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Atmospheric Rivers (ARs), corridors of intensive water vapor transport, have received much attention recently due to their potentially large economic and societal impacts. However, a comprehensive understanding of the interannual variability of ARs over East Asia is still lacking. This study examines the frequency of AR occurrence over East Asia based on the EAR5 reanalysis. It is shown that the frequency of AR occurrence over East Asia displays obvious seasonality, with its peak in early boreal summer. ARs contribute to up to 50% of total precipitation and largely modulate the interannual variability of precipitation in East Asia during early boreal summer. The interannual variability of ARs over East Asia during early boreal summer is largely controlled by the Indo-western Pacific Ocean Capacitor (IPOC) effect, which is mainly triggered by El Niño-Southern Oscillation and sometimes by Indian Ocean dipole. During early boreal summer, a higher frequency of AR occurrence over East Asia is associated with anomalous anticyclonic circulation (AAC) over the Indo-western Pacific region. The AAC, the atmospheric signal of the IPOC mode, indicates an enhanced East Asian summer monsoon and a northwestward shift of the western North Pacific subtropical high, favoring the AR occurrence over East Asia. This study illustrates that a strong positive Indian Ocean dipole event could impact AR activity over East Asia in the following summer by triggering the IPOC effect. The IPOC-related signals 2–4 months ahead could be useful to predict the AR occurrence over East Asia during early boreal summer. [ABSTRACT FROM AUTHOR]

The 2019 positive Indian Ocean Dipole (IOD) was the strongest event since the 1960s which developed independently without coinciding El Niño. The dynamics is not fully understood. Here we show that in March–May, westward propagating oceanic Rossby waves, a remnant consequence of the weak 2018 Pacific warm condition, led to anomalous sea surface temperature warming in the southwest tropical Indian Ocean (TIO), inducing deep convection and anomalous easterly winds along the equator, which triggered the initial cooling in the east. In June–August, the easterly wind anomalies continued to evolve through ocean‐atmosphere coupling involving Bjerknes feedback and equatorial nonlinear ocean advection, until its maturity in September–November. This study clarifies the contribution of oceanic Rossby waves in the south TIO in different dynamic settings and reveals a new triggering mechanism for extreme IOD events that will help to understand IOD diversity. Plain Language Summary: The Indian Ocean Dipole (IOD) is an ocean‐atmosphere coupled climatic phenomenon which can cause severe social and economic losses in the surrounding regions such as drought in the Maritime Continent/Australia and flooding in East Africa. The IOD features a see‐saw structure accompanied by an anomalous sea surface temperature gradient, winds, and oceanic adjustments. The El Niño–Southern Oscillation in the Pacific is an important trigger to a strong IOD event. However, an extreme positive IOD event occurred in 2019 without a concurrent or ensuing El Niño. We show that the thermocline warming associated with anomalous ocean downwelling in the southwest tropical Indian Ocean triggered atmospheric convection, inducing anomalous easterly winds along the equator and hence, positive feedbacks associated with an IOD event. This study may help to understand the evolution of extreme IOD and improve IOD predictability. Key Points: The oceanic downwelling Rossby waves in the south tropical Indian Ocean is key to the 2019 extreme positive Indian Ocean Dipole (IOD)The Rossby waves induced thermocline warming, triggering wind‐evaporation‐SST feedback thus anomalous easterly winds along the equatorThe easterly wind anomalies further triggered the Bjerknes feedback and other positive feedbacks and established an extreme IOD [ABSTRACT FROM AUTHOR]

