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El Niño–Southern Oscillation (ENSO) is the leading mode of interannual ocean–atmosphere coupling in the tropical Pacific, greatly influencing the global climate system. Seasonal phase locking, which means that ENSO events usually peak in boreal winter, is a distinctive feature of ENSO. In model future projections, the ENSO sea surface temperature (SST) amplitude in winter shows no significant change with a large intermodel spread. However, whether and how ENSO phase locking will respond to global warming are not fully understood. In this study, using Community Earth System Model Large Ensemble (CESM-LE) projections, we found that the seasonality of ENSO events, especially its peak phase, has advanced under global warming. This phenomenon corresponds to the seasonal difference in the changes in the ENSO SST amplitude with an enhanced (weakened) amplitude from boreal summer to autumn (winter). Mixed layer ocean heat budget analysis revealed that the advanced ENSO seasonality is due to intensified positive meridional advective and thermocline feedback during the ENSO developing phase and intensified negative thermal damping during the ENSO peak phase. Furthermore, the seasonal variation in the mean El Niño–like SST warming in the tropical Pacific favors a weakened zonal advective feedback in boreal autumn–winter and earlier decay of ENSO. The advance of the ENSO peak phase is also found in most CMIP5/6 models that simulate the seasonal phase locking of ENSO well in the present climate. Thus, even though the amplitude response in the winter shows no model consensus, ENSO also significantly changes during different stages under global warming. [ABSTRACT FROM AUTHOR]

The North Pacific Oscillation (NPO) is an important intrinsic atmospheric pattern over the North Pacific. Observations have shown that the boreal winter NPO is a crucial precursor to the El Niño-Southern Oscillation (ENSO), with many ENSO events being preceded by the NPO. It is therefore imperative to assess the ability of current coupled climate models to reproduce the relationship between winter NPO and the subsequent winter ENSO. Previous studies have shown that most coupled climate models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) underestimate the influence of winter NPO on the subsequent winter ENSO. Simulations from CMIP6, representing the latest generation of climate models, are now available. This study shows a remarkable improvement of the CMIP6 models in simulating the influence of winter NPO on the subsequent ENSO development compared to the CMIP5 models. This improvement is due to the improved air-sea interaction over the subtropical North Pacific in CMIP6. The enhanced air-sea interaction over the subtropical North Pacific promotes the equatorward propagation of the NPO-induced wind and SST anomalies, resulting in enhanced surface zonal wind anomalies over the tropical western Pacific, which further exert a stronger influence on the subsequent winter ENSO development by triggering the tropical Bjerknes positive air-sea interaction. The enhanced subtropical air-sea interaction is thought to be related to a southward shift of the North Pacific intertropical convergence zone in CMIP6 compared to CMIP5. [ABSTRACT FROM AUTHOR]

As the primary interannual signal of variability in the tropical ocean-atmosphere interaction, the El Niño-Southern Oscillation has a considerable impact on tropical cyclone (TC) activity over the western North Pacific (WNP). Both 2018 and 2021 were La Niña decay years, but TC activity over the WNP during the two summers (June–August) showed notable differences. In 2018, summer TC activity was unusually high with a total of 18 TCs, and the region of TC genesis was mainly in the central and eastern WNP. In contrast, only 9 TCs were generated in summer 2021, and the region of TC genesis was primarily in the western WNP. By comparing the characteristics of the large-scale environmental conditions over the regions of TC genesis, the thermal factors of the tropical oceans, and the activity of the Madden-Julian Oscillation (MJO), this study revealed the possible causes for the marked differences in TC genesis over the WNP during the two summers, which both had a similar background of La Niña decay. The Indian Ocean Basin Mode (IOBM) transitioned of a cold anomaly in the winter of 2017/2018 and persisted until summer 2018. At the same time, the Pacific Meridional Mode (PMM) maintained a positive phase, leading to eastward and northward displacement of the Western Pacific Subtropical High in summer, and eastward extension of the tropical monsoon trough, which presented conditions conducive to TC genesis over the Northwest Pacific. Moreover, the days when the MJO stagnated in phases 5 and 6 in the summer of 2018 increased by approximately 150% relative to climatological state, providing dynamic conditions favorable for TC formation. In 2021, the IOBM quickly turned to a warm anomaly in March and persisted until summer, whereas the PMM became a negative phase in January and remained so until summer. At the same time, the MJO stagnated in phases 2 and 3 for up to 47 days, with the center of convection located over the western Maritime Continent, producing conditions unconducive to TC genesis over the Northwest Pacific. Thus, despite being under a similar background of La Niña decaying year, the distinct evolutions of the IOBM, PMM, and MJO in spring and summer of 2018 and 2021 were the main causes of the notable differences in TC activity over the WNP during these two summers, and the anomalies in IOBM and MJO contributed more significantly than those of the PMM. [ABSTRACT FROM AUTHOR]

The Pacific meridional mode (PMM) is one of the dominant coupled modes in the northeastern tropical Pacific (NETP), characterized by strip-like sea surface temperature (SST) anomalies spanning from Baja California to the central equatorial Pacific. While the majority of the El Niño events follow a positive PMM, only a few La Niña events are preceded by a negative PMM. Such an asymmetric activity of PMM before the onset of El Niño–Southern Oscillation (ENSO) was previously attributed to the inherent nonlinear response of the wind–evaporation–SST (WES) feedback to trade winds in the NETP. Through data analysis and coupled model experiments, we pointed out that PMM is in fact a highly symmetric phenomenon, and the asymmetry of PMM before the ENSO onset thus must be associated with ENSO. On the one hand, the nonlinear response of deep convection over the equator to symmetric ENSO forcing in the central equatorial Pacific permits a stronger Pacific–North America (PNA) pattern in El Niño years than in La Niña years. On the other hand, since the majority of La Niña events are preceded by a sharp decay of an El Niño, the warm equatorial SST anomalies associated with the preceding El Niño provide another source to trigger PNA before the La Niña onset. The two mechanisms modulate the trade winds and heat fluxes in NETP more heavily before the La Niña onset than the El Niño onset and equally contribute to PMM asymmetry before the ENSO onset. [ABSTRACT FROM AUTHOR]

