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Pantropical climate interactions across ocean basins operate on a wide range of timescales and can improve the accuracy of climate predictions. Here, we show in observations that Central Pacific (CP) El Niño-like sea surface temperature (SST) anomalies have coevolved with tropical South Atlantic SST anomalies on a quasi-decadal (~10-year) timescale over the past seven decades. During the austral autumn–winter season, decadal warm SSTs in the tropical CP effectively induce tropical SST cooling in the South Atlantic, mainly by strengthening the South Atlantic subtropical anticyclone via an extratropical atmospheric wave teleconnection in the southern hemisphere. Partially coupled pacemaker simulations corroborate the observational findings, indicating that tropical CP decadal SSTs play a primary pacing role, while Atlantic feedback is of secondary importance throughout the study period. Our results suggest that the tropical CP could be an important source of decadal predictability for tropical South Atlantic SST and the surrounding climate. [ABSTRACT FROM AUTHOR]

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

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

El Niño–Southern Oscillation (ENSO) exhibits highly asymmetric temporal evolutions between its warm and cold phases. While El Niño events usually terminate rapidly after their mature phase and show an already established transition into the cold phase by the following summer, many La Niña events tend to persist throughout the second year and even reintensify in the ensuing winter. While many mechanisms were proposed, no consensus has been reached yet and the essential physical processes responsible for the multiyear behavior of La Niña remain to be illustrated. Here, we show that a unique ocean physical process operates during multiyear La Niña events. It is characterized by rapid double reversals of zonal ocean current anomalies in the equatorial Pacific and exhibits a fairly regular near-annual periodicity. Mixed-layer heat budget analyses reveal comparable contributions of the thermocline and zonal advective feedbacks to the SST anomaly growth in the first year of multiyear La Niña events; however, the zonal advective feedback plays a dominant role in the reintensification of La Niña events. Furthermore, the unique ocean process is identified to be closely associated with the preconditioning heat content state in the central to eastern equatorial Pacific before the first year of La Niña, which has been shown in previous studies to play an active role in setting the stage for the future reintensification of La Niña. Despite systematic underestimation, the above oceanic process can be broadly reproduced by state-of-the-art climate models, providing a potential additional source of predictability for the multiyear La Niña events. [ABSTRACT FROM AUTHOR]

As the dominant form of mesoscale variability in the equatorial eastern Pacific, Tropical Instability Waves (TIWs) are known to interact with the El Niño and Southern Oscillation (ENSO) in complex ways. TIWs activity is modulated by the ENSO state and also provide significant feedback on ENSO via nonlinear dynamic heating (NDH), acting as a source of asymmetry between the El Niño and La Niña phases. In this work, we show that the interannual variability of TIWs-induced heat flux and NDH can be approximately expressed in terms of the mean meridional temperature gradient as TIWs tend to transport heat downgradient of the temperature anomalies along the Sea Surface Temperature (SST) front. The TIWs-induced NDH can be quantified as an asymmetric negative feedback on ENSO by a nonlinear thermal eddy diffusivity which depends on the background TIWs pattern and the ENSO-related linear and nonlinear processes. This proposed parameterization scheme can capture well the direct ENSO modulation on TIWs activity, the combination effect arising from the nonlinear interaction between ENSO and the cold tongue annual cycle, and associated ENSO nonlinearity. This parameterization scheme is effectively tested using four ocean reanalysis datasets with different horizontal resolutions that exhibit contrasted patterns of TIWs activity. This scheme may be useful for assessing the TIWs-induced feedback on ENSO in mechanistic ENSO models to better understand the dynamics of ENSO complexity. [ABSTRACT FROM AUTHOR]

In observations, the boreal winter El Niño—Southern Oscillation (ENSO) phase-locking phenomenon is evident in the central-eastern Pacific. In the far eastern equatorial Pacific (FEP) and South American coastal regions, however, the peak of sea surface temperature anomalies (SSTA) tends to occur in the boreal summer, with fewer winter peak events. By separating the direct ENSO forcing from the FEP SSTA, we found that the summer peak preference is contributed by the residual SSTA component, while the ENSO forcing provides only a small probability of winter peak. The dynamics of FEP SSTA phase-locking in observations and its biases in the climate models are investigated by adopting a linear stochastic-dynamical model. In observations, the summer phase-locking of FEP SSTA is controlled by the seasonal modulation of the SSTA damping process. In contrast, in the climate models the strength of FEP SSTA phase-locking is much smaller than observed due to the overly negative SSTA damping rate. [ABSTRACT FROM AUTHOR]

El Niño-Southern Oscillation (ENSO) sea surface temperature (SST) anomaly skewness encapsulates the nonlinear processes of strong ENSO events and affects future climate projections. Yet, its response to CO2 forcing remains not well understood. Here, we find ENSO skewness hysteresis in a large ensemble CO2 removal simulation. The positive SST skewness in the central-to-eastern tropical Pacific gradually weakens (most pronounced near the dateline) in response to increasing CO2, but weakens even further once CO2 is ramped down. Further analyses reveal that hysteresis of the Intertropical Convergence Zone migration leads to more active and farther eastward-located strong eastern Pacific El Niño events, thus decreasing central Pacific ENSO skewness by reducing the amplitude of the central Pacific positive SST anomalies and increasing the scaling effect of the eastern Pacific skewness denominator, i.e., ENSO intensity, respectively. The reduction of eastern Pacific El Niño maximum intensity, which is constrained by the SST zonal gradient of the projected background El Niño-like warming pattern, also contributes to a reduction of eastern Pacific SST skewness around the CO2 peak phase. This study highlights the divergent responses of different strong El Niño regimes in response to climate change. [ABSTRACT FROM AUTHOR]

