Search

Introduction. Today, the study of climate is one of the most important tasks of modern science. Particular attention is paid to issues of climate variability on different time scales. The El Niño – Southern Oscillation (ENSO) is the leading signal of interannual climate variability that influences the weather and climate of the globe. It has been revealed that during the onset of its different phases, weather and climate anomalies (up to extreme ones) are observed not only over the tropical zone of the Pacific Ocean, but also in extratropical latitudes, which are transmitted through teleconnections in the ocean-atmosphere system. Numerous studies have shown that ENSO modulates tropical cyclone activity in the northwest Pacific Ocean. The purpose of the study is to determine the relationship between sea surface temperature (SST) changes in the equatorial Pacific Ocean and the frequency of tropical cyclones of varying intensity in the northwestern Pacific Ocean under current climatic conditions. Materials and methods of research. The work uses data of the number of cases and intensity of tropical cyclones (TCs) for the period 1991-2020, and series of indices ONI and SOI. Using these data, statistics on the frequency of TCs in the study area were calculated and Pearson correlation coefficients were calculated. The scientific novelty lies in obtaining quantitative characteristics of the relationship between SST and the number of TCs of varying intensity in modern climatic conditions. The practical significance of the work lies in the fact that TCs are associated with the occurrence of severe weather conditions, which, when a TC reaches the densely populated coast of Southeast Asia, can pretend to be significant losses. Therefore, information that can be used in forecasting TCs and their trajectories is very high importance. Research results. Analysis of the frequency of TCs of different intensity in the northwestern Pacific Ocean during the 1991-2020 showed that there was a decrease in the number of tropical depressions and typhoons. However, there has been a slight increase in the occurrence of tropical storms. The results of the analysis of the influence of SST variability in the equatorial Pacific Ocean on the frequency of TCs, showed that there is a statistically significant relationship only with the frequency of typhoons. The establishment of positive SST in the center of the equatorial Pacific Ocean, associated with the El Niño episode, is associated with an increase in typhoons in the northwestern Pacific Ocean.

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]

El Niño‐Southern Oscillation (ENSO) is the leading mode of interannual climate variability that affects climate and society across the world. However, our understanding of ENSO variability is currently lacking due to short instrumental observations, which limit our confidence in predicting its future occurrence and impact. In this study, we use the nested principal component regression method to reconstruct the ENSO index based on a global‐scale proxy network of stable oxygen isotopes (δ18O). We account for the teleconnection changes that influence proxy records corresponding to different ENSO phases by using the isotope‐enabled Community Earth System Model. According to the precipitation δ18O–ENSO relationship, we reconstruct the ENSO index over the last eight hundred years based on the first two leading modes of the selected proxy records. The skill of ENSO reconstruction improves with an increased number of δ18O series distributed over the ENSO‐affected regions. Our new reconstruction shows high consistency with existing ENSO reconstructions during periods of strong ENSO variability, indicating that the global proxy records capture enhanced teleconnection signals during the phase of strong ENSO amplitude. There is a weak El Niño response 1 year after the large volcanic eruptions in our reconstruction, and the amplitude of ENSO is significantly correlated with volcanic intensity. The strengthened energetic interactions between the ocean and atmosphere following large volcanic eruptions result in enhanced sea surface temperature variability. Plain Language Summary: El Niño‐Southern Oscillation (ENSO) is a mode of interannual climate variability originating in the tropical Pacific region. Our understanding of the range of natural variability in ENSO and the long‐term relationships between ENSO and climate change is still limited due to short instrumental observations. Global proxy records preserved in tree rings and caves are a common data source for ENSO variability over the preindustrial period. We use a climate model to interpret the patterns of proxy records associated with different ENSO phases. According to the proxy–ENSO relationship, we reconstruct the ENSO index over the last eight hundred years based on global proxy records. They could reasonably represent ENSO variability when distributed around the ENSO‐affected regions, including the Asian–Australian monsoon region, the tropical Pacific, northern South America, and Africa. Our reconstruction shows high consistency with existing ENSO reconstructions during periods of strong ENSO amplitude and reveals a weak El Niño‐like response 1 year after large volcanic eruptions. We further show a tight coupling between volcanic intensity and ENSO amplitude, with large volcanic eruptions corresponding to strong ENSO amplitudes. Key Points: We account for the teleconnection changes in a global‐scale proxy network of δ18O corresponding to different El Niño‐Southern Oscillation (ENSO) phasesThe new ENSO reconstruction shows high consistency with existing ENSO reconstructions during periods of strong ENSO variabilityWe reveal an El Niño‐like response to large volcanic eruptions and the amplitude of ENSO is significantly correlated with volcanic intensity [ABSTRACT FROM AUTHOR]

The El Niño-Southern Oscillation (ENSO) stands out as the most significant tropical phenomenon in terms of climatic magnitude resulting from ocean–atmosphere interaction. Due to its atmospheric teleconnection mechanism, ENSO influences various environmental variables across distinct atmospheric scales, potentially impacting the spatiotemporal distribution of atmospheric aerosols. Within this context, this study aims to evaluate the relationship between ENSO and atmospheric aerosols across the entire Legal Amazon during the period from 2006 to 2011. Over this five-year span, four ENSO events were identified. Concurrently, an analysis of the spatiotemporal variability of aerosol optical depth (AOD) and Black Carbon radiation extinction (EAOD-BC) was conducted alongside these ENSO events, utilizing data derived from the Aerosol Robotic Network (AERONET), MERRA-2 model, and ERSSTV5. Employing the Windowed Cross-Correlation (WCC) approach, statistically significant phase lags of up to 4 to 6 months between ENSO indicators and atmospheric aerosols were observed. There was an approximate 100% increase in AOD immediately after El Niño periods, particularly during intervals encompassing the La Niña phase. The analysis of specific humidity anomaly (QA) revealed that, contrary to expectations, positive values were observed throughout most of the El Niño period. This result suggests that while there is a suppression of precipitation events during El Niño due to the subsidence of drier air masses in the Amazon, the region still exhibits positive specific humidity (Q) conditions. The interaction between aerosols and humidity is intricate. However, Q can exert influence over the microphysical and optical properties of aerosols, in addition to affecting their chemical composition and aerosol load. This influence primarily occurs through water absorption, leading to substantial alterations in radiation scattering characteristics, and thus affecting the extinction of solar radiation. [ABSTRACT FROM AUTHOR]

Introduction. Today, the study of climate is one of the most important tasks of modern science. Particular attention is paid to issues of climate variability on different time scales. The El Niño – Southern Oscillation (ENSO) is the leading signal of interannual climate variability that influences the weather and climate of the globe. It has been revealed that during the onset of its different phases, weather and climate anomalies (up to extreme ones) are observed not only over the tropical zone of the Pacific Ocean, but also in extratropical latitudes, which are transmitted through teleconnections in the ocean-atmosphere system. Numerous studies have shown that ENSO modulates tropical cyclone activity in the northwest Pacific Ocean. The purpose of the study is to determine the relationship between sea surface temperature (SST) changes in the equatorial Pacific Ocean and the frequency of tropical cyclones of varying intensity in the northwestern Pacific Ocean under current climatic conditions. Materials and methods of research. The work uses data of the number of cases and intensity of tropical cyclones (TCs) for the period 1991-2020, and series of indices ONI and SOI. Using these data, statistics on the frequency of TCs in the study area were calculated and Pearson correlation coefficients were calculated. The scientific novelty lies in obtaining quantitative characteristics of the relationship between SST and the number of TCs of varying intensity in modern climatic conditions. The practical significance of the work lies in the fact that TCs are associated with the occurrence of severe weather conditions, which, when a TC reaches the densely populated coast of Southeast Asia, can pretend to be significant losses. Therefore, information that can be used in forecasting TCs and their trajectories is very high importance. Research results. Analysis of the frequency of TCs of different intensity in the northwestern Pacific Ocean during the 1991-2020 showed that there was a decrease in the number of tropical depressions and typhoons. However, there has been a slight increase in the occurrence of tropical storms. The results of the analysis of the influence of SST variability in the equatorial Pacific Ocean on the frequency of TCs, showed that there is a statistically significant relationship only with the frequency of typhoons. The establishment of positive SST in the center of the equatorial Pacific Ocean, associated with the El Niño episode, is associated with an increase in typhoons in the northwestern Pacific Ocean. [ABSTRACT FROM AUTHOR]

Marine Heat Waves (MHWs) can cause significant distress to marine environment and modulate air-sea interaction, which in turn can have economic and societal impacts. This study aims to identify surface and subsurface MHWs in the Bay of Bengal using available sea-surface temperature data and buoy observations spanning four decades. The results show significant increase in the number, frequency, duration, and intensity of surface MHWs in recent years. To better understand the relationship between MHW occurrences and different phases of two climate phenomena, namely, the Indian Ocean Dipole (IOD) and the El Niño-Southern Oscillation, multiple extreme MHW event years between 2008 and 2018 are analyzed. The findings show that the surface MHWs become more prominent during El Niño/positive IOD events due to weakened winds, increased net heat flux input towards the ocean, increased stratification and warming tendency through vertical processes in the presence of inversion. This, in turn, affects the surface biological productivity in the region. Additionally, surface MHWs were also found to be driven by surface currents and eddies. On the other hand, subsurface MHWs develop during negative IOD/La Niña conditions due to the deepening of the thermocline, which is forced by coastally trapped downwelling Kelvin waves and reflected downwelling Rossby waves, and is followed by substantial freshening of deeper layers. This study provides a deeper understanding of the causes of surface and subsurface MHWs in the Bay of Bengal, which is a crucial basin in influencing monsoon and cyclonic events affecting its surrounding landmasses. [ABSTRACT FROM AUTHOR]