The present study uses reanalysis data to investigate how the East Asian cold surges are linked to the initiation of westerly wind anomalies prior to El Niño events and easterly wind anomalies prior to La Niña events in the western equatorial Pacific. Our analysis shows that East Asian cold surge events can be associated with the formation of an anomalous anticyclone that moves eastward across the North Pacific and cools the ocean surface prior to El Niño events. The outflow of the anomalous anticyclonic circulation is responsible for the initiation of westerly wind anomalies in the western equatorial Pacific. We also find that East Asian cold surge events are involved in the development of an anomalous cyclone that moves northeastward along the subpolar North Pacific and reinforces the horizontal temperature gradient prior to La Niña events. The cross-equatorial flow into the anomalous cyclone in the North Pacific contributes to the development of easterly wind anomalies in the western equatorial Pacific. We show that air-sea interactions can play a role in the propagation of anomalous circulations across the North Pacific, eventually initiating zonal wind anomalies in the western equatorial Pacific. [ABSTRACT FROM AUTHOR]

The super long La Niña phenomenon, which has an extremely long duration, like the recent 2020–2023 La Niña event, is less concerned than the super El Niño. In this study, we identify five super long La Niña events after 1950 and investigate roles of the 2–3-year quasi-biennial (QB) and 3–7-year low-frequency (LF) ocean–atmosphere coupled processes of El Niño–Southern Oscillation (ENSO), and the interdecadal background in forming the large-scale prolonged negative sea surface temperature anomalies (SSTAs) in the central to eastern equatorial Pacific during these events. We group the five events into the thermocline-driven type (the 1983–1986 and 1998–2002 events) and the wind-driven type (the 1954–1957, 1973–1976, and 2020–2023 events). The former inherited a sufficiently discharged state of equatorial upper-ocean heat content from the preceding super El Niño and dominated by the thermocline feedback, leading to a LF oceanic dynamical adjustment to support the maintenance of negative ENSO SSTAs. The latter were promoted by the relatively more important zonal advective feedback and Ekman pumping feedback and deeply affected by a strongly negative equatorial zonal wind stress background state that sourced from the strong negative phase of the Interdecadal Pacific Oscillation. Besides, the QB ENSO variability with casual contributions during these events is less important. Results show that both the LF ENSO variability and the interdecadal Pacific background could assist in generating prolonged La Niñas. [ABSTRACT FROM AUTHOR]

Knowledge of spring warm pool in the Bay of Bengal (BoBWP) is key for further understanding the climate variability in this region and beyond, but little is known about the BoBWP climatological state and the related mechanisms. In this study, we investigate the spatial structure and evolution of the BoBWP using daily Optimum Interpolation SST data from 1982-2022 in combination with multi-source data. Our analysis shows that the BoBWP is located in the central bay (6°-13°N) with a thickness around 20m~ 40 m. Composite analysis indicates that the BoBWP emerges in early April, peaks in early May and dissipates in early June. During the developing period, the net heat flux dominates the formation of spring warm pool through significant air-sea coupling processes, and induces the warming rate of 0.27°C/10d in the mixed layer, which is far larger than the contribution of oceanic dynamical processes (0.01°C/10d). During the decaying period, the net heat flux also plays a dominant role, with a cooling rate of -0.21°C/10d, meanwhile ocean dynamical processes contribute to the cooling of the warm pool with a rate of -0.01°C/10d. Additionally, the SST and the area of the BoBWP are significantly correlated with ENSO (r=0.66 and 0.73, p=0.05). During El Niño decaying year, the BoBWP primarily expands in a southward direction, with a 75% increase in area. Conversely, during La Niña decaying year, the BoBWP almost disappears, with a 52% decrease. [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.

The Pacific/North American (PNA) pattern is a major low‐frequency variability in boreal winter. A recent modeling study suggested that PNA variability increases through extratropical atmosphere‐ocean coupling, but the effect was not fully extracted due to a particular experimental design. By comparing coupled and two sets of uncoupled large‐ensemble global model simulations, here we show that the PNA‐induced horseshoe‐shaped sea‐surface temperature (SST) anomaly in the North Pacific returns a non‐negligible influence on the PNA itself. Its magnitude depends on the presence or absence of atmosphere‐ocean coupling. The coupling accounts for ∼16% of the PNA variance, while the horseshoe‐shaped SST anomaly explains only 5% under the uncoupled condition. The coupling reduces the damping of available potential energy by modulating turbulent heat fluxes and precipitation, magnifying the PNA variance. Precipitation processes in the extratropics as well as tropics are therefore important for realistically representing PNA variability and thereby regional weather and climate. Plain Language Summary: Atmospheric flow is not entirely random; patterns of circulation variability appear recurrently in the same regions, known as teleconnection patterns. A major wintertime teleconnection pattern over the North Pacific‐North American sector is called the Pacific/North American (PNA) pattern. It causes strong fluctuations in precipitation, air temperature, and pressure over North America through persistent strengthening or meandering of the jet stream. While the influence of tropical ocean variability, such as El Niño/La Niña, on the formation and persistence of the PNA has been known, the role of the extratropical ocean remains unclear. Here we perform a vast number of numerical model simulations to detect the influence of the extratropical ocean on PNA. We show that the atmosphere‐ocean coupling (two‐way interaction between the ocean and atmosphere) enhances the PNA variability (i.e., the magnitude of meandering and strengthening of the westerlies) compared to the uncoupled condition. Furthermore, we propose possible mechanisms behind this enhancement. The findings of this study are expected to contribute to improving the accuracy of long‐term forecasts, such as one‐month predictions, and reducing uncertainty in future climate change projections through the improvement of numerical models. Key Points: Extratropical air‐sea coupling enhances the variance of the Pacific/North American (PNA) pattern, which explains 16% of the total varianceThe enhancement is due to the reduced damping of available potential energy through modulations of turbulent heat fluxes and precipitationAtmosphere‐only simulation is likely to underestimate the impact of extratropical sea surface temperature anomalies on the PNA variability [ABSTRACT FROM AUTHOR]

