Central-eastern Equatorial Pacific Research Articles

Possible factors responsible for the withdrawal process of the South Asian high during summer-to-winter transition

This study investigates the possible factors responsible for interannual variations in the withdrawal process of the South Asian high (SAH) during its summer-to-winter transition. The SAH is more obviously intensified from July to September and lasts longer over the Tibetan Plateau in late-withdrawal years (L_SAH) than early-withdrawal years (E_SAH). The L_SAH tends to appear during the stage when an El Niño has faded away and a La Niña begins to emerge, with warm SST anomalies occurring over the northern Indian Ocean and tropical northwestern Pacific, and cold anomalies existing across the equatorial central eastern Pacific. The influences of these anomalous SST variations on the SAH's withdrawal process take place primarily through two pathways. On the one hand, warm SST anomalies over the northern Indian Ocean and tropical western Pacific can warm the local tropospheric column above via a Matsunuo-Gill response to moisture adjustment and subsequently lead to an intensified and later withdrawal of the SAH. On the other hand, accompanied by warm SST anomalies, an abnormally intensified anticyclone is observed over the northwestern Pacific that shifts southwestwards and brings abundant rainfall to the region between the Yellow River and Yangtze River in July and August via southwesterly water vapor transport. Consequently, latent heating will generate positive geopotential height anomalies on its northwest side in the upper troposphere and ultimately result in an intensified and later withdrawal of the SAH. In addition, warm surface air temperature over the Tibetan Plateau in September also plays a significant role in lengthening the maintenance of the SAH over the Tibetan Plateau. However, the E_SAH tends be weak and lasts for a shorter period over the Tibetan Plateau. The E_SAH tends to occur together with a developing El Niño event, with cold SST anomalies existing over the northern Indian Ocean and tropical northwestern Pacific. Negative SST anomalies, together with a weak and northeastward-shifting subtropical western Pacific high, result in a weak SAH and quicken its withdrawal process from the Tibetan Plateau via cooling the local tropospheric column and reducing rainfall over East Asia, respectively.

ENSO Spread Caused by Uncertainties in Initial Condition and High-Frequency Perturbations in the Tropical Pacific

Abstract Uncertainties both in the initial condition (IC) and surface wind high-frequency perturbations (HFPs) contribute to the prediction spread of El Niño–Southern Oscillation (ENSO), yet the relative roles of the two are hard to separate, mostly due to the nonlinearity in ENSO dynamics. In this study, we conducted two ensemble experiments using the CESM model to analyze the source of ENSO spread arising from the tropical Pacific. The HFPs were precluded using an online low-pass-filtering scheme in one experiment. Results showed that the uncertainty in ENSO prediction depends on both the IC and HFPs. By comparing simulations with and without HFPs, it is found that the predicted ENSO spread decreased in magnitude when HFPs are removed, especially in the first four prediction months. At integration time longer than that, the additional impact of HFPs cannot be detected on top of that of the uncertainty in IC. To the ENSO spread anomaly, we found that it is tightly associated with uncertainty in the sea surface temperature and surface wind in the central-eastern equatorial Pacific at the short integration time, and associated with the HFPs in the western equatorial Pacific at the long integration time. When the HFPs are removed, the precursor of the ENSO spread anomaly at the long lead time emerges in the northeastern tropical Pacific, the area where the Pacific meridional mode originates from. Our results indicate that the uncertainty in ENSO prediction depends primarily on IC, and the HFPs in the western equatorial Pacific play the secondary role.

Sea surface temperature predictability assessment with an ensemble machine learning method using climate model simulations

Ensemble models, statistical analysis and machine learning (ML) can be used to predict novel conditions in a rapidly changing ocean. Traditionally, ML has been understood as a purely data-driven approach and has been used on both observational and model data to forecast Sea Surface Temperature (SST) anomalies. Here we use ML trained only on climate model simulations to predict regional SST variations, thereby suggesting a novel role for ML as an ensemble model interpolator. We propose a measure of the predictability provided by different ML implementations as well as by standard time series analysis methods. Weighting each forecast by this predictability measure computed on model data only, provides a significant improvement in forecast skill. We demonstrate the performance of this approach for regions around Australia, the Nino3.4 region (central-eastern equatorial Pacific) and in the eastern equatorial Pacific. These analyses show that SST predictability varies as a function of geographical location, area size, seasonality, proximity to the coast and model data quality.