Tropical cyclones (TC) are one of the deadliest natural meteorological hazards with destructive winds and heavy rains, resulting losses often reach billions of dollars, imposing a substantial and long-lasting burden on both local and national economies. The El-Niño Southern Oscillation (ENSO), a tropical ocean–atmosphere interaction, is known to significantly impact cyclonic systems over global ocean basins. This study investigates the variability of TC activity in the presence of ENSO over the North Indian Ocean (NIO), comprising the Arabian Sea (ARB) and the Bay of Bengal (BOB) basins during the pre- and post-monsoon season, using accumulated cyclone energy (ACE) over the last 29 years. Our analysis reveals a significant rise in tropical cyclone energy intensity over the past two decades, with eight of the ten most active years occurring since the 2000s. Total ACE over the NIO is found to be higher in La-Niña. Higher ACE observed over ARB is strongly associated with a combination of elevated sea surface height (SSH) anomaly and low vertical wind shear during the El-Niño episodes, with higher sea surface temperatures (SST) during the post-monsoon season. Whereas in the BOB, El Niño not only reduces ACE, but also decreases basin-wide variability, and more pronounced effects during the post-monsoon season, coinciding with warmer SST and higher SSH along the coast during La-Niña. [ABSTRACT FROM AUTHOR]

El Niño Southern Oscillation (ENSO) is a significant form of internal climate variability resulting from the interactions between the atmosphere and ocean in the tropical Pacific. It is a main driver of interannual rainfall variability in Northeast Thailand, where rainfed agriculture is one of the largest economic sectors. Therefore, it is essential to understand the ability of climate models to simulate the basic characteristics of ENSO phenomena and project its impacts in the region. We evaluated the ability of 12 climate models from the sixth phase of the Coupled Model Intercomparison Project (CMIP6) and their 12 predecessor models from the fifth phase (CMIP5) to simulate ENSO and its impact on rainfall over Northeast Thailand using thirteen performance metrics under seven criteria considering the simulation of observed indices, pattern, variability, peaks and seasonal phase locking of ENSO. The intensity and frequency of the ENSO events were projected for the near future (2021–2050). Although six CMIP5 and eight CMIP6 models performed well in simulating half of the ENSO evaluation metrics (performance score > 40%) such as ENSO variability, seasonal phase locking, location of ENSO variability, and dominant secondary peak in SST variability, we did not find very compelling evidence of improved ENSO simulation in CMIP6 compared to CMIP5 models. However, high rainfall in extreme La Niña events and low rainfall in extreme El Niño events with rainfall anomalies of 0.4 mm/day and − 0.3 mm/day, respectively were observed over Northeast Thailand due to the interaction of the easterly winds from the Pacific Ocean with the south-westerly flow from the Indian Ocean. The corresponding sea level pressure maps further confirmed the mechanisms associated with this phenomenon over the region. The ensemble averages of models from both phases predicted an increase in intensity (by 32–47%) and frequency (by 10–50%) in the near-future (2021–2050) with a slightly higher increase by CMIP6 models compared to CMIP5 models under medium and high emission scenarios. [ABSTRACT FROM AUTHOR]

This study identifies the relationship between tropical southern Atlantic (TSA) sea surface temperature anomaly (SSTA) and the El Niño‐Southern Oscillation (ENSO) and focuses on how the Pacific Decadal Oscillation (PDO) modulates this relationship. Results suggest a significant but non‐stationary interannual TSA‐ENSO relationship which undergoes a significant decadal shift. A strong TSA‐ENSO relationship is observed during the positive PDO phase, while this relationship is weak during the negative PDO phase. Two processes, involving the anomalous Pacific Walker circulation (PWC) and the intensity of air‐sea interactions over the Pacific, are proposed for this decadal shift. During the positive PDO phase, the weak and variable PWC and strong air‐sea interaction facilitate the development of SSTA in the tropical Pacific triggered by TSA SSTA, resulting in a strong TSA‐ENSO relationship and vice versa. These findings emphasize the important role of the modulation of PDO on the TSA‐ENSO relationship. Plain Language Summary: The present study finds a significant TSA‐ENSO relationship that anomalous warm (cold) TSA sea surface temperature (SST) in the preceding winter is closely linked to the cold (warm) ENSO in the subsequent summer and winter. However, the TSA‐ENSO interannual relationship is non‐stationary, which is strong during the positive PDO phase and weak during the negative PDO phase. Two processes responsible for this non‐stationary relationship are proposed. During the positive PDO phase, the weak and variable PWC is more susceptible to the TSA forcing, resulting in a strong TSA‐ENSO relationship. Meanwhile, the strong air‐sea interaction in the tropical Pacific also facilitates the development of SSTA triggered by TSA SSTA, leading to a strong and robust TSA‐ENSO relationship. During the negative PDO phase, the situations of PWC and air‐sea interaction are opposite and thus lead to a weak TSA‐ENSO relationship. These findings could help to improve our understanding of the decadal variability of the ENSO evolution. Key Points: The relationship between tropical southern Atlantic (TSA) SST and subsequent El Niño‐Southern Oscillation (ENSO) is stronger in positive Pacific Decadal Oscillation (PDO) than in negative PDOIn positive PDO, the TSA SST anomaly easily exerts an impact on ENSO because of a weaker and more variable Walker circulationIn positive PDO, the stronger air‐sea interaction over the tropical Pacific facilitates the maintenance of local SST and wind anomalies [ABSTRACT FROM AUTHOR]

This study investigates the temporal variability of the northeast monsoon rainfall (NEMR) in India and its relationship with El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) using wavelet analysis. The results show that NEMR is highly variable, with a coefficient of variation of about 25%, which is due to its dependence on dynamic systems with a wide range of spatial and temporal scales. The temporal variability of NEMR is at different time scales ranging from intra-annual to interannual to decadal time scales. The large-scale circulation features, ENSO and IOD, also exhibit variability at different time scales. El Niño events have a typical frequency range of 2 to 7 years, with a prolonged impact that can last for several months, spreading across the globe. Wavelet transforms of SOI and Nino 3.4 have revealed variability at the time scale of 2–8 years, which is in line with the observed ENSO cycle. The wavelet transform of DMI displays significant variability in short time scales, typically less than 2–4 years. The study also finds a strong negative relationship between NEMR and SOI, recorded in September, especially over Tamil Nadu, Kerala, and Coastal Andhra Pradesh during the periods 1900–1920 and 1980–2000. This correlation is strong at time scales of 2–4 years and 4–8 years during the periods 1880–1920, possibly related to the ENSO cycle and the oscillation of anomalous SST over the Indian Ocean. The study highlights the need for further studies to understand the physical processes that contribute to the observed variability in El Niño and other ocean–atmosphere interactions. [ABSTRACT FROM AUTHOR]

The El Niño‐Southern Oscillation (ENSO) is a natural climate phenomenon that alters the biogeochemical and physical dynamics of the Eastern Tropical Pacific Ocean. Its two phases, El Niño and La Niña, are characterized by decreased and increased coastal upwelling, respectively, which have cascading effects on primary productivity, organic matter supply, and ocean‐atmosphere interactions. The Eastern Tropical South Pacific oxygen minimum zone is a source of nitrous oxide (N2O), a potent greenhouse gas, to the atmosphere. Here, we present the first study to directly compare N2O sources during opposing ENSO phases using N2O isotopocule analyses. Our data show that during La Niña, N2O accumulation increased six‐fold in the upper 100 m of the water column, and N2O fluxes to the atmosphere increased up to 20‐fold. N2O isotopocule data demonstrated substantial increases in δ18O up to 60.5‰ and decreases in δ15Nβ down to −10.3‰ in the oxycline, signaling a shift in N2O cycling during La Niña compared to El Niño. During El Niño, N2O production was primarily due to ammonia‐oxidizing archaea, whereas during La Niña, N2O production by incomplete denitrification supplemented that from ammonia‐oxidation, with N2O consumption likely maintaining the high site preference values (up to 26.7‰). Ultimately, our results illustrate a strong connection between upwelling intensity, biogeochemistry, and N2O flux to the atmosphere. Additionally, they highlight the combined power of N2O isotopocule analysis and repeat measurements in the same region to constrain N2O interannual variability and cycling dynamics under different climate scenarios. Plain Language Summary: Nitrous oxide is a greenhouse gas that is 300 times more potent than carbon dioxide in its ability to warm the Earth, and it can be produced and consumed through the activities of microorganisms in the ocean. The low‐oxygen waters of the tropical Pacific are a known global "hotspot" for nitrous oxide cycling as the chemical and physical conditions there provide fuel for nitrous oxide production. A natural climate cycle known as the El Niño‐Southern Oscillation, which has a warm and a cold phase, can affect the conditions that raise or lower the speed of nitrous oxide cycling. We used isotopes—small, but measurable natural differences in the masses of molecules and atoms—to understand how nitrous oxide cycling changes during each phase. We found a decrease in nitrous oxide accumulation during the warm phase, and enhanced production during the cold phase. This enhanced production during the cold phase is connected to the release of more nitrous oxide from the ocean to the atmosphere. Key Points: La Niña in the Eastern Tropical South Pacific led to a six‐fold increase in nitrous oxide concentrations compared to El NiñoIsotopocule analysis indicated that incomplete denitrification and archaeal ammonia oxidation both contributed to nitrous oxide accumulationNumerous co‐occurring production and consumption pathways likely explain the nitrous oxide distributions during La Niña [ABSTRACT FROM AUTHOR]