This study identifies a new triggering mechanism of the Indian Ocean Dipole (IOD) from the Southern Hemisphere. This mechanism is independent from the El Niño‐Southern Oscillation (ENSO) and tends to induce the IOD before its canonical peak season. The joint effects of this mechanism and ENSO may explain different lifetimes and strengths of the IOD. During its positive phase, development of sea surface temperature cold anomalies commences in the southern Indian Ocean, accompanied by an anomalous subtropical high system and anomalous southeasterly winds. The eastward movement of these anomalies enhances the monsoon off Sumatra‐Java during May–August, leading to an early positive IOD onset. The pressure variability in the subtropical area is related with the Southern Annular Mode, suggesting a teleconnection between high‐latitude and midlatitude climate that can further affect the tropics. To include the subtropical signals may help model prediction of the IOD event. Plain Language Summary: An Indian Ocean Dipole (IOD) at its positive phase featuring anomalously high and low sea surface temperature (SST) in the west and east equatorial Indian Ocean, respectively, shifts atmosphere convection westward, causing severe floods and droughts in surrounding west and east Indian Ocean‐rim regions. Known triggering mechanisms, such as the external El Niño‐Southern Oscillation (ENSO), cannot explain development or intensity of many IOD events. Here we find a novel triggering mechanism of the IOD from the Southern Hemisphere, in which the subtropical high pressure and wind anomalies forcing cool SST anomalies evolve to trigger onset of an IOD event via enhancing the monsoon off Sumatra‐Java. This Southern Hemisphere Mechanism can operate independently from the ENSO and commences earlier than the ENSO forcing, thus providing explanation of different IOD characteristics and a longer prediction lead time. Key Points: A triggering mechanism of the IOD originated from the Southern Hemisphere is identifiedThe Southern Hemisphere Mechanism is independent from ENSO and associated with subtropical and high‐latitude climatic variabilityThe joint effects of such mechanism and ENSO produce different characteristics of the IOD [ABSTRACT FROM AUTHOR]

This study investigates the influence of southeast tropical Indian Ocean (SETIO) sea surface temperature (SST) warming on Indo-Pacific climate during the decaying phase of the 2015/16 El Niño by using observations and model experiments. The results show that the SETIO SST warming in spring 2016 enhanced local convection and forced a “C-shape” wind anomaly pattern in the lower troposphere. The “C-shape” wind anomaly pattern over the eastern tropical Indian Ocean consists of anomalous westerly flow south of the equator and anomalous easterly flow north of the equator. The anomalous easterly flow then extended eastward into the western North Pacific (WNP) and facilitates the development or the maintenance of an anomalous anticyclone over the South China Sea (SCS). Correspondingly, the eastern part of the Bay of Bengal, the SCS and the WNP suffered less rainfall. Such precipitation features and the associated “C-shape” wind anomaly pattern shifted northward about five latitudes in summer 2016. Additionally, the SETIO warming can induce local meridional circulation anomalies, which directly affect Indo-Pacific climate. Numerical model experiments further confirm that the SETIO SST warming plays an important role in modulating Indo-Pacific climate. [ABSTRACT FROM AUTHOR]

The Indo-Pacific Convergence Zone (IPCZ) has a complex ocean dynamical system. All scale processes are active and interplay from small-scale turbulent mixing to basin-scale circulation. The IPCZ acts as an "oceanic bridge" for the inter-basin mass transports and basin-scale planetary waves, closely linking basin-scale circulations in the Pacific and Indian Oceans. Numerous straits in the Indonesian Seas provide oceanic channels for planetary waves propagating between the tropical Pacific and the southeast Indian Ocean. On a large scale, the inter-basin mass transports and planetary waves change the ocean thermal structure, triggering strong air-sea interactions, and further regulating the variability of Walker and Hadley Circulations. The latter form an "atmospheric bridge", which significantly affects the Asian and Australian monsoon systems and a series of global climate events like El Niño-Southern Oscillation (ENSO), Indian Ocean Dipole, and Indian Ocean Basinwide warming. On meso- and small- scales, eddy, submesoscale processes, and turbulent motions mix the water mass from different sources, dramatically stirring the upper ocean in the IPCZ. Ocean dynamics have essential impacts on the ecological system in the IPCZ. On the one hand, these processes directly affect biodiversity by controlling the genetic exchanges and species dispersals between basins. On the other hand, the ocean dynamical processes influence biogeochemical cycling by controlling the trophic transfer, which further impacts primary productivity. With the intensified climate variations under global warming, e.g., marine heatwaves and extreme events, the ocean dynamical processes turn more active and enhance the influence on the ecosystem. This work overviews a series of studies focusing on the multi-scale ocean dynamical and environmental processes in the IPCZ, including the climatic signatures in coral records and the ecological response. [ABSTRACT FROM AUTHOR]

