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The 2015/16 El Niño event ranks among the top three of the last 100 years in terms of intensity, but most dynamical models had a relatively low prediction skill for this event before the summer months. Therefore, the attribution of this particular event can help us to understand the cause of super El Niño–Southern Oscillation events and how to forecast them skillfully. The present study applies attribute methods based on a deep learning model to study the key factors related to the formation of this event. A deep learning model is trained using historical simulations from 21 CMIP6 models to predict the Niño-3.4 index. The integrated gradient method is then used to identify the key signals in the North Pacific that determine the evolution of the Niño-3.4 index. These crucial signals are then masked in the initial conditions to verify their roles in the prediction. In addition to confirming the key signals inducing the super El Niño event revealed in previous attribution studies, we identify the combined contribution of the tropical North Atlantic and the South Pacific oceans to the evolution and intensity of this event, emphasizing the crucial role of the interactions among them and the North Pacific. This approach is also applied to other El Niño events, revealing several new precursor signals. This study suggests that the deep learning method is useful in attributing the key factors inducing extreme tropical climate events. [ABSTRACT FROM AUTHOR]

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

Abstract 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.

The moistening impact of El Niño on the tropical lower stratosphere has been extensively studied, yet a long-standing challenge is its potential nonlinearities regarding the strength of El Niño. Extreme El Niño's hydration in 2015/2016 was unprecedented in the satellite era, providing a great opportunity to distinguish the differential response of water vapor to extreme and moderate El Niño. Using ERA5 and MERRA-2 reanalysis data from 1979–2019, we compare the composite tropical lower stratospheric water vapor anomalies throughout all extreme and moderate El Niño episodes since the satellite era. We validate the variations in the lower stratospheric water vapor during the two distinct El Niño episodes using a three-dimensional chemistry transport model simulating the same period. The model reproduces the observed pattern in lower stratospheric water vapor. Both demonstrate that robust moistening during extreme El Niño events occurs throughout the tropical lower stratosphere. However, moderate El Niño events seem to have a weak effect on lower stratospheric water vapor. In comparison to moderate El Niño, the strong convective activities induced by extreme El Niño release large amounts of latent heat, causing extensive and intense warming in the tropical upper troposphere and lower stratosphere, thus greatly increasing the water vapor content in the tropical lower stratosphere. Additionally, moderate El Niño events have strong seasonality in their hydration effect in the tropics, whereas the intense moistening effect of extreme El Niño events prevails in all seasons during their episodes. [ABSTRACT FROM AUTHOR]

We show that the well-known failure of any single index to capture the diversity and extremes of El Niño-Southern Oscillation (ENSO) results from the inability of existing indices to uniquely characterize the average longitude of deep convection in the Walker Circulation. We present a simple sea surface temperature (SST)-based index of this longitude that compactly characterizes the different spatial patterns, or flavors of observed and projected ENSO events. It recovers the familiar global responses of temperature, precipitation, and tropical cyclones to ENSO and identifies historical extreme El Niño events. Despite its simplicity, the new longitude index describes the nonlinear relationship between the first two principal components of SST, and unlike previous indices, accounts for background SST changes associated with the seasonal cycle and climate change. The index reveals that extreme El Niño, El Niño Modoki, and La Niña events are projected to become more frequent in the future at the expense of neutral ENSO conditions.

The moistening impact of El Niño on the tropical lower stratosphere has been extensively studied, yet a long-standing challenge is its potential nonlinearities regarding the strength of El Niño. Extreme El Niño’s hydration in 2015/2016 was unprecedented in the satellite era, providing a great opportunity to distinguish the differential response of water vapor to extreme and moderate El Niño. Using ERA5 and MERRA-2 reanalysis data from 1979–2019, we compare the composite tropical lower stratospheric water vapor anomalies throughout all extreme and moderate El Niño episodes since the satellite era. We validate the variations in the lower stratospheric water vapor during the two distinct El Niño episodes using a three-dimensional chemistry transport model simulating the same period. The model reproduces the observed pattern in lower stratospheric water vapor. Both demonstrate that robust moistening during extreme El Niño events occurs throughout the tropical lower stratosphere. However, moderate El Niño events seem to have a weak effect on lower stratospheric water vapor. In comparison to moderate El Niño, the strong convective activities induced by extreme El Niño release large amounts of latent heat, causing extensive and intense warming in the tropical upper troposphere and lower stratosphere, thus greatly increasing the water vapor content in the tropical lower stratosphere. Additionally, moderate El Niño events have strong seasonality in their hydration effect in the tropics, whereas the intense moistening effect of extreme El Niño events prevails in all seasons during their episodes.