Error Evolutions and Analyses on Joint Effects of SST and SL via Intermediate Coupled Models and Conditional Nonlinear Optimal Perturbation Method

A seasonal predictability barrier has long been noticed in ENSO forecasting with numerical models. Previous studies explored the impact of seasonal optimal initial perturbation evolutions in sea surface temperature anomalies (SSTA) on ENSO forecasting using the intermediate coupled model (ICM) via the conditional nonlinear optimal perturbation (CNOP) method. In this paper, we investigate the joint effects of SSTA and sea level anomalies (SLA) from the perspective of the optimal growth initial error (OGE). After determining the four seasonal OGEs about SSTA and SLA (i.e., SSTA-OGE, SLA-OGE and Joint-OGE), we first demonstrate the patterns, evolutions and the resulting spring predictability barrier (SPB) of the above OGEs. Then, we analyze the mechanism of OGE evolutions and SPB. Finally, we conduct observing system simulation experiments to determine the best (economic) observation network. Our experimental results indicate that the ENSO evolution error induced by SSTA-OGE and Joint-OGE presents season dependency, but SLA-OGE has no impact on ENSO evolution. Moreover, Joint-OGEs induce error evolutions and the SPB with more significant intensity than SSTA-OGEs and SLA-OGEs. From mechanism analyses, the evolutions of SSTA-OGEs are mainly dominated by Bjerknes feedback. Further, the evolution dynamics of Joint-OGEs primarily contain the continuous heating between the upper ocean combined with Bjerknes feedback and thermal diffusion in response to the discharge process. In addition, comprehensive and economical sensitive areas are identified through Joint-OGE, including the central-eastern equatorial Pacific and the western and north-eastern tropical Pacific boundary, which contribute to the ENSO prediction benefits reaching 58.31% on average.

Impacts of the SSTs over Equatorial Central–Eastern Pacific and Southeastern Indian Ocean on the Cold and Rainy/Snowy/Icy Weather in Southern China

Impacts of the SSTs over Equatorial Central–Eastern Pacific and Southeastern Indian Ocean on the Cold and Rainy/Snowy/Icy Weather in Southern China

A spatiotemporal 3D convolutional neural network model for ENSO predictions: A test case for the 2020/21 La Niña conditions