The Indo-western Pacific Ocean capacitor (IPOC) effect induces coherent climate variations over the Indo-western Pacific Ocean with significant sea-surface temperature (SST) anomalies over the South China Sea (SCS). Most IPOC events are related to El Ni n ∼ o-Southern Oscillation (ENSO), occurring in the post-ENSO years. The coral δ18O and Sr/Ca precisely recorded climate variations as proxies for SST. This study investigated the IPOC via a reduced major axis (RMA) regression analysis based on coral records in the SCS. The coral records of Yongxing Island show a double SST peak during El Niño events after the 1960s, one in the mature phase and the other in the post-El Ni n ∼ o summer. Tropical Indian Ocean warming persists into summer and anchors an anomalous anticyclonic circulation over the western North Pacific, extending the ENSO effect on the SCS SST through the IPOC effect, despite the decay of El Ni n ∼ o. The corals from Yongle Atoll, Dongsha Atoll, and the region off central Vietnam well recorded the IPOC impacts associated with two strong El Ni n ∼ o events (2009–2010 and 1997–1998) on SST anomalies in summer. According to coral records off central Vietnam, the 1959 IPOC effect occurred without an antecedent El Ni n ∼ o event. The results show that coral proxies could capture the IPOC events in the SCS and prove that IPOC could act as an intrinsic climate mode in the Indo-western Pacific Ocean. • Coral proxies from Yongxing Island record the IPOC events since 1960. • Most IPOC events recorded by corals are associated with the El Ni n ∼ o events. • Some IPOC events, e.g., recorded off central Vietnam in 1959, occur without antecedent El Ni n ∼ o events. [ABSTRACT FROM AUTHOR]

In 2015, the Indian Ocean exhibits an exceptionally weakened CO2 uptake, highlighting strong interannual variability of ocean carbon sink. By utilizing multiple ocean CO2 partial pressure (pCO2) data and a state‐of‐the‐art ocean biogeochemical model, we show that the 2015 ocean CO2 anomaly is characterized by a basin‐wide amplification of ocean pCO2, differing from ocean pCO2 responses to other Indian Ocean Dipole events (e.g., 1997 and 2019). The distinct ocean pCO2 anomaly is attributed to an amplified warming and an unprecedented weakening Indonesian Throughflow under the influence of co‐occurrence of positive IOD and extreme El Niño in 2015. The amplified warming drives higher ocean pCO2 in the western and central Indian Ocean, while the ITF transports anomalously high ocean pCO2 water from the Pacific Ocean to the southeastern Indian Ocean. This newly identified ocean carbon response provides deeper insights into the Indian Ocean carbon interannual variability. [ABSTRACT FROM AUTHOR]

Using Sea-viewing Wide Field-of-view Sensor (SeaWiFS) ocean color data, we investigated the biological responses to oceanic Rossby waves in the southern tropical Indian Ocean (TIO) during 1997–2007. The findings indicate that, during the developing phase of El Niño/La Niña events, usually in boreal fall, and triggered by anomalous anticyclonic/cyclonic wind circulations in the southeast TIO, downwelling/upwelling Rossby waves form and then propagate westward. After a few months, downwelling/upwelling Rossby waves interface with the thermocline dome in the southern TIO, and suppress/enhance the upwelling. Correspondingly, less/more nutrient-rich waters enter the mixed layer and result in lower/higher chlorophyll concentrations. Due to the asymmetric effects on the thermocline dome between downwelling and upwelling Rossby waves, higher chlorophyll concentrations appear earlier and in the eastern part of the dome, whereas lower chlorophyll concentrations appear later and in the central part of the dome. Moreover, when El Niño/La Niña–Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) events coincide, the biological responses are stronger. [ABSTRACT FROM AUTHOR]