The 2015/16 El Niño displayed a distinct feature in the SST anomalies over the far eastern Pacific (FEP) compared to the 1997/98 extreme case. In contrast to the strong warm SST anomalies in the FEP in the 1997/98 event, the FEP warm SST anomalies in the 2015/16 El Niño were modest and accompanied by strong southeasterly wind anomalies in the southeastern Pacific. Exploring possible underlying causes of this distinct difference in the FEP may improve understanding of the diversity of extreme El Niños. Here, we employ observational analyses and numerical model experiments to tackle this issue. Mixed-layer heat budget analysis suggests that compared to the 1997/98 event, the modest FEP SST warming in the 2015/16 event was closely related to strong vertical upwelling, strong westward current, and enhanced surface evaporation, which were caused by the strong southeasterly wind anomalies in the southeastern Pacific. The strong southeasterly wind anomalies were initially triggered by the combined effects of warm SST anomalies in the equatorial central and eastern Pacific (CEP) and cold SST anomalies in the southeastern subtropical Pacific in the antecedent winter, and then sustained by the warm SST anomalies over the northeastern subtropical Pacific and CEP. In contrast, southeasterly wind anomalies in the 1997/98 El Niño were partly restrained by strong anomalously negative sea level pressure and northwesterlies in the northeast flank of the related anomalous cyclone in the subtropical South Pacific. In addition, the strong southeasterly wind and modest SST anomalies in the 2015/16 El Niño may also have been partly related to decadal climate variability. [ABSTRACT FROM AUTHOR]

Intensive air‐sea interaction and the formation of the salinity barrier layer (BL) in the Pacific has fundamental importance to the El Niño evolution. The structure and formation of the BL in the equatorial Pacific Ocean during moderate and extreme El Niños over the past 30 years are investigated using in situ temperature and salinity data measured by the TAO/TRITON array and the data‐assimilating ECCO2 product. In the western and central Pacific Ocean, the BL is thicker during moderate El Niños compared to extreme El Niños due to a deeper isothermal layer depth compared to the density defined mixed layer depth. Moreover, in the western and central Pacific Ocean, the anomalous zonal eastward current related to the westerly wind event that initiates El Niños is found to be stronger during extreme El Niños, advecting the thicker BLs from west to east. A salinity budget suggests that during both moderate and extreme El Niño events, surface freshwater flux dominates at the equator. During extreme El Niños, the change in the freshwater flux drives a strong surface jet in the far western Pacific at 1°S, 156°E. North of the equator, the surface freshwater flux largely dampens this advective impact. Thus during the different El Niño strengths, the BL distribution, evolution and impact are also different. This suggests that climate models need to better distinguish different types of El Niño events in order to simulate the ENSO dynamics correctly. Plain Language Summary: During the different El Niño scenarios, the barrier layer (BL) distribution, evolution and impact are different. Observations from TAO/TRITON array shows that the BL is thicker during moderate El Niños than during extreme El Niños in the western and central Pacific Ocean. Less heat input leads to a thinner isothermal layer depth during extreme El Niños. Stronger anomalous zonal eastward current during extreme El Niños advects the thicker BLs from west to east, dampens thick BL maintaining in the western and central Pacific Ocean. During extreme El Niños, the diagnosis shows that the change in the freshwater flux further drives a strong eastward surface jet mainly through salinity effect in the far western Pacific, further advect the thick BL to the east. Climate models need to better distinguish the evolution of BL during different types of El Niño events. Key Points: Salinity stratified barrier layers (BLs) are thicker in the western Pacific Ocean during moderate El Niños compared to extreme El NiñosDifferences in the freshwater flux and advection are the main reasons for the discrepancies during the two types of El Niño eventsDuring extreme El Niños, the freshwater flux results in a salinity‐dominated zonal current anomaly advecting BLs eastward [ABSTRACT FROM AUTHOR]