Many coupled models are unable to accurately depict the multi-year La Niña conditions in the tropical Pacific during 2020–22, which poses a new challenge for real-time El Niño–Southern Oscillation (ENSO) predictions. Yet, the corresponding processes responsible for the multi-year coolings are still not understood well. In this paper, reanalysis products are analyzed to examine the ocean–atmosphere interactions in the tropical Pacific that have led to the evolution of sea surface temperature (SST) in the central-eastern equatorial Pacific, including the strong anomalous southeasterly winds over the southeastern tropical Pacific and the related subsurface thermal anomalies. Meanwhile, a divided temporal and spatial (TS) 3D convolution neural network (CNN) model, named TS-3DCNN, was developed to make predictions of the 2020/21 La Niña conditions; results from this novel data-driven model are compared with those from a physics-based intermediate coupled model (ICM). The prediction results made using the TS-3DCNN model for the 2020–22 La Niña indicate that this deep learning–based model can capture the two-year La Niña event to some extent, and is comparable to the IOCAS ICM; the latter dynamical model yields a successful real-time prediction of the Niño3.4 SST anomaly in late 2021 when it is initiated from early 2021. For physical interpretability, sensitivity experiments were designed and carried out to confirm the dominant roles played by the anomalous southeasterly wind and subsurface temperature fields in sustaining the second-year cooling in late 2021. As a potential approach to improving predictions for diversities of ENSO events, additional studies on effectively combining neural networks with dynamical processes and mechanisms are expected to significantly enhance the ENSO prediction capability.摘要2020–22年间热带太平洋经历了持续性多年的拉尼娜事件, 多数耦合模式都难以准确预测其演变过程, 这为厄尔尼诺-南方涛动(ENSO)的实时预测带来了很大的挑战. 同时, 目前学术界对此次持续性双拉尼娜事件的发展仍缺乏合理的物理解释, 其所涉及的物理过程和机制有待于进一步分析. 本研究利用再分析数据产品分析了热带东南太平洋东南风异常及其引起的次表层海温异常在此次热带太平洋海表温度(SST)异常演变中的作用, 并构建了一个时空分离(Time-Space)的三维(3D)卷积神经网络模型(TS-3DCNN)对此次双拉尼娜事件进行实时预测和过程分析. 通过将TS-3DCNN与中国科学院海洋研究所(IOCAS)中等复杂程度海气耦合模式(IOCAS ICM)的预测结果对比, 表明TS-3DCNN模型对2020–22年双重拉尼娜现象的预测能力与IOCAS ICM相当, 二者均能够从2021年初的初始场开始较好地预测2021年末 El Niño3.4区SST的演变. 此外, 基于TS-3DCNN和IOCAS ICM的敏感性试验也验证了赤道外风场异常和次表层海温异常在2021年末赤道中东太平洋海表二次变冷过程中的关键作用. 未来将神经网络与动力 模式模式间的有效结合, 进一步发展神经网络与物理过程相结合的混合建模是进一步提高ENSO事件预测能力的有效途径.

Causal relationship between sea surface temperature and precipitation revealed by information flow

The atmosphere and the ocean are coupled with each other through various processes. Therefore, it is of great importance to understand the causality relationship between the atmosphere and the ocean for predicting their states. Here we apply the normalized information flow (NIF) to sea surface temperature (SST) and precipitation data to investigate the causality from the atmosphere to the ocean and from the ocean to the atmosphere. When the global spatial structure of the local NIFs is calculated for both directions, it is found that the ocean affects the atmosphere more in the tropics, while the atmosphere affects the ocean more in the extratropics. This causality relationship between the ocean and the atmosphere agrees with previous studies. To examine the teleconnections, the remote NIFs are then calculated and compared with the local NIFs. The local impact from SST to precipitation is dominant in almost all tropical regions, while the relative importance of remote impacts is higher for the precipitation-to-SST NIFs, except for a small area in the central-eastern equatorial Pacific and southeastern tropical Indian Ocean. Regional analyses for six selected areas within the tropics are also presented. This study suggests that NIFs may be a powerful tool to study ocean-atmosphere interactions.

Seasonal Prediction for May Rainfall over Southern China Based on the NCEP CFSv2

In this study, we assess the prediction for May rainfall over southern China (SC) by using the NCEP CFSv2 outputs. Results show that the CFSv2 is able to depict the climatology of May rainfall and associated circulations. However, the model has a poor skill in predicting interannual variation due to its poor performance in capturing related anomalous circulations. In observation, the above-normal SC rainfall is associated with two anomalous anticyclones over the western tropical Pacific and northeastern China, respectively, with a low-pressure convergence in between. In the CFSv2, however, the anomalous circulations exhibit the patterns in response to the El Niño-Southern Oscillation (ENSO), demonstrating that the model overestimates the relationship between May SC rainfall and the ENSO. Because of the onset of the South China Sea monsoon, the atmospheric circulation in May over SC is more complex, so the prediction for May SC rainfall is more challenging. In this study, we establish a dynamic-statistical forecast model for May SC rainfall based on the relationship between the interannual variation of rainfall and large-scale ocean-atmosphere variables in the CFSv2. The sea surface temperature anomalies (SSTAs) in the northeastern Pacific and the central-eastern equatorial Pacific, and the 500-hPa geopotential height anomalies over western Siberia in previous April, which exert great influence on the SC rainfall in May, are chosen as predictors. Furthermore, multiple linear regression is employed between the predictors obtained from the CFSv2 and observed May SC rainfall. Both cross validation and independent test show that the hybrid model significantly improve the model's skill in predicting the interannual variation of May SC rainfall by two months in advance.