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

El Niño events exhibit rich diversity in their spatial patterns, which can lead to distinct global impacts. Therefore, how El Niño pattern diversity will change in a warmer climate is one of the most critical issues for future climate projections. Based on the sixth Coupled Model Intercomparison Project simulations, we report an inter-model consensus on future El Niño diversity changes. Central Pacific (CP) El Niño events are projected to occur more frequently compared to eastern Pacific (EP) El Niño events. Concurrently, EP El Niño events are projected to increase in amplitude, leading to higher chances of extreme EP El Niño occurrences. We suggest that enhanced upper-ocean stability due to greenhouse warming can lead to a stronger surface-layer response for increasing positive feedbacks, more favorable excitation of CP El Niño. Whereas, enhanced nonlinear atmospheric responses to EP sea surface temperatures can lead to a higher probability of extreme EP El Niño. [ABSTRACT FROM AUTHOR]

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

In many regions the projected future sea surface temperature (SST) response to greenhouse warming is larger in summer than in winter. What causes this amplification of the SST seasonal cycle has remained unclear. To determine robustness of the projected seasonal cycle intensification and ascertain underlying physical mechanisms we analyze a suite of historical and greenhouse warming simulations conducted with 13 coupled general circulation models in the Coupled Model Intercomparison Project Phase 5. In the Representative Concentration Pathway 8.5 scenario, the amplitude of SST seasonal cycle, defined as the difference between climatological maximum and minimum temperature, increases by 30% ± 20% on average by the end of 21st century. Analysis of a simplified mixed layer heat budget demonstrates that the amplification can be attributed to the increasing upper ocean stratification and hence shoaling of the annual‐mean mixed layer. The projected intensification of SST seasonality may have important implications for future changes in marine ecosystems. Plain Language Summary: One of the robust projected climate changes in response to global warming is the amplification of seasonal cycle of sea surface temperature (SST) with larger warming occurring in summer than in winter, especially in the North Atlantic, North Pacific and South Indian Ocean. Here we investigate the underlying physical mechanisms using a suite of future greenhouse warming simulations. We show that for the high greenhouse gas emission scenario the amplitude of SST seasonality increases over the next 80 years by 30% ± 20% globally. Overall mean ocean warming increases the upper ocean stratification, which leads to a shoaling of the mixed layer. This implies that climatological air‐sea heat fluxes impact a smaller ocean volume, which then leads to an increased SST response. Other heat budget terms play only a secondary role. The increased temperature seasonality could further impact plankton phenology and the climatology of upper ocean CO2. Key Points: Under the high greenhouse gas emission scenario, the seasonal amplitude of sea surface temperature (SST) is projected to increase by 30% ± 20% by the end of 21st centuryThe intensification of SST seasonality is largely due to the shoaling of the annual mean mixed layer depthThe advection term explains about 10%–40% of the change in SST seasonality depending on the region [ABSTRACT FROM AUTHOR]

The Tropical North Atlantic (TNA) is characterized by significant interannual variability in sea surface temperature (SST), which is phase‐locked to the boreal spring. In this study, the phase‐locking of TNA is investigated by adopting a linear stochastic‐dynamical model (SDM) using seasonally modulated TNA feedbacks together with the seasonal modulation of ENSO forcing. In the observations, the role of local TNA feedbacks and ENSO forcing in TNA phase‐locking are equivalently important with both preferring the peak of TNA variability to appear in the boreal spring. Besides, the seasonal modulation of TNA feedbacks and ENSO forcing strength are both mainly controlled by thermodynamic processes. In most climate models, the contribution of ENSO on TNA phase‐locking is weaker than that in observations. The strength of ENSO‐related TNA phase‐locking is highly correlated with the relationship between ENSO and TNA, which is mainly determined by the amplitude of ENSO and its teleconnection patterns. Plain Language Summary: The interannual variability of sea surface temperature (SST) in the Tropical North Atlantic (TNA) is a prominent climate mode occurring in the Atlantic. TNA SST anomalies usually grow during boreal winter, reach their maximum amplitude in boreal spring and then decay in the summer and fall, called TNA phase‐locking phenomenon. Although understanding the phase‐locking of TNA is important for predicting the TNA and determining the global TNA teleconnection, there is very little discussion of the mechanisms of TNA phase‐locking in the literature. In this study, we develop a conceptual stochastic‐dynamical model (SDM) using seasonally modulated TNA feedbacks with seasonal modulation of ENSO forcing to investigate the features and mechanisms of TNA phase‐locking. The SDM simulations can well reproduce the TNA phase‐locking for observations and CMIP models. The mechanisms of TNA phase‐locking and the contribution of ENSO on TNA phase‐locking are discussed. Key Points: By considering the seasonal modulation of local feedbacks and remote ENSO forcing, the SDM model can well reproduce the TNA phase‐lockingIn observations, the role of local TNA feedbacks and ENSO forcing are equivalently important in TNA phase‐lockingThe TNA phase‐locking strength associated with ENSO is highly correlated with the relationship between ENSO and TNA [ABSTRACT FROM AUTHOR]