The zonal wind stress () over the equatorial Pacific plays a vital role in the air-sea interaction associated with the El Niño-Southern Oscillation (ENSO). The equatorial westerly wind stress (w ) anomalies during the El Niño (EN) decay phase propagate eastward and decrease rapidly, while the equatorial easterly wind stress (e ) anomalies during the La Niña (LN) decay phase do not display obvious zonal movement and remain near the dateline. The cause of the asymmetry in anomalies and its contribution to ENSO asymmetric decays are investigated in this study. The difference in anomalies is due to the interaction between the atmospheric response to sea surface temperature anomalies (SSTAs) and the forcing of the wind field on subsurface temperature. The results of numerical experiments demonstrate that the e anomalies during the LN decay phase have a strong and prolonged suppressive effect on the zonal advective feedback and thermocline feedback, resulting in the weakness of the delayed negative feedback. However, due to the eastward propagation and rapid decay of w anomalies, the suppressive effect during the EN decay phase is weak and favors its rapid decay. Previous studies have extensively analyzed the asymmetric response of the atmosphere to SSTAs. The difference in zonal displacement of air-sea couplings during ENSO decay phases leads to the decay asymmetry between EN and LN through weakening the negative air-sea feedback during LN, in which our results highlight that the zonal propagation asymmetry of anomalies plays a crucial role. [ABSTRACT FROM AUTHOR]

El Niño-Southern Oscillation (ENSO) is the strongest interannual climate mode influencing the coupled ocean-atmosphere system in the tropical Pacific, and numerous dynamical and statistical models have been developed to simulate and predict it. In some simplified coupled ocean-atmosphere models, the relationship between sea surface temperature (SST) anomalies and wind stress (τ) anomalies can be constructed by statistical methods, such as singular value decomposition (SVD). In recent years, the applications of artificial intelligence (AI) to climate modeling have shown promising prospects, and the integrations of AI-based models with dynamical models are active areas of research. This study constructs U-Net models for representing the relationship between SSTAs and τ anomalies in the tropical Pacific; the UNet-derived τ model, denoted as τUNet, is then used to replace the original SVD-based τ model of an intermediate coupled model (ICM), forming a newly AI-integrated ICM, referred to as ICM-UNet. The simulation results obtained from ICM-UNet demonstrate their ability to represent the spatiotemporal variability of oceanic and atmospheric anomaly fields in the equatorial Pacific. In the ocean-only case study, the τUNet-derived wind stress anomaly fields are used to force the ocean component of the ICM, the results of which also indicate reasonable simulations of typical ENSO events. These results demonstrate the feasibility of integrating an AI-derived model with a physics-based dynamical model for ENSO modeling studies. Furthermore, the successful integration of the dynamical ocean models with the AI-based atmospheric wind model provides a novel approach to ocean-atmosphere interaction modeling studies. [ABSTRACT FROM AUTHOR]

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

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]

Studies have shown that the springtime Arctic Oscillation (AO) has a strong influence on the subsequent Indian Ocean Dipole (IOD) and El Niño-Southern Oscillation (ENSO). Nevertheless, not every AO is followed by IOD and ENSO. This study divides the spring AO into three types: followed by both IOD and ENSO (AO-Both), followed by ENSO only (AO-ENSO), and followed by IOD only (AO-IOD). In the AO-Both type, the Pacific component of the AO, which is characterized by a dipole pattern, induces sea surface temperature (SST) anomalies in the subtropical North Pacific that extend southward to the tropical Pacific via a wind-evaporation-SST (WES) feedback. The generated tropical SST anomalies influence subsequent ENSO via tropical air-sea interaction and impact autumn IOD via modulation of the tropical Walker circulation. In the AO-ENSO type, the Pacific component of the AO is featured by a mono-pole anomaly pattern with a center over the mid-latitude northeast Pacific. The northeast Pacific atmospheric anomalies propagate southward and lead to surface zonal wind anomalies in the tropical central Pacific via the WES feedback that influence subsequent ENSO via tropical air-sea interaction. In the AO-IOD type, the Pacific component of the AO is weak. Spring AO-related atmospheric heating anomalies over the North Atlantic excite a wave train extending from the North Atlantic to the western Pacific. The associated low-level wind anomalies induce SST anomalies around the Maritime Continent, which play a crucial role in influencing subsequent IOD. This study suggests that the spatial pattern of the AO plays a critical role in determining its impact on tropical climate variability. [ABSTRACT FROM AUTHOR]

The present study investigates the lead relationship of the April–May (AM) Southern Annular Mode (SAM) with the northern Australia rainfall (NAR) variability in the subsequent December–February (DJF). The AM SAM was revealed to exhibit a considerable negative relationship with the NAR in the following DJF, which was still significant after removing the possible influence of El Niño–Southern Oscillation (ENSO). Further analysis revealed that the AM SAM can trigger the dipole sea surface temperature anomalies (SSTA) in the southern Indian Ocean through air-sea interactions. The dipole SSTA persists across the following months from June to September, and then it evolves to form the dipole SSTA at locations to the east of previous definitions of the Subtropical Indian Ocean Dipole from October to February of the following year. Based on the circulation anomalies, the AM SAM-related dipole SSTA in DJF is associated with an eastward shift of the Walker circulation between the tropical eastern Indian Ocean and the western Pacific Ocean, resulting in upward (downward) motion over the Indian (Pacific) Ocean and easterly surface wind anomalies across northern Australia. The anomalous easterlies are part of a localized anticyclonic circulation centred over the study region, which indicates diminished moisture influx into the northern parts of the continent, resulting in dry conditions over northern Australia. Besides, model results based on the preceding AM SAM and September ENSO indicate enhanced predictive skill of the NAR associated with combined indices compared to either single index. [ABSTRACT FROM AUTHOR]

The boreal winter North Pacific Oscillation (NPO) has a significant impact on the tropical Pacific large scale air-sea interaction in the following spring and summer. But it is unclear whether the boreal winter NPO is associated with the El Niño development or decaying during following summer. This study confirms the relationship between the winter NPO-like sea level pressure anomalies and following summertime El Niño and reveals an interdecadal change in this relationship. It tends to promote the development of sea surface temperature anomaly in central equatorial Pacific in the following summer when the subtropical lobe of the NPO located west of its average position. But after a significant eastward shift of NPO's subtropical lobe, it is more conducive to the development of sea surface temperature anomaly in eastern equatorial Pacific. The eastward shift of the NPO's subtropical lobe altered both the seasonal foortprint mechanism and the trade wind charging mechanism associated with the NPO, thus profoundly influenced the summertime El Niño. The findings in the present study have implications for a better understanding of the relationship between the boreal winter NPO and the El Niño in developing summer. [ABSTRACT FROM AUTHOR]