Recent Decadal Weakening of the Summertime Rainfall Interannual Variability Over Yellow-Huaihe River Valley Attributable to the Western Pacific Cooling

This study focuses on the interdecadal transition of summer rainfall interannual variability over the Yellow-Huaihe River valley (YHRV). It is found that the interannual variability of summer rainfall over the YHRV becomes significantly weakened after the late 2000s. In the decade before the late 2000s (hereafter P1), the variance is 1.28 mm d−1 and the mean value of interannual variability is 1.49 mm d−1. In the decade after the late 2000s (P2), the variance is 0.35 mm d−1 and the mean value of interannual variability is 0.62 mm d−1. The variance and mean value of summer rainfall interannual variability have considerably decreased by 72.7% and 58.4% since the late 2000s, respectively. The reasons for the aforementioned interdecadal transition are explored. The results show that the interdecadal shift of the East Asian jet stream axis and the western Pacific subtropical high (WPSH) are two major factors leading to the interdecadal transition of summer rainfall interannual variability over the YHRV. The northward shift of the jet stream axis, which resembles the Pacific-Japan (PJ) pattern, suppresses the development of summer rainfall over the YHRV. The interannual oscillation of WPSH in P1 is more significant than that in P2, which is consistent with the weakened interannual variability of summer rainfall in P2. Further analysis reveals that the modulation factors responsible for the above changes are El Niño in its decaying phase in the equatorial central eastern Pacific and the warming in the Maritime continent and the western Pacific (WP) in P1. While in P2, the primary modulation factor is La Niña in its developing phase in the equatorial central eastern Pacific. As for the mechanism of the growth of summer rainfall in P1 positive years, the WP warm pool and El Niño in its decaying phase strengthen the Walker circulation in the tropical Pacific and induce Hadley circulation, whose ascending branch over the YHRV provides a favorable condition for the development of rainfall. At the same time, the northward propagation of Rossby waves in response to the above SSTA pattern suppresses convective activities over the Northwestern Pacific (NWP), which indirectly intensifies the WPSH. As a result, the PJ pattern is triggered and provides a favorable large-scale circulation condition for the growth of rainfall.

Comparison of East Asian Summer Monsoon Simulation between an Atmospheric Model and a Coupled Model: An Example from CAS-ESM

In this study, the Chinese Academy of Sciences’ Earth System Model Version 2 (CAS-ESM2) and its atmospheric component were evaluated for the ability to simulate the East Asian summer monsoon (EASM), in terms of climatology and composites in El Niño decaying years (EN) and La Niña years (LN). The results show that the model can realistically simulate the El Niño Southern Oscillation (ENSO) annual cycle, the interannual variation, the evolution process, and the prerequisites of ENSO, but the trend of developing and decaying is faster than that of the observations. With regard to the climatological mean state in the EASM, the coupled model run can largely improve the precipitation and 850 hPa wind simulated in the atmospheric model. Moreover, the coupled run can also reduce the mid-latitude bias in the atmospheric model simulation. Composite methods were then adopted to examine performance in different phases of the ENSO, from a mature winter to a decaying summer. The atmospheric model can well reproduce the Western North Pacific Anomalous Anticyclone (WNPAC)/Western North Pacific Anomalous Cyclone (WNPC) during EN/LN well, but the westerly/easterly anomalies and the associated precipitation anomalies over the equatorial Central Eastern Pacific are somewhat overestimated. Compared with the atmospheric model, these anomalies are all underestimated in the coupled model, which may be related to the ENSO-related SST bias appearing in the Eastern Indian Ocean. Due to the ENSO and ITCZ bias in the historical simulations, the simulated ENSO-related SST and the precipitation anomaly are too equator-trapped in comparison with the observations, and the cold tongue overly extends westward. This limits the ability of the model to simulate ENSO-related EASM variability. For the subseasonal simulations, though atmospheric model simulations can reproduce the westward extension of the Western Pacific subtropic high (WPSH) in EN decaying summers, the eastward retreat of the WPSH in LN is weak. The historical simulations show limited improvement, indicating that the subseasonal variation in the EASM is still a considerable challenge for current generation models.