The equatorial Pacific warm water volume (WWV) and western equatorial Pacific WWV (WWVw) are two important long‐lead time El Niño‐Southern Oscillation (ENSO) event precursors. Here, we utilize the recharge‐discharge oscillator framework to obtain the analytical solution of WWV and WWVw lead times to ENSO sea surface temperatures (SSTs). Both WWV and WWVw lead times show a strong dependence on the inherent ENSO periodicity, whereas WWV lead time is further modulated by the air‐sea coupling strength and ENSO SST spatial pattern. The derived solution is able to explain observed decadal variations in lead times of WWV to SST in terms of variations in ENSO periodicity and air‐sea coupling strength. Plain Language Summary: Predicting El Niño‐Southern Oscillation (ENSO) events in advance has profound socio‐economic benefits because of ENSO's impacts on global weather and climate. The basin‐wide equatorial Pacific heat content (also known as warm water volume, WWV) and western Pacific WWV (WWVw) are well‐known predictors of ENSO events several seasons ahead. In this study, we present an analytical expression of WWV and WWVw lead times to ENSO sea surface temperature (SST) anomalies. The analytical solution can explain the observed decadal changes in WWV lead times. We demonstrate that the recent shortened WWV lead time observed after the year 2000 can be largely explained by a decrease in the inherent ENSO periodicity, and second by an increase in the air‐sea coupling strength. The latter is associated with an ENSO SST spatial pattern change with more frequent occurrences of Central Pacific El Niño events in recent decades. Key Points: Theoretical lead times of basin‐wide warm water volume (WWV) and western Pacific WWV (WWVw) to El Niño‐Southern Oscillation (ENSO) events are validatedWWV lead time depends on both ENSO periodicity and the ENSO spatial pattern, whereas WWVw lead time depends solely on ENSO periodicityObserved decadal variations of WWV lead times result from variations in ENSO periodicity and air‐sea coupling strength [ABSTRACT FROM AUTHOR]

The Interdecadal Pacific Oscillation (IPO) features a basin‐scale horseshoe‐like sea surface temperature (SST) anomaly pattern. Its cold‐phase shift around 1999, implicated as a driver for the early‐2000s global warming slowdown, has been potentially linked to the Atlantic warming during the positive‐phase Atlantic Multidecadal Variability (AMV). However, the key mechanism for the trans‐basin Atlantic‐Pacific teleconnection remains debatable. Here, we show that an AMV‐SST cooling can initiate a pan‐Pacific response. The North Atlantic cooling induces westerly wind anomalies over the central‐western equatorial Pacific as Kelvin‐wave responses and easterly wind anomalies over the far‐eastern equatorial Pacific as Rossby‐wave responses. Additionally, anomalous lows are generated over the extratropical North and South Pacific through the midlatitude Rossby wave propagation. The tropical and midlatitude teleconnections act together to develop into a warm‐phase IPO‐like pattern through the wind‐induced latent heat. Our results suggest a potential of advancing the predictability of IPO through a skillful simulation of AMV. Plain Language Summary: The Interdecadal Pacific Oscillation (IPO) is the most prominent mode of decadal climate variability over the whole Pacific basin, characterized by a horseshoe‐shaped sea surface temperature (SST) anomaly pattern. Its Atlantic counterpart, the Atlantic Multidecadal Variability (AMV) is characterized by a basin‐wide SST warming or cooling pattern across the North Atlantic. In recent decades, the Atlantic warming strongly tied to the positive phase of AMV has significantly strengthened the tropical Pacific cooling, contributing to the recent global warming hiatus. Yet, the root cause of the trans‐basin Atlantic‐Pacific teleconnection on decadal timescales remains unclear. In this study, using the idealized pacemaker simulations with an imposed AMV‐SST anomaly pattern in a fully coupled model, we rigorously investigate how an AMV‐SST cooling impacts the Pacific‐basin decadal‐scale climate change. We found that an AMV cooling initiates a tropical Gill‐type response and midlatitude Rossby wave trains. The tropical and midlatitude teleconnections work together to form a warm‐phase IPO‐like SST anomaly pattern. Our results highlight that realistic simulation of the AMV may improve the predictability of the IPO state and predictions of global warming rates. Key Points: The Atlantic Multidecadal Variability cooling can drive an Interdecadal Pacific Oscillation‐like warm‐phase patternThe North Atlantic cooling excites a tropical Gill‐type response and midlatitude Rossby wave train responsesBoth the tropical and midlatitude dynamical pathways are central to the decadal‐scale trans‐basin Atlantic‐Pacific connection [ABSTRACT FROM AUTHOR]

