime precipitation prediction and disaster mitigation in the country. [ABSTRACT FROM AUTHOR]

Many previous studies have shown that an Indian Ocean basin warming (IOBW) occurs usually during El Niño–Southern Oscillation (ENSO) decaying spring to summer seasons through modifying the equatorial zonal circulation. Decadal modulation associated with the interdecadal Pacific oscillation (IPO) is further investigated here to understand the nonstationary ENSO–IOBW relationship during ENSO decaying summer (July–September). During the positive IPO phase, significant warm sea surface temperature (SST) anomalies are observed over the tropical Indian Ocean in El Niño decaying summers and vice versa for La Niña events, while these patterns are not well detected in the negative IPO phase. Different decaying speeds of ENSO associated with the IPO phase, largely controlled by both zonal advective and thermocline feedbacks, are suggested to be mainly responsible for these different ENSO–IOBW relationships. In contrast to ENSO events in the negative IPO phase, the ones in the positive IPO phase display a slower decaying speed and delay their transitions both from a warm to a cold state and a cold to a warm state. The slower decay of El Niño and La Niña thereby helps to sustain the teleconnection forcing over the equatorial Indian Ocean and corresponding SST anomalies there can persist into summer. This IPO modulation of the ENSO–IOBW relationship carries important implications for the seasonal prediction of the Indian Ocean SST anomalies and associated summer climate anomalies. [ABSTRACT FROM AUTHOR]

Many previous studies have demonstrated that the boreal winters of super El Niño events are usually accompanied by severely suppressed Madden-Julian oscillation (MJO) activity over the western Pacific due to strong descending motion associated with a weakened Walker Circulation. However, the boreal winter of the 2015/16 super El Niño event is concurrent with enhanced MJO activity over the western Pacific despite its sea surface temperature anomaly (SSTA) magnitude over the Niño 3.4 region being comparable to the SSTA magnitudes of the two former super El Niño events (i.e., 1982/83 and 1997/98). This study suggests that the MJO enhanced over western Pacific during the 2015/16 super El Niño event is mainly related to its distinctive SSTA structure and associated background thermodynamic conditions. In comparison with the previous super El Niño events, the warming SSTA center of the 2015/16 super El Niño is located further westward, and a strong cold SSTA is not detected in the western Pacific. Accordingly, the low-level moisture and air temperature (as well as the moist static energy, MSE) tend to increase in the central-western Pacific. In contrast, the low-level moisture and MSE show negative anomalies over the western Pacific during the previous super El Niño events. As the MJO-related horizontal wind anomalies contribute to the further westward warm SST-induced positive moisture and MSE anomalies over the western tropical Pacific in the boreal winter of 2015/16, stronger moisture convergence and MSE advection are generated over the western Pacific and lead to the enhancement of MJO convection. [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]

Previous studies have shown that nonlinear atmospheric interactions between ENSO and the warm pool annual cycle generates a combination mode (C-mode), which is responsible for the termination of strong El Niño events and the development of the anomalous anticyclone over the western North Pacific (WNP). However, the C-mode has experienced a remarkable decadal change in its characteristics around the early 2000s. The C-mode in both pre- and post-2000 exhibits its characteristic anomalous atmospheric circulation meridional asymmetry but with somewhat different spatial structures and time scales. During 1979–99, the C-mode pattern featured prominent westerly surface wind anomalies in the southeastern tropical Pacific and anticyclonic anomalies over the WNP. In contrast, the C-mode-associated westerly anomalies were shifted farther westward to the central Pacific and the WNP anticyclone was farther westward extended and weaker after 2000. These different C-mode patterns were accompanied by distinct climate impacts over the Indo-Pacific region. The decadal differences of the C-mode are tightly connected with the ENSO regime shift around 2000; that is, the occurrence of central Pacific (CP) El Niño events with quasi-biennial and decadal periodicities increased while the occurrence of eastern Pacific (EP) El Niño events with quasi-quadrennial periodicity decreased. The associated near-annual combination tone periodicities of the C-mode also changed in accordance with these changes in the dominant ENSO frequency between the two time periods. Numerical model experiments further confirm the impacts of the ENSO regime shift on the C-mode characteristics. These results have important implications for understanding the C-mode dynamics and improving predictions of its climate impacts. [ABSTRACT FROM AUTHOR]

