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Distinctive Precursory Air-Sea Signals between Regular and Super El Niños

doi: 10.1007/s00376-016-5250-8

  • Statistically different precursory air-sea signals between a super and a regular El Niño group are investigated, using observed SST and rainfall data, and oceanic and atmospheric reanalysis data. The El Niño events during 1958-2008 are first separated into two groups: a super El Niño group (S-group) and a regular El Niño group (R-group). Composite analysis shows that a significantly larger SST anomaly (SSTA) tendency appears in S-group than in R-group during the onset phase [April-May(0)], when the positive SSTA is very small. A mixed-layer heat budget analysis indicates that the tendency difference arises primarily from the difference in zonal advective feedback and the associated zonal current anomaly (u'). This is attributed to the difference in the thermocline depth anomaly (D') over the off-equatorial western Pacific prior to the onset phase, as revealed by three ocean assimilation products. Such a difference in D' is caused by the difference in the wind stress curl anomaly in situ, which is mainly regulated by the anomalous SST and precipitation over the Maritime Continent and equatorial Pacific.
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  • Balmaseda M. A., K. Mogensen, and A. T. Weaver, 2013: Evaluation of the ECMWF ocean reanalysis system ORAS4. Quart. J. Roy. Meteor. Soc., 139, 1132- 1161.10.1002/ A new operational ocean reanalysis system (ORAS4) has been implemented at ECMWF. It spans the period 1958 to the present. This article describes its main components and evaluates its quality. The adequacy of ORAS4 for the initialization of seasonal forecasts is discussed, along with the robustness of some prominent climate signals. ORAS4 has been evaluated using different metrics, including comparison with observed ocean currents, RAPID-derived transports, sea-level gauges, and GRACE-derived bottom pressure. Compared to a control ocean model simulation, ORAS4 improves the fit to observations, the interannual variability, and seasonal forecast skill. Some problems have been identified, such as the underestimation of meridional overturning at 26, the magnitude of which is shown to be sensitive to the treatment of the coastal observations. ORAS4 shows a clear and robust shallowing trend of the Pacific Equatorial thermocline. It also shows a clear and robust nonlinear trend in the 0–700 m ocean heat content, consistent with other observational estimates. Some aspects of these climate signals are sensitive to the choice of sea-surface temperature product and the specification of the observation-error variances. The global sea-level trend is consistent with the altimeter estimate, but the partition into volume and mass variations is more debatable, as inferred by discrepancies in the trend between ORAS4- and GRACE-derived bottom pressure.
    Carton J. A., B. S. Giese, 2008: A Reanalysis of ocean climate using simple ocean data assimilation (SODA). Mon. Wea. Rev., 136, 2999-
    Chao J. P., R. H. Zhang, 1990: The air-sea interaction waves in the tropics and their instabilities. Acta Meteorologica Sinica, 48, 46- 54. (in Chinese) using a simple air-sea coupled model,the interaction of Rossby waves between the air and sea inthe tropics is discussed.It is shown that the coupling of Rossby waves in the two media produces notonly the westward propagating waves,but also a type of new wave which moves eastward.The eastwardpropagating waves exist in the scope of comparatively long wavelengths and this scope is governed bythe intensity of the air-sea interaction.In addition,instability may appear in both the eastward and west-ward propagating waves,and the wave amplifying rates are also governed by the intensity of the air-seainteraction.In the end,a possible explanation to ENSO events is given in terms of the air-sea interactionwaves.
    Chen, D. K., Coauthors, 2015a: Strong influence of westerly wind bursts on El Niño diversity. Nature Geosci., 8, 339- 345.10.1038/ the tremendous progress in the theory, observation and prediction of El Ni09o over the past three decades, the classification of El Ni09o diversity and the genesis of such diversity are still debated. This uncertainty renders El Ni09o prediction a continuously challenging task, as manifested by the absence of the large warm event in 2014 that was expected by many. We propose a unified perspective on El Ni09o diversity as well as its causes, and support our view with a fuzzy clustering analysis and model experiments. Specifically, the interannual variability of sea surface temperatures in the tropical Pacific Ocean can generally be classified into three warm patterns and one cold pattern, which together constitute a canonical cycle of El Ni09o/La Ni09a and its different flavours. Although the genesis of the canonical cycle can be readily explained by classic theories, we suggest that the asymmetry, irregularity and extremes of El Ni09o result from westerly wind bursts, a type of state-dependent atmospheric perturbation in the equatorial Pacific. Westerly wind bursts strongly affect El Ni09o but not La Ni09a because of their unidirectional nature. We conclude that properly accounting for the interplay between the canonical cycle and westerly wind bursts may improve El Ni09o prediction.
    Chen L., T. Li, and Y. Q. Yu, 2015b: Causes of strengthening and weakening of ENSO amplitude under global warming in four CMIP5 models. J.Climate, 28, 3250- 3274.10.1175/ Available
    Chen L., Y. Q. Yu, and W.-P. Zheng, 2016: Improved ENSO simulation from climate system model FGOALS-g1.0 to FGOALS-g2, Climate Dyn., 1-18, doi: 10.1007/s00382-016-2988-8.10.1007/ study presents an overview of the improvement in the simulation of El Ni甯給-Southern Oscillation (ENSO) in the latest generation of the Institute of Atmospheric Physics' coupled general circulation model (CGCM), the Flexible Global Ocean-Atmosphere-Land System model Grid-point Version 2 (FGOALS-g2; hereafter referred to as "g2") from its predecessor FGOALS-g1.0 (referred to as "g1"), including the more realistic amplitude, irregularity, and ENSO cycle. The changes have been analyzed quantitatively based on the Bjerknes stability index, which serves as a measure of ENSO growth rate. The improved simulation of ENSO amplitude is mainly due to the reasonable representation of the thermocline and thermodynamic feedbacks: On the one hand, the deeper mean thermocline results in a weakened thermocline response to the zonal wind stress anomaly, and the looser vertical stratification of mean temperature leads to a weakened response of anomalous subsurface temperature to anomalous thermocline depth, both of which cause the reduced thermocline feedback in g2; on the other hand, the alleviated cold bias of mean sea surface temperature leads to more reasonable thermodynamic feedback in g2. The regular oscillation of ENSO in g1 is associated with its unsuccessful representation of the role of atmospheric noise over the western-central equatorial Pacific (WCEP) in triggering ENSO events, which arises from the weak synoptic-intraseasonal variability of zonal winds over the WCEP in g1. The asymmetric transition of ENSO in g1 is attributed to the asymmetric effect of thermocline feedback, which is due to the annual cycle of mean upwelling in the eastern Pacific. This study highlights the great impact of improving the representation of mean states on the improved simulation of air-sea feedback processes and ultimately more reasonable depiction of ENSO behaviors in CGCMs.