Tropical precipitation anomalies show very different patterns over the central‐eastern equatorial Pacific between the developing and decay phases of extreme El Niño despite similar patterns of sea surface temperature (SST) anomalies. Using observational data and atmospheric simulations for 1979–2018, we identified a meridional dipole mode of tropical precipitation variability, called the Pacific precipitation dipole mode (PPDM), which is critical for seasonally varying rainfall patterns related to extreme El Niño. We examined the mechanism of PPDM using a numerical model and found that the seasonal shift of the warm SST band in the eastern equatorial Pacific, when coupled with extreme SST anomalies, causes phase reversal of the PPDM. The PPDM can be regarded as a different manifestation of the El Niño/Southern Oscillation combination mode, as revealed by the surface wind anomalies, which also explains the distinct impact of extreme El Niño events over East Asia. Plain Language Summary: The rainfall patterns over the tropical Pacific are altered by the underling sea surface temperatures (SSTs). The patterns of rainfall anomalies, that is, deviation from climatological mean states, vary between the developing and decay seasons of extreme El Niño, despite similarity in patterns of SST anomalies. Using observational data and atmospheric simulations driven by observed SST for 1979–2018, we identified a meridional dipole mode of tropical rainfall variability that was asymmetric about the equator, called the Pacific precipitation dipole mode (PPDM). PPDM changes the polarity between autumn and spring, before and after the peak of extreme El Niño, and therefore is critical for seasonally varying rainfall patterns related to extreme El Niño. We examined factors that control the polarity of the PPDM by using a mechanistic model of the atmosphere and found that a coupling between SST anomalies associated with extreme El Niño and the seasonal cycle of the warm SST band in the eastern Pacific, migrating between the two hemispheres in a year, causes PPDM to change polarity between autumn and spring. The excitation of PPDM explains a distinct climatic impact over East Asia during extreme El Niño and also a feedback to extreme El Niño. Key Points: The second leading mode of tropical Pacific precipitation variability shows a meridional dipole pattern called the Pacific precipitation dipole mode (PPDM)PPDM amplifies and reverses its sign between developing and decay phases of extreme El Niño, indicating a nonlinear feature of ENSOCoupling between sea surface temperature anomalies and seasonal cycle in the eastern Pacific excites PPDM, which is a manifestation of ENSO combination mode [ABSTRACT FROM AUTHOR]

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

Recent classification of El Nino‐Southern Oscillation (ENSO) into two types, Eastern (EP) and Central (CP) events, has highlighted the importance of the central Pacific. We show here that the local correlation between ENSO subsurface temperatures (Tsub) in the upper 100‐m and thermocline depth anomalies breaks down in the central equatorial Pacific, whereas Tsub remains well correlated with sea surface height anomalies. This observed difference in the central equatorial Pacific is simulated by almost all climate models. It arises from a vertically slanted Tsub anomaly structure that is unique to the central equatorial Pacific. We show that this feature is an adiabatic response to wind‐driving that is even present in a linear dynamic model, as long as the model has enough baroclinic modes to adequately represent the observed vertical complexity. Our findings have implications for better understanding the different importance of thermocline feedback in CP and EP events. Plain Language Summary: The El Niño‐Southern Oscillation (ENSO) is the most prominent interannual variability of the Earth climate system, strongly affecting the global climate. The variations of subsurface temperature, which is often represented by the fluctuations of thermocline depth and sea surface height interchangeably, play a controlling role in the evolutions of sea surface temperature of ENSO cycle. Here, we show that in the central equatorial Pacific ENSO subsurface temperature correlates poorly with thermocline but well with sea surface height. This observed difference occurs commonly in almost all CMIP6 models. We demonstrate that it arises from a unique vertically slanted distribution of the subsurface temperature in the central Pacific in response to winds associated with ENSO. Our findings may have implications for better understanding diverse behaviors of ENSO events occurring in central and eastern equatorial Pacific. Key Points: In the central equatorial Pacific, El Nino‐Southern Oscillation subsurface temperature (Tsub) correlates poorly with thermocline depth but well with sea levelThis kind of difference in Tsub correlations is a robust feature captured by nearly all climate modelsWind‐driven vertically slanted Tsub distribution is largely responsible for this unique feature in the central equatorial Pacific [ABSTRACT FROM AUTHOR]

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

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

A profound change in the variations of warm water volume (WWV) of the tropical Pacific occurred around 1999/2000, coincided with the interdecadal shift of the El Niño–Southern Oscillation (ENSO) properties to a higher frequency and smaller amplitude regime. The persistence of WWV is decreased for all calendar months since 2000, associated with a decrease in its variance and an increase in its frequency. Moreover, the WWV‐sea surface temperature relationship associated with the ENSO cycle is weakened during 2000–2019, implying that the recharge/discharge process is less efficient in driving the phase transition of an ENSO cycle and ENSO became less periodical compared with that during 1979–1999. Corresponding to the weaker precursory connection of WWV to ENSO, the geographical origin of precedent thermocline anomalies has shown a northwestward migration since 2000. This is consistent with the suppression of the equatorial air‐sea feedback and the systematical northwestward shift of the atmosphere‐ocean coupling center in the tropical Pacific. Changed annual cycle and enhanced zonal contrast of sea surface temperature in the equatorial Pacific may be connected with the interdecadal variations of the ENSO and WWV features. Key Points: Amplitude and persistence decreases and northwestward shift of WWV since 2000Less efficient of the recharge/discharge process in driving the phase transition of an ENSO cycle since 2000Less periodical and more irregular ENSO cycle since 2000 [ABSTRACT FROM AUTHOR]