The Equatorial Pacific Cold Tongue Bias in CESM1 and Its Influence on ENSO Forecasts

Abstract The mean-state bias and the associated forecast errors of the El Niño–Southern Oscillation (ENSO) are investigated in a suite of 2-yr-lead retrospective forecasts conducted with the Community Earth System Model, version 1, for 1954–2015. The equatorial Pacific cold tongue in the forecasts is too strong and extends excessively westward due to a combination of the model’s inherent climatological bias, initialization imbalance, and errors in initial ocean data. The forecasts show a stronger cold tongue bias in the first year than that inherent to the model due to the imbalance between initial subsurface oceanic states and model dynamics. The cold tongue bias affects not only the pattern and amplitude but also the duration of ENSO in the forecasts by altering ocean–atmosphere feedbacks. The predicted sea surface temperature anomalies related to ENSO extend to the far western equatorial Pacific during boreal summer when the cold tongue bias is strong, and the predicted ENSO anomalies are too weak in the central-eastern equatorial Pacific. The forecast errors of pattern and amplitude subsequently lead to errors in ENSO phase transition by affecting the amplitude of the negative thermocline feedback in the equatorial Pacific and tropical interbasin adjustments during the mature phase of ENSO. These ENSO forecast errors further degrade the predictions of wintertime atmospheric teleconnections, land surface air temperature, and rainfall anomalies over the Northern Hemisphere. These mean-state and ENSO forecast biases are more pronounced in forecasts initialized in boreal spring–summer than other seasons due to the seasonal intensification of the Bjerknes feedback.

Impacts of Interannual Variations of Chlorophyll on Seasonal Predictions of the Tropical Pacific

In this study, we explored impacts of interannual variations of chlorophyll on seasonal predictions of the tropical Pacific by the SINTEX-F2 dynamical climate prediction system, which is highly skillful at predicting El Niño/Southern Oscillation (ENSO) and other tropical climate phenomena. We conducted twin re-forecast experiments; one system used the observed climatology of chlorophyll to compute the shortwave absorption in the upper ocean, while the other used the observed chlorophyll with year-to-year variations. Although the chlorophyll impacts on predictions of the Niño 3.4 index were limited, improvements are noticed in the predictions of sea surface temperature over the eastern edge of the Western Pacific Warm Pool. This region corresponds to the separation between warm, low-salinity waters of the warm Pool and cold, high-salinity upwelled waters of the Pacific cold tongue in the central-eastern equatorial Pacific. The improvement was very striking in the 2015 case, when a super El Nino occurred.

El Ni\xf1o/Southern Oscillation inhibited by submesoscale ocean eddies

The El Niño/Southern Oscillation is characterized by irregular warm (El Niño) and cold (La Niña) events in the tropical Pacific Ocean, which have substantial global environmental and socioeconomic impacts. These events are generally attributed to the instability of basin-scale air–sea interactions in the equatorial Pacific. However, the role of sub-basin-scale processes in the El Niño/Southern Oscillation life cycle remains unknown due to the scarcity of observations and coarse resolution of climate models. Here, using a long-term high-resolution global climate simulation, we find that equatorial ocean eddies with horizontal wavelengths less than several hundred kilometres substantially inhibit the growth of La Niña and El Niño events. These submesoscale eddies are regulated by the intensity of Pacific cold-tongue temperature fronts. The eddies generate an anomalous surface cooling tendency during El Niño by inducing a reduced upward heat flux from the subsurface to the surface in the central-eastern equatorial Pacific; the opposite occurs during La Niña. This dampening effect is missing in the majority of state-of-the-art climate models. Our findings identify a pathway to resolve the long-standing overestimation of El Niño and La Niña amplitudes in climate simulations.