This study investigates the impact of boreal winter–peaked La Niña on the following-summer precipitation in East Asia through intermediate sea surface temperature (SST) anomalies playing the role of relay in observation and numerical models. There are widespread dry conditions in both North and South China and wet conditions in coastal areas of central and eastern China. Such a pattern is mainly attributed to an anomalous low pressure over the western tropical Pacific and an anomalous anticyclone over northeast Asia. It is found that the delayed impact of La Niña on the East Asian climate is operated through intermediate SST anomalies—the Z-shape cold SST anomalies in the tropical North Pacific. There might be three ways for the SST anomalies to operate. First, they produce tropical atmospheric perturbations that can penetrate into the subtropical jet through the westerly trough over the northeast subtropical Pacific, the wave train being then excited along the jet. Second, perturbations created through the monsoon trough over the western Pacific can directly stimulate northward-propagating Rossby waves along the East Asian coast, mainly at low level. And third, perturbations over the tropical Atlantic–northwest Africa can also trigger downstream propagating waves along the subtropical jet. The observation effects of the intermediate SST anomalies and their possible impact mechanisms on atmospheric circulation are largely reproduced within numerical simulations performed with the Community Earth System Model. [ABSTRACT FROM AUTHOR]

Tropical dendrochronology has gained significant attention in recent years, particularly with the dendrochronological study of new species that produce annual growth rings and are responsive to environmental changes. Despite the progress, the extent to which ocean–atmosphere interactions influence regional climate and, consequently, tree growth, is not fully understood. Among the new species, Ocotea porosa (Nees & Mart.) Barroso (also known as Imbuia) has shown excellent potential for climate research. This study investigates the climatic and solar influences on a chronology of 41 Imbuia tree samples. Pearson's correlation was used alongside Wavelet transform to evaluate periodicities between the tree-ring chronology and climatic parameters such as the southern-oscillation index (SOI), annual precipitation, El Niño 3.4 (PACE), and the South Atlantic Index (ATLS). Our analysis revealed evidence of the influence of the El Niño Southern Oscillation (SOI) on rainfall variability in the region, the Hale and Gleissberg solar cycles causing precipitation variation, likely due to the influence of the Atlantic Ocean, and the Brückner-Egeson-Lockyer climatic cycle, which is correlated with sunspot activity. Furthermore, our wavelet analysis identified possible connections to the Eastern Pacific-type El Niño events during five specific periods: 1911–1912, 1918–1919, 1976–1977, 1982–1983, and 1986–1987. The results indicate that southern Brazil is affected by several climatic and geophysical parameters from both the Atlantic and Pacific oceans, which directly affect the growth of Imbuia trees as their tree-ring series display sensitivity to these parameters. [ABSTRACT FROM AUTHOR]

The Kuroshio Extension (KE) is known to undergo large decadal variations, especially after the 1976/77 climate regime shift. Due to strong ocean‐atmosphere interactions over the KE, the region may be a key region for basin‐scale Pacific decadal climate variability. Although baroclinic Rossby waves are suggested to influence the KE decadal variability, it remains unclear where and how decadal atmospheric forcing anomalies that induce the Rossby waves are generated. Using a linear Rossby wave model forced by reanalysis surface wind stress, the relative contribution of wind stress at each longitude is quantified for the first time. It is shown that the contribution is largest at 160°W, and 40% of the total variance in the decadal KE variability is explained by the wind forcing around 170°–150°W. Further analyses suggest that the atmospheric forcing anomalies are generated by the El Niño/Southern Oscillation Modoki through an atmospheric teleconnection after the regime shift. Plain Language Summary: The Kuroshio Extension (KE) is an intense eastward ocean flow in the western North Pacific formed after the Kuroshio separates from the Japanese coast. Satellite altimeter records and reanalysis products constrained by observational data have revealed that the KE strongly fluctuates with the decadal timescale between a stable and an unstable state, characterized by positive and negative sea surface height (SSH) anomalies to the south of the KE axis, respectively. A westward propagating oceanic wave with an SSH anomaly is considered as a primary cause of the phase reversal of the KE decadal variability. However, it has not been quantified to what extent wind forcing at each longitude contributes to the KE decadal variability through the ocean wave. In our study, we find that the largest contribution of wind forcing to the KE decadal variability is at 160°W, and 40% of the total variance in the KE variability is owing to wind forcing around 170°–150°W by simulation of westward propagating oceanic waves forced by reanalysis surface wind stress. Furthermore, it is suggested that the surface wind anomalies at 170°–150°W are remotely forced by a mode of climate variability in the tropical Pacific via an atmospheric bridge. Key Points: Wind forcing at 150°–170°W explains 40% of the total variance in Kuroshio Extension (KE) decadal variability via Rossby wavesThe dominant forcing at 150°–170°W is due to an atmospheric teleconnection of El Niño/Southern Oscillation (ENSO) ModokiThe statistically significant relation between KE decadal variability and ENSO Modoki is found only after the 1976/77 climate regime shift [ABSTRACT FROM AUTHOR]

Drought is one natural disaster with the greatest impact worldwide. Southern Africa (SA) is susceptible and vulnerable to drought due to its type of climate. In the last four decades, droughts have occurred more frequently, with increasing intensity and impacts on ecosystems, agriculture, and health. The work consists of a systematic literature review on the drought regime's characteristics in the SA under current and future climatic conditions, conducted on the Web of Science and Scopus platforms, using the PRISMA2020 methodology, with usual and appropriate inclusion and exclusion criteria to minimize/eliminate the risk of bias, which lead to 53 documents published after the year 1987. The number of publications on the drought regime in SA is still very small. The country with the most drought situations studied is South Africa, and the countries with fewer studies are Angola and Namibia. The analysis revealed that the main driver of drought in SA is the ocean–atmosphere interactions, including the El Niño Southern Oscillation. The documents used drought indices, evaluating drought descriptors for some regions, but it was not possible to identify one publication that reports the complete study of the drought regime, including the spatial and temporal distribution of all drought descriptors in SA. [ABSTRACT FROM AUTHOR]

The positive phase of the Pacific meridional mode (PMM) is closely related to the onset of El Niño. Previous studies have indicated that positive sea surface temperature anomalies (SSTAs) in the central equatorial Pacific (CEP) during the spring and summer of positive PMM years primarily originate from the northeastern tropical Pacific (NETP) via positive wind–evaporation–SST feedback. We review the evolution of PMM and find weak evidence to support such a linkage. Coupled model experiments show that the positive PMM-regressed SSTAs in the NETP only account for ∼24% of those in the CEP from winter to spring, illustrating the principle that correlation does not necessarily mean causality. The strongest positive PMM SSTAs in the NETP and CEP increase El Niño intensity by 1.07°C, whereas that in the NETP alone increase El Niño intensity by 0.69°C. When the composite SSTAs in the NETP during positive PMM years are used, however, the El Niño intensity is increased merely by 0.17°C. The change in the subsurface temperature in the equatorial Pacific is curtailed for the NETP SSTAs to trigger El Niño, while the wind–evaporation–SST feedback plays a less important role. Our results indicate that the impact of PMM on El Niño might be overestimated by ∼55%. Moreover, a comprehensive understanding about the role of the tropical North Pacific on El Niño can be obtained only when the impact from the western North Pacific is considered. [ABSTRACT FROM AUTHOR]

El Niño‐Southern Oscillation (ENSO) triggers variations of the global Hadley circulation (HC), while the latter may potentially feedback to ENSO events. Previous studies mainly investigated the interactions between ENSO and the global zonal‐mean HC. Here, we present a regional perspective of HC variability by introducing zonal variations. Results show that El Niño intensifies the regional HC over the central‐eastern Pacific, while weakening the regional HC over both the Indo‐Pacific warm pool and the tropical Atlantic. The background seasonal cycle modulates the equatorial‐asymmetric component of HC, with an anticlockwise (clockwise) asymmetric circulation over the central equatorial Pacific before (after) El Niño peaks. Remarkably, the asymmetric HC in boreal spring leads ENSO with a lead correlation of up to 0.68, mediated by the wind‐evaporation‐sea surface temperature (SST) feedback and other atmosphere‐ocean dynamics. The antecedent HC anomaly may contribute to ENSO predictability. Plain Language Summary: By decomposing the atmospheric circulation to its irrotational and nondivergent components, we integrated the three‐dimensional structure of the Hadley circulation (HC), and investigate its interactions with El Niño‐Southern Oscillation (ENSO). In particular, we focus on the zonal variation of the HC and its equatorial‐symmetric and asymmetric components associated with ENSO variability. We find that an El Niño event may intensify the HC over the central‐eastern Pacific, but weaken the circulation over the Indo‐Western Pacific and the tropical Atlantic. The intensity of the ENSO‐related asymmetric HC anomaly is as large as that of the symmetric HC. The asymmetric HC anomaly is characterized by a clear seasonal feature. In boreal spring (MAM) before an El Niño signal becomes clear, a strong anticlockwise (asymmetric) HC across the central equatorial Pacific usually lead the El Niño event, with an 8‐month lead correlation with the November‐December‐January (NDJ, the peak season) Niño‐3.4 index of up to 0.68, implying that the MAM asymmetric HC and its associated atmosphere‐ocean interactions may largely contribute to the development of ENSO events. Further analysis indicated that the wind‐evaporation‐sea surface temperature feedback and the Bjerknes feedback play important roles in the interactions between the ENSO events and HC. Key Points: The three‐dimensional structure of El Niño‐related Hadley circulation (HC) anomaly shows a strong zonal variationThe direction of the equatorial‐asymmetric HC associated with El Niño events shows a clear seasonal featureThe asymmetric HC over equatorial central Pacific in spring may contribute to the development of an El Niño event [ABSTRACT FROM AUTHOR]