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

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

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

The role of El Niño in influencing the response of the Hadley circulation (HC) to different tropical sea surface temperature (SST) meridional structures is investigated over the period 1979–2016. The explained variance of the equatorially asymmetric principal mode of HC variability is enhanced during El Niño events that are characterized by equatorially symmetric El Niño SST anomalies. It is shown that the response of the HC to different SST meridional structures is greatly enhanced in El Niño events; this is because the SST over the Indo-Pacific warm pool (IPWP) shows inhomogeneous warming in El Niño events, with greater values in the southern IPWP. The asymmetric warming over the IPWP is opposite to the climatological meridional structure of the SST over the IPWP, which intensifies the equatorially symmetric variation of SST, and contributes to the enhanced response of the HC to SST. This point is further verified by the suppression of this enhanced response in El Niño events of the HC to SST when the effects of SST over the IPWP are removed, implying that the different warming amplitude within the IPWP during the El Niño events contributes to the enhanced response. The response in La Niña events is also explored, indicating a difference between El Niño and La Niña events. These results may help explain the corresponding variations of the HC during the El Niño events, and highlight the different influences of the El Niño and La Niña events on the response of the HC to SST. [ABSTRACT FROM AUTHOR]

In this study, the persistence barrier (PB) of the El Niño–Southern Oscillation (ENSO) phenomenon is investigated using reanalysis data and historical simulations of the Coupled Model Intercomparison Project Phase 5 (CMIP5) models. Both the timing and intensity of the ENSO PB can be quantified using the maximum gradient of autocorrelation decline of Niño sea surface temperature (SST) anomaly indices. Most of the CMIP5 models were found to reasonably reproduce the observed timing of the ENSO PB that typically occurs during the boreal late spring to early summer, and underestimated the PB intensity compared to observations. Furthermore, the PB properties of the Eastern Pacific (EP) ENSO indices were much better represented by the models than those of the Central Pacific (CP) ENSO indices, probably because CP ENSO events are more challenging to simulate than their counterparts. Approximately half of the models can satisfyingly reflect the intensity and timing of PB for indices of EP ENSO and their distinctions from those of the CP ENSO, with a larger uncertainty for the modeled PB timing than intensity. Further diagnosis has revealed the relationship between the ENSO PB intensity and the factors associated with the tropical Pacific background state. The PB intensity exhibits a stronger relationship with the seasonality of the SST amplitude in CP, compared to the SST amplitude, and the strength of seasonal synchronization of EP SST anomalies is highly correlated with the PB intensity. These results suggest that the seasonality of tropical SST variability may fundamentally contribute to the ENSO PB. [ABSTRACT FROM AUTHOR]

The relationship between the equatorial Pacific warm water volume (WWV) and El Niño–Southern Oscillation (ENSO) sea surface temperature (SST) has varied considerably on decadal timescales. These changes are strongly related to the occurrence frequency of central Pacific (CP) ENSO events. While both eastern Pacific (EP) and CP ENSO events show clear signatures of WWV recharge/discharge, their phase‐lag relationships between WWV and Niño3.4 SST are different. The WWV usually leads the Niño3.4 SST by two to three seasons during EP ENSO, while the lead time is reduced to one season during CP ENSO. The different phase‐lag relationships can be explained by distinct periodicities of the two ENSO types. Hence, ENSO regime changes associated with decadal predominance of either EP or CP ENSO events can give rise to decadal variations in the statistical WWV‐ENSO SST relationship. We emphasize the importance of identifying these different ENSO types and potentially different ENSO regimes to assess ENSO predictability. Key Points: Relationship between WWV and Niño3.4 SST varies on decadal timescales, corresponding to the occurrence frequency of central Pacific ENSOBoth eastern and central Pacific ENSO events show recharge/discharge signatures but with different WWV/Niño3.4 phase‐lag relationshipsThis difference can be explained by the existence of two different ENSO types characterized by distinct intrinsic periodicities [ABSTRACT FROM AUTHOR]

Arabian Sea (AS) warming has been significantly accelerated since the 1990s, in particular in the spring season. Here we link the AS warming changes to the Atlantic multidecadal oscillation (AMO). A set of Atlantic pacemaker experiments with a slab mixed‐layer ocean model successfully reproduces the AS spring multidecadal variability and its connection with the AMO. An atmospheric teleconnection from the Atlantic to the AS in the preceding winter and associated thermodynamic air‐sea feedback is found to be important. The teleconnection can be reestablished by the atmospheric model when the AMO sea surface temperature (SST) and its trans‐basin footprint over the western Pacific are prescribed simultaneously. The western Pacific SST warming associated with the AMO positive phase induces a Gill‐type Rossby wave over the AS, showing anomalously low pressures and converging southerlies that weaken winter northerlies. Thus, the wind‐evaporation‐SST feedback results in and maintains the AS warm SST anomalies to the subsequent spring. Plain Language Summary: The rapid warming of the Arabian Sea (AS) since the 1990s not only has significant impacts on the monsoon climate change and extreme weather events (flood, heat wave, and cyclone) in Arabian Peninsula and Indian subcontinent but also poses increasingly severe risks of damage to the coastal and marine ecosystems. However, the cause of this recent acceleration of AS warming remains unclear. Here using observations and atmosphere‐ocean coupled models, we link the observed AS warming changes to the Atlantic multidecadal oscillation (AMO) and show that more than 70% of AS multidecadal variability can be explained by the AMO. This Atlantic‐AS teleconnection involves atmosphere‐ocean interactions across multiple ocean basins, with a contribution from the western Pacific (WP). The SST footprint of the AMO over the WP acts as a relay for the effect of the AMO on the AS by exciting an atmospheric teleconnection and subsequently thermodynamic feedbacks over the AS. The concurrent cold‐to‐warm phase shift of the AMO and its WP SST footprint since the 1990s contribute constructively to the rapid warming of the AS. Our results highlight an unexpected multiple‐basin interaction at decadal timescales, which plays a key role in the attribution of historical regional SST warming. Key Points: The observed Arabian Sea warming changes (slowdown and acceleration) are linked to the Atlantic multidecadal oscillation for the first timeThe teleconnection involves air‐sea interactions across multiple ocean basins, with a contribution from western Pacific that acts as a relayThe decadal phase shift of Atlantic‐Western Pacific trans‐basin variability since the 1990s contributes to the rapid warming of Arabian Sea [ABSTRACT FROM AUTHOR]