Previous studies have indicated that the extratropics can influence ENSO via specific processes. However, it is still unclear to what extent ENSO is influenced by the extratropics in observation. Now we assess this issue by applying the regional data assimilation (RDA) approach in an advanced model, the GFDL CM2.1. Our study confirms a strong extratropical impact on observed ENSO. Quantitatively, the extratropical atmospheric variability poleward of 20° explains 56% of the observed variance of ENSO and greatly influences ∼67% of observed El Niño events during 1969–2008. This extratropical impact is still significant even as far as poleward of 30°. Furthermore, the impact from the southern extratropics is slightly stronger than that from the northern extratropics, partly caused by the Pacific ITCZ location north of the equator and different mixed-layer depth along the northern Pacific meridional mode (NPMM) and the southern Pacific meridional mode (SPMM). Our study further shows that all of three super El Niño events, those in 1972/73, 1982/83, and 1997/98, are influenced greatly by both hemispheric extratropics, with NPMM and SPMM interfering constructively, while most weak and moderate El Niño events are triggered by only one hemispheric extratropics, with NPMM and SPMM interfering destructively. Besides the extratropical Pacific influence on ENSO via NPMM/SPMM, the extratropics also has a potential impact on ENSO by influencing other tropical oceans and then by interbasin interactions. [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]

Understanding the variability in the Hadley circulation (HC) changes is crucial for understanding ocean‐atmosphere interactions. In this study, the variability in the boreal winter HC in the last 4 decades is explored using multiple reanalyzes and model simulations. The results show that regime shift occurred in the leading mode of HC variability. The primary mode of the recent HC is dominated by an equatorially symmetrical pattern, which was considered the second mode in previous studies. The regime shift in HC variability is mainly due to the El Niño–Southern Oscillation (ENSO), which explains both the spatiotemporal variation and formation of HC variability. Moreover, the abilities of the models to reproduce HC variability is subject to their ability to simulate ENSO variability, suggesting that the ENSO has become a more important modulator of the HC variability in recent decades, and additional research is warranted to evaluate future climate changes and potential effects on the HC. Plain Language Summary: The Hadley circulation (HC) has significant regulatory impacts on tropical and extratropical interactions. As a thermal‐dynamical circulation, the variability in the HC exhibits a substantial correlation with the underlying thermal conditions. However, strong interdecadal variations have been observed in the tropical sea surface temperature (SST) around 1976/1977, particularly in the SST related to the El Niño–Southern Oscillation (ENSO). The HC variability after 1977 is still unknown. We investigate the spatial‐temporal variation in the boreal winter HC during the period of 1980–2020. The results show that the primary dominant mode (EOF1) of the HC presents an equatorially symmetrical structure with ascension around the equator. This pattern corresponds to the second dominant mode in preceding studies. The different result indicates that a regime shift in HC variability has occurred. We find that the formation of EOF1 is due to the enhanced variation in ENSO, which explains the spatial‐temporal variations in EOF1. This result is further confirmed in model simulations, highlighting a more important role of the ENSO on the HC in recent decades. These results deepen our understanding of HC variability and emphasize the important climatic effects of the ENSO, which is of great interest because more severe ENSO events are projected for the future. Key Points: The principal mode of boreal winter Hadley circulation (HC) during recent 4 decades shows a regime shift and is dominated by an equatorially symmetrical patternThe strengthened El Niño–Southern Oscillation (ENSO) explains the spatiotemporal variation and regime shift of the HC variabilityModel simulations indicate that their ability to reproduce the ENSO variability is a key factor in determining HC variability [ABSTRACT FROM AUTHOR]

El Niño can be categorized into long and short decaying cases in terms of the lengths of decaying phases. We find that the difference between long and short decaying El Niño emerges as early as the decaying spring, rather than the decaying summer mentioned in previous studies. Observational evidence suggests that this difference corresponds to the different positions of the spring Aleutian low (AL). Further analysis using 15 Coupled Model Intercomparison Project phase 6 (CMIP6) model simulations suggests that the AL that extends southeastward into the northeastern subtropical Pacific contributes to the long decaying El Niño. The northeastern subtropical Pacific AL, with an anomalous southwesterly on the southeast edge, is associated with warming in situ, which further induces cyclonic circulation anomalies over the eastern tropical Pacific as Gill's response. The anomalous westerly at the equator along the southern side of the cyclonic circulation anomalies is related to positive SST anomalies over the central and eastern tropical Pacific and therefore the persistence of El Niño in the decaying spring. On the other hand, the AL restricted to the North Pacific contributes to anticyclonic circulation anomalies over the central Pacific and favors the decline of El Niño through the seasonal footprint mechanism. Moreover, the models that reproduce many more long (short) decaying El Niño cases tend to simulate the AL southeastward extension (restriction over the North Pacific) during the El Niño decaying spring. The results imply that the AL position in the El Niño decaying spring could be a precursor to the length of El Niño decaying phase. [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]

Prediction of El Niño–Southern Oscillation (ENSO) is hindered by a spring predictability barrier (SPB). In this paper, we investigate the effects of the Indian Ocean (IO) on the SPB. Using a seasonally varying extended IO–ENSO recharge oscillator model, we find that the SPB is much weakened when IO is coupled with ENSO. To gauge the relative role of the Indian Ocean dipole (IOD) and the Indian Ocean Basin (IOB) modes in weakening ENSO SPB, we develop an empirical dynamical model, the linear inverse model (LIM). By coupling/decoupling the IOB or IOD mode with ENSO, we show that the IOB significantly weakens eastern Pacific and central Pacific ENSO SPBs, while the IOD plays a weaker role. The evolution of the optimum initial structures also illustrates the importance of the IOB in ENSO SPB. Moreover, the IOB strongly influences the forecast skill of La Niña SPB rather than El Niño SPB. This point is also identified through six coupled models from the North American multimodel ensemble. It may be related to the role of the IO in the asymmetry in the duration of El Niño and La Niña. The IOB-induced easterly wind anomalies are conducive to the development of La Niña and thus the prediction of La Niña events, whereas these anomalous easterlies are less important during the development of El Niño and the related forecast of El Niño events. [ABSTRACT FROM AUTHOR]