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    Ding R. Q., J. P. Li, and Y.-H. Tseng, 2015: The impact of South Pacific extratropical forcing on ENSO and comparisons with the North Pacific. Climate Dyn., 44, 2017- 2034.10.1007/ studies suggest that North Pacific extratropical atmospheric variability influences ENSO via the seasonal footprinting mechanism (SFM). This study confirms that quadrapole sea surface temperature (SST) variability in the extratropical South Pacific triggered by mid-latitude South Pacific atmospheric variability may also have an additional influence on ENSO. The response of the evolution of the ENSO-related zonal wind and SST anomalies in the tropics to the South Pacific extratropical forcing is consistent with the SFM hypothesis. That is, the Pacific outh American (PSA) pattern of the South Pacific extratropical sea level pressure (SLP) anomalies imparts an SST footprint (i.e., a quadrapole SST pattern) onto the ocean during austral summer. This SST footprint subsequently forces the zonal wind anomalies along the equator in the following austral winter that ultimately result in ENSO events during the following austral summer via ocean-tmosphere coupling in the tropics. The present study demonstrates that the influences of extratropical atmospheric variability in the South Pacific and North Pacific on ENSO are different and relatively independent. It is possible that they may, together or separately, influence the occurrence of ENSO events, and the importance of the South Pacific forcing in initiating ENSO events is comparable with that of the North Pacific forcing. An empirical model was established to predict the Ni09o3.4 index based on the combined South Pacific and North Pacific signals, and results show that it can be used to produce skillful forecasts of the Ni09o3.4 index with a leading time of up to 102year.
    Eisenman I., L. Yu, and E. Tziperman, 2005: Westerly wind bursts: ENSO's tail rather than the dog? J.Climate, 18, 5224- 5238.10.1175/ wind bursts (WWBs) in the equatorial Pacific occur during the development of most El Ni09o events and are believed to be a major factor in ENSO- dynamics. Because of their short time scale, WWBs are normally considered part of a stochastic forcing of ENSO, completely external to the interannual ENSO variability. Recent observational studies, however, suggest that the occurrence and characteristics of WWBs may depend to some extent on the state of ENSO components, implying that WWBs, which force ENSO, are modulated by ENSO itself. Satellite and in situ observations are used here to show that WWBs are significantly more likely to occur when the warm pool is extended eastward. Based on these observations, WWBs are added to an intermediate complexity coupled ocean-tmosphere ENSO model. The representation of WWBs is idealized such that their occurrence is modulated by the warm pool extent. The resulting model run is compared with a run in which the WWBs are stochastically applied. The modulation of WWBs by ENSO results in an enhancement of the slow frequency component of the WWBs. This causes the amplitude of ENSO events forced by modulated WWBs to be twice as large as the amplitude of ENSO events forced by stochastic WWBs with the same amplitude and average frequency. Based on this result, it is suggested that the modulation of WWBs by the equatorial Pacific SST is a critical element of ENSO- dynamics, and that WWBs should not be regarded as purely stochastic forcing. In the paradigm proposed here, WWBs are still an important aspect of ENSO- dynamics, but they are treated as being partially stochastic and partially affected by the large-scale ENSO dynamics, rather than being completely external to ENSO. It is further shown that WWB modulation by the large-scale equatorial SST field is roughly equivalent to an increase in the ocean-tmosphere coupling strength, making the coupled equatorial Pacific effectively self-sustained.
    Fedorov A. V., S. N. Hu, M. Lengaigne, and E. Guilyardi, 2015: The impact of westerly wind bursts and ocean initial state on the development, and diversity of El Niño events. Climate Dyn., 44, 1381- 1401.10.1007/ wind bursts (WWBs) that occur in the tropical Pacific near the Dateline are believed to play an important role in the development of El Ni09o events, even though the direct link is sometimes difficult to establish. Here, following the study of Lengaigne et al. (2004), we conduct numerical simulations in which we reexamine the response of the climate system to an observed wind burst added to a coupled model. Two sets of ensemble experiments are conducted. In the first set, the initial ocean heat content of the system is higher than the model climatology (or recharged), while in the second set it is nearly normal (neutral). For the recharged state, in the absence of WWBs, a moderate central Pacific El Ni09o (CP) develops in about a year. In contrast, for the neutral state, there develops a weak La Ni09a. However, when the WWB is imposed, the situation changes dramatically: the recharged state slides into an eastern Pacific El Ni09o (EP), while the neutral set shifts into a weak CP El Ni09o instead of previous La Ni09a conditions. The different response of the system to the exact same perturbations is controlled by the initial state of the ocean and the subsequent ocean-atmosphere interactions involving an interplay between the eastward shift of the warm pool and the warming of the eastern equatorial Pacific. Consequently, the occurrence of different flavors of El Ni09o, including extreme events, may depend on stochastic atmospheric processes, modulating El Ni09o properties within a broad continuum.
    Gebbie G., I. Eisenman, A. Wittenberg, and E. Tziperman, 2007: Modulation of westerly wind bursts by sea surface temperature: a semistochastic feedback for ENSO. J. Atmos. Sci., 64, 3281- 3295.10.1175/ - Document Details (Isaac Councill, Lee Giles, Pradeep Teregowda): Westerly wind bursts (WWBs) in the equatorial Pacific are known to play a significant role in the development of El Ni09o events. They have typically been treated as a purely stochastic external forcing of ENSO. Recent observations, however, show that WWB characteristics depend upon the large-scale SST field. The consequences of such a WWB modulation by SST are examined using an ocean general circulation model coupled to a statistical atmosphere model (i.e., a hybrid coupled model). An explicit WWB component is added to the model with guidance from a 23-yr observational record. The WWB parameterization scheme is constructed such that the likelihood of WWB occurrence increases as the western Pacific warm pool extends: a -渟emistochastic -� formulation, which has both deterministic and stochastic elements. The location of the WWBs is parameterized to migrate with the edge of the warm pool. It is found that modulation of WWBs by SST strongly affects the characteristics of ENSO. In particular, coupled feedbacks between SST and WWBs may be sufficient to transfer the system from a damped regime to one with self-sustained oscillations. Modulated WWBs also play a role in the irregular timing of warm episodes and the asymmetry in the size of warm and cold events in this ENSO model. Parameterizing the modulation of WWBs by an increase of the linear air-搒ea coupling coefficient seems to miss important dynamical processes, and a purely stochastic representation of WWBs elicits only a weak ocean response. Based upon this evidence, it is proposed that WWBs may need to be treated as an internal part of the coupled ENSO system, and that the detailed knowledge of wind burst dynamics may be necessary to explain the characteristics of ENSO. 1.
    Gill A. E., 1980: Some simple solutions for heat-induced tropical circulation. Quart. J. Roy. Meteor. Soc., 106, 447- 462.10.1002/ - Scientific documents that cite the following paper: Some simple solutions for heat-induced tropical circulation