The Intraseasonal Variations of the Leading Mode of Summer Precipitation Anomalies in Meiyu Area of East Asia

The intraseasonal variations of summer precipitation anomalies in the Meiyu area of East Asia are analyzed by applying a combined empirical orthogonal function (CEOF) of the latest meteorological reanalysis data ERA5 of European Center for Medium-Range Weather Forecasts for the period from 1991 to 2020, and the circulation structures and sources of variability of CEOF are also investigated. The first mode of the intraseasonal variations shows an in-phase pattern over the Meiyu area in June, July, and August, accounting for 22.2% of the total variance in the intraseasonal variations of summer precipitation anomalies. The positive (negative) CEOF1 is accompanied by the negative (positive) East Asia/Pacific pattern, including strong westerly wind anomalies in the upper troposphere and southwest monsoon in the lower troposphere, and the Western Pacific Subtropical High extending westward and its ridge line slightly south. The positive CEOF1 is preceded by decay of El Niño episodes, including the abnormal warm sea surface temperature anomalies (SSTAs) in the equatorial Central-Eastern Pacific in spring and warm SSTAs in the equatorial Indian Ocean in summer. The second mode shows an opposite precipitation anomaly in June and July, and the distribution in August is not significant. The corresponding geopotential height circulation of positive CEOF2 shows the large negative anomaly in the region north of 40° N and a positive anomaly over Japan in June, whereas the pattern reverses in July. At the same time, there is a radical reversion from abnormal eastly to westly wind in the upper troposphere. The structure of CEOF2 is somewhat induced by local SSTAs over the Northern Indian Ocean and South China Sea.

Dynamic Causes of ENSO Decay and Its Asymmetry

AbstractEl Niño and La Niña exhibit asymmetric evolution characteristics during their decay phases. The decay speed of El Niño is significantly greater than that of La Niña. This study systematically and quantitatively investigates the relative contributions of the equatorial western Pacific (WP) and central-eastern Pacific (CEP) wind stress anomalies to ENSO decay and its asymmetry through data analysis, numerical experiments, and dynamic and thermodynamic diagnoses. It is demonstrated that the sea surface temperature anomalies (SSTAs) forced by the wind stress anomalies in the equatorial CEP play a dominant role in ENSO decay and contribute to ENSO decay asymmetry, while the forcing by the equatorial WP wind stress anomalies has a small contribution. Diagnoses of the oceanic mixed layer heat budget indicate that anomalous zonal advection term and vertical advection term forced by the wind stress anomalies in the equatorial CEP are the most important dynamic terms contributed to ENSO decay. Both terms in El Niño decay phase are much larger than in La Niña decay phase, resulting in a larger decay speed in El Niño than in La Niña. The contributions of these two terms do not depend on the equatorial WP wind field, confirming that the equatorial WP wind stress anomalies do not act as a pivotal part in ENSO asymmetric decay. Moreover, it is demonstrated that within the equatorial CEP, dominant contribution comes from the wind stress anomalies in the equatorial central Pacific, in which those in the equatorial southern central Pacific play a major role.Significance StatementPrevious studies proposed why wind fields in the equatorial western Pacific (WP) or central-eastern Pacific (CEP) are asymmetric and how the asymmetric wind fields affect ENSO decay and decay asymmetry. By using an oceanic general circulation model, we quantitatively estimate the relative contributions of the wind stress anomalies over the equatorial WP and CEP. It is demonstrated that the wind stress anomalies over the equatorial CEP and the associated ocean response play a dominant role in the asymmetric decay. Additionally, it is further illustrated the predominant role comes from the wind stress anomalies in the equatorial southern central Pacific within the equatorial CEP. Our study provides a physical explanation on the ENSO decay and its asymmetry.