This study evaluates the simulation of the Indian Ocean Basin (IOB) mode and relevant physical processes in models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). Historical runs from 20 CMIP5 models are available for the analysis. They reproduce the IOB mode and its close relationship to El Niño-Southern Oscillation (ENSO). Half of the models capture key IOB processes: a downwelling oceanic Rossby wave in the southern tropical Indian Ocean (TIO) precedes the IOB development in boreal fall and triggers an antisymmetric wind anomaly pattern across the equator in the following spring. The anomalous wind pattern induces a second warming in the north Indian Ocean (NIO) through summer and sustains anticyclonic wind anomalies in the northwest Pacific by radiating a warm tropospheric Kelvin wave. The second warming in the NIO is indicative of ocean-atmosphere interaction in the interior TIO. More than half of the models display a double peak in NIO warming, as observed following El Niño, while the rest show only one winter peak. The intermodel diversity in the characteristics of the IOB mode seems related to the thermocline adjustment in the south TIO to ENSO-induced wind variations. Almost all the models show multidecadal variations in IOB variance, possibly modulated by ENSO. [ABSTRACT FROM AUTHOR]

The tropical Indian Ocean dipole/zonal mode (IOD) is phase locked with the austral winter and spring seasons. This study describes three types of the IOD in terms of their peak time and duration. In particular, the authors focus on a new type that develops in May-June and matures in July-August, which is distinctively different from the canonical IOD, which may develop later and peak in September-November or persist from June to November. Such 'unseasonable' IOD events are only observed since the mid-1970s, a period after which the tropical Indian Ocean has a closer relationship with the Pacific Ocean. The unseasonable IOD is an intrinsic mode of the Indian Ocean and occurs without an ensuing El Niño. A change in winds along the equator is identified as a major forcing. The wind change is in turn related to a weakening Walker circulation in the Indian Ocean sector in austral winter, which is in part forced by the rapid Indian Ocean warming. Thus, although the occurrence of the unseasonable IOD may be partially influenced by oceanic variability, the authors' results suggest an influence from the Indian Ocean warming. This suggestion, however, awaits further investigation using fully coupled climate models. [ABSTRACT FROM AUTHOR]

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

El Niño induces a basin-wide increase in tropical Indian Ocean (TIO) sea surface temperature (SST) with a lag of one season. The north IO (NIO), in particular, displays a peculiar double-peak warming with the second peak larger in magnitude and persisting well through the summer. Motivated by recent studies suggesting the importance of the TIO warming for the Northwest Pacific and East Asian summer monsoons, the present study investigates the mechanisms for the second peak of the NIO warming using observations and general circulation models. This analysis reveals that internal air–sea interaction within the TIO is key to sustaining the TIO warming through summer. During El Niño, anticyclonic wind curl anomalies force a downwelling Rossby wave in the south TIO through Walker circulation adjustments, causing a sustained SST warming in the tropical southwest IO (SWIO) where the mean thermocline is shallow. During the spring and early summer following El Niño, this SWIO warming sustains an antisymmetric pattern of atmospheric anomalies with northeasterly (northwesterly) wind anomalies north (south) of the equator. Over the NIO as the mean winds turn into southwesterly in May, the northeasterly anomalies force the second SST peak that persists through summer by reducing the wind speed and surface evaporation. Atmospheric general circulation model experiments show that the antisymmetric atmospheric pattern is a response to the TIO warming, suggestive of their mutual interaction. Thus, ocean dynamics and Rossby waves in particular are important for the warming not only locally in SWIO but also on the basin-scale north of the equator, a result with important implications for climate predictability and prediction. [ABSTRACT FROM AUTHOR]