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    Hu S.-N., A. V. Fedorov, 2016: Exceptionally strong easterly wind burst stalling El Niño of 2014. Proc. Natl. Acad. Sci. U. S. A., 113, 2005- 2010.10.1073/ wind bursts in the tropical Pacific are believed to affect the evolution and diversity of El Ni09o events. In particular, the occurrence of two strong westerly wind bursts (WWBs) in early 2014 apparently pushed the ocean-tmosphere system toward a moderate to strong El Ni09o-攑otentially an extreme event according to some climate models. However, the event- progression quickly stalled, and the warming remained very weak throughout the year. Here, we find that the occurrence of an unusually strong basin-wide easterly wind burst (EWB) in June was a key factor that impeded the El Ni09o development. It was shortly after this EWB that all major Ni09o indices fell rapidly to near-normal values; a modest growth resumed only later in the year. The easterly burst and the weakness of subsequent WWBs resulted in the persistence of two separate warming centers in the central and eastern equatorial Pacific, suppressing the positive Bjerknes feedback critical for El Ni09o. Experiments with a climate model with superimposed wind bursts support these conclusions, pointing to inherent limits in El Ni09o predictability. Furthermore, we show that the spatial structure of the easterly burst matches that of the observed decadal trend in wind stress in the tropical Pacific, suggesting potential links between intraseasonal wind bursts and decadal climate variations.
    Hu S.-N., A. V. Fedorov, M. Lengaigne, and E. Guilyardi, 2014: The impact of westerly wind bursts on the diversity and predictability of El Niño events: an ocean energetics perspective. Geophys. Res. Lett.,41, 4654-4663, doi: 10.1002/2014 GL059573.10.1002/ this study, we apply ocean energetics as a diagnostic tool to investigate the impact of westerly wind bursts (WWBs) on the evolution, diversity, and predictability of El Ni09o events. Following Fedorov et al. (2014), we add an observed WWB to simulations within a comprehensive coupled model and explore changes in the available potential energy (APE) of the tropical Pacific basin. We find that WWB impacts strongly depend on the ocean initial state and can range from a Central Pacific (CP) to Eastern Pacific (EP) warming, which is closely reflected by the ocean energetics. Consequently, the APE can be used to quantify the diversity of El Ni09o events within this continuum-igher negative APE values typically correspond to EP events, lower values to CP events. We also find that a superimposed WWB enhances El Ni09o predictability even before the spring predictability barrier, if one uses the APE as a predictor.
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    Jin F. F., S. I. An, A. Timmermann, and J. X. Zhao, 2003: Strong El Niño events and nonlinear dynamical heating. Geophys. Res. Lett., 30(4),1120, doi: 10.1029/2002GL016356.10.1029/[1] We present evidence showing that the nonlinear dynamic heating (NDH) in the tropical Pacific ocean heat budget is essential in the generation of intense El Ni09o events as well as the observed asymmetry between El Ni09o (warm) and La Ni09a (cold) events. The increase in NDH associated with the enhanced El Ni09o activity had an influence on the recent tropical Pacific warming trend and it might provide a positive feedback mechanism for climate change in the tropical Pacific.
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    Kumar A., Z. Z. Hu, 2012: Uncertainty in the ocean-atmosphere feedbacks associated with ENSO in the reanalysis products. Climate Dyn., 39, 575- 588.10.1007/ evolution of El Ni09o-Southern Oscillation (ENSO) variability can be characterized by various ocean-tmosphere feedbacks, for example, the influence of ENSO related sea surface temperature (SST) variability on the low-level wind and surface heat fluxes in the equatorial tropical Pacific, which in turn affects the evolution of the SST. An analysis of these feedbacks requires physically consistent observational data sets. Availability of various reanalysis data sets produced during the last 15years provides such an opportunity. A consolidated estimate of ocean surface fluxes based on multiple reanalyses also helps understand biases in ENSO predictions and simulations from climate models. In this paper, the intensity and the spatial structure of ocean-tmosphere feedback terms (precipitation, surface wind stress, and ocean surface heat flux) associated with ENSO are evaluated for six different reanalysis products. The analysis provides an estimate for the feedback terms that could be used for model validation studies. The analysis includes the robustness of the estimate across different reanalyses. Results show that one of the -渃oupled-� reanalysis among the six investigated is closer to the ensemble mean of the results, suggesting that the coupled data assimilation may have the potential to better capture the overall atmosphere-搊cean feedback processes associated with ENSO than the uncoupled ones.
    Kumar A., Z.-Z. Hu, 2014: Interannual and interdecadal variability of ocean temperature along the equatorial Pacific in conjunction with ENSO. Climate Dyn., 42, 1243- 1258.10.1007/ this paper, the leading modes of ocean temperature anomalies (OTA) along the equatorial Pacific Ocean are analyzed and their connection with El Ni09o-Southern Oscillation (ENSO) and interdecadal variation is investigated. The first two leading modes of OTA are connected with the different phases of the canonical ENSO and display asymmetric features of ENSO evolution. The third leading mode depicts a tripole pattern with opposite variation of OTA above the thermocline in the central Pacific to that along the thermocline in the eastern and western Pacific. This mode is found to be associated with so-called ENSO-Modoki. Insignificant correlations of this mode with the first two leading modes suggest that ENSO-Modoki may be a mode that is independent to the canonical ENSO and also has longer time scales compared with the canonical ENSO. The fourth mode reflects a warming (cooling) tendency above (below) the thermocline since 2000. Both the first and second modes have a large contribution to the interdecadal change in thermocline during 1979-�2012. Also, the analysis also documents that both ENSO and OTA shifted into higher frequency since 2000 compared with that during 1979-�1999. Interestingly, the ENSO-Modoki related OTA mode does not have any trend or significant interdecadal shift during 1979-�2012. In addition, it is shown that first four EOF modes seem robust before and after 1999/2000, suggesting that the interdecadal shift of the climate system in the tropical Pacific is mainly a frequency shift and the changes in spatial pattern are relatively small, although the mean states over two periods experienced some significant changes.
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    Lengaigne M., E. Guilyardi, J. P. Boulanger, C. Menkes, P. Delecluse, P. Inness, J. Cole, and J. Slingo, 2004: Triggering of El Niño by westerly wind events in a coupled general circulation model. Climate Dyn., 23, 601- 620.10.1007/ ten-members ensemble experiments using a coupled ocean-atmosphere general circulation model are performed to study the dynamical response to a strong westerly wind event (WWE) when the tropical Pa
    Levine A. F., F.-F. Jin, 2010: Noise-induced instability in the ENSO recharge oscillator. J. Atmos. Sci., 67, 529- 542.10.1175/ conceptual El Ni甯給 Southern Oscillation (ENSO) recharge oscillator model is used to study the linear stability of ENSO under state-dependent noise forcing. The analytical framework developed by Jin et al. (2007) is extended to more fully study noise-induced instability of ENSO. It is shown that in addition to the noise-induced positive contribution to the growth rate of the ensemble mean (first moment) evolution of the ENSO cycle, there is also a noise-induced instability for the ensemble spread (second moment). These growth rates continue to increase as the strength of the multiplicative noise increases. In both the analytical solution and the numerical model, the criticality threshold for instability of the second moment occurs at a lower value of the parameter that measures multiplicative forcing than the threshold for the first moment. The noise-induced instability not only enhances ENSO activity but also results in a large ensemble spread and thus may reduce the effectiveness of ENSO prediction. As in the additive noise forcing case, the low frequency variability in thee forcing is the important part for forcing El Ni甯給 events and the high frequency forcing alone cannot effective excite ENSO.