The spatiotemporal variability of equatorial Pacific upper ocean heat content (HC) and subsurface heat during two types of El Niño-Southern Oscillation (ENSO), namely eastern and central Pacific (EP and CP) types, is investigated using subsurface ocean heat budget analysis. Results show that HC tendencies during both types of ENSO are mainly controlled by oceanic heat advection beneath the mixed layer to the thermocline, and the role of net surface heat flux can be neglected. The most important three terms are the zonal and vertical advections of anomalous heat by climatological currents (QU0T′, QW0T′) and zonal advection of climatological heat by anomalous current (QU′T0). The large contribution of QU0T′ extends from west to east along the equatorial Pacific. The considerable contribution of QU′T0 is confined to the east of 160°W, and that of the QW0T′ is observed in the central Pacific between 180°E and 120°W. In particular, a major contribution of QW0T′ is also observed in the far eastern Pacific east of 100°W during EP ENSO. There is also a small contribution from meridional advection of climatological heat by anomalous current (QV′T0). In contrast, the meridional advection of anomalous heat by climatological currents (QV0T′) and vertical advection of climatological heat by anomalous current (QW′T0) are two damping factors in the HC tendency, with the former dominating. Differences in spatial distribution of the heat advection associated with the two types of ENSO are also presented. We define a warm water heat index (WWH) as integrated heat content above 26 kg m−3 potential density (26σɵ) isopycnal depth within 130°E–80°W and 5°S–5°N. Further examination suggests that the recharge–discharge of WWH is involved in both types of El Niño, though with some differences. First, it takes about 42 (55) months for the evolution of a recharge–discharge cycle during an EP (CP) ENSO. Second, the EP El Niño event peaks during the discharge phase, 7–8 months after the recharge time. The CP El Niño peaks during the recharge phase, 4–5 months before the recharge time. The locations of HC anomalies in the El Niño mature phase relative to those at recharged time explain why the EP and CP El Niño peak in different stages of the recharge–discharge process. [ABSTRACT FROM AUTHOR]

In this study, a simple coupled framework established in Part І is utilized to investigate inter-model diversity in simulating the equatorial Pacific SST annual cycle (SSTAC). It demonstrates that the simulated amplitude and phase characteristics of SSTAC in models are controlled by two internal dynamical factors (the damping rate and phase speed) and two external forcing factors (the strength of the annual and semi-annual harmonic forcing). These four diagnostic factors are further condensed into a dynamical response factor and a forcing factor to derive theoretical solutions of amplitude and phase of SSTAC. The theoretical solutions are in remarkable agreement with observations and CMIP5 simulations. The great diversity in the simulated SSTACs is related to the spreads in these dynamic and forcing factors. Most models tend to simulate a weak SSTAC, due to their weak damping rate and annual harmonic forcing. The latter is due to bias in the meridional asymmetry of the annual mean state of the tropical Pacific, represented by the weak cross-equatorial winds in the cold tongue region. [ABSTRACT FROM AUTHOR]

Tropical Pacific climate varies at interannual, decadal and centennial time scales, and exerts a significant influence on global climate. Climate model projections exhibit a large spread in the magnitude and pattern of tropical Pacific warming in response to greenhouse-gas forcing. Here, we show that part of this spread can be explained by model biases in the simulation of interannual variability, namely the El Niño/Southern Oscillation (ENSO) phenomenon. We show that models that exhibit strong ENSO nonlinearities simulate a more accurate balance of ENSO feedbacks, and their projected tropical Pacific sea surface temperature warming pattern is closely linked to their projected ENSO response. Within this group, models with ENSO nonlinearity close to observed project stronger warming of the cold tongue, whereas models with stronger than observed ENSO nonlinearity project a more uniform warming of the tropical Pacific. These differences are also manifest in the projected changes of precipitation patterns, thereby highlighting that ENSO simulation biases may lead to potentially biased projections in long-term precipitation trends, with great significance for regional climate adaptation strategies. [ABSTRACT FROM AUTHOR]