This study investigates the interannual modes of the tropical Pacific using salinity from observations, ocean reanalysis output and CMIP6 products. Here we propose two indices of sea surface salinity (SSS), a monopole mode and a dipole mode, to identify the El Niño—South Oscillation (ENSO) and its diversity, respectively. The monopole mode is primarily controlled by atmospheric forcing, namely, the enhanced precipitation that induces negative SSS anomalies across nearly the entire tropical Pacific. The dipole mode is mainly forced by oceanic dynamics, with zonal current transporting fresh water from the western fresh pool into the western-central and salty water from the subtropics into the eastern tropical Pacific. Under a global warming condition, an increase in the monopole and dipole mode variance indicates an increase in both the central and eastern Pacific El Niño variability. The increase in central Pacific El Niño variability is largely due to enhanced vertical stratification during global warming in the upper layer, with intensified zonal advection. An eastern Pacific El Niño-like warming pattern contributes to the increase in eastern Pacific El Niño, with enhanced precipitation over the central-eastern tropical Pacific. [ABSTRACT FROM AUTHOR]

This study examines the climate response to a sea surface temperature (SST) warming imposed over the southwest tropical Indian Ocean (TIO) in a coupled ocean–atmosphere model. The results indicate that the southwest TIO SST warming can remotely modulate the atmospheric circulation over the western North Pacific Ocean (WNP) via interbasin air–sea interaction during early boreal summer. The southwest TIO SST warming induces a "C shaped" wind response with northeasterly and northwesterly anomalies over the north and south TIO, respectively. The northeasterly wind anomalies contribute to the north TIO SST warming via a positive wind–evaporation–SST (WES) feedback after the Asian summer monsoon onset. In June, the easterly wind response extends into the WNP, inducing an SST cooling by WES feedback on the background trade winds. Both the north TIO SST warming and the WNP SST cooling contribute to an anomalous anticyclonic circulation (AAC) over the WNP. The north TIO SST warming, WNP SST cooling, and AAC constitute an interbasin coupled mode called the Indo-western Pacific Ocean capacitor (IPOC), and the southwest TIO SST warming could be a trigger for IPOC. While the summertime southwest TIO SST warming is often associated with antecedent El Niño, the warming in 2020 seems to be related to extreme Indian Ocean dipole during the autumn of 2019. The strong southwest TIO SST warming seems to partly explain the strong summer AAC of 2020 over the WNP even without a strong antecedent El Niño. [ABSTRACT FROM AUTHOR]

Indian summer monsoon (ISM) rainfall during 2016 exhibited a prominent month-to-month fluctuations over India, with below normal rainfall in June and August and above normal rainfall in July. The factors determining the month-to-month fluctuations in ISM rainfall during 2016 are investigated with main focus on the Indo-Pacific climatic anomalies. Warm sea surface temperature (SST) anomalies associated with super El Niño 2015 disappeared by early summer 2016 over the central and eastern Pacific. On the other hand, negative Indian Ocean dipole (IOD) like SST anomaly pattern over the equatorial Indian Ocean and anomalous anticyclonic circulation over the western North Pacific (WNP) are reported in summer 2016 concurrently with decaying El Niño/developing La Niña phase. Observations revealed that the low rainfall over central north India in June is due to moisture divergence caused by the westward extension of ridge corresponding to WNP anticyclone and subsidence induced by local Hadley cell partly related to negative IOD. Low level convergence of southeasterly wind from Bay of Bengal associated with weak WNP anticyclone and northwesterly wind corresponding to anticyclonic circulation over the northwest India remarkably contributed to positive rainfall in July over most of the Indian subcontinent. While reduced rainfall over the Indian subcontinent in August 2016 is associated with the anomalous moisture transport from ISM region to WNP region, in contrast to July, due to local cyclogenesis corroborated by number of tropical cyclones in the WNP. In addition to this, subsidence related to strong convection supported by cyclonic circulation over the WNP also resulted in low rainfall over the ISM region. Coupled General Circulation model sensitivity experiments confirmed that strong convective activities associated with cyclonic circulation over the WNP is primarily responsible for the observed negative ISM rainfall anomalies in August 2016. It is noted that the Indo-Western Pacific circulation anomalies in August 2016 are well predicted when the coupled model is initiated with initial conditions from end of July and beginning of August compared to May. This analysis suggests the importance of the WNP circulation in forcing strong sub-seasonal/month to month rainfall variations over India. [ABSTRACT FROM AUTHOR]