Interaction between the Tropical Atlantic and Pacific Oceans on an Interannual Time Scale

ABSTRACT Based on the fifth-generation reanalysis of the European Centre for Medium-range Weather Forecasts (ECMWF), the Hadley Centre sea surface temperature (SST) data, and the Global Precipitation Climatology Project (GPCP) precipitation data, the interaction between the tropical Atlantic and Pacific Oceans on an interannual time scale is investigated using correlation, regression, and composite analyses, and the quantitative estimates of the relative and collective contribution of the tropical North Atlantic (NTA) and equatorial Atlantic (ETA) SST anomalies to the El Niño–Southern Oscillation (ENSO) are presented. An El Niño or La Niña event can cause a same-sign SST anomaly in the NTA in the following boreal spring, accounting for 33% of the interannual variability of the NTA SST. The NTA SST anomaly may excite a same-sign SST anomaly of the ETA in boreal summer with an ETA SST anomaly of 0.44°C per 1°C NTA SST anomaly. Moreover, an SST anomaly in the NTA and ETA can cause an opposite-sign SST anomaly in the equatorial Pacific during the following winter. The NTA SST anomaly influences the central equatorial Pacific with a Niño 3.4 SST anomaly of −0.85°C per 1°C NTA SST anomaly, while the ETA SST anomaly exerts an impact on the central-eastern equatorial Pacific with a Niño 3.4 SST anomaly of −1.85°C per 1°C ETA SST anomaly based on a multiple regression analysis. The composite analyses based on Ensemble-Based Predictions of Climate Changes and Their Impacts (ENSEMBLES) hindcasts show that the ETA SST anomaly in summer acts as a primary contributor to the interaction between the tropical Atlantic and Pacific Oceans. The ETA impact can relay and amplify the NTA SST anomaly such that it affects the equatorial Pacific. The collective impact of spring NTA and summer ETA SST anomalies can explain 33% of the total variance of the winter Niño 3.4 index. This study deepens our understanding of the interannual interaction between the tropical Atlantic and Pacific Oceans and highlights the role of the tropical Atlantic Ocean, especially the ETA region, in the pan-tropical air–sea interaction.

Mechanisms linking multi-year La Ni\xf1a with preceding strong El Ni\xf1o

El Niño-Southern Oscillation (ENSO), characterized by anomalous sea surface temperature in the central-eastern equatorial Pacific, is a dominant interannual variability, impacting worldwide weather and socioeconomics. The ENSO cycle contains irregularity, in which La Niña often persists for more than two years, called “multi-year La Niña”. Observational records show that multi-year La Niña tends to accompany strong El Niño in the preceding year, but their physical linkage remains unclear. Here we show using reanalysis data that a strong El Niño excites atmospheric conditions that favor the generation of multi-year La Niña in subsequent years. Easterly wind anomalies along the northern off-equatorial Pacific during the decay phase of the strong El Niño are found crucial as they act to discharge ocean heat content (OHC) via an anomalous northward Ekman transport. The negative OHC anomaly is large enough to be restored by a single La Niña and, therefore, causes another La Niña to occur in the second year. Furthermore, analyses of the Coupled Model Intercomparison Project Phase 6 (CMIP6) climate models support the abovementioned mechanisms and indicate that the occurrence frequencies of multi-year La Niña and strong El Niño are highly correlated.

The role of background SST changes in the ENSO-driven rainfall variability revealed from the atmospheric model experiments in CMIP5/6