High-resolution oceanic precipitation estimates are needed to increase our understanding of and ability to monitor ocean–atmosphere coupled processes. Satellite multisensor precipitation products such as IMERG provide global precipitation estimates at relatively high resolution (0.1°, 30 min), but the resolution at which IMERG precipitation estimates are considered reliable is coarser than the nominal resolution of the product itself. In this study, we examine the ability of the Rainfall Autoregressive Model (RainFARM) statistical downscaling technique to produce ensembles of precipitation fields at relatively high spatial and temporal resolution when applied to spatially and temporally coarsened precipitation fields from IMERG. The downscaled precipitation ensembles are evaluated against in situ oceanic rain-rate observations collected by passive aquatic listeners (PALs) in 11 different ocean domains. We also evaluate IMERG coarsened to the same resolution as the downscaled fields to determine whether the process of coarsening then downscaling improves precipitation estimates more than averaging IMERG to coarser resolution only. Evaluations were performed on individual months, seasons, by ENSO phase, and based on precipitation characteristics. Results were inconsistent, with downscaling improving precipitation estimates in some domains and time periods and producing worse performance in others. While the results imply that the performance of the downscaled precipitation estimates is related to precipitation characteristics, it is still unclear what characteristics or combinations thereof lead to the most improvement or consistent improvement when applying RainFARM to IMERG. [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]

Profound changes in Earth's biogeochemical cycles are recorded in the sedimentology and geochemistry of carbonate sediments, with much of the early geological history of our planet recorded in carbonate mud precipitated from seawater. Despite their relevance for understanding early Earth, the trigger for whitings—carbonate spontaneously precipitated from the water column—remains intensely debated. Here, we used a neural network to map whitings imaged by daily MODIS Aqua satellite overpasses between 2003 and 2018 on Great Bahama Bank (GBB). Our results show that the frequency of whitings after 2011 increased by 3.3× compared to those recorded between 2003 and 2010, with the frequency of winter whitings increasing more than those in the summer. We contend that the period of increased whitings frequency was spurred by concurrent perturbations to the North Atlantic Oscillation, Atlantic Multidecadal Oscillation, and the El Niño Southern Oscillation in 2010 that facilitated enhanced hydrodynamic mixing between the waters atop GBB and those that surround it. We conclude that whitings can exhibit a sensitive but non‐linear response to ocean‐atmosphere dynamics which is unlikely to be resolved in the geological record using conventional stratigraphic techniques. This pronounced fluctuation in mud production on short timescales contrasts with expectations of persistent tranquility in the depositional environments that produce lime mud. Our results suggest that long‐established links between ocean‐atmosphere dynamics and the lime mud budget through Earth history may require reevaluation. Plain Language Summary: The size, composition, and distribution of carbonate sediment produced in the oceans are quantifiable features that enable reconstructions of environmental conditions throughout Earth history. Lime mud is a fine‐grained carbonate sediment that is commonly found in ancient rocks, but relatively rare in the modern oceans. However, Great Bahama Bank (GBB) has significant accumulations of lime mud thought to be produced by ongoing whitings. Whitings are a geological phenomenon that can be observed from orbit as milky white patches of seawater. Using a combination of satellite‐based remote sensing observations and deep learning, we created a two‐decade record of the frequency of whitings. Our results show, for the first time, that a pronounced change in ocean‐atmosphere dynamics more than tripled the frequency of whitings on GBB. Conventional geological methods cannot resolve such short‐time scale dependencies, so our study of modern lime mud formation provides useful insights into processes that happened in the ancient geological record, facilitating more accurate interpretations of environmental conditions throughout Earth history. Key Points: A perturbation to North Atlantic ocean‐climate dynamics in 2010 spurred a 3.3× increase in whitings lime mud productionLime mud production by whitings responds non‐linearly and disproportionately to subtle ocean‐atmosphere interactions on decadal timescalesClimate control on lime mud production provides new insight into one of the principal sedimentary archives of Earth's biogeochemical cycles [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]

The tropical Pacific plays an important role in modulating the global climate through its prevailing sea-surface temperature spatial structure and dominant climate modes like El Niño-Southern Oscillation (ENSO), the Madden-Julian Oscillation (MJO), and their teleconnections. These modes of variability, including their oceanic anomalies, are considered to provide sources of prediction skill on subseasonal timescales in the Tropics. Therefore, this study aims to examine how assimilating in situ ocean observations influences the initial ocean sea-surface temperature (SST) and mixed-layer depth (MLD) and their subseasonal forecasts. We analyze two subseasonal forecast systems generated with the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecast System (IFS), where the ocean states were initialized using two Observing-System Experiment (OSE) reanalyses. We find that the SST differences between forecasts with and without ocean data assimilation grow with time, resulting in a reduced cold-tongue bias when assimilating ocean observations. Two mechanisms related to air-sea coupling are considered to contribute to this growth of SST differences. One is a positive feedback between the zonal SST gradient, pressure gradient, and surface wind. The other is the difference in Ekman suction and mixing at the Equator due to surface wind-speed differences. While the initial mixed-layer depth (MLD) can be improved through ocean data assimilation, this improvement is not maintained in the forecasts. Instead, the MLD in both experiments shoals rapidly at the beginning of the forecast. These results emphasize how initialization and model biases influence air-sea interaction and the accuracy of subseasonal forecasts in the tropical Pacific. [ABSTRACT FROM AUTHOR]