The stratospheric pathway is a major driver of El Niño–Southern Oscillation (ENSO) impacts on mid-latitude tropospheric circulation and winter weather. The weak vortex induced by El Niño conditions has been shown to increase the risk of cold spells, especially over Eurasia, but its role for North American winters is less clear. This study involved idealized experiments with the Whole Atmosphere Community Climate Model to examine how the weak winter vortex induced by extreme El Niño events is linked to North American coldness in spring. Contrary to the expected mid-latitude cooling associated with a weak vortex, extreme El Niño events do not lead to North American cooling overall, with daily cold extremes actually decreasing, especially in Canada. The expected cooling is absent in most of North America because of the advection of warmer air masses guided by an enhanced ridge over Canada and a trough over the Aleutian Peninsula. This pattern persists in spring as a result of the trapping of stationary waves from the polar stratosphere and troposphere, implying that the stratospheric influence on North America is sensitive to regional downward wave activities.

Why did the predicted "super El Niño" fade out in the summer 2014 and the following year develop into one of the three strongest El Niño on record? Although some hypotheses have been proposed in previous studies, the quantitative contribution of oceanic processes to these events remains unclear. We investigated the role of various oceanic feedbacks, especially in response to intra-seasonal westerly wind busts, in the evolution of the 2014–2016 El Niño events, through a detailed heat budget analysis using high temporal resolution Estimating the Circulation and Climate of the Ocean—Phase II (ECCO2) simulation outputs and satellite-based observations. Results show that the Ekman feedback and zonal advective feedback were the two dominant oceanic processes in the developing phase of the warm event in the spring of 2014 and its decay in June. In the 2015–2016 super El Niño event, the zonal advective feedback and thermocline feedback played a significant role in the eastern Pacific warming. Moreover, the thermocline feedback tended to weaken in the central Pacific where the zonal advection feedback became the dominant positive feedback. [ABSTRACT FROM AUTHOR]

The stratosphere is a key link between El Niño and Eurasian surface climate in winter. Instead of the amplitude of the stratospheric response discussed by many previous studies, we focus on the persistence of wintertime extreme El Niño impacts through the stratospheric pathway to midlatitude Eurasia in spring. A novel approach is used by running WACCM4 with El Niño forcing imposed only during winter to isolate the stratospheric role in the following spring. We show that a descending signal with a strong deceleration of zonal wind throughout the atmosphere could reach the surface and persist in spring during extreme El Niño events, whereas during moderate El Niño events the signal is generally confined to the stratosphere. Two different mechanisms are involved in maintaining the tropospheric signal of extreme El Niño. Under the westerly regime, the planetary Rossby waves from the wave centers over East Asia and the North Pacific transmit the signals from the stratosphere to the troposphere; and under the easterly regime, strong tropospheric eddy feedback over the midlatitude Atlantic favors the persistence of tropospheric responses during extreme El Niño events. The prolonged stratospheric pathway of extreme El Niño events amplifies and extends the equatorward shift of the midlatitude jet, allowing southward intrusions of cold polar air into Eurasia. Such circulation changes induced by a prolonged stratospheric pathway can cause springtime coldness and daily cold extremes in midlatitude Eurasia. Key Points: We found greater persistence of extreme El Niño signals through the stratospheric pathway to midlatitude EurasiaThe contribution of the stratospheric pathway is isolated with model and is found to be critical in maintaining the signalsThe prolonged stratospheric pathway of extreme El Niño events can increase the risk of cold spells especially over Eurasia [ABSTRACT FROM AUTHOR]