The late-summer western North Pacific subtropical high (WNPSH) has been known to exhibit a strong lagged connection with the El Niño Southern Oscillation (ENSO) of preceding winter. We find that this lagged relationship has been substantially weakened in the recent decades and that, instead, the WNPSH-ENSO connection has become pronounced during early summer. The processes that lead to the changes in the connection are examined by comparing those of the past (1982–1995) and those of the recent (2006–2020) decades. In the earlier period, the sea surface temperature (SST) and the atmospheric responses associated with the WNPSH show a strong resemblance to the decaying ENSO for both early and late summer. Local air-sea interaction mainly regulates the early-summer WNPSH, whereas the Kelvin wave response to Indian Ocean warming largely contributes to the intensification of the late-summer WNPSH. The latter is more related to the delayed ENSO effect, and the strengthening of this remote effect may bring a high correlation between the late-summer WNPSH and ENSO. In contrast, during the recent period, the regional Hadley circulation, associated with the contrasting SST anomalies over the maritime continent (MC) and tropical Central Pacific (TCP), plays a role in maintaining the WNPSH. However, the ENSO-induced SST pattern accompanies the WNPSH only during early summer, while the SST anomalies over the MC and TCP in late summer are too weak to influence the variability of the WNPSH. [ABSTRACT FROM AUTHOR]

This study reveals that the impact of the spring North Pacific meridional mode (PMM) on the following-winter El Niño–Southern Oscillation (ENSO) shows a continuing increase in the past. A comparative analysis is conducted for the high- and low-correlation periods to understand the factors for the strengthened impact of the PMM. The spring PMM-related sea surface temperature (SST) and atmospheric anomalies over the subtropical northeastern Pacific propagate southwestward to the tropical central Pacific via wind–evaporation–SST feedback in the high-correlation period. The tropical SST and atmospheric anomalies further develop to an ENSO-like pattern via positive air–sea interaction. In the low-correlation period, SST and atmospheric anomalies over the subtropical northeastern Pacific related to the PMM cannot extend to the deep tropics. Therefore, the spring PMM has a weak impact on ENSO. The extent to which the PMM-related SST and atmospheric anomalies extend toward the tropics is related to the background flow. The stronger mean trade winds in the high-correlation period lead to an increase in the air–sea coupling strength over the subtropical northeastern Pacific. As such, the spring PMM-related SST and atmospheric anomalies can more efficiently propagate southwestward to the tropical Pacific and exert stronger impacts on the succeeding ENSO. In addition, the southward shifted intertropical convergence zone in the high-correlation period also favors the southward extension of the PMM-related SST anomalies to the tropics and contributes to a stronger PMM–ENSO relation. The variation and its formation mechanism of the spring PMM–winter ENSO relationship appear in both the observations and the long historical simulation of Earth system models. Significance Statement: The North Pacific meridional mode (PMM) is the leading atmosphere–ocean coupling pattern over the subtropical northeastern Pacific after removing the ENSO variability, with maximum variance during boreal spring. Previous studies indicated that the PMM plays an important role in relaying the impact of the atmosphere–ocean forcings over the extratropics on the tropical ENSO. This study reveals that the impact of the spring PMM on the following winter ENSO shows a continuing increase in the past 70 years. The physical mechanisms for this strengthened impact are further examined. Results obtained in this study have important implications for improving the prediction of the tropical ENSO variability. [ABSTRACT FROM AUTHOR]

As an important external forcing, the effect of the 11-yr solar cycle on the tropical Pacific decadal variability is an interesting question. Here, we systematically investigate the phase-locking of the atmosphere and ocean covariations to the solar cycle in the tropical Pacific and propose a new mechanism to explain these decadal covariations. In both observation/reanalysis datasets and a solar cycle forced sensitivity experiment (named the SOL experiment), the ocean heat content anomalies (OHCa; 300 m) resemble a La Niña–like pattern in the solar cycle ascending phase, and the Walker circulation shifts westward. In the declining phase, the opposite is true. The accumulative solar irradiation directly contributes to this coherent decadal variability via changing the warm water volume and the solar-related heat is redistributed by the ocean dynamic processes. During the 11-yr solar cycle, the Pacific Walker circulation anomalies maintain the OHCa in the western equatorial Pacific and work as negative feedback for the eastern Pacific to help the OHCa phase transition. In addition, oceanic meridional heat transport via the subtropical cells and the propagation of off-equatorial Rossby waves also provide a lagged negative feedback to the OHCa phase transition according to the 11-yr solar cycle. The decadal coupled responses of the tropical Pacific climate system are 2 years more lag in the SOL experiment than in the observation/reanalysis. Significance Statement: Here, we propose a new mechanism that the heating effect of the accumulative solar irradiation during the 11-yr solar cycle can be "integrated" into the tropical Pacific OHC and then provide a bottom-up effect on the atmosphere at decadal time scales. The strongly coupled processes in this region amplify the decadal phase-locking of the covariations to the 11-yr solar cycle. Our study demonstrates the role of the 11-yr solar cycle in the tropical Pacific decadal variability and provides a new explanation for the "bottom-up" mechanism of the solar cycle forcing. Our results update the understanding of the tropical Pacific decadal variability and may help to improve climate predictions at decadal time scales. [ABSTRACT FROM AUTHOR]

Despite an insignificant linear relation between the boreal summer equatorial Atlantic sea surface temperature anomaly (EA SSTA) and preceding winter El Niño–Southern Oscillation (ENSO), an EA warming event was found to be mostly preceded by either phase of ENSO. Physical mechanisms of this asymmetric impact of El Niño and La Niña on EA warming were investigated through observational and modeling analyses. For the El Niño–induced EA warming group, the El Niño–related SSTA induces a warm SSTA in the South Atlantic during developing fall and winter via the Pacific–South American (PSA) pattern. The induced extratropical SSTA extends to the equator through local air–sea interaction processes and favors EA warming development. In contrast, for the La Niña–induced EA warming group, there is no significant SSTA in the South Atlantic from developing fall to winter. It is not until La Niña decaying spring that a significant westerly anomaly is generated and thus an EA warming through the remote Gill-type response. The asymmetric South Atlantic response is attributed to the stronger (weaker) heating strength over the central Pacific and Maritime Continent for El Niño (La Niña). The distinctive evolutions of El Niño (fast transition) and La Niña (long persistence) also contribute to the asymmetric EA response. The persistent La Niña makes a stronger equatorial Atlantic response during ENSO decaying spring. Our results suggest that the South Atlantic SSTA and ENSO temporal evolution are important in explaining the asymmetric impacts of the El Niño and La Niña on EA warming. Furthermore, the Indian Ocean SSTA is also suggested to contribute to the asymmetric impacts. [ABSTRACT FROM AUTHOR]

A prominent change occurred in the relationship between August Indochina Peninsula (ICP) precipitation and tropical Pacific sea surface temperature (SST) around 1980. More August ICP precipitation falls in La Niña developing years during 1959–1979, but in El Niño developing years during 1983–2003. The relation change is associated with a change in the evolution of equatorial Pacific SST anomalies. During 1959–1979, negative SST anomalies develop in the equatorial eastern Pacific (EEP) in early spring and extend westward. They cause anomalous southwesterly and reduce surface latent heat flux (LHF) over the tropical western North Pacific (WNP), inducing positive SST anomalies there in early summer. Negative SST anomalies form in the tropical South Indian Ocean (SIO) due to LHF increase attributed to a local air-sea interaction process in preceding winter and a remote forcing of negative EEP SST anomalies in late spring-early summer. More August ICP precipitation is due to anomalous ascent induced by a Rossby wave response to positive WNP SST anomalies and a cross-equatorial vertical circulation due to negative SIO SST anomalies. During 1983–2003, positive SST anomalies form in the equatorial central Pacific (ECP) in late spring and extend eastward. Negative SST anomalies develop in the tropical southeastern Indian Ocean (SEIO) due to sustained LHF increase induced by both preceding winter negative SST anomalies and late spring-early summer positive SST anomalies in the EEP. More August ICP precipitation is induced by a Rossby wave response to positive ECP SST anomalies and a cross-equatorial vertical circulation due to negative SEIO SST anomalies. [ABSTRACT FROM AUTHOR]

The southern Bay of Bengal (BOB) cold pool (SCP) plays an important role in the regional climate fluctuation of the BOB. However, the interannual variability in the SCP is still unknown. Multisource satellite remote sensing data and assimilation have been applied to explore the interannual variability in the SCP and its relationship with El Niño–Southern Oscillation (ENSO) events for the period 1982–2020. The anomalous SST of the SCP in the summer following the peak phase (i.e., winter) of the ENSO was closely related to the ENSO events. El Niño (La Niña)-induced the warm (cold) anomaly of the SCP starting from May and persisted throughout August with a peak value appearing in June during the El Niño (La Niña) decaying years. In the El Niño decaying years, the southwest monsoon current (SMC) was weakened, forced locally by the weakening southwesterly wind and remotely by the easterly wind anomaly at the equator associated with El Niño. The El Niño-related weakening SMC and the associated less cold advection led to the warm anomaly of the SCP. In addition, El Niño-related atmospheric heating also made a comparable contribution to the evolution of the SCP's SST. In the early stage (15 May to 10 June), its contribution to the warming of the SCP was much larger than that of the SMC, whereas from mid-June to August, it reversed to have a cooling effect and partially offset the advection heating induced by the SMC on the SCP. In the La Niña decaying years, similar oceanic and atmospheric processes operated but in an opposite way. [ABSTRACT FROM AUTHOR]