The tropical Pacific rainfall variability driven by El Niño–Southern Oscillation (ENSO rainfall) is the crucial bridge across which ENSO exerts its influence on global climate. Previous modeling studies have documented a robust intensification and eastward shift of ENSO-driven rainfall under global warming despite large inter-model biases in ENSO-related sea surface temperature variability (ENSO SST), but the formation mechanisms of the robust ENSO rainfall changes are not well clarified. Here, we utilized a set of atmospheric model experiments—namely, “amip”, forced by observed SST; “amip-p4K”, forced by observed SST plus uniform 4-K warming, and “amip-future4K”, forced by observed SST plus spatially patterned SST warming, in CMIP5/CMIP6—to disentangle the role of the changes in mean-state SST (uniform and spatially patterned SST warming) with an unchanged ENSO SST. Relative to the weak ENSO rainfall changes in amip-p4K, those in amip-future4K forced by an El Niño-like SST warming pattern show an increase and eastward shift over the equatorial central–eastern Pacific. The results in amip-future4K without ENSO SST changes coincide well with previous results of the coupled models in CMIP5/3, suggesting a dominant role played by the mean-state SST warming pattern in ENSO rainfall changes. By diagnosing the moisture budget decomposition and moist static energy transport, we revealed that the structural and amplitude changes in ENSO rainfall are mainly controlled by the dynamic and thermodynamic components, respectively. In addition, the El Niño-like SST warming pattern in amip-future4K can increase low-level specific humidity with a remarkable spatial gradient, particularly in the equatorial central–eastern Pacific. As a result, the low-level specific humidity dominates the pattern of changes in the vertical transport of moist static energy, and subsequently the ENSO rainfall changes, in amip-future4K. This study further verified the importance of background SST changes to ENSO rainfall under global warming.

The impact of tropical Atlantic SST variability on the tropical atmosphere during boreal summer

AbstractThe interannual variability of boreal summer sea surface temperature (SST) in the tropical Atlantic displays two dominant modes, the Atlantic zonal mode highlighting SST variations in the equatorial–southern tropical Atlantic (ESTA) region and the northern tropical Atlantic (NTA) mode focusing on SST fluctuations in the NTA region except in the Gulf of Guinea. Observational evidence indicates that both the boreal summer ESTA and NTA warming are accompanied by a pair of anomalous low-level anti-cyclones over the western tropical Pacific, and the NTA-related anti-cyclone is more obvious than the ESTA-related one. Both atmosphere-only and partially coupled experiments conducted with the Community Earth System Model Version 1.2 support the observed NTA–Pacific teleconnection. In contrast, the ESTA-induced atmospheric circulation response is negligible over the tropical Pacific in the atmosphere-only experiments, and though the response becomes stronger in the partially coupled experiments, obvious difference still exists between the simulations and observation. The ESTA-induced atmospheric circulation response is featured by an anomalous low-level cyclone over the western tropical Pacific in the partially coupled experiments, opposite to its observed counterpart. It is found that the ESTA warming coincides with significantly La Niña-like SST anomalies in the central–eastern equatorial Pacific, the influence of which on the tropical atmospheric circulation is opposite to that of the ESTA warming, and therefore contributes to difference between the ESTA-related simulations and observation. Moreover, the cold climatological mean SST in the ESTA region is unfavourable to enhancing the ESTA–Pacific teleconnection during boreal summer.

Linking the Madden–Julian Oscillation, tropical cyclones and westerly wind bursts as part of El Niño development

Westerly wind bursts (WWBs) that occur over the western and central equatorial Pacific are believed to play an important role in ENSO dynamics; however, the mechanisms of WWB generation are still debated. In this study we investigate a link between the Madden–Julian Oscillation (MJO) and WWBs that involves tropical cyclones (TCs) generated within the MJO sufficiently close to the equator. Using an atmospheric reanalysis and WWB composites, we first isolate WWBs that occur (1) between December and April and (2) between May and November, corresponding to the onset and development stages of El Nino. We find that during the onset stage, a warm background sea surface temperature (SST) anomaly develops over the central-western equatorial Pacific with anomalous westerly winds to its west, which draws the MJO convective activity in the southern hemisphere closer to the equator. As a result, westerly wind anomalies associated with the MJO together with tropical cyclones embedded in the MJO induce strong WWBs (during neutral conditions the MJO usually takes a more southerly route and cannot induce strong bursts). Subsequently, during the development stage of El Nino, there develops an anomalous warming in the central-eastern equatorial Pacific, also with corresponding westerly winds, which steers the MJO path, now in the northern hemisphere, toward the equator, strengthening the MJO signal over the central equatorial Pacific. Consequently, tropical cyclones modulated by the MJO move closer to the equator and farther east, facilitating WWBs. Thus, MJO events with embedded tropical cyclones are crucial for the generation process of WWBs during the onset and development of El Nino.