The warm center of the 2015/2016 extreme El Niño was located further to the west compared to previous extreme events, causing vastly different social‐economic impacts. Though many studies investigated ocean‐atmosphere couplings responsible for its large magnitude, the dynamics that govern the unique structure of the event remain elusive. Here we show that a local ocean‐atmosphere feedback among convection‐induced surface winds, oceanic upwelling and sea surface temperature in the far‐eastern Pacific, resulted in a boreal summer cooling tendency along the South American coast. It canceled warming in the far‐eastern equatorial Pacific that would otherwise develop, leading to the distinctive anomaly pattern of the 2015/2016 El Niño. These processes were initially triggered by the leftover anomalous warm sea surface temperature and positive moist static energy over the northeastern tropical Pacific in the following boreal spring of the 2014 warm event. Plain Language Summary: Extreme El Niño events exert severe impacts on society, but predicting extreme events is challenging. These events are rare and distinctive from one to another in their spatial pattern, generating different regional impacts and disasters. In particular, the 2015/2016 extreme El Niño features a more westward warm center of sea surface temperature anomalies, compared to that of 1982/1983 or 1997/1998. Our analysis of observations relates this unique anomaly pattern to the preceding warm tropical Pacific in 2014. That is, rainfall anomalies over the eastern Pacific were initially fueled by the leftover unstable energy over the northeastern tropical Pacific. Then the center of anomalous rainfall was restricted to the north of the equator through a local ocean‐atmosphere feedback among convection‐induced surface flows, oceanic upwelling and sea surface temperature. The positive feedback resulted in a summer cooling tendency along the South American coast, which offset warm SST anomalies and caused the unusual pattern of the 2015/2016 extreme El Niño. Our further investigation suggested that the state of the eastern Pacific before the developing summer is also essential for the development of other El Niño events. Key Points: The South American coastal cooling process led to a westward propagation of the 2015/2016 El Niño warm center after the boreal springConvective activity relating to the 2014 warm condition induced cooling tendency along the South American coast through a feedback loopThis antecedent state of the eastern Pacific played a crucial role in the unique spatial pattern of the 2015/2016 El Niño [ABSTRACT FROM AUTHOR]

The El Niño-Southern Oscillation (ENSO) is a quasi-periodical natural phenomenon occurring in the tropical Pacific whose characteristics can be influenced by global warming. Using outputs of 14 general circulation models (GCMs) participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5), the numbers of extreme El Niño and La Niña events are analyzed for the 50-year period in the future (2050–2099) under the RCP4.5 and RCP8.5 scenarios relative to those during the historical period 1950–1999. Analyses are based on the Oceanic Niño Index (ONI) and the modified Cai index that is defined in this study. Our approach deviates from that of Cai et al. (Nat Clim Change 4:111–116, 2014, Nat Clim Change 5:132–137, 2015b), who used rainfall thresholds to identify extreme El Niño events but sea surface temperature (SST) thresholds to identify extreme La Niña events. Analysis of SST and rainfall in the tropical Pacific indicated that under global warming the eastern equatorial Pacific warms faster than the surrounding ocean waters, which is accompanied by an increase of rainfall in the eastern equatorial Pacific. Thus, changes in the mean state of the tropical Pacific climate will be like an El Niño pattern under global warming. By applying the ONI, it is found that the number of very strong El Niño events slightly increases under global warming, while the rate of increase in the number of very strong La Niña events is greater than that of very strong El Niño events. Analysis based on the modified Cai index indicates a slight decrease in the number of extreme El Niño events and a slight increase in the number of extreme La Niña events under global warming. Thus, results of several previous studies which concluded that the number of extreme El Niño events nearly doubles under global warming are not supported by results of this study. [ABSTRACT FROM AUTHOR]

Strong El Niño events have had significant impacts on society through their association with extreme events, such as droughts and floods. However, questions remain as to the robustness of strong El Niño events in forcing regional climate variability. The strong 1982, 1997 and 2015 El Niño events were of similar type and strength, but in eastern Australia they were associated with differing spring rainfall anomaly magnitudes and patterns. To understand these differences, we first determined the most important processes for teleconnecting the El Niño signal to east Australian spring rainfall using historical relationships with winds and sea level pressure. Then, using a 60-member atmospheric model ensemble, we estimated the influence of sea surface temperatures (SSTs) on Australian atmospheric circulation and rainfall during these three El Niño events relative to internal variability. We found that the different east Australian spring rainfall anomalies for each of the three El Niño events are best explained by differences in the strength of the meridional wind component of the regional circulation. All three El Niño events exhibited a positive sea level pressure anomaly to the south of Australia, which was associated with rainfall deficits along the southeast Australian coast. The experiments indicate the regional atmospheric circulation and rainfall differences were forced by SSTs during spring of 1982 and 1997, with their influence on the circulation in 2015 remaining unclear. We also show that SSTs adjacent to Australia further contributed to the modelled rainfall differences mainly by regulating moisture availability. [ABSTRACT FROM AUTHOR]