Following triple La Niña events during 2020–2022, the future evolution of climate conditions over the tropical Pacific has been a focused interest in ENSO-related communities. Observations and modeling studies indicate that an El Niño event is occurring in 2023; however, large uncertainties remain in terms of its detailed evolution, and the factors affecting its resultant amplitude remain to be understood. Here, a novel deep learning-based Transformer model is adopted to make real-time predictions for the 2023–2024 climate conditions in the tropical Pacific. Several key fields vital to the El Niño and Southern Oscillation (ENSO) in the tropical Pacific are collectively and simultaneously utilized in model training and in making predictions; therefore, this purely data-driven model is configured in both training and predicting procedures such that the coupled ocean-atmosphere interactions are adequately represented. Also similar to dynamic models, the prediction procedure is executed in a rolling manner to allow ocean-atmosphere anomaly exchanges month by month; the related key fields during multi-month time intervals (TIs) prior to prediction target months are taken as input predictors, serving as initial conditions to precondition the future evolution more effectively. Real-time predictions indicate that the climate conditions in the tropical Pacific are surely to develop into an El Niño state in late 2023. Furthermore, sensitivity experiments are conducted to examine how prediction skills are affected by the input predictor specifications, including TIs during which information on initial conditions is retained for making predictions. A comparison with other dynamic coupled models is also made to demonstrate the prediction performance for the 2023–2024 El Niño event. [ABSTRACT FROM AUTHOR]

This study investigates the roles of water vapor transport and sea surface temperature (SST) warming in the tropical Indian Ocean (TIO) on the heavy rainfall in central China during the boreal early summer (May–June–July). In the past four decades, four significant rainfall events, in 1983, 1998, 2016, and 2020, occurred in central China and caused severe floods, and the year 2020 has the most extreme event. All four events are associated with significant TIO SST warming and a strong anomalous anticyclone on the western North Pacific (WNPAC). The anomalous winds in the northwestern flank of the WNPAC bring excess water vapor into central China. The water vapor, mainly carried from the western tropical Pacific, converges in central China and result in heavy rainfall. A theory of regional ocean–atmosphere interaction can well explain the processes, called the Indo-Western Pacific Ocean Capacitor (IPOC) effect. The WNPAC is usually associated with strong El Niño-Southern Oscillation (ENSO), except for the 2020 case. The 2020 event is extraordinary without a significant El Niño occurred in the previous winter. In 2020, the significant TIO warming sustained the anomalous WNPAC, inducing the most significant extreme rainfall event in central China. This study reveals that the IPOC effect can dramatically influence the East Asian climate even without involving the ENSO in the Pacific. [ABSTRACT FROM AUTHOR]

Boreal summer intraseasonal oscillation (BSISO) strongly interacts with background mean fields and tends to be stronger and longer in its northward propagation during La Niña than El Niño summers. However, BSISO dependence on El Niño‐Southern Oscillation phase has not been well understood. Here, we show that BSISO‐related convections are stronger and more organized with northward propagation on 30–60‐day timescales during El Niño developing (E‐DV) than decaying (E‐DC) summers over the western Pacific. The easterly vertical shear, responsible for the next vorticity generation north of the BSISO convection, in the E‐DV (E‐DC) remarkably increases (decreases) due to the enhanced (weakened) upper‐level easterly anomaly. The air‐sea interaction in E‐DV gets stronger due to the larger meridional gradient of sea surface temperature, amplifying northward propagation than in E‐DC. Plain Language Summary: In boreal summer, convective activity over the western Pacific on the intraseasonal time scale is primarily located to the north of the equator and exhibits characteristics of propagating northward. The northward propagation of convective activity can modulate the subtropical variability and typhoon activity and has significant impacts on the extreme weather and climate in Asia. The intensity and propagation of the convective activity are considerably influenced by the El Niño‐Southern Oscillation. In this study, we find that the northward propagation of convective activity over the western Pacific is significantly strengthened by easterly wind shear related with the positive vorticity generation as well as air‐sea interaction associated with the meridional gradient of the warm Sea Surface Temperature anomalies in the El Niño developing summers than in the decaying summers. Better understanding the mechanisms of the complicated convective activity is the key to reduce the climate‐related socio‐economic risks and disasters by improving our capability for accurate prediction of BSISO and the related risks. Key Points: Convective activity and northward propagation of boreal summer intraseasonal oscillation (BSISO) in El Niño developing summers are stronger and more distinct than decaying summersDynamical and air‐sea interaction processes leading to northward propagation of BSISO are more active in El Niño developing summersKey factors driving the phase dependence are the increase in upper‐level easterly and meridional Sea Surface Temperature (SST) gradient during developing summers [ABSTRACT FROM AUTHOR]

Maritime Continent (MC) rainfall plays an important role in global climate variability, but its prediction remains extremely challenging. Based on a long-term state-of-the-art hindcast product recently completed by the authors' group, this work investigates the decadal variation of the MC rainfall predictability in the wet season for the first time. The prediction skills were relatively high before 1940 and after 1980, but relatively low between these years. In a diagnostic analysis of the controlling factors of the decadal variation, the signal strength represented by the variance of the rainfall variability was identified as the dominant factor. Further analysis concluded that the phases of El Niño–Southern Oscillation (ENSO) are the key controlling sources. The MC rainfall was more predictable during periods dominated by El Niño events than during periods dominated by La Niña events because El Niño elicits stronger ocean–atmosphere interactions in the tropics, providing a stronger signal of MC rainfall than La Niña events. [ABSTRACT FROM AUTHOR]

Indian Ocean Dipole (IOD) is the major interannual ocean-atmosphere interaction phenomena in the Indian Ocean (IO) and is influenced by external forcing such as El Nino Southern Oscillation (ENSO). Meanwhile, its co-occurrence and stronger relationship with ENSO have decreased during recent decades. The IOD variability also reduced after 2000, accompanied by a shift of the western pole to central IO extending further southward. The study reports that the boreal fall (SON, September, October, November) season IOD has an intensified relationship with the previous winter Subtropical Indian Ocean dipole (SIOD) and spring season equatorial north tropical Atlantic (NTA) SST anomalies., while the ENSO relationship is reduced from its pre-2000 value. It is found that the persistent warming (cooling) in the western side of positive (negative) SIOD during the previous winter induces easterly (westerly) wind anomalies in the equatorial IO during the following summer and fall, leading to positive (negative) IOD events. These IOD events have the western pole shifted to south-central IO instead of the canonical northwestern warming. The spring season NTA SST anomalies induce stronger summer season circulation and SST gradient in the equatorial Pacific, like ENSO. During the SON season, this pattern is associated with cooling and easterly wind anomalies in the tropical eastern IO and IOD. These two patterns explain the major mode of IOD variability after 2000. While the IOD associated with NTA have co-occurring ENSO during summer and fall, the SIOD-induced IOD events are independent of ENSO in the Pacific. Thus, these two predictors provide long-lead predictability (2–3 seasons ahead) of IOD for both ENSO co-occurring and non-ENSO IOD events. However, the IOD predictability of seasonal prediction models mainly depends on the ENSO-IOD relationship, resulting in reduced IOD skills for many of them after 2000. The models such as COLA-CCSM4, which has improved skill have stronger than observed ENSO-IOD relationship than the pre-2000 period. A linear regression model including SIOD and NTA SST indices of the previous winter and spring season respectively as predictors simulates IOD with a skill of around 0.65 during the recent period, indicating the necessity of seasonal prediction models to capture the variability in the southern IO and NTA and their teleconnections for better prediction of IOD in the recent period of reduced ENSO skill. [Display omitted] • ENSO predictability and ENSO-IOD relationship weakened in the recent decades. • Subtropical IOD in the previous winter strongly influence ENSO-independent IOD. • North tropical Atlantic SST in boreal spring induces IOD through Pacific modification. • Subtropical IOD and North Atlantic SST role in IOD is prominent after 2000. • A model with these two predictors induces IOD skill of above 0.65 at 2-3 seasons ahead. [ABSTRACT FROM AUTHOR]

With their complex narrow passages and vigorous mixing, the Indonesian seas provide the only low-latitude pathway between the Pacific and Indian Oceans and thus play an essential role in regulating Pacific-Indian Ocean exchange, regional air-sea interaction, and ultimately, global climate phenomena. While previous investigations using remote sensing and numerical simulations strongly suggest that this mixing is tidally driven, the impacts of monsoon and El Niño Southern Oscillation (ENSO) on tidal mixing in the Indonesian seas must play an important role. Here we use high-resolution sea surface temperature from June 2002 to June 2021 to reveal monsoon and ENSO modulations of mixing. The largest spring-neap (fortnightly) signals are found to be localized in the narrow passages/straits and sills, with more vigorous tidal mixing during the southeast (boreal summer) monsoon and El Niño than that during the northwest (boreal winter monsoon) and La Niña. Therefore, tidal mixing, which necessarily responds to seasonal and interannual changes in stratification, must also play a feedback role in regulating seasonal and interannual variability of water mass transformations and Indonesian throughflow. The findings have implications for longer-term variations and changes of Pacific–Indian ocean water mass transformation, circulation, and climate. [ABSTRACT FROM AUTHOR]