This work investigates the decadal modulation of the El Niño-Southern Oscillation (ENSO)-East Asian winter monsoon (EAWM) relationship by the Atlantic Multidecadal Oscillation (AMO). A stable ENSO-EAWM relationship is found during the positive AMO phase but not during the negative phase. While the impact of El Niño events on the EAWM does not depend on the AMO phase, a different picture is observed for La Niña events. The La Niña boreal winter season coincides with a strengthened EAWM during a positive AMO phase and a weakened EAWM during a negative AMO phase. We suggest that the AMO's modulating effect mainly comprises two pathways that influence ENSO's impact on the EAWM. On one hand, when La Niña coincides with a positive AMO, the warm SST anomalies over the western North Pacific (WNP) are amplified both in intensity and spatial extent, which favors strengthened WNP cyclonic anomalies and an enhanced EAWM. During La Niña with a negative AMO, only very weak SST anomalies occur over the WNP with reduced WNP cyclonic anomalies that are confined to the tropics, thus having little effect on the EAWM. On the other hand, an eastward-propagating Rossby wavetrain across the mid-high latitudes of Eurasia during a warm AMO phase strengthens the Siberian high and thus leads to a strengthened EAWM, while during a cold AMO phase the Siberian high is weakened, leading to a reduced EAWM. In contrast, El Niño and its associated atmospheric responses are relatively strong and stable, independent of the AMO phase. These results carry important implications to the seasonal-to-interannual predictability associated with ENSO. [ABSTRACT FROM AUTHOR]

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

In this study, using the Bjerknes stability (BJ) index analysis, we estimate the overall linear El Niño-Southern Oscillation (ENSO) stability and the relative contribution of positive feedbacks and damping processes to the stability in historical simulations of Coupled Model Intercomparison Project Phase 5 (CMIP5) models. When compared with CMIP3 models, the ENSO amplitudes and the ENSO stability as estimated by the BJ index in the CMIP5 models are more converged around the observed, estimated from the atmosphere and ocean reanalysis data sets. The reduced diversity among models in the simulated ENSO stability can be partly attributed to the reduced spread of the thermocline feedback and Ekman feedback terms among the models. However, a systematic bias persists from CMIP3 to CMIP5. In other words, the majority of the CMIP5 models analyzed in this study still underestimate the zonal advective feedback, thermocline feedback and thermodynamic damping terms, when compared with those estimated from reanalysis. This discrepancy turns out to be related with a cold tongue bias in coupled models that causes a weaker atmospheric thermodynamical response to sea surface temperature changes and a weaker oceanic response (zonal currents and zonal thermocline slope) to wind changes. [ABSTRACT FROM AUTHOR]

The El Niño/Southern Oscillation (ENSO) is the Earth's most prominent source of interannual climate variability, exerting profound worldwide effects. Despite decades of research, its behaviour continues to challenge scientists. In the eastern equatorial Pacific Ocean, the anomalously cool sea surface temperatures (SSTs) found during La Niña events and the warm waters of modest El Niño events both propagate westwards, as in the seasonal cycle. In contrast, SST anomalies propagate eastwards during extreme El Niño events, prominently in the post-1976 period, spurring unusual weather events worldwide with costly consequences. The cause of this propagation asymmetry is currently unknown. Here we trace the cause of the asymmetry to the variations in upper ocean currents in the equatorial Pacific, whereby the westward-flowing currents are enhanced during La Niña events but reversed during extreme El Niño events. Our results highlight that propagation asymmetry is favoured when the westward mean equatorial currents weaken, as is projected to be the case under global warming. By analysing past and future climate simulations of an ensemble of models with more realistic propagation, we find a doubling in the occurrences of El Niño events that feature prominent eastward propagation characteristics in a warmer world. Our analysis thus suggests that more frequent emergence of propagation asymmetry will be an indication of the Earth's warming climate. [ABSTRACT FROM AUTHOR]

A severe drought struck southwest China during autumn 2009, which had a huge impact on productivity and the lives of the affected population. A nonconventional El Niño, the so-called warm pool (WP) El Niño, was supposed to be a principal factor of this strong autumn drought. In sharp contrast to a conventional El Niño, in the 2009 WP El Niño year the maximum sea surface temperature (SST) anomalies are confined to the central equatorial Pacific Ocean. Moreover, this WP El Niño was characterized by the relatively farther westward location and the strongest intensity among the WP El Niño events in the past 60 years. Observations and modeling studies both indicate that the rainfall deficits over southwest China are significantly influenced by the combined effects of the location and intensity of the WP El Niño. That is, the drought over southwest China tends to be more severe when the warming SST anomalies associated with the WP El Niño are located farther westward and are stronger. Therefore, the strong autumn drought over southwest China in 2009 can be largely attributed to the concurrent distinctive WP El Niño, which generates a strongly anomalous cyclone over the west North Pacific and leads to a serious reduction in rainfall over southwest China. The influence of the Indian Ocean warming on autumn rainfall over southwest China was also examined but seems to have little contribution to this drought. [ABSTRACT FROM AUTHOR]