At the beginning of 2015, as one year earlier in 2014, the scientific community anticipated that El Niño conditions could develop in the tropical Pacific by year-end. Such projections were related to the occurrence of westerly wind bursts during winter–spring of each year that generated strong downwelling Kelvin waves indicative of an emerging El Niño. However, the event's progression quickly stalled in 2014, but actively continued in 2015, leading to an extreme warm event (comparable to 1997 or 1982). Here, we compare climate evolution during these two years using satellite observations and numerical simulations. We show that during 2014, El Niño development was interrupted mid-year by an exceptionally strong easterly wind burst, whereas during the second year it continued through the summer. Further, we show that the failed 2014 event created favorable conditions for El Niño development during the next year, as it kept ocean heat content recharged and the western Pacific warm pool extended eastward. Subsequently, the winter–spring westerly wind bursts in 2015 were followed by a series of state-dependent westerly bursts as part of a strong positive Bjerknes feedback. Analogue simulations with a coupled GCM wherein we superimpose the observed sequences of westerly and easterly wind bursts support these conclusions, stressing the role of the failed 2014 event in preconditioning the ocean–atmosphere system for the development of an extreme El Niño. In our simulations the probability of an extreme event following early-year westerly wind bursts increases from 14% to nearly 60% due to this preconditioning. Thus, the interplay between westerly and easterly wind bursts shapes El Niño development and diversity. [ABSTRACT FROM AUTHOR]

The recent 2015-16 El Niño was of comparable magnitude to the two previous record-breaking events in 1997-98 and 1982-83. To better understand how this event became an extreme event, we examine the underlying processes leading up to the peak of the event in comparison to those occurring in the 1997-98 and 1982-83 events. Differences in zonal wind stress anomalies are found to be an important factor. In particular, the persistent location of the zonal wind stress anomalies north of the equator during the two years prior to the 2015-16 peak contrasts the more symmetric pattern and shorter duration observed during the other two events. By using linear equatorially trapped wave theory, we determine the effect of these off-equatorial westerly winds on the amplitude of the forced oceanic Rossby and Kelvin wave response. We find a stronger upwelling projection onto the asymmetric Rossby wave during the 2-year period prior to the peak of the most recent event compared to the two previous events, which might explain the long-lasting onset. Here we also examine the ocean advective heat fluxes in the surface mixed layer throughout the event development phase. We demonstrate that, although zonal advection becomes the main contributor to the heat budget across the three events, meridional and vertical advective fluxes are significantly larger in the most recent event compared to those in 1997-98 and 1982-83. We further highlight the key role of advective processes during 2014 in enhancing the sea surface temperature anomalies, which led to the big El Niño in the following year. [ABSTRACT FROM AUTHOR]

Abstract: The extraordinarily strong El Niño events, such as those of 1982/1983, 1997/1998, and 2015/2016, are known to cause critical socio‐economic impacts worldwide by disrupting global weather patterns, cyclones, drought/floods, and ecosystems. Therefore, it is a critical question how often such extreme El Niño will take place in the future. Although many climate models tend to simulate more frequency extreme El Niño under the greenhouse warming, there is a large intermodel diversity, with a range as large as the multimodel averaged change. The cause for this intermodel uncertainty is not known. Here we show that the north tropical Atlantic (NTA) mean precipitation plays a significant role in controlling changes in the extreme El Niño frequency under global warming; that is, much frequent extreme El Niño events under the greenhouse warming are simulated in the models whose climatological precipitation over the NTA is largely decreased. Relatively drier climatology over the NTA can induce a wetter climatology over the equatorial eastern Pacific through an atmospheric teleconnection, and it provides background condition that the El Niño‐related convective responses are amplified to push El Niño to the extremes. [ABSTRACT FROM AUTHOR]