Previous studies have shown that models overestimate the strength of ENSO teleconnections to the North Pacific during springtime, but the underlying reasons for this bias remain unknown. In this work, the relative contributions from basic-state and thermodynamic/dynamic forcing factors are disentangled through idealized experiments with the Community Earth System Model and a range of stationary wave modeling experiments. It is revealed that in CESM1 the diabatic heating biases over the tropical Indian Ocean and tropical central-western Pacific jointly favor a cyclonic (anticyclonic) circulation bias to occur in the North Pacific during the springtime of El Niño (La Niña) events. On one hand, the difference in the modeled and observed climatological basic state does not lead to the bias formation directly, as the diabatic heating biases are the primary cause. On the other hand, the springtime basic state is conducive to a more vigorous stationary wave response to the biased diabatic heating than the wintertime state, and this explains why the teleconnection bias occurs during springtime but not in winter. An iterative bias-correction approach is then implemented in the atmospheric model component of CESM1 to verify the linkage between the tropical diabatic heating bias and the teleconnection bias. Moreover, this explanation is shown to be relevant in other models of phase 5 of the Coupled Model Intercomparison Project (CMIP5) as a strong relationship is found between biases in ENSO-related tropical central-western Pacific/Indian Ocean precipitation and North Pacific circulation across models in spring. Significance Statement: The purpose of this study is to explain why climate models tend to overestimate the springtime ENSO teleconnection to the North Pacific. Through both simplified and comprehensive model experiments, we found that the diabatic heating biases over the tropical Indian Ocean and central-western Pacific basins are the main cause behind the circulation bias. Although similar heating biases also occur in winter, the spring mean climate state is more sensitive to the biased heating than the winter mean state. These findings are useful for developing future climate models that would better simulate the springtime climate response during the ENSO events, as the same problem can be found in many other models. [ABSTRACT FROM AUTHOR]

Interannual-to-decadal variability and trends of sea level in the South China Sea (SCS) are studied using altimetric data during 1993-2012 and reconstructed sea level data from 1950-2009. The interannual variability shows a strong seasonality. Surface wind anomalies associated with El Niño-Southern Oscillation explain the sea-level anomaly pattern in the interior SCS, while Rossby waves radiated from the eastern boundary dominate the sea-level variability in the eastern SCS. Decadal variability of sea level in the SCS follows that in the western tropical Pacific, with large variance found west of Luzon Island. Local atmospheric forcing makes a negative contribution to decadal variability in the central SCS, and Rossby waves radiated from the eastern boundary appear to be important. During 1993-2012, decadal sea level averaged in the SCS is significantly correlated with the Pacific Decadal Oscillation (PDO) ( r = −0.96). The decadal variability associated with the PDO accounts for most part of sea-level trends in the SCS in the last two decades. [ABSTRACT FROM AUTHOR]

ENSO induces coherent climate anomalies over the Indo-western Pacific, but these anomalies outlast SST anomalies of the equatorial Pacific by a season, with major effects on the Asian summer monsoon. This review provides historical accounts of major milestones and synthesizes recent advances in the endeavor to understand summer variability over the Indo-Northwest Pacific region. Specifically, a large-scale anomalous anticyclone (AAC) is a recurrent pattern in post-El Ni˜no summers, spanning the tropical Northwest Pacific and North Indian oceans. Regarding the ocean memory that anchors the summer AAC, competing hypotheses emphasize either SST cooling in the easterly trade wind regime of the Northwest Pacific or SST warming in the westerly monsoon regime of the North Indian Ocean. Our synthesis reveals a coupled ocean-atmosphere mode that builds on both mechanisms in a two-stage evolution. In spring, when the northeast trades prevail, the AAC and Northwest Pacific cooling are coupled via wind-evaporation-SST feedback. The Northwest Pacific cooling persists to trigger a summer feedback that arises from the interaction of the AAC and North Indian Ocean warming, enabled by the westerly monsoon wind regime. This Indo-western Pacific ocean capacitor (IPOC) effect explains why El Ni˜no stages its last act over the monsoonal Indo-Northwest Pacific and casts the Indian Ocean warming and AAC in leading roles. The IPOC displays interdecadal modulations by the ENSO variance cycle, significantly correlated with ENSO at the turn of the 20th century and after the 1970s, but not in between. Outstanding issues, including future climate projections, are also discussed. [ABSTRACT FROM AUTHOR]