The El Niño/Southern Oscillation (ENSO) is characterized by two main states: El Niño events defined by positive sea surface temperature anomalies in the eastern tropical Pacific Ocean and La Niña events marked by cooler surface temperatures in the same region. ENSO is broadly considered to be an oscillatory instability of the coupled ocean-atmosphere system in the tropical Pacific that shows a tight interaction with the seasonal cycle. El Niño events typically peak in the boreal winter, but the mechanism governing this phase synchronization is unclear. Here we show, using observational data and climate model experiments, that the nonlinear atmospheric response to combined seasonal and inter-annual sea surface temperature changes gives rise to a near-annual combination climate mode with periods of 10 and 15 months. Specifically, we find that the associated southward shift of westerly wind anomalies during boreal winter and spring triggers the termination of large El Niño events. We conclude that combination mode dynamics and related shifts in western tropical Pacific rainfall patterns occur most prominently during strong El Niño events. [ABSTRACT FROM AUTHOR]

In this study, the authors demonstrate that the two types of El Niño-Southern Oscillation (ENSO) have asymmetric features with respect to the impact of their positive and negative phases on boreal summer rainfall over the Yangtze River Valley (YRV) and South China (SC). The relationship between rainfall over the YRV and the warm pool (WP) La Niña is positive and significant, whereas the relationship with the WP El Niño is not. In the case of the cold tongue (CT) ENSO, its positive phase has a positive influence, while there is no significant relationship with the negative phase. In contrast, rainfall over SC has a significant positive relationship with WP El Niño, but a nonsignificant relationship with WP La Niña. The positive phase of the CT ENSO has a significant negative influence on SC rainfall, while the negative phase has a nonsignificant impact. An asymmetric atmospheric response to the asymmetric sea surface temperature anomalies (SSTAs) was also observed in the lower troposphere. The location of the center of the anomalous circulations over the study region differs during the opposite phases of the two types of ENSO. This asymmetric response is likely to be linked to the different spatial patterns of the two types of El Niño and La Niña. Atmospheric general circulation models confirm the authors' analysis of the observed data. Numerical simulations show that the asymmetric response of the lower atmosphere is driven mainly by differing SSTA patterns in the equatorial Pacific Ocean. [ABSTRACT FROM AUTHOR]

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

An interdecadal shift in the variability and mean state of the tropical Pacific Ocean is investigated within the context of changes in El Niño-Southern Oscillation (ENSO). Compared with 1979-99, the interannual variability in the tropical Pacific was significantly weaker in 2000-11, and this shift can be seen by coherent changes in both the tropical atmosphere and ocean. For example, the equatorial thermocline tilt became steeper during 2000-11, which was consistent with positive (negative) sea surface temperature anomalies, increased (decreased) precipitation, and enhanced (suppressed) convection in the western (central and eastern) tropical Pacific, which reflected an intensification of the Walker circulation. The combination of a steeper thermocline slope with stronger surface trade winds is proposed to have hampered the eastward migration of the warm water along the equatorial Pacific. As a consequence, the variability of the warm water volume was reduced and thus ENSO amplitude also decreased. Sensitivity experiments with the Zebiak-Cane model confirm the link between thermocline slope, wind stress, and the amplitude of ENSO. [ABSTRACT FROM AUTHOR]

In this study, two types of El Niño events are classified based on spatial patterns of the sea surface temperature (SST) anomaly. One is the cold tongue (CT) El Niño, which can be regarded as the conventional El Niño, and the other the warm pool (WP) El Niño. The CT El Niño is characterized by relatively large SST anomalies in the Niño-3 region (5°S–5°N, 150°–90°W), while the WP El Niño is associated with SST anomalies mostly confined to the Niño-4 region (5°S–5°N, 160°E–150°W). In addition, spatial patterns of many atmospheric and oceanic variables are also distinctively different for the two types of El Niño events. Furthermore, the difference in the transition mechanism between the two types of El Niño is clearly identified. That is, the discharge process of the equatorial heat content associated with the WP El Niño is not efficient owing to the spatial structure of SST anomaly; as a result, it cannot trigger a cold event. It is also demonstrated that zonal advective feedback (i.e., zonal advection of mean SST by anomalous zonal currents) plays a crucial role in the development of a decaying SST anomaly associated with theWP El Niño, while thermocline feedback is a key process during the CT El Niño. [ABSTRACT FROM AUTHOR]

Over the period from 1971-2010 global oceans have warmed by about 0.11°C per decade (upper 75 m). This warming is neither spatially, nor seasonally uniform. Here we address the question whether future greenhouse warming will bring about changes in the amplitude and phase of the seasonal cycle of sea surface temperatures (SST). We analyse RCP8.5 scenario simulations conducted with 5 Coupled General Circulation Models, as part of Coupled Model Intercomparison Project Phase 5 (CMIP5). Our results show that the amplitude of the SST seasonal cycle (defined here as the difference between climatological maximum and minimum temperature) increases by a factor of 1.2 to 2.5 towards the end of this Century (2080-2100). The amplification is not homogeneous with the largest changes occurring in the North Atlantic and North Pacific and smallest anomalies in the tropics. The key mechanisms behind this amplification is further analyzed by performing an upper ocean heat budget analysis. Our results suggest that the shoaling of the mean mixed layer depth in response to future warming is a primary driver for the amplification in the North Atlantic. In the North Pacific, the amplification can be traced back to an intensification of the seasonal amplitude of surface heat fluxes. More detailed regional analysis of the amplification mechanisms will be presented.Keywords: Sea surface temperature, Mixed layer depth, seasonal cycle amplification, mixed layer temperature equation, greenhouse warming. [ABSTRACT FROM AUTHOR]