Extratropical air–sea interactions have become increasingly involved in promoting the transition to El Niño–Southern Oscillation (ENSO) with climate change. In this study, we break down the effects of future warming on the 1-year lead relationship between a cold western North Pacific (WNP) phase and El Niño development the following winter. We apply a conditional probability approach and sea surface temperature (SST) budget analysis on historical and Shared Socioeconomic Pathway 3–7.0 (SSP370) model runs. With enhanced warming, cold WNP SST anomalies in the boreal winter further strengthen summer westerly anomalies in the western equatorial Pacific, which promote the intensification of surface convergence and anomalous Ekman and geostrophic advection, and positive SST anomalies in the central equatorial Pacific in the seasons prior to the El Niño. Downwelling equatorial Kelvin waves induced by the westerly wind stress facilitate entrainment of subsurface water into the mixed layer during the transition period to trigger stronger thermocline feedback in the central–eastern equatorial Pacific. As a result, the amplitude and frequency of El Niño and its tropical precursors are projected to increase with warming under the WNP influence. ENSO diversity modulated by this relationship depends on the relative strength of advective and thermocline feedbacks, as well as the background state at the time of the event. The intensification of positive Pacific Meridional Mode (PMM) southwesterlies during the WNP–ENSO transition suggests a strengthened three-way link between WNP, PMM and ENSO under enhanced warming that may promote stronger and/or more frequent El Niños. [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]

Different evolutionary patterns of the East Asian trough (EAT) anomaly could cause different climate patterns over East Asia. In this study, the leading mode of month-to-month variation in winter (December–January–February) EAT during 1979/80–2018/19 is investigated using the extended empirical orthogonal function (EEOF). The first EEOF mode (EEOF1) shows a persistent anomalous EAT pattern throughout the three winter months, which results in persistent air temperature anomalies over East Asia. Mechanism analyses indicate that the El Niño-Southern Oscillation (ENSO) negatively relates to the EEOF1, but its significant influence on the EAT is only in December. In contrast, a dipole pattern of sea surface temperature (SST) anomalies over the Eastern North Atlantic-Eastern Mediterranean Sea closely links with the EAT EEOF1. Further analyses indicate that the connection of the dipole SST pattern with the EAT EEOF1 depends on its persistence. When there is positive feedback between the local atmosphere and sea, the dipole SST pattern is well-sustained in the three winter months and further leads to a persistent EAT anomaly by exciting the Rossby wave train over mid-latitude Eurasia. Numerical simulations by the linear baroclinic model further validate the role of dipole SST pattern-related diabatic and vorticity forcing in air-sea interactions. In contrast, in the SST poorly-sustained years, the dipole SST pattern has a weak connection with the EAT EEOF1. Based on the aforementioned analysis, we propose a combined index using the dipole SST pattern and anomalous Northeast Atlantic cyclone in December, which could provide valuable information to predict the dipole SST pattern persistence and EAT EEOF1 in winter. [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]

We investigated the relationship between the North Atlantic Oscillation (NAO) and Indian Ocean Dipole (IOD), which has remained unknown to date. Reanalysis data and linear baroclinic model experiments were employed in our study. The results showed significant correlation between the March NAO and the boreal summer and autumn IOD, independent of the El Niño–Southern Oscillation signal, verified by partial correlation analysis. Air–sea interaction over the western North Pacific (WNP) is a significant aspect of the physical mechanism through which the March NAO affects the subsequent IOD. A strong positive March NAO induces equivalent barotropic cyclonic circulation over the WNP through a steady Rossby wave, accompanied by a local tripole sea surface temperature (SST) anomaly pattern. Facilitated by local air–sea positive feedback, the low-level cyclonic circulation and associated precipitation anomalies over the WNP persist from early spring to summer and shift equatorward. During May–June, the WNP anomalous cyclone strengthens the southeasterly wind and enhances cooling off Sumatra–Java through local meridional circulation. Such circulation ascends over the WNP and descends over the tropical southeastern Indian Ocean and Maritime Continent. Subsequently, wind–evaporation–SST and wind–thermocline–SST positive feedback in the tropical Indian Ocean contribute to IOD development. A diagnosis of ocean mixed-layer heat budget indicated that the ocean dynamic process associated with the NAO contributes more to IOD development than does atmospheric thermal forcing. Determining the influence mechanism of the March NAO on the subsequent IOD is considered useful in advancing the seasonal prediction of IOD. [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]

The western-central equatorial Pacific (WCEP) zonal wind affects El Niño-Southern Oscillation (ENSO) by involving a series of multiscale air-sea interactions. Its interannual variation contributes the most to ENSO amplitude. Thus, understanding the predictability of the WCEP interannual wind is of great importance for better predictions of ENSO. Here, we show that the North Pacific Oscillation (NPO) and the South Pacific Oscillation (SPO) alternate in fueling this interannual wind from late boreal winter to austral winter in the presence of background trade winds in different hemispheres. During the boreal winter-spring, the NPO registers footprints in the tropics by benefiting from the Pacific meridional mode and modulating the northwestern Pacific intertropical convergence zone (NITCZ). However, as austral winter approaches, the SPO takes over the role of the NPO in maintaining the anomalous NITCZ. Moreover, the interannual wind is further driven to the east in the positive phase of the SPO, by intensified central-eastern equatorial Pacific convection resulting from tropical-extratropical heat flux adjustments. A reconstructed WCEP interannual wind index involving only the NPO and the SPO possesses a long lead time for ENSO prediction of nearly one year. These two extratropical boosters enhance the viability of equatorial Pacific zonal wind anomalies associated with the large growth rate of ENSO, and the one in the winter hemisphere seems to be more efficient in forcing the tropics. Our result further indicates that the NPO benefits a long-lead prediction of the WCEP interannual wind and ENSO, while the SPO is the dominant extratropical predictor of ENSO amplitude. [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]