Abstract: A robust impact of El Niño on the Northern Hemisphere (NH) polar stratosphere has been demonstrated by previous studies, although whether this applies to extreme El Niño is uncertain. The time evolution of the response of the NH stratospheric vortex to extreme El Niño, compared with that to moderate eastern Pacific El Niño, is addressed by means of composite analysis using the National Centers for Environmental Prediction/Department of Energy reanalysis data set from 1980 to 2016. Lead‐lag analysis indicates that the El Niño signal actually leads the stratospheric response by ~2 months. Considering the time lag, the signal of December‐January‐February El Niño in the NH stratospheric vortex should mature in the February‐March‐April season (late winter/early spring). The patterns of circulation and temperature for late winter/early spring during extreme and moderate El Niño events are significant, exhibiting similar structure. The results are confirmed with the Whole Atmosphere Community Climate Model version 4 model, which is forced with observed SSTs of extreme and moderate El Niño in winter (December‐January‐February) to analyze the day‐to‐day propagation of their signals. Note that the magnitudes of the stratospheric responses are much larger in the case of extreme El Niño, as stronger upward propagation of planetary waves leads to a weaker northern polar vortex than during moderate El Niño events. [ABSTRACT FROM AUTHOR]

During extreme El Niño events, the Intertropical Convergence Zone moves to the normally cold and dry east equatorial Pacific, resulting in a nonlinear rainfall increase with sea surface temperature in the region. An arbitrary threshold value of boreal winter total rainfall (e.g., 5 mm d−1) in the east equatorial Pacific was used by previous studies to capture this feature. Under greenhouse warming, the frequency of extreme El Niño events is projected to increase, so is the mean east equatorial Pacific rainfall. Is the projected frequency increase a consequence of the mean rainfall increase? We show that the projection is not significantly influenced by the increased mean rainfall. Instead, the increased frequency accounts for approximately 50% of the mean rainfall increase. Using upward atmospheric vertical velocity for defining extreme El Niño reaffirms the conclusion that the increased frequency results from increased probability of atmospheric deep convection, as the eastern equatorial Pacific warms faster than the surrounding regions. [ABSTRACT FROM AUTHOR]

Characteristics of El Niño-Southern Oscillation (ENSO) have changed since the late 1970s as it synchronized with the Pacific Decadal Oscillation (PDO). In order to investigate the primary feedback process responsible for the interdecadal change in ENSO characteristics according to the PDO, using the ocean assimilation data (SODA) and the reanalysis data (NCEP/NCAR), we performed Bjerknes linear stability index (BJ index) analysis of two decadal periods: one before the late 1970s (the nPDO period) and the other after the late 1970s (the pPDO period). The BJ index for the pPDO period (−0.07 year for the growth rate of the eastern Pacific SST anomaly) is significantly larger than that for the nPDO period (−0.25 year). The larger BJ index value is primarily due to the enhanced zonal advection feedback (ZA; +0.44 year), thermocline feedback (TH; +0.33 year), and the reduced damping by the mean meridional current (MD; +0.16 year). The increases in ZA and TH are mainly attributed to the shoaling of the mean thermocline depth, which increased the sensitivity of the ocean dynamic fields to the wind forcing; and the reduced MD is related to the reduced mean meridional current associated with the weakened trade wind. The enhanced positive feedback is partly compensated by the enhanced thermodynamic damping including the shortwave, sensible heat flux and latent heat flux (collectively, −0.88 year). Interestingly, the change in air-sea coupling strength from the nPDO to the pPDO period was small. Without the two extreme El Niño events (1982-1983 and 1997-1998) in the pPDO period (pPDO_noBIG), the difference in BJ index between nPDO and pPDO_noBIG periods became smaller (~0.07 year), indicating that the two extreme El Niño events largely contribute to the larger ENSO variability of the pPDO period, possibly due to nonlinear feedback processes. Nevertheless, qualitative similarity in each of the feedback and damping components of BJ index exists between the pPDO and pPDO_noBIG periods, which suggests that the tropical climate states of the pPDO period provided more favorable conditions for the emergence of extreme El Niño events by intensifying the linear feedback processes. [ABSTRACT FROM AUTHOR]