El Niño‐Southern Oscillation (ENSO), which dominates variability on interannual timescale in the climate system, is known to exhibit various spatio‐temporal characteristics. Recent studies show that in additional to a canonical El Niño with its major center of sea surface temperatures (SST) anomalies in the equatorial Pacific cold‐tongue region, a different type of El Niño with its major action center shifted to the warm‐pool edge has emerged and become more common during the past two decades. Because the SST patterns of these two types of El Niño events are highly correlated, neither of the traditional Niño3 and Niño4 SST indices alone is effective in representing the new‐type El Niño. Through a simple transformation of the Niño3 and Niño4 indices, we devised two new indices that separately identify the two types of ENSO events. Unlike the Niño3 and Niño4 indices, the two new indices are of little simultaneous correlation. The SST patterns associated with these new indices capture SST characteristics of the two types of ENSO. Their running lagged‐correlations capture different ENSO‐phase propagations and ENSO regime changes associated with the climate shift in 1976/77.

Atmospheric circulation anomalies associated with the interannual El Niño-Southern Oscillation (ENSO) phenomenon 1 exert global impacts on the climate system 2. El Niño events are characterised by positive sea surface temperature anomalies in the eastern equatorial Pacific, whereas La Niña events exhibit an anomalously cold sea surface. ENSO is considered an oscillatory instability of the tropical Pacific coupled ocean-atmosphere system 1, 3-6. The boreal winter peak of El Niño events and the seasonal variance modulation of associated eastern equatorial sea surface temperature anomalies 2 , often referred to as phase-locking 7 , document ENSO's tight interaction with the seasonal cycle. To date there exists no established theory for ENSO's synchronisation with the annual cycle. In the present study, we show that seasonal changes in the western tropical Pacific warm pool region interact with El Niño, giving rise to a near-annual combination climate mode with periods of 10 and 15 months. Associated wind changes trigger the termination of large El Niño events 8 , thereby controlling ENSO's seasonal synchronisation and predictability. This combination mode is shown to cause massive shifts of Earth's largest rainbands, impacting human livelihoods across the Asia-Pacific region and beyond. Current ENSO theories, such as the Recharge Oscillator Paradigm 9 , while very successful in explaining some observational features of ENSO, do not account for the interaction between interannual and seasonal timescales. In particular, they do not provide any insight into why El Niño events peak toward the end of the calendar year (in December-January-February: DJF) and terminate in the subsequent months. Previous extensions to these theories that rely on nonlinear concepts 10-13 , such as subharmonic frequency locking and frequency entrainment, nonlinear resonance, the quasi-periodic transition to chaos or parametric excitation 7, 14, 15 , capture some aspects of ENSO / annual cycle interactions. None of these extended dynamical systems' concepts, however, describe the observational finding 16 that a weakening and southward shift of westerly wind anomalies on the equator accompanies the termination of strong El Niño events. Numerous modelling studies 8, 17-20 have confirmed this observational evidence, thus supporting the notion of strong annual cycle / ENSO interactions originating in the tropical western Pacific. Here, we set out to provide a simple unifying dynamical framework to understand various aspects of ENSO, such as the physics of seasonallypaced El Niño transitions, spectral characteristics and ENSO's hydroclimatic impacts. The seasonal weakening and southward shift of westerly wind anomalies that contributes to the transition between El Niño and La Niña can be readily described in terms of an Empirical

The El Niño–Southern Oscillation (ENSO) tends to behave arguably as two different “types” or “flavors” in recent decades. One is the canonical cold-tongue-type ENSO with major sea surface temperature anomalies (SSTA) positioned over the eastern Pacific. The other is a warm-pool-type ENSO with SSTA centered in the central Pacific near the edge of the warm pool. In this study, the basic features and main feedback processes of these two types of ENSO are examined. It is shown that the interannual variability of upper-ocean heat content exhibits recharge–discharge processes throughout the life cycles of both the cold tongue (CT) and warm pool (WP) ENSO types. Through a heat budget analysis with focus on the interannual frequency band, the authors further demonstrate that the thermocline feedback plays a dominant role in contributing to the growth and phase transitions of both ENSO types, whereas the zonal advective feedback contributes mainly to their phase transitions. The westward shift of the SSTA center of the WP ENSO and the presence of significant surface easterly wind anomalies over the far eastern equatorial Pacific during its mature warm phase are the two main factors that lead to a reduced positive feedback for the eastern Pacific SSTA. Nevertheless, both the WP and CT ENSO can be understood to a large extent by the recharge oscillator mechanism.

This work contrasts the climatic impacts of so-called warm-pool (WP) and cold-tongue (CT) El Niño on the atmospheric circulation over the western North Pacific (WNP). It is found that the anomalous atmospheric circulation over the WNP is nearly opposite in response to these two types of El Niño events in developing autumn. A weak anomalous anticyclone appears over the WNP during CT El Niño, whereas a weak anomalous cyclone emerges in the same region during WP El Niño. These nearly opposite autumn responses of atmospheric circulation have a significant impact on East Asian climate, and southern China autumn rainfall in particular, although this contrast tends to diminish as El Niño events enter their mature phase.

An interdecadal shift in the variability and mean state of the tropical Pacific Ocean is investigated within the context of changes in El Niñ o-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.

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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.

ENSO is the strongest interannual signal in the global climate system with worldwide climatic, ecological and societal impacts. Over the past decades, the research about ENSO prediction and predictability has attracted broad attention. With the development of coupled models, the improvement in initialization schemes and the progress in theoretical studies, ENSO has become the most predictable climate mode at the time scales from months to seasons. This paper reviews in detail the progress in ENSO predictions and predictability studies achieved in recent years. An emphasis is placed on two fundamental issues: the improvement in practical prediction skills and progress in the theoretical study of the intrinsic predictability limit. The former includes progress in the couple models, data assimilations, ensemble predictions and so on, and the latter focuses on efforts in the study of the optimal error growth and in the estimate of the intrinsic predictability limit.