The correlation between sea surface temperature (SST) and El Niño-Southern Oscillation (ENSO) persists into post-ENSO September over the South China Sea (SCS), the longest correlation in the World Ocean. Slow modulations of this correlation are analyzed by using the International Comprehensive Ocean-Atmosphere Dataset (ICOADS). ENSO's influence on SCS SST has experienced significant interdecadal changes over the past 138 years (1870-2007), with a double-peak structure correlation after the 1960s compared to a single-peak before the 1940s. According to the ENSO correlation character, the analysis period is divided into four epochs. In epoch 3, 1960-83, the SST warming and enhanced precipitation over the southeastern tropical Indian Ocean, rather than the Indian Ocean basinwide warming, induce easterly wind anomalies and warm up the SCS in the summer following El Niño. Besides the Indian Ocean effect, during epochs 2 (1930-40) and 4 (1984-2007), the Pacific-Japan (PJ) pattern of atmospheric circulation anomalies helps sustain the SCS SST warming through summer (June-August) with easterly wind anomalies. The associated increase in shortwave radiation and decrease in upward latent heat flux cause the SCS SST warming to persist into the summer. Meanwhile, the rainfall response around the SCS to ENSO shows interdecadal variability, with stronger variability after the 1980s. The results suggest that both the remote forcing from the tropical Indian Ocean and the PJ pattern are important for the ENSO teleconnection to the SCS and its interdecadal modulations. [ABSTRACT FROM AUTHOR]

Reports on a warm event during spring 1997 to spring 1999 in the South China Sea. The intensity and duration of the record event; Association with the severe flood over the Yangtze River valley; Assessment of the evolution process with several data sets, such as sea surface temperature, height and wind stress, as well as subsurface temperature; How the warm event's onset almost teleconnects with El Niño; Effect on the summer and winter monsoons in 1997; Presence of westerly anomalies in the South China Sea; How southerly anomalies affect thermocline adjustment and the downwelling pattern; End of the warm event in May 1999.

El Niño induces an anomalous lower-tropospheric anticyclone over the tropical Northwest Pacific (NWP), accompanied by suppressed local convection and rainfall. The tropical NWP anomalies persist until the following summer, with major effects on the Asian summer monsoons. Based on the phase 5 of the Coupled Model Intercomparison Project (CMIP5) multi-model ensemble, this study finds that climate models commonly underestimate this El Niño-NWP teleconnection with too weak tropical NWP anticyclone and rainfall anomalies in post-El Niño summers, potentially limiting the models' skill in seasonal prediction of the Asian summer monsoons. The analyses show that such underestimated NWP anomalies in post-El Niño summers in CMIP5 models can be traced back to the well-known excessive equatorial Pacific cold tongue error in the mean. Models with an excessive westward extension of Pacific cold tongue tend to displace westward the simulated pattern of El Niño-related warm SST anomalies along the equator. The warm SST biases over the western Pacific in CMIP5 models would enhance the local atmospheric convection/rainfall and induce low-level cyclonic circulation anomalies over the tropical NWP with a Gill-type Rossby wave response, resulting in the commonly underestimated NWP anticyclone and rainfall anomalies during post-El Niño summers. The present results, along with our previous finding that the equatorial cold tongue bias would distort the projections of tropical Pacific warming pattern under increased greenhouse gas scenario, imply that reducing equatorial cold tongue bias in models can substantially improve climate simulation and prediction/projection for the tropical Pacific and Asian monsoons. [ABSTRACT FROM AUTHOR]