We examine the projected changes in Indian summer monsoon rainfall (ISMR) and El Niño Southern Oscillation (ENSO) along with their changing relationships by the end of 21st century using Coupled Model Inter Comparison Project Phase5 (CMIP5) models. The historical (1951–2005) and future (2050–2099) projections of ISMR were assessed in addition to the changes in the ENSO–ISMR relationship during both the periods. The future period was selected as 2050–2099 in order to understand how the increase in greenhouse gas emissions and global warming by the end of 21st century will affect the ocean atmosphere interactions over the tropical Pacific Ocean and the associated rainfall characteristics over the Indian region. For the future projection, we considered Representative Concentration Pathways (RCP) 4.5 and 8.5 scenarios for the five models (CNRM‐CM5, GISS‐E2‐R, GISS‐E2‐R‐CC, HadGEM2‐CC and IPSL‐CM5A‐MR), which were selected from the 25 CMIP5 models based on their ability to capture the seasonal cycle and variability in the historical period. Although, all the five models simulate an increase in ISMR in both the RCPs, the increase is well simulated by CNRM‐CM5, HadGEM2‐CC and IPSL‐CM5A‐MR models mainly over the northeast, west coast, foothills of Himalayas, central India and the peninsular Indian regions. Substantial differences in simulating the ENSO–ISMR relationship are observed among the models in both historical and future RCP scenarios, and the observed relationship is well represented in GISS‐E2‐R‐CC, HadGEM2‐CC and IPSL‐CM5A‐MR models. The rainfall over most of the Indian regions shows a significant decrease (increase) during El Niño (La Niña) events in RCP 4.5. The changes in sea surface temperature (SST) anomalies and Walker circulations over the Indo‐Pacific region during El Niño and La Niña events are connected with the ISMR characteristics during these events in the RCP 4.5 scenario. On the other hand, the Indo‐Pacific region undergoes total warming (cooling) during El Niño (La Niña) events, and the Indo‐Pacific SST does not exhibit significant variability in the RCP 8.5 scenario. Hence, they are not well related with the observed ISMR patterns in that scenario. [ABSTRACT FROM AUTHOR]

The Central Pacific (CP) and Eastern Pacific (EP) types of El Niño‐Southern Oscillation events are distinctly different in terms of their forming locations and mechanisms. Using an updated physical‐base statistical forecast model, the forecast of CP indices has overall higher skills than that of EP indices due to the higher persistent forecast skill. At lead time beyond 6‐month, the predictability of CP indices can be greatly enhanced by including the extratropical precursor from the North Pacific, confirming the ocean‐atmosphere interaction associated with North Pacific Oscillation (NPO)/Victoria Mode (VM) evolution. The predictability of EP indices can be moderately enhanced by including the extratropical precursor from the south, resulting from the charging of the equatorial Pacific. We also find that the predictability of CP indices does not significantly decrease after 2000 while the predictability of EP indices drops dramatically, suggesting the strengthening impacts of NPO/VM on the CP‐ENSO in the recent two decades. Plain Language Summary: The predictability of different types of El Niño‐Southern Oscillation (ENSO) events can be significantly enhanced by including extratropical signals using a physical‐base statistical forecast model. At a lead time longer than 6‐month, the predictability of Central Pacific ENSO can be greatly enhanced by including the extratropical signal from the North Pacific, confirming a well‐known ocean‐atmosphere interaction process proposed earlier. The predictability of Eastern Pacific ENSO can be moderately enhanced by including the extratropical precursor from the south, resulting from the charging of the equatorial Pacific. We also find that the predictability of CP indices does not significantly decrease after 2000 while the predictability of EP indices drops dramatically, suggesting the enhanced north hemispheric impacts in the recent two decades. Key Points: Both predictabilities of Central Pacific (CP)/Eastern Pacific (EP) El Niño‐Southern Oscillation (ENSO) can be greatly enhanced by including the Pacific extratropical precursorsThe distinct roles of North and South Pacific precursors on the CP and EP ENSO, respectively, are clarifiedThe stable/decreased predictability of CP/EP indices after 2000 confirms the recent enhanced NPO‐relevant impacts [ABSTRACT FROM AUTHOR]

The interannual and interdecadal variabilities of Indonesian rainfall in dry seasons (June-November) are investigated by using rainfall data from the Climate Research Unit (CRU) from 1939 to 2016 and from the Global Precipitation Climatology Project (GPCP) from 1979 to 2016. The first principal component (PC1) of both the CRU and GPCP data shows that the canonical El Niño-Southern Oscillation (ENSO), ENSO Modoki, and Indian Ocean Dipole (IOD) are major climate modes influencing the interannual variability of rainfall in Indonesia, and the Interdecadal Pacific Oscillation (IPO) is the major decadal phenomenon affecting the decadal variability of the rainfall. Furthermore, the IPO modulates the influence of IOD on Indonesian rainfall, with a weaker influence during the positive IPO phase during 1979 - 1997 and a stronger influence during the negative IPO phases during 1939 - 1978 and 1998 - 2016. The dependency of Indonesian rainfall response to the canonical ENSO and ENSO Modoki on IPO phases is not significant, although the response to the ENSO Modoki (canonical ENSO) becomes significant (less significant) in the negative IPO phase during 1998 - 2016 when compared with earlier periods. [ABSTRACT FROM AUTHOR]

Atlantic Niño is the Atlantic equivalent of El Niño-Southern Oscillation (ENSO), and it has prominent impacts on regional and global climate. Existing studies suggest that the Atlantic Niño may arise from local atmosphere-ocean interaction and is sometimes triggered by the Atlantic Meridional Mode (AMM), with overall weak ENSO contribution. By analyzing observational datasets and performing numerical model experiments, here we show that the Atlantic Niño can be induced by the Indian Ocean Dipole (IOD). We find that the enhanced rainfall in the western tropical Indian Ocean during positive IOD weakens the easterly trade winds over the tropical Atlantic, causing warm anomalies in the central and eastern equatorial Atlantic basin and therefore triggering the Atlantic Niño. Our finding suggests that the cross-basin impact from the tropical Indian Ocean plays a more important role in affecting interannual climate variability than previously thought. The Atlantic Niño is an important mode of tropical Atlantic variability that influences the climate conditions in surrounding areas. Here, the authors use observational data and model simulations to show that positive phases of the Indian Ocean Dipole can trigger Atlantic Niño events. [ABSTRACT FROM AUTHOR]

The Victoria mode (VM) is the second dominant sea surface temperature mode in the North Pacific, forced by North Pacific Oscillation–like extratropical atmospheric variability. Observational studies have shown that the boreal spring VM is closely connected to the following winter El Niño, with the VM efficiently acting as a precursor signal to El Niño events. This study evaluates the relationship of the spring VM with subsequent winter El Niño in the preindustrial simulations of phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP5 and CMIP6). We found that most CMIP5 and CMIP6 models can simulate the basic characteristics of the VM reasonably well. The current CMIP6 models simulate the VM–El Niño connections more realistically as compared to the earlier CMIP5 models. The analysis further suggests that the improved capability of the CMIP6 models to simulate the VM–El Niño relationship is because the CMIP6 models are better able to capture the VM-related surface air–sea thermodynamic coupling process over the subtropical/tropical Pacific and the seasonal evolution of VM-related anomalous subsurface ocean temperature in the equatorial Pacific. [ABSTRACT FROM AUTHOR]

The North Pacific Oscillation (NPO), a primary atmospheric mode over the North Pacific Ocean in boreal winter, is known to trigger El Niño–Southern Oscillation (ENSO) in the following winter, the process of which is recognized as the seasonal footprinting mechanism (SFM). On the basis of the analysis of model simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5), we found that the SFM acts differently among models, and the correlation between the NPO and subsequent ENSO events, called the SFM efficiency, depends on the background mean state of the model. That is, SFM efficiency becomes stronger as the climatological position of the Pacific intertropical convergence zone (ITCZ) moves poleward, representing an intensification of the northern branch of the ITCZ. When the Pacific ITCZ is located poleward, the wind–evaporation–sea surface temperature (SST) feedback becomes stronger as the precipitation response to the SST anomaly is stronger in higher latitudes than that in lower latitudes. In addition, such active ocean–atmosphere interactions enhance NPO variability, favoring the SFM to operate efficiently and trigger an ENSO event. Consistent with the model results, the observed SFM efficiency increased during the decades in which the northern branch of the climatological ITCZ was intensified, supporting the importance of the tropical mean state of precipitation around the Pacific ITCZ. [ABSTRACT FROM AUTHOR]

The surface heat flux anomalies during El Niño events have always been treated as an atmospheric response to sea surface temperature anomalies (SSTAs). However, whether they play roles in the formation of SSTAs remains unclear. In this study, we find that the surface net heat flux anomalies in different El Niño types have different effects on the development of the spatial pattern of SSTAs. By applying the fuzzy clustering method, El Niño events during 1982–2018 are classified into two types: 1) extreme El Niños with strong positive SSTAs, with the largest SSTAs in the eastern equatorial Pacific, and 2) moderate El Niños with moderate positive SSTAs, with the largest SSTAs in the central equatorial Pacific. The surface net heat flux anomalies in extreme El Niños generally display a "larger warming gets more damping" zonal paradigm, and essentially do not impact the formation of the spatial pattern of SSTAs. Those in moderate El Niños, however, can impact the formation of the spatial pattern of SSTAs by producing more damping effects in the eastern than in the central equatorial Pacific, thus favoring the largest SSTAs being confined to the central equatorial Pacific. More damping effects of net heat flux anomalies in the eastern equatorial Pacific in moderate El Niños are contributed by the surface latent heat flux anomalies, which are mainly regulated by the negative relative humidity–SST feedback and the positive wind–evaporation–SST feedback. Therefore, we highlight that these two atmospheric adjustments should be considered during the development of moderate El Niños in order to obtain a comprehensive understanding of the formation of El Niño diversity. [ABSTRACT FROM AUTHOR]