Based on 12 years (2007–2018) of salinity data from the Array for Real-Time Geostrophic Oceanography (Argo) dataset, we found significant positive salinity anomalies in the upper layer of the central tropical Indian Ocean from autumn 2010 to spring 2011 and from autumn 2016 to spring 2017. We used wind, precipitation, outgoing longwave radiation and ocean current data from satellites and reanalysis datasets to analyze the atmospheric conditions, ocean dynamic processes and salinity budget associated with these high salinity events. Our results suggest that surface buoyancy fluxes are not the dominant factor affecting the positive salinity anomalies and that ocean dynamic processes have a more important role. Under the influence of the La Niña events and strong negative Indian Ocean dipole in 2010 and 2016, positive salinity anomalies appeared in the eastern Indian Ocean at the end of 2010 and 2016 as a result of strong westerlies and positive zonal currents. However, because the La Niña event in 2010 was stronger than that in 2016, the salinity anomalies in 2010 were also stronger and the decrease in the following year was both stronger and lasted longer, meaning that the salinity anomalies weakened only gradually. The maximum value of the salinity anomalies in 2011 therefore appeared in January, whereas in 2017 the salinity anomalies first decreased and then increased, with the largest anomalies in March. Salinity budget analyses showed that ocean advection was the main factor leading to the variations in the salinity anomaly during these two periods and that the changes in the zonal velocity in the zonal advection anomalies had the greatest impact. Zonal advection was positive and strongest at the end of 2010 and negative in early 2011, but weakly positive at the end of 2016. In early 2017, the zonal advection was first negative, then became positive and strengthened in spring, so the salinity anomalies in spring 2017 were higher than those in 2011. The freshwater flux had a small, negative effect on the positive salinity anomalies for these two events. The mutual effects of the horizontal advection and the freshwater flux led to high salinity anomalies. The high salinity anomalies reflect the response of the upper ocean to climate events and may influence regional air–sea interactions and large-scale processes. [ABSTRACT FROM AUTHOR]

El Niño‐Southern Oscillation (ENSO) can effectively modulate global tropical cyclone (TC) activity, but the role TCs may play in determining ENSO characteristics remains unclear. Here we investigate the impact of TC winds on ENSO using a suite of Earth system model experiments where we insert TC winds, extracted from a TC‐permitting high‐resolution simulation, into a low‐resolution model configuration with nearly no intrinsic TCs. The presence of TC winds in the model increases ENSO power and shifts ENSO frequency closer to what we observe. TCs lead to an increase of strong to extreme El Niño events seen in observations and not simulated in the low‐resolution model without intrinsic TCs, mainly through enhanced zonal advection feedback and thermocline feedback. Our results indicate that TCs play a fundamental role in producing the ENSO characteristics we experience today in the climate system and point to a two‐way climatological interaction between TCs and ENSO. Plain Language Summary: El Niño‐Southern Oscillation (ENSO) can influence global tropical cyclone (TC) activity by altering the large‐scale conditions. TCs, as transient yet powerful weather events, can cause strong air‐sea interactions over the tropical ocean that may consequently influence the climate mean state. Here we show how TCs could be essential to the characteristics of ENSO using a suite of Earth system model experiments where we insert TC winds, extracted from a high‐resolution model simulation, into a low‐resolution model simulation with nearly no intrinsic TCs. The added TC winds in the model increases ENSO power and shifts ENSO frequency closer to the observations. TCs lead to an increase of strong to extreme El Niño events seen in observations and not simulated in the low‐resolution model without intrinsic TCs. Our results indicate that TCs play a fundamental role in producing the ENSO characteristics we experience today in the climate system and point to a previously unidentified two‐way climatological interaction between TCs and ENSO. Key Points: The climatological impacts of tropical cyclone (TC) winds on El Niño‐Southern Oscillation (ENSO) are investigated for the first time in a fully coupled Earth system modelTC winds impact ENSO magnitude, frequency, and the occurrence of strong to extreme El Niño eventsModel proved that TC winds affect El Niño by enhancing the thermocline feedback and the zonal advection feedback [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]

The onset of the Asian summer monsoon (ASM) starts the rainy season in one of the most world populous regions. The present study found that the stepwise ASM onset process (ASMOP) in the Bay of Bengal, South China Sea (SCS), and India exhibits three distinct interannual modes induced by different sea surface temperature anomalies (SSTAs). The first mode features are in‐phase variation of ASMOP in southern Asia, which originates from the ENSO‐related SSTAs and anomalous circulations in the tropical Indo‐Pacific Ocean. The second mode indicates the anomalous ASMOP in India and depends on the southwestern Indian SSTAs that alter the low‐level cross‐equatorial flow over the Arabian Sea. In the third mode, the western North Pacific SSTAs and their resultant circulation anomalies regulate ASMOP in the northern SCS. These extra‐equatorial SSTAs and their associated air‐sea interaction serve as an additional source of seasonal predictability for ASMOP besides El Niño–Southern Oscillation events. Plain Language Summary: The onset of the Asian summer monsoon (ASM) begins the rainy season in South and East Asia, one of the world's most populous regions. In general, the ASM onset process (ASMOP) follows a stepwise progression over the Bay of Bengal (BOB), South China Sea (SCS), and India, in turn. The year‐by‐year relationship between the monsoon onset dates over these areas helps to predict the arrival of the monsoon in Asian countries. However, there is no consensus on the year‐by‐year relationship of ASMOP among these areas. The present study used a unified definition to revisit the distinct modes of interannual variation in ASMOP. The El Niño–Southern Oscillation (ENSO)–associated tropical sea surface temperature anomalies (SSTAs) in the Indian and Pacific oceans can modify ASMOP over BOB, the southern SCS, and India simultaneously. Besides the influence of ENSO, the SSTAs in the southwestern Indian Ocean can regulate ASMOP in India. At the same time, the SSTAs modulate ASMOP in the northern SCS in the western North Pacific. Results from numerical experiments verified that air–sea interaction is critical for the effect of extra‐equatorial SSTAs on ASMOP, which provides an additional source of seasonal predictability of ASMOP besides ENSO factors. Key Points: The Asian summer monsoon onset process (ASMOP) shows distinct interannual modes in the tropicsEl Niño–Southern Oscillation determines the interannual variability in ASMOP over India, the Bay of Bengal, and the southern South China Sea (SCS)The southwestern Indian and western Pacific sea surface temperature anomalies modulate ASMOP over India and the northern SCS via air–sea interaction, respectively [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]