Recent studies demonstrated the existence of a conspicuous atmospheric combination mode (C-mode) originating from nonlinear interactions between El Niño–Southern Oscillation (ENSO) and the Pacific warm pool annual cycle (AC). Here we find that the C-mode exhibits prominent decadal amplitude variations during the ENSO decaying boreal spring season. It is revealed that the Atlantic multidecadal oscillation (AMO) can largely explain this waxing and waning in amplitude. A robust positive correlation between ENSO and the C-mode is detected during a negative AMO phase but not during a positive phase. Similar results can also be found in the relationship of ENSO with 1) the western North Pacific (WNP) anticyclone and 2) spring precipitation over southern China, both of which are closely associated with the C-mode. We suggest that ENSO property changes due to an AMO modulation play a crucial role in determining these decadal shifts. During a positive AMO phase, ENSO events are distinctly weaker than those in an AMO negative phase. In addition, El Niño events concurrent with a positive AMO phase tend to exhibit a westward-shifted sea surface temperature (SST) anomaly pattern. These SST characteristics during the positive AMO phase are both not conducive to the development of the meridionally asymmetric C-mode atmospheric circulation pattern and thus reduce the ENSO/C-mode correlation on decadal time scales. These observations can be realistically reproduced by a coupled general circulation model (CGCM) experiment in which North Atlantic SSTs are nudged to reproduce a 50-yr sinusoidally varying AMO evolution. Our conclusion carries important implications for understanding seasonally modulated ENSO dynamics and multiscale climate impacts over East Asia. [ABSTRACT FROM AUTHOR]

Here we investigate the response of boreal spring precipitation over southern China (SPSC) to central Pacific (CP) El Niño based on observational datasets. While there is enhanced precipitation over southern China during the decaying boreal spring of eastern Pacific (EP) El Niño events, so far no clear precipitation response has been detected during the same decaying stage for CP El Niño composites. Here we show that around half of the CP El Niño events coincide with enhanced SPSC (wet CP El Niño), while the other half are accompanied by reduced SPSC (dry CP El Niño). These two types of CP El Niño events bear dramatically different evolution features in their respective tropical sea surface temperature anomaly (SSTA) patterns. Wet CP El Niño events are characterized by an SSTA longitudinal position confined to the tropical central-eastern Pacific. In contrast, dry CP El Niño events exhibit a clear westward propagation of SSTAs during their evolution, with maximum SSTAs located to the west of the date line after their mature phase. These different longitudinal positions of positive SSTAs during their decaying phase result in distinct meridional structures of the tropical Pacific convection anomalies as well as the ENSO combination mode (C-mode) response. An anomalous low-level anticyclone is evident over the western North Pacific during wet CP El Niño events during their decaying phase, while an anomalous cyclonic circulation is found for dry CP El Niño events. We emphasize that the impacts of CP El Niño on the SPSC depend crucially on the simultaneous zonal location of warm SSTAs in the tropical Pacific. [ABSTRACT FROM AUTHOR]

The present work investigates different responses of Arctic surface air temperature (SAT) to two ENSO types based on reanalysis datasets and model experiments. We find that eastern Pacific (EP) ENSO events are accompanied by statistically significant SAT responses over the Barents–Kara Seas in February, while central Pacific (CP) events coincide with statistically significant SAT responses over northeastern Canada and Greenland. These impacts are largely of opposite sign for ENSO warm and cold phases. During EP El Niño in February, the enhanced tropospheric polar vortex over Eurasia and associated local low-level northeasterly anomalies over the Barents–Kara Seas lead to anomalously cold SAT in this region. Simultaneously, the enhanced tropospheric polar vortex leads to enhanced sinking air motion and consequently reduced cloud cover. This in turn reduces downward infrared radiation (IR), which further reduces SAT in the Barents–Kara Seas region. Such a robust response cannot be detected during other winter months for EP ENSO events. During CP El Niño, the February SATs over northeastern Canada and Greenland are anomalously warm and coincide with a weakened tropospheric polar vortex and related local low-level southwesterly anomalies originating from the Atlantic Ocean. The anomalous warmth can be enhanced by the local positive feedback. Similar SAT signals as in February during CP ENSO events can also be seen in January, but they are less statistically robust. We demonstrate that these contrasting Arctic February SAT responses are consistent with responses to the two ENSO types with a series of atmospheric general circulation model experiments. These results have implications for the seasonal predictability of regional Arctic SAT anomalies. [ABSTRACT FROM AUTHOR]

Many previous studies have demonstrated a high uncertainty in the relationship between El Niño–Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO). In the present work, decadal modulation by the Atlantic multidecadal oscillation (AMO) is investigated as a possible cause of the nonstationary ENSO–NAO relationship based on observed and reanalysis data. It is found that the negative ENSO–NAO correlation in late winter is significant only when ENSO and the AMO are in phase (AMO+/El Niño and AMO−/La Niña). However, no significant ENSO-driven atmospheric anomalies can be observed over the North Atlantic when ENSO and the AMO are out of phase (AMO−/El Niño and AMO+/La Niña). Further analysis indicates that the sea surface temperature anomaly (SSTA) in the tropical North Atlantic (TNA) plays an essential role in this modulating effect. Because of broadly analogous TNA SSTA responses to both ENSO and the AMO during late winter, a warm SSTA in the TNA is evident when El Niño occurs during a positive AMO phase, resulting in a significantly weakened NAO, and vice versa when La Niña occurs during a negative AMO phase. In contrast, neither the TNA SSTA nor the NAO shows a prominent change under out-of-phase combinations of ENSO and AMO. The AMO modulation and the associated effect of the TNA SSTA are shown to be well reproduced by historical simulations of the HadCM3 coupled model and further verified by forced experiments using an atmospheric circulation model. These offer hope that similar models will be able to make predictions for the NAO when appropriately initialized. [ABSTRACT FROM AUTHOR]