Super El Niño has been a research focus since the first event occurred. On the basis of observations and models, we propose that a super El Niño emerges if El Niño is an early-onset type coincident with the distribution of an Atlantic Niña (AN) in summer and a positive Indian Ocean dipole (IOD) in autumn, conditions referred to as the Indo-Atlantic Booster (IAB). The underlying physical mechanisms refer to three-ocean interactions with seasonality. Early onset endows super El Niño with adequate strength in summer to excite wind-driven responses over the Indian and Atlantic Oceans, which further facilitate IAB formation by coupling with the seasonal cycle. In return, IAB alternately produces additional zonal winds U over the Pacific Ocean, augmenting super El Niño via the Bjerknes feedback. Adding AN and IOD indices into the regression model of U leads to a better performance than the single Niño-3.4 model, with a rise in the total explained variances by 10%–20% and a reduction in the misestimations of super El Niños by 50%. Extended analyses using Coupled Model Intercomparison Project models further confirm the sufficiency and necessity of early onset and IAB on super El Niño formation. Approximately 70% of super El Niños are early-onset types accompanied by IAB and 60% of early-onset El Niños with IAB finally grow into extreme events. These results highlight the super El Niño as an outcome of pantropical interactions, so including both the Indian and Atlantic Oceans and their teleconnections with the Pacific Ocean will greatly improve super El Niño prediction. [ABSTRACT FROM AUTHOR]

The impact of the El Niño‐Southern Oscillation (ENSO) on the extratropics is investigated in an idealized, reduced‐order model that has a tropical and an extratropical module. Unidirectional ENSO forcing is used to mimick the atmospheric bridge between the tropics and the extratropics. The variability of the coupled ocean‐atmosphere extratropical module is then investigated through the analysis of its pullback attractors (PBAs). This analysis focuses on two types of ENSO forcing generated by the tropical module, one periodic and the other aperiodic. For a substantial range of the ENSO forcing, two chaotic PBAs are found to coexist for the same set of parameter values. Different types of extratropical low‐frequency variability (LFV) are associated with either PBA over the parameter ranges explored. For periodic ENSO forcing, the coexisting PBAs exhibit only weak nonlinear instability. For chaotic forcing, though, they are quite unstable and certain extratropical perturbations induce transitions between the two PBAs. These distinct stability properties may have profound consequences for extratropical climate predictions: in particular, ensemble averaging may no longer help isolate the LFV signal. Plain Language Summary: The authors have investigated the variability of a simplified coupled ocean‐atmosphere model for the Earth's midlatitudes, subject to the influence of the El Niño‐Southern Oscillation (ENSO). This study reveals that multiple climates may coexist, each of which is characterized by distinct types of low‐frequency variability (LFV) and predictability properties. When the ENSO forcing is periodic, these climates are fairly robust against perturbations, but when it is chaotic, small perturbations induce transitions between the different climates. These properties could have profound consequences for extratropical climate predictions, since ensemble averaging may no longer be a valid approach to ascertain the LFV signal. Key Points: Two pullback attractors (PBAs) coexist in the model's midlatitudes for both periodic and chaotic El Niño‐Southern Oscillation (ENSO) forcingThese local PBAs are nonlinearly unstable, with some trajectories that visit both of themThe ENSO forcing synchronizes the midlatitude behavior in unexpected ways [ABSTRACT FROM AUTHOR]

The complex interaction between the Indian Ocean dipole (IOD) and El Niño–Southern Oscillation (ENSO) is further investigated in this study, with a focus on the impacts of the IOD on ENSO in the subsequent year [ENSO(+1)]. The interaction between the IOD and the concurrent ENSO [ENSO(0)] can be summarized as follows: ENSO(0) can trigger and enhance the IOD, while the IOD can enhance ENSO(0) and accelerate its demise. Regarding the impacts of IOD(0) on the subsequent ENSO(+1), it is revealed that the IOD can lead to anomalous SST cooling patterns over the equatorial Pacific after the winter following the IOD, indicating the formation of a La Niña–like pattern in the subsequent year. While the SST cooling tendency associated with a positive IOD is attributable primarily to net heat flux (thermodynamic processes) from autumn to the ensuing spring, after the ensuing spring the dominant contribution comes from oceanic processes (dynamic processes) instead. From autumn to the ensuing spring, the downward shortwave flux response contributes the most to SST cooling over the central and eastern Pacific, due to the cloud–radiation–SST feedback. From the ensuing winter to the ensuing summer, changes in latent heat flux (LHF) are important for SST cooling, indicating that the release of LHF from the ocean into the atmosphere increases due to strong evaporation and leads to SST cooling through the wind–evaporation–SST feedback. The wind stress response and thermocline shoaling verify that local Bjerknes feedback is crucial for the initiation of La Niña in the later stage. [ABSTRACT FROM AUTHOR]

The loss of autocorrelations of tropical sea surface temperatures (SST) during late spring, also called the spring predictability barrier (SPB), is a factor that strongly limits the predictability of El Nino Southern Oscillation (ENSO), and especially the statistical SST‐based ENSO forecasts starting from the winter‐spring season. Recent studies show that Pacific atmospheric circulation anomalies in winter‐spring may have a long‐term impact on the summer tropical climate via the SST footprint. Here, we infer an index based on sea level pressure (SLP) data from February to March in a single area surrounding Hawaii, and show that this area is the most informative part of the large SLP pattern initiating the SST footprinting mechanism. We then construct a statistically optimal linear model of the Nino 3.4 index taking this atmospheric index as a forcing. We find that this forcing efficiently lowers the SPB and provides significant improvements of interseasonal Niño 3.4 forecasts. Plain Language Summary: Interseasonal forecasting of El Niño Southern Oscillation (ENSO) is in high demand due to the impacts of ENSO on regional climatic conditions around the world as well as the global climate. Improvements in the quality of climate data in recent decades have led to the active use of statistical ENSO models, which compete with physical models in predictive power. The main disadvantage of statistical forecasts is the pronounced seasonal growth of uncertainty when predicting the upcoming summer‐fall ENSO conditions from winter‐spring months; this phenomenon is called the spring predictability barrier (SPB). A number of recent works revealed that winter‐spring atmospheric anomalies can substantially impact the ENSO system through the SPB via a complex atmosphere‐ocean interaction mechanism. Here, we introduce a reliable ENSO predictor constructed from sea level pressure data relating to this mechanism and show that the predictor significantly improves the multimonth (up to 1 year) ENSO forecast by lowering the SPB in a statistical model of the key ENSO index. Key Points: A novel early El Nino Southern Oscillation (ENSO) predictor based on the February–March sea level pressure is introducedSignificant correlations of the predictor with the upcoming summer – next spring ENSO conditions are shownThe predictor significantly improves the interseasonal forecast skills of the statistical Nino 3.4 index model [ABSTRACT FROM AUTHOR]

Research on Atmospheric Rivers (ARs) has focused primarily on AR (thermo)dynamics and hydrological impacts over land. However, the evolution and potential role of nearshore air‐sea fluxes during landfalling ARs are not well documented. Here, we examine synoptic evolutions of nearshore latent heat flux (LHF) during strong late‐winter landfalling ARs (1979–2017) using 138 overshelf buoys along the U. S. west coast. Composite evolutions show that ARs typically receive upward (absolute) LHF from the coastal ocean. LHF is small during landfall due to weak air‐sea humidity gradients but is strongest (30–50 W/m2 along the coast) 1–3 days before/after landfall. During El Niño winters, southern‐coastal LHF strengthens, coincident with stronger ARs. A decomposition of LHF reveals that sea surface temperature (SST) anomalies modulated by the El Niño Southern Oscillation dominate interannual LHF variations under ARs, suggesting a potential role for nearshore SST and LHF influencing the intensity of landfalling ARs. Plain Language Summary: Atmospheric Rivers (ARs) are elongated streams of enhanced water vapor transport, contributing to a substantial fraction of total wintertime precipitation and extreme streamflow events along the U.S. west coast. Thus, better understanding the processes contributing to the intensity of landfalling ARs is of broad scientific and societal interests. Considerable efforts have been directed at their meteorological structures and hydrological impacts over land, but it remains unclear if and how the severity of landfalling ARs is influenced by their interactions with coastal oceans. Here, we use in situ near‐surface atmospheric and sea surface temperature measurements from 138 overshelf buoys along the U.S. west coast to characterize variations of latent heat flux during strong late‐winter landfalling ARs from 1979 to 2017. We find that the coastal ocean is an important heat and moisture source for ARs in the days before landfall. During El Niño events, the oceanic heat and moisture input to the atmosphere increases along the southern coast, coincident with more intense ARs. Further analysis shows that surface ocean temperature anomalies related to El Niño Southern Oscillation dominantly affect latent heat flux during landfalling ARs. Our results suggest a potential role for the coastal ocean in influencing the intensity of landfalling ARs. Key Points: Atmospheric Rivers (ARs) experience upward latent heat flux (LHF) over the coastal ocean, strongest 1–3 days before and after landfallIn El Niño winters, LHF during ARs is enhanced along the southern coast and reduced along the northern coast by ∼70% relative to La NiñaLHF decomposition reveals sea surface temperature variability via ENSO as a dominant contributor to interannual coastal LHF variations [ABSTRACT FROM AUTHOR]

Compound weather and climate events, which may cause devastating impacts on economic and social development, have become major threats in the twenty‐first century, especially to countries with complex climate and terrain like China. A better understanding of the driving factors of compound weather and climate events is essential for reducing their impacts, which remains overall lacking. In this study, we evaluate the potential influence of large‐scale circulation patterns on compound dry and hot events (CDHEs) during summer for the period 1921–2016 in mainland China using the composite analysis. Furthermore, the logistic regression model and correlation analysis are also used to confirm the result. The Atlantic Multidecadal Oscillation is found to affect CDHEs for about 18.52% of mainland China, followed by North Atlantic Oscillation and Pacific Decadal Oscillation accounting for about 14.64% and 12.96%, respectively. El Niño–Southern Oscillation affects CDHEs for small areas (about 5.27%). In general, regions with significant impacts of these indices from the three methods are overall consistent (mainly shown in northern parts of mainland China) with relatively larger areas estimated from the correlation analysis. These results are expected to be helpful for understanding occurrences of CDHEs and mitigating their negative impacts in China. Key Points: Compare impacts of climate indices on compound dry and hot events using different methodsInfluence from Atlantic Multidecadal Oscillation (AMO) and North Atlantic Oscillation (NAO) is mainly shown in northern parts of mainland ChinaInfluence from AMO is shown in larger areas than that from the other three climate indices [ABSTRACT FROM AUTHOR]