Using 20 models of the Coupled Model Intercomparison Project Phase 5 (CMIP5), the simulation of the Southwest Indian Ocean (SWIO) thermocline dome is evaluated and its role in shaping the Indian Ocean Basin (IOB) mode following El Ni˜no investigated. In most of the CMIP5 models, due to an easterly wind bias along the equator, the simulated SWIO thermocline is too deep, which could further influence the amplitude of the interannual IOB mode. A model with a shallow (deep) thermocline dome tends to simulate a strong (weak) IOB mode, including key attributes such as the SWIO SST warming, antisymmetric pattern during boreal spring, and second North Indian Ocean warming during boreal summer. Under global warming, the thermocline dome deepens with the easterly wind trend along the equator in most of the models. However, the IOB amplitude does not follow such a change of the SWIO thermocline among the models; rather, it follows future changes in both ENSO forcing and local convection feedback, suggesting a decreasing effect of the deepening SWIO thermocline dome on the change in the IOB mode in the future. [ABSTRACT FROM AUTHOR]

The present study investigates interdecadal modulations of the El Niño-Southern Oscillation (ENSO) influence on the climate of the northwest Pacific (NWP) and East Asia (EA) in early boreal summer following a winter ENSO event, based on 19 simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5). In the historical run, 8 out of 19 models capture a realistic relationship between ENSO and NWP early summer climate-an anomalous anticyclone develops over the NWP following a winter El Niño event-and the interdecadal modulations of this correlation. During periods when the association between ENSO and NWP early summer climate is strong, ENSO variance and ENSO-induced anomalies of summer sea surface temperature (SST) and tropospheric temperature over the tropical Indian Ocean (TIO) all strengthen relative to periods when the association is weak. In future projections with representative concentration pathways 4.5 and 8.5, the response of TIO SST, tropospheric temperature, and NWP anomalous anticyclone to ENSO all strengthen regardless of ENSO amplitude change. In a warmer climate, low-level specific humidity response to interannual SST variability strengthens following the Clausius-Clapeyron equation. The resultant intensification of tropospheric temperature response to interannual TIO warming is suggested as the mechanism for the strengthened ENSO effect on NWP-EA summer climate. [ABSTRACT FROM AUTHOR]

Significant climate anomalies persist through the summer (June–August) after El Niño dissipates in spring over the equatorial Pacific. They include the tropical Indian Ocean (TIO) sea surface temperature (SST) warming, increased tropical tropospheric temperature, an anomalous anticyclone over the subtropical northwest Pacific, and increased mei-yu–baiu rainfall over East Asia. The cause of these lingering El Niño effects during summer is investigated using observations and an atmospheric general circulation model (GCM). The results herein indicate that the TIO warming acts like a capacitor anchoring atmospheric anomalies over the Indo–western Pacific Oceans. It causes tropospheric temperature to increase by a moistadiabatic adjustment in deep convection, emanating a baroclinic Kelvin wave into the Pacific. In the northwest Pacific, this equatorial Kelvin wave induces northeasterly surface wind anomalies, and the resultant divergence in the subtropics triggers suppressed convection and the anomalous anticyclone. The GCM results support this Kelvin wave–induced Ekman divergence mechanism. In response to a prescribed SST increase over the TIO, the model simulates the Kelvin wave with low pressure on the equator as well as suppressed convection and the anomalous anticyclone over the subtropical northwest Pacific. An additional experiment further indicates that the north Indian Ocean warming is most important for the Kelvin wave and northwest Pacific anticyclone, a result corroborated by observations. These results have important implications for the predictability of Indo–western Pacific summer climate: the spatial distribution and magnitude of the TIO warming, rather than simply whether there is an El Niño in the preceding winter, affect summer climate anomalies over the Indo–western Pacific and East Asia. [ABSTRACT FROM AUTHOR]