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    Li J. Y., B. Q. Liu, J. D. Li, and J. Y. Mao, 2015: A comparative study on the dominant factors responsible for the weaker-than-expected El Niño event in 2014. Adv. Atmos. Sci.,32, 1381-1390, doi: 10.1007/s00376-015-4269-6.10.1007/ warming occurred in the equatorial central-eastern Pacific in early May 2014,attracting much attention to the possible occurrence of an extreme El Ni?o event that year because of its similarity to the situation in early 1997. However,the subsequent variation in sea surface temperature anomalies(SSTAs) during summer 2014 in the tropical Pacific was evidently different to that in 1997,but somewhat similar to the situation of the 1990 aborted El Ni?o event. Based on NCEP(National Centers for Environmental Prediction) oceanic and atmospheric reanalysis data,the physical processes responsible for the strength of El Ni?o events are examined by comparing the dominant factors in 2014 in terms of the preceding instability of the coupled ocean-tmosphere system and westerly wind bursts(WWBs) with those in 1997 and 1990,separately. Although the unstable ocean-tmosphere system formed over the tropical Pacific in the preceding winter of 2014,the strength of the preceding instability was relatively weak. Weak oceanic eastward-propagating downwelling Kelvin waves were forced by the weak WWBs over the equatorial western Pacific in March 2014,as in February 1990. The consequent positive upper-oceanic heat content anomalies in the spring of 2014 induced only weak positive SSTAs in the central-eastern Pacific-搖nfavorable for the subsequent generation of summertime WWB sequences. Moreover,the equatorial western Pacific was not cooled,indicating the absence of positive Bjerknes feedback in early summer 2014. Therefore,the development of El Ni?o was suspended in summer 2014.
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    Yu Y., D.-Z. Sun, 2009: Response of ENSO and the mean state of the tropical Pacific to extratropical cooling and warming: A study using the IAP coupled model. J.Climate, 22, 5902- 5917.10.1175/ The coupled model of the Institute of Atmospheric Physics (IAP) is used to investigate the effects of extratropical cooling and warming on the tropical Pacific climate. The IAP coupled model is a fully coupled GCM without any flux correction. The model has been used in many aspects of climate modeling, including the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) climate change and paleoclimate simulations. In this study, the IAP coupled model is subjected to cooling or heating over the extratropical Pacific. As in an earlier study, the cooling and heating is imposed over the extratropical region poleward of 108N–108S. Consistent with earlier findings, an elevated (reduced) level of ENSO activity in response to an increase (decrease) in the cooling over the extratropical region is found. The changes in the time-mean structure of the equatorial upper ocean are also found to be very different between the case in which ocean–atmosphere is coupled over the equatorial region and the case in which the ocean–atmosphere over the equatorial region is decoupled. For example, in the uncoupled run, the thermocline water across the entire equatorial Pacific is cooled in response to an increase in the extratropical cooling. In the corresponding coupled run, the changes in the equatorial upper-ocean temperature in the extratropical cooling resemble a La Nin 09 a situation—a deeper thermocline in the western and central Pacific accompanied by a shallower thermocline in the eastern Pacific. Conversely, with coupling, the response of the equatorial upper ocean to extratropical cooling resembles an El Nin 09o situation. These results ascertain the role of extratropical ocean in determining the amplitude of ENSO. The results also underscore the importance of ocean–atmosphere coupling in the interaction between the tropical Pacific and the extratropical Pacific.
    Zebiak S. E., M. A. Cane, 1987: A model El Niño-southern oscillation. Mon. Wea. Rev., 115, 2262- 2278.10.1038/ intervals that vary from 2 to 10 yr sea-surface temperatures and rainfall are unusually high and the tradewinds are unusually weak over the tropical Pacific Ocean. These Southern Oscillation El Niño events which devastate the ecology of the coastal zones of Ecuador and Peru, which affect the global atmospheric circulation and which can contribute to severe winters over northern America, often develop in a remarkably predictable manner. But the event which began in 1982 has not followed this pattern.
    Zhang Y. C., W. B. Rossow, A. A. Lacis, V. Oinas, and M. I. Mishchenko, 2004: Calculation of radiative fluxes from the surface to top of atmosphere based on ISCCP and other global data sets: Refinements of the radiative transfer model and the input data. J. Geophys. Res., 109,D19105, doi: 10.1029/2003 JD004457.10.1029/ We continue reconstructing Earth's radiation budget from global observations in as much detail as possible to allow diagnosis of the effects of cloud (and surface and other atmospheric constituents) variations on it. This new study was undertaken to reduce the most noticeable systematic errors in our previous results (flux data set calculated mainly using International Satellite Cloud Climatology Project-C1 input data (ISCCP-FC)) by exploiting the availability of a more advanced NASA Goddard Institute for Space Studies (GISS) radiative transfer model and improved ISCCP cloud climatology and ancillary data sets. The most important changes are the introduction of a better treatment of ice clouds, revision of the aerosol climatology, accounting for diurnal variations of surface skin/air temperatures and the cloud-radiative effects on them, revision of the water vapor profiles used, and refinement of the land surface albedos and emissivities. We also extend our previous flux results, limited to the top of atmosphere (TOA) and surface (SRF), to also include three levels within the atmosphere, forming one integrated vertical atmospheric flux profile from SRF to TOA, inclusive, by combining a new climatology of cloud vertical structure with the ISCCP cloud product. Using the new radiative transfer model and new input data sets, we have produced an 18-year at 3-hour time steps, global at 280-km intervals, radiative flux profile data set (called ISCCP-FD) that provides full- and clear-sky, shortwave and longwave, upwelling and downwelling fluxes at five levels (SRF, 680 mbar, 440 mbar, 100 mbar, and TOA). Evaluation is still only possible for TOA and SRF fluxes: Comparisons of monthly, regional mean values from FD with Earth Radiation Budget Experiment, Clouds and the Earth's Radiant Energy System and Baseline Surface Radiation Network values suggest that we have been able to reduce the overall uncertainties from 10-15 to 5-10 W/m2 at TOA and from 20-25 to 10-15 W/m2 at SRF. Annual mean pressure-latitude cross sections of the cloud effects on atmospheric net radiative fluxes show that clouds shift the longwave cooling downward in the Intertropical Convergence Zone, acting to stabilize the tropical atmosphere while increasing the horizontal heating gradient forcing the Hadley circulation, and shift the longwave cooling upward in the midlatitude storm zones, acting to destabilize the baroclinic zones while decreasing the horizontal heating gradient there.
    Zheng F., L. H. Feng, and J. Zhu, 2015: An incursion of off-equatorial subsurface cold water and its role in triggering the "double dip" La Niña event of 2011. Adv. Atmos. Sci.,32, 731-742, doi: 10.1007/s00376-014-4080-9.10.1007/
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Manuscript received: 27 November 2015
Manuscript revised: 23 March 2016
Manuscript accepted: 11 April 2016
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Distinctive Precursory Air-Sea Signals between Regular and Super El Niños