Analysis of observational data and a long control simulation of the Community Earth System Model, version 1 (CESM1), shows that El Niño events developing in boreal spring to early summer usually terminate after peaking in winter, whereas those developing after summer tend to persist into the second year. To test the predictability of El Niño duration based on the onset timing, perfect model predictions were conducted for three El Niño events developing in April or September in the CESM1 control simulation. For each event, 30-member ensemble simulations are initialized with the same oceanic conditions in the onset month but with slightly different atmospheric conditions and integrated for 2 years. The CESM1 successfully predicts the termination of El Niño after the peak in 95% of the April-initialized simulations and the continuation of El Niño into the second year in 83% of the September-initialized simulations. The predictable component of El Niño duration arises from the initial oceanic conditions that affect the timing and magnitude of negative feedback within the equatorial Pacific, as well as from the Indian and Atlantic Oceans. The ensemble spread of El Niño duration, on the other hand, originates from surface wind variability over the western equatorial Pacific in spring following the peak. The wind variability causes a larger spread in the September-initialized than the April-initialized ensemble simulations due to weaker negative feedback in spring. These results indicate potential predictability of El Niño events beyond the current operational forecasts by 1 year. [ABSTRACT FROM AUTHOR]

The Asian–Australian monsoon (AAM) exerts a strong impact on the regional climate. The ability of climate models to simulate the AAM is of great importance. This study evaluates the performance of 26 models, which participate in the Coupled Model Intercomparison Project phase 5 (CMIP5), in representing features of the leading modes of the AAM interannual variability. This is achieved by diagnosing quantitative metrics derived from a Season‐reliant Empirical Orthogonal Function (S‐EOF) analysis, and plausible external forcing contributing to the simulation results are presented. Our results show that the CMIP5 models can generally capture the spatial patterns of the leading modes of AAM interannual variability during El Niño development periods because of strong air–sea interactions. The variance percentage explained by the first two S‐EOF modes is found to be closely related to the amplitude of the El Niño‐Southern Oscillation (ENSO) in the models. In contrast, simulation of the periodicity remains challenging, and the models show deficiencies in the accurate capture of periodic characteristics. Models with realistic ENSO simulations typically perform well in simulating S‐EOF1, and the key to improving the simulation of S‐EOF2 is the greater intensity of the simulated field of background heating over the Northwest Pacific warm pool. The multimodel ensemble (MME) performs better than most of the 26 individual models. Models in the MME with better ENSO simulation and stronger warm pool heating are found to be better able to represent the relationships between the first two leading S‐EOF modes and ENSO, although biases remained. [ABSTRACT FROM AUTHOR]

In contrast to the temporal evolution of forecast ensemble mean (signal) and spread (noise) in an ensemble of seasonal forecasts, the spatial patterns of signal and noise components for sea surface temperature (SST) predictions have not been analyzed. In this work, we examine the leading patterns of signal and noise components of SST forecasts by the National Centers for Environmental Prediction Climate Forecast System version 2. It is noted that the leading empirical orthogonal function pattern of SST is similar between the signal and the noise with maximum loading in the central and eastern tropical Pacific associated with El Niño–Southern Oscillation (ENSO) variability. The similarity implies that while some members of the forecasts predict a stronger (weaker) ENSO than others, the dominant pattern of SST anomalies from all members still resembles the ENSO SST pattern. This reflects the notion that for each forecast ensemble member, the evolution of ENSO is governed by the similar air–sea coupled interactions, the strength of which, however, differs due to unpredictable noise. On the other hand, the leading mode of the signal and the noise are found temporally independent. Thus, it is concluded that although the largest variability in the signal and the noise is spatially collocated, their temporal evolution is independent. [ABSTRACT FROM AUTHOR]

The Indian summer monsoon rainfall (ISMR) during June to September contributes most of the annual rainfall over India and plays an important role in Indian agriculture and thus the economy. It exhibits high spatio-temporal variabilities forced from both internal and external factors, which are important for better understanding and prediction of ISMR. Since the internal factors, mainly in the form of intraseasonal oscillations set a limit to the predictability, the major focus is given to the external forcing factors including the coupled air–sea interactions, sea surface temperature variations, snow cover, etc. This paper mainly aims to review the results of recent research analysis on ISMR variability and the major climate factors that determine the variability. Focus is given on the contributions from the coupled ocean–atmosphere processes in the Indian and Pacific Oceans to the ISMR variability [(primarily the El Niño Southern Oscillation (ENSO)] and Indian Ocean Dipole (IOD). Several studies were carried out in recent decades to explore the ISMR variabilities and their influences from tropical oceans. The studies, which focused the impact of ENSO and IOD on the ISMR variability have been considered in exploring their relationships and observed changes in recent decades. In the backdrop of varying relationship of ISMR with ENSO and IOD in the regional scale, it is important to study further the regional teleconnection of ISMR variabilities with oceanic factors, especially from the Indian and Pacific Ocean basin. [ABSTRACT FROM AUTHOR]

Wave direction and wave power are the main climate‐related drivers of large‐scale coastal morphological hazards on open sandy coastlines. El Niño‐Southern Oscillation (ENSO) is the main climate driver affecting global wave climates on interannual timescales. Hence, a quantification of wave climate variability with ENSO would aid the characterization of coastal risk as a first‐pass assessment tool. However, no such data are available on the global scale at present. To address this, this work (1) quantifies ENSO impacts on global mean wave direction and wave power, and (2) classifies the global coast according to the ENSO wave climate hazard. The correlation coefficient (R) of the Multivariate ENSO Index between wave power and mean wave direction was calculated. Then, employing a multivariate technique, the resulting R values were used to classify the global coastlines. The results show that ENSO has distinct latitudinal impacts around the world. El Niño increases the wave power of swells generated in the extra‐tropics which propagate toward the tropics, a phenomenon which is particularly notable in the Pacific Basin, while the most significant rotations in wave direction with ENSO occur at tropical and subtropical latitudes. Based on this new information, the global coastline was classified into five distinct types, depending on the directional wave climate response to ENSO. Plain Language Summary: High ocean waves can cause erosion and flooding of coastal regions. Wave climate—the term used to describe the characteristics of ocean waves at a certain location—varies on timescales of seasons to decades. This variability can increase or reduce the amount of wave power received at the coast, affecting the probability of coastal flooding or erosion. One of the principal drivers of long‐term wave climate variability is El Niño‐Southern Oscillation (ENSO). This work undertakes a global analysis of wave climate response to El Niño and La Niña events along the world's coastlines. Results show that wave power is most increased/decreased during El Niño/La Niña at high latitudes (extra‐tropical zone), while wave direction is most impacted at low latitudes (tropical zone). As the wave climate response to ENSO is different around the world, the global coast was classified into five types, a criterion that can be used for regional coastal planning and management. Key Points: El Niño increases the wave power of swells generated in the extra‐tropics which propagate toward the tropicsThe most significant rotations in wave direction with El Niño‐Southern Oscillation (ENSO) occur at tropical and subtropical latitudesThe global coastline was classified into five distinct types, depending on the directional wave climate response to ENSO [ABSTRACT FROM AUTHOR]

Mooring measurements at ;1408E in the western equatorial Pacific Ocean documented greatly intensified eastward subsurface currents, which largely represent the nascent Equatorial Undercurrent, to ~67 cm s-1 in boreal summer of 2016. The eastward currents occupied the entire upper 500m while the westward surface currents nearly disappeared. Historical in situ data observed similar variations after most El Niño events. Further analysis combining satellite and reanalysis data reveals that the eastward currents observed at ~1408E are a component of an anomalous counterclockwise circulation straddling the equator, with westward current anomalies retroflecting near the western boundary and feeding southeastward current anomalies along the New Guinea coast. A 1.5-layer reduced-gravity ocean model is able to crudely reproduce these variations, and a hierarchy of sensitivity experiments is performed to understand the underlying dynamics. The anomalous circulation is largely the delayed ocean response to equatorial wind anomalies over the centralto- eastern Pacific basin emerging in the mature stage of El Niño. Downwelling Rossby waves are generated by the reflection of equatorial Kelvin waves and easterly winds in the eastern Pacific. Upon reaching the western Pacific, the southern lobes of Rossby waves encounter the slanted New Guinea island and deflect to the equator, establishing a local sea surface height maximum and leading to the detour of westward currents flowing from the Pacific interior. Additional experiments with edited western boundary geometry confirm the importance of topography in regulating the structure of this cross-equatorial anomalous circulation. [ABSTRACT FROM AUTHOR]

The effect of the Pacific Decadal Oscillation (PDO) phase on the intensity and localization of anomalies in sea surface temperature and meteorological parameters associated with two types of El Niño is analyzed. The analysis is based on reanalysis and CMIP5 model data for modern and future climate (RCP 8.5 scenario). It is found that El Niño does not exhibit robust intensification during the positive PDO phase, as mentioned in the previous studies, but the PDO influence is highly dependent on the El Niño type. The canonical El Niño is intensified during the positive PDO phase, while, on the contrary, El Niño Modoki weakens. In future climate the interaction between PDO and El Niño is modified, but the significant intermodel spread does not allow an unambiguous determination of trends. [ABSTRACT FROM AUTHOR]