Based on the updates of the Climate Prediction Center and International Research Institute for Climate and Society (CPC/IRI) and the China Multi-Model Ensemble (CMME) El Niño-Southern Oscillation (ENSO) Outlook issued in April 2022, La Niña is favored to continue through the boreal summer and fall, indicating a high possibility of a three-year La Niña (2020–23). It would be the first three-year La Niña since the 1998–2001 event, which is the only observed three-year La Niña event since 1980. By examining the status of air—sea fields over the tropical Pacific in March 2022, it can be seen that while the thermocline depths were near average, the southeasterly wind stress was at its strongest since 1980. Here, based on a quaternary linear regression model that includes various relevant air-sea variables over the equatorial Pacific in March, we argue that the historic southeasterly winds over the equatorial Pacific are favorable for the emergence of the third-year La Niña, and both the anomalous easterly and southerly wind stress components are important and contribute ∼50% of the third-year La Niña growth, respectively. Additionally, the possible global climate impacts of this event are discussed. [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]

Interannual variability of surface mixed‐layer near‐inertial energy (NIE, representing the intensity of near‐inertial waves) in the South China Sea and western North Pacific (WNP) is investigated using satellite‐tracked surface drifter data set. It is found that NIE in the study region correlates negatively with El Niño‐Southern Oscillation (ENSO) with a correlation coefficient of R = −0.44 and a time lag of 5 months, mainly because the variation of local wind stress lags behind El Niño by 4 months. By separating summer and winter seasons, the correlation is significantly improved. The summer NIE correlates positively with El Niño (R = 0.62), since tropical cyclones over the WNP tend to be stronger and longer‐lived during the El Niño developing phase. The winter NIE correlates negatively with El Niño (R = −0.65), since the winter monsoon is weakened by the ENSO‐related WNP anomalous anticyclone. This is the first time that interannual variability of NIE is studied by direct current velocity observations. Plain Language Summary: Interannual climate variability, especially El Niño‐Southern Oscillation (ENSO), can trigger a series of flood, drought, and wildfires, resulting in a huge loss of people's property around the world. It has been demonstrated that ENSO can modulate large‐scale oceanic and atmospheric processes, such as sea surface temperature, circulation, monsoon, tropical cyclones (TCs), etc. Near‐inertial waves (NIWs) are ubiquitous in the upper ocean and usually contain the most energy of internal waves. Since NIWs are generated by time‐varying wind stress, they could also be influenced by ENSO. Using current velocity observations from satellite‐tracked surface drifter data set with hourly resolution, interannual variability of the surface mixed‐layer near‐inertial energy (NIE) is investigated. It is found that NIE in the western North Pacific (WNP) is indeed modulated by ENSO, with the monthly NIE lagging behind ENSO by 5 months. Since the monsoon reverses direction seasonally, summer and winter NIEs are discussed separately. The summer NIE is mainly generated by TCs while the winter NIE is mainly generated by winter monsoon, which are all tightly connected to and modulated by ENSO. Since NIWs play an important role in upper‐ocean turbulent mixing and air‐sea interaction, their interannual variations have important implications for the climate system. Key Points: Mixed‐layer near‐inertial energy in the study region correlates negatively with El Niño‐Southern Oscillation (ENSO) with a time lag of 5 monthsSummer near‐inertial energy is mainly generated by tropical cyclones and correlates positively with ENSOWinter near‐inertial energy is mainly generated by the winter monsoon and correlates negatively with ENSO [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]

Continuing the tropical Pacific multivariate air‐sea coupler proposed by us before, we design the Global spatial‐temporal Teleconnection Coupler (GTC), which is modeled to discover the latent teleconnections among global sea surface temperature (SST). To this end, Pacific, Indian, and Atlantic oceans are divided into small ocean patches that compose a dynamics graph, in which the adjacent relationships are artificially constructed by prior knowledge and the non‐adjacent relationships are learned from the data by deep learning methods. Based on GTC, an El Niño‐Southern Oscillation (ENSO) deep learning forecast model (ENSO‐GTC) is established, where probability graph convolution layers are designed to learn spatial‐temporal teleconnection, that is, non‐adjacent relationships in the dynamics graph. A loss function with a graph total variations penalty term is remarkably proposed to maintain physical consistency. We tune ENSO‐GTC to the optimal, which has Niño3.4 index correlation skills of 0.79/0.66/0.51 at 6‐/12‐/18‐month forecasts with iterative strategy, and above 0.6 within 20‐month forecasts with direct strategy, outperforming the other state‐of‐the‐art models. We explore the forecast skill of ENSO‐GTC on the effective forecast lead time, improvements of persistence barrier, and analysis of forecast errors. Moreover, we find that what GTC has learned exactly matches multiple ENSO theories. The oscillations of Kelvin and Rossby waves make a 6‐month lagged correlation on Pacific SST. The north and south Pacific meridional modes (NPMM and SPMM) are strongly linked to the evolutions of ENSO and should be monitored more. The teleconnections between equatorial Indian/Atlantic and Pacific are very important and usually have 2‐month/8‐month lagged correlations before ENSO. Plain Language Summary: El Niño‐Southern Oscillation (ENSO) is the most dominant inter‐annual climate cycle over the tropical Pacific. ENSO deep learning forecast model with the coupler considering the multivariate interactions in the tropical Pacific has achieved a significant performance. Despite ENSO being majorly in the tropical Pacific, the other ocean basins (such as the Indian and Atlantic oceans) play important roles in the occurrence, development, and decay of ENSO. In this paper, we construct a Global spatial‐temporal Teleconnection Coupler (GTC) to capture crucial teleconnections to ENSO from the aspect of sea surface temperature (SST) interaction among the global three oceans. We construct a dynamics graph to describe SST interactions by cutting global oceans into small ocean patches. In the graph, the adjacent relationships are artificially constructed by prior knowledge, and the non‐adjacent relationships are learned from the data by using deep learning methods. Based on this coupler, we construct an ENSO deep learning forecast model (named ENSO‐GTC) and design the probability graph convolution layer to learn spatial‐temporal teleconnections, that is, non‐adjacent relationships in the dynamics graph. We first tune ENSO‐GTC to the optimal and measure its forecast skills. Then, we carry out the physical interpretability of what the model learns. Key Points: Continuing the tropical Pacific multivariate air‐sea coupler, we propose the Global spatial‐temporal Teleconnection CouplerThis coupler models the teleconnections of adjacent and non‐adjacent small ocean patches by deep learning to learn the interactions of sea surface temperatureThe proposed El Niño‐Southern Oscillation deep learning forecast model based on this coupler possesses 0.68 correlation skills in 18 lead months [ABSTRACT FROM AUTHOR]