  • 1. Key Laboratory of Meteorological Disaster, Joint International Research Laboratory of Climate and Environmental Change, and Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science and Technology, Nanjing 210044
  • 2. International Pacific Research Center, School of Ocean and Earth Science and Technology, University of Hawaii, 2525 Correa Rd., Honolulu, HI 96822, USA
  • 3. Application Laboratory, Japan Agency for Marine-Earth Science and Technology, Yokohama, 236-0001, Japan

Abstract: Statistically different precursory air-sea signals between a super and a regular El Niño group are investigated, using observed SST and rainfall data, and oceanic and atmospheric reanalysis data. The El Niño events during 1958-2008 are first separated into two groups: a super El Niño group (S-group) and a regular El Niño group (R-group). Composite analysis shows that a significantly larger SST anomaly (SSTA) tendency appears in S-group than in R-group during the onset phase [April-May(0)], when the positive SSTA is very small. A mixed-layer heat budget analysis indicates that the tendency difference arises primarily from the difference in zonal advective feedback and the associated zonal current anomaly (u'). This is attributed to the difference in the thermocline depth anomaly (D') over the off-equatorial western Pacific prior to the onset phase, as revealed by three ocean assimilation products. Such a difference in D' is caused by the difference in the wind stress curl anomaly in situ, which is mainly regulated by the anomalous SST and precipitation over the Maritime Continent and equatorial Pacific.

1. Introduction
  • ENSO is one of Earth's most important climate variabilities (Philander et al., 1984; Chao and Zhang, 1990), and its accurate prediction carries considerable socioeconomic impacts (Huang and Wu, 1989; Li, 1990; McPhaden, 1999). In early 2014, many climate models around the world predicted the occurrence of a super El Niño [here, this term stands for an extraordinarily strong El Niño (Hong et al., 2014; Latif et al., 2015), such as the 1997-98 El Niño] by the end of 2014 (e.g., Tollefson, 2014). However, the forecast of a 2014 super El Niño did not materialize.