Previous studies documented that a distinct southward shift of central Pacific low-level wind anomalies occurring during the ENSO decaying phase is caused by an interaction between the western Pacific annual cycle and El Niño-Southern Oscillation (ENSO) variability. The present study finds that the meridional movement of the central Pacific wind anomalies appears only during traditional eastern Pacific El Niño (EP El Niño) events rather than in central Pacific El Niño (CP El Niño) events in which sea surface temperature (SST) anomalies are confined to the central Pacific. The zonal structure of ENSO-related SST anomalies therefore has an important effect on meridional asymmetry in the associated atmospheric response and its modulation by the annual cycle. In contrast to EP El Niño events, the SST anomalies of CP El Niño events extend farther west toward the warm pool region with its climatological warm SSTs. In the warm pool region, relatively small SST anomalies are thus able to excite convection anomalies on both sides of the equator, even with a meridionally asymmetric SST background state. Therefore, almost meridionally symmetric precipitation and wind anomalies are observed over the central Pacific during the decaying phase of CP El Niño events. The SST anomaly pattern of La Niña events is similar to CP El Niño events with a reversed sign. Accordingly, no distinct southward displacement of the atmospheric response occurs over the central Pacific during the La Niña decaying phase. These results have important implications for ENSO climate impacts over East Asia, since the anomalous low-level anticyclone over the western North Pacific is an integral part of the annual cycle-modulated ENSO response. [ABSTRACT FROM AUTHOR]

The fidelity of coupled climate models simulating El Niño-Southern Oscillation (ENSO) patterns has been widely examined. Nevertheless, a systematical narrow bias in the simulated meridional width of the sea surface temperature anomaly (SSTA) of ENSO has been largely overlooked. Utilizing the preindustrial control simulations of 11 coupled climate models from phase 3 of the Coupled Model Intercomparison Project (CMIP3), it was shown that the simulated width of the ENSO SSTA is only about two-thirds of what is observed. Through a heat budget analysis based on simulations and ocean reanalysis datasets, it is demonstrated that the SSTA outside of the equatorial strip is predominantly controlled by the anomalous meridional advection by climatological currents and heat-flux damping. The authors thus propose a simple damped-advective conceptual model to describe ENSO width. The simple model indicates that this width is primarily determined by three factors: meridional current, ENSO period, and thermal damping rate. When the meridional current is weak, it spreads the equatorial SSTA away from the equator less effectively and the ENSO width thus tends to be narrow. A short ENSO period allows less time to transport the equatorial SSTA toward the off-equatorial region, and strong damping prevents expansion of the SSTA away from the equator, both of which lead to the meridional width becoming narrow. The narrow bias of the simulated ENSO width is mainly due to a systematical bias in weak trade winds that lead to weak ocean meridional currents, and partly due to a bias toward short ENSO periods. [ABSTRACT FROM AUTHOR]

The persistence barrier of sea surface temperature anomalies (SSTAs) in the North Pacific was investigated and compared with the ENSO spring persistence barrier. The results show that SSTAs in the central western North Pacific (CWNP) have a persistence barrier in summer: the persistence of SSTAs in the CWNP shows a significant decline in summer regardless of the starting month. Mechanisms of the summer persistence barrier in the CWNP are different from those of the spring persistence barrier of SSTAs in the central and eastern equatorial Pacific. The phase locking of SSTAs to the annual cycle does not explain the CWNP summer persistence barrier. Remote ENSO forcing has little linear influence on the CWNP summer persistence barrier, compared with local upper-ocean process and atmospheric forcing in the North Pacific. Starting in wintertime, SSTAs extend down to the deep winter mixed layer then become sequestered beneath the shallow summer mixed layer, which is decoupled from the surface layer. Thus, wintertime SSTAs do not persist through the following summer. Starting in summertime, persistence of summer SSTAs until autumn can be explained by the atmospheric forcing through a positive SSTAs-cloud/radiation feedback mechanism because the shallow summertime mixed layer is decoupled from the temperature anomalies at depth, then the following autumn-winter-spring, SSTAs persist. Thus, summer SSTAs in the CWNP have a long persistence, showing a significant decline in the following summer. In this way, SSTAs in the CWNP show a persistence barrier in summer regardless of the starting month. [ABSTRACT FROM AUTHOR]