    The failure of the 2014 forecast poses an important issue for the ENSO research community; namely, what makes a super El Niño? Previous studies have proposed various hypotheses to explain what causes the different growth rate of such an El Niño event. For example, (McPhaden, 1999) suggested that atmospheric "noise" on synoptic-intraseasonal timescales may have played a role in the rapid onset of the 1997-98 El Niño. A series of modeling studies support the notion that westerly wind events (WWEs) help set up the favorable condition for a super El Niño to occur (Lengaigne et al., 2004; Vecchi and Harrison, 2006), with their role in the development of El Niño events having been further tested in recent studies (Hu et al., 2014; Menkes et al., 2014; Fedorov et al., 2015; Li et al., 2015; Chen et al., 2016). However, the occurrence of WWEs is not a solely sufficient condition for the occurrence of super El Niños, because WWEs also occur during regular and non-El Niño years. In fact, it has been argued that WWEs are not even a necessary condition for the occurrence of El Niño, because many ocean-atmosphere coupled models simulate ENSO variability without well-simulated WWEs (McPhaden, 1999). Moreover, WWEs tend to be more frequent and stronger during large warm events, implying that they are not purely stochastic——at least some WWEs depend strongly on the low-frequency state of ENSO (Eisenman et al., 2005; Gebbie et al., 2007; Rong et al., 2011; Chen et al., 2015a).

    It is widely accepted that the buildup of heat content in the western Pacific is a precursor to El Niño events (Wyrtki, 1975; Wyrtki, 1985; Clarke, 2010; Kumar and Hu, 2014). However, it is not clear whether or not there is a statistically significant difference in the precursory signal between super and regular El Niños. (Ramesh and Murtugudde, 2013) argued that similar precursor subsurface signals appear in the western equatorial Pacific prior to nearly all El Niño events. However, they only concentrated on the subsurface signals at the equator, ignoring the impact of off-equatorial signals on ENSO (Yu and Sun, 2009; Ding et al., 2015; Zheng et al., 2015).

    Other factors that may affect El Niño intensity include the internal nonlinearity (Timmermann et al., 2003), oceanic nonlinear advection (Jin et al., 2003; Su et al., 2010), atmospheric nonlinearity associated with the SST threshold for deep convection (Takahashi and Dewitte, 2016) and noise-induced instability (Jin et al., 2007; Levine and Jin, 2010). It is not clear, however, how these nonlinear processes operate differently during regular and strong El Niños. (Hong et al., 2014) suggested that anomalous low-level flow induced by a high pressure anomaly in the Southern Hemisphere (SH) is critical in accelerating El Niño growth in boreal summer. However, it is difficult to identify if such low-level wind anomalies are a cause or a result of a super El Niño, as by the boreal summer of super El Niño development, the warming in the eastern equatorial Pacific is already quite strong.

    Most previous studies have focused on examining an individual El Niño case. As each El Niño event is very different (Chen et al., 2015a), it is necessary to examine their common features and the statistically significant differences between super and regular El Niño groups. In this respect, we focus in the present work on investigating the distinctive signals in both the atmosphere and ocean prior to the onset of El Niño events, as a small SST anomaly (SSTA) tendency difference during the onset phase can make a huge difference in the later developing stage, due to various positive atmosphere-ocean feedbacks (e.g., Philander et al., 1984; Hirst, 1988; Li, 1997).

    The main objective of the current study is to reveal the fundamental differences in the precursory atmospheric and oceanic signals between super and regular El Niño groups. In section 2, the data and method are described. In section 3, we present the statistically significant signals for the two groups. A summary and discussion are given in section 4.

2. Data and method
  • The observational SST data used in this study are from ERSST.v3b (Smith et al., 2008). To obtain a sufficient amount of observed El Niño samples, the SODA (version 2.1.6) data (Carton and Giese, 2008) for the period 1958-2008 are used. SODA provides oceanic 3D temperature and velocity fields, the surface wind stress field, and sea surface height fields. The sea surface height datasets from the ECMWF's Ocean Reanalysis System 4 (ORA-S4; Balmaseda et al., 2013), covering the period 1958-2008, along with the NCEP GODAS data (Saha et al., 2006) for the period 1980-2008, are also used. To reduce the uncertainty of the surface heat flux data (Kumar and Hu, 2012), they are derived from the ensemble mean of the NCEP-NCAR Reanalysis-1 data (Kalnay et al., 1996) for the period 1958-2008, and the WHOI OAFlux data (Yu et al., 2008) for the period 1984-2008, in which the shortwave and longwave radiation flux are acquired from the ISCCP (Zhang et al., 2004). The composite analysis associated with precipitation data is derived from the ensemble mean of three datasets, including the precipitation reconstruction (PREC; Chen et al., 2002) dataset for the period 1958-2008, and the CMAP (Xie and Arkin, 1997) and GPCP (version 2.1; Huffman et al., 2009) data for the period 1979-2008.

    Monthly anomalies are obtained by first subtracting the monthly mean climatology for the period 1958-2008; then, the Butterworth band-pass filter (Russell, 2006) is used to remove the high-frequency (<6 months) and low-frequency (>8 years) components.

    The SSTA over the Niño3 region of (5°S-5°N, 150°-90°W) during November-January [ND(0)J(2)] exceeding 0.75 standard deviations (STDs) is considered as an El Niño event. Here, year(0) indicates the year of an El Niño event, and year(-1) and year(+1) indicate the preceding and following year, respectively. As shown in the time series of normalized Niño3 index (Fig. 1a), three El Niño events (72/73, 82/83, and 97/98) with amplitudes greater than 2.5 STDs can be classified into a super El Niño group, and the other eleven El Niño events (63/64, 65/66, 69/70, 76/77, 86/87, 87/88, 91/92, 94/95, 02/03, 04/05, and 06/07) into a regular El Niño group.

    Figure 1.  (a) Time series of normalized Niño3 index obtained by using 6 months to 8 years band-pass-filtered SSTA derived from ERSST.v3b for the period 1958-2008. The green dashed line indicates the magnitude of 2.5 times STD. The red coloring indicates the super El Niño events (72/73, 82/83, and 97/98), and the blue indicates regular El Niño events. (b, c) Temporal evolutions of composite Niño3 SSTA (units: K) and SSTA tendency (units: K month$^-1$). The red (blue) line indicates the results derived from S-group (R-group), with shading representing the 1 STD spread of the samples within each group. Respectively, the green dots and thick lines indicate the difference between the two groups exceeding the 90% and 95% confidence level using the $t$-test. Here, year(0) indicates the El Niño event year, e.g., November-December(0)January(+1) corresponds to the peak phase of an El Niño event, with year($-1$) or year($+1$) indicating the preceding or following year, respectively.

    To investigate the specific dynamic and thermodynamic air-sea coupling processes in causing different SSTA tendencies during the onset phase, a mixed-layer heat budget analysis is conducted. The mixed-layer temperature tendency equation (Li et al., 2002; Wang et al., 2012) is

    $$ \dfrac{\partial{T}'}{\partial t}=-{u}'\dfrac{{\partial \overline {T}}}{\partial x}-\bar{u}\dfrac{{\partial{T}'}}{\partial x} -{u}'\dfrac{{\partial{T}'}}{\partial x}-{w}'\dfrac{{\partial\overline {T}}}{\partial z} -\bar{w}\dfrac{{\partial{T}'}}{\partial z}-{w}'\dfrac{{\partial{T}'}}{\partial z}\\$$ $$ \ \ \ term 1 \ \ \ term 2 \ \ \ \ term 3 \ \ \ \ term 4 \ \ \ \ term 5 \ \ \ \ term 6 \\ -{v}'\dfrac{{\partial\overline {T}}}{\partial y}-\bar{v}\dfrac{{\partial{T}'}}{\partial y}-{v}'\dfrac{{\partial{T}'}}{\partial y}+\dfrac{{Q}'_{{net}}}{\rho c_pH}+R\\term 7 \ \ \ term 8 \ \ \ term 9 \ \ \ term 10 , \ \ \ (1) $$

    where u,v and w represent the three-dimensional oceanic current; T is the mixed-layer temperature; ( )' denotes the interannual anomaly variables; (\(\bar\ \bar\ \bar\ \)) denotes the climatological mean variables; Q net is the summation of net downward shortwave radiation absorbed in the mixed layer (Q sw), net downward surface longwave radiation, and surface latent and sensible heat fluxes (positive heat flux indicates heating moving into the ocean); R represents the residual term; ρ is the density of seawater; cp is the specific heat of seawater; and H is the mixed-layer depth that varies in time and space. H is defined as the depth where ocean temperature is 0.8°C lower than the surface, following (Wang et al., 2012). All the budget terms in Eq. (2) are integrated from the surface to the mixed-layer depth. Considering the shortwave penetration below the mixed layer, the Q sw absorbed in the mixed layer can be written as (Wang et al., 2012) \begin{eqnarray} \label{eq2} Q_{{sw}}=Q_{{surf}}-0.47Q_{{surf}}e^{-0.04H} , \ \ (2)\\[-4mm]\nonumber \end{eqnarray} where Q surf is net downward surface shortwave radiation.

    To understand the variation of the upper-ocean current anomaly, the anomalous geostrophic current and anomalous Ekman current are diagnosed. The zonal geostrophic current anomaly (u' g) and zonal Ekman current anomaly (u' e) are estimated as (Chen et al., 2015b) \begin{eqnarray} \label{eq3} u'_{g}&=&-\dfrac{g\partial^2{D}'}{\beta\partial y^2} ,\ \ (3)\\[1mm] \label{eq4} u'_{e}&=&\dfrac{1}{\rho H}\dfrac{r_{s}\tau'_x+\beta y\tau'_y}{r_{s}^2+(\beta y)^2} ,(4) \end{eqnarray} where D', g and β are the thermocline depth anomaly, the reduced gravity, and the planetary vorticity gradient, respectively; τ'x and τ'y are the anomalous zonal and meridional wind stress; and r s is the Rayleigh damping coefficient (0.5 d-1) (Zebiak and Cane, 1987).

    Figure 2.  The composite mixed-layer heat budget terms (units: K month$^-1$) during the onset phase [April-May(0)] from (a) S-group, (b) R-group and (c) the difference between S-group and R-group (using S minus R). Bar 12 denotes the mixed-layer temperature tendency $\partial T'/\partial t$, and bar 11 is the sum of all first 10 terms. The remaining terms are indicated by bar 1: $-u'\partial\overline T/\partial x$, bar 2: $-\bar u\partial T'/\partial x$, bar 3: $-u'\partial T'/\partial x$, bar 4: $-w'\partial\overline T/\partial z$, bar 5: $-\bar w\partial T'/\partial z$, bar 6: $-w'\partial T'/\partial z$, bar 7: $-v'\partial\overline T/\partial y$, bar 8: $-\bar v\partial T'/\partial y$, bar 9: $-v'\partial T'/\partial y$, and bar 10: $Q'_net/\rho_oC_pH$. See the mixed-layer temperature tendency equation in section 2 for more details. (d) Temporal evolution of the composite zonal current anomaly ($u'$; m s$^-1$) averaged over 0-50 m, zonal geostrophic current anomaly ($u_g'$; m s$^-1$), and zonal Ekman current anomaly ($u'_e$; m s$^-1$) along the equator (averaged over 2$^\circ$S-2$^\circ$N). The red and blue curves indicate S-group and R-group, respectively.

3. Results
  • Figure 1a shows that, among the 14 El Niño events during 1958-2008, three super El Niño events (72/73, 82/83, 97/98) are apparent, and their amplitudes are much stronger than the average of the rest of the El Niño events. Figures 1b and c show the composite evolutions of the SSTA and SSTA tendency for the super El Niño group (S-group) and the regular El Niño group (R-group). It is noted that while the positive SSTAs are nearly zero in April(0) in both the S-group and R-group, there is a considerable difference in the SSTA tendencies between the two groups (Figs. 1b and c). Because of such a difference, the SSTAs in the two groups begin to bifurcate from April(0); that is, the positive SSTA increases rapidly in the following months and ultimately attains more than 2°C at the end of year(0) in S-group, whereas the positive

    SSTA increases at a much slower rate and hardly exceeds 1°C by end of year(0) in R-group.

    Figure 1c shows that the average SSTA tendency in April-May(0) in S-group is more than two times greater than that in R-group. It is such a tendency difference during the onset phase that leads to the subsequent evolution difference. Thus, a key issue that needs to be addressed is the cause of the significantly large SSTA tendency difference during the onset phase [April-May(0)].

    The tendencies of the mixed-layer temperature anomaly (MLTA) during the onset phase of the composite super and regular El Niño events are diagnosed, based on Eq. (2). Figures 2a and b show the mixed-layer heat budget terms for S-group and R-group. Here, the estimated MLTA tendency (i.e., term 11, which is the sum of terms 1-10), approximates the actual MLTA tendency (i.e., term 12), implying that the mixed-layer heat budget is approximately balanced. Note that the heat budget results by using different data and different approaches may produce different residual terms (Huang et al., 2010). Although the eddy process is not considered in this study, the dominant heat budget terms contributing to the El Niño development are similar to those revealed in previous studies (e.g., Huang et al., 2010). Figure 2c shows the difference in these budget terms between S-group and R-group (S minus R). Note that the most important term that contributes to the SSTA tendency difference between these two groups is term 1 (\(-u'\partial\overline T/\partial x\)), followed by term 5 (\(-\bar w\partial T'/\partial z\)). Respectively, these terms are the zonal advection of mean temperature by the zonal current anomaly, and the vertical advection of the anomalous temperature by the mean upwelling, denoting the well-known zonal advective feedback and thermocline feedback.

    As both the zonal advective feedback and thermocline feedback involve the product of the mean and anomalous parts, we further examine their relative roles, using the total differentiation analysis approach proposed by (Chen et al., 2015b). The result indicates that the main contributor is the anomaly part; that is, the zonal current anomaly (u') and the vertical gradient of anomalous temperature (\(\partial T'/\partial z\)). Thus, in the following analysis, we focus on examining the cause of the difference in u' and \(\partial T'/\partial z\) during the onset phase between the two groups.

    Figure 3.  The evolution of the composite SODA sea surface height anomaly (SSH$'$, units: m; a proxy of the thermocline depth anomaly, $D'$) for June-July($-1$), August-September($-1$), October-November($-1$), December($-1$)January(0), February-March(0) and April-May(0), derived from (a) S-group, (b) R-group and (c) the difference between S-group and R-group. The stippling in each panel indicates the difference between S-group and R-group exceeding the 95% confidence level using the $t$-test. (d) As in (c) except for the ORA-S4 SSH$'$ data covering the period 1958-2008. (e) As in (c) except for the GODAS SSH$'$ data covering the period 1980-2008. Here, a linear $D'$-SSH$'$ relationship is applied.

    To investigate what causes the difference in u' between the two groups, we diagnose the zonal geostrophic current anomaly (u' g) and the zonal Ekman current anomaly (u' e) in an equatorial β-plane framework. Figure 2d displays the evolution of u', u' g and u' e at the equator derived from S-group (red curves) and R-group (blue curves). Indeed, u' during the onset phase is stronger in S-group than in R-group. Note that the u' is mainly determined by u' g, whereas the contribution of u' e is much weaker. This is consistent with previous studies (e.g., Su et al., 2010, 2014; Chen et al., 2015b). Based on Eq. (4), u' g is associated with the second meridional derivative of the thermocline depth anomaly (D'). Thus, a local maximum of D' at the equator would lead to an anomalous eastward geostrophic current. Additionally, previous studies have pointed out a close relationship between D' and \(\partial T'/\partial z\); that is, a deepened thermocline (positive D') at the equator could induce the anomalous subsurface warming and thus a weakened stratification (e.g., Zebiak and Cane, 1987). This prompts us to further examine the difference in D' during the onset phase.

  • To understand the cause of the D' difference during the onset phase, we investigate the evolution of D' from the pre-onset stage to the onset phase. Figure 3 shows the evolution of D' from June-July(-1) to April-May(0) in S-group and R-group, as well as their difference (S minus R). During JJ(-1) to ON(-1), strong positive D' appears over the off-equatorial (10°-20°N and 10°-20°S) western Pacific region in S-group, while the D' over the off-equatorial western Pacific is significantly weak in R-group (Figs. 3a and b). In the subsequent months, the different D' signals over the off-equatorial region gradually propagate westward as Rossby waves and are reflected in the western boundary, resulting in a marked difference in the magnitude of D' at the equator in February-March(0) and April-May(0), i.e., much larger positive D' at the equator in S-group than R-group (Fig. 3c). Such a pronounced difference in the precursory D' signal is further confirmed by two other ocean products——ORA-S4 and GODAS (Figs. 3d and e). Thus, the accumulation of a deepened thermocline depth anomaly in the off-equatorial western Pacific in preceding months [June-September(-1)] holds the key for the differences in the thermocline depth, zonal current and vertical velocity anomalies in the subsequent months [i.e., February-March(0) and April-May(0)].

  • Further observational analysis shows that the significant difference in off-equatorial D' is primarily related to the difference in surface wind stress curl anomaly (C' url) between the two groups. During the pre-onset stage [June-July(-1) and August-September(-1)], anomalous negative C' url appears over the off-equatorial western Pacific region of (8°-20°N, 130°E-160°W) in the Northern Hemisphere (NH), and anomalous positive C' url appears over the off-equatorial western Pacific region of (8°-25°S, 150°E-160°W) in the SH. Here, "anomalous" stands for the difference between S-group and R-group (S minus R). Given the sign change of vorticity north and south of the equator, the opposite sign in C' url between the NH and SH implies a pair of anomalous anticyclonic wind stress anomalies on both sides of the equator. As pointed out by (Kessler, 2006), C' url is a major factor regulating D' in the off-equatorial western Pacific. Therefore, it is the distinctive wind stress anomaly that causes the marked difference in the thermocline depth anomaly between S-group and R-group in the off-equatorial western Pacific.

    The twin anomalous anticyclonic wind stress fields shown in Fig. 4a during June-September(-1) are caused by the difference in the precipitation anomaly (P' r) and SST anomaly fields between S-group and R-group (Figs. 4b and c). Note that in June-September(-1), an anomalous positive P' r appears over the Maritime Continent, and an anomalous negative P' r appears in the western equatorial Pacific (near the dateline). The positive P' r over the Maritime Continent, collocated with an underlying positive SSTA, can induce an atmospheric Kelvin wave response to its east (Wang and Li, 1993). As the amplitude of easterly anomalies associated with the Kelvin wave response decreases with latitude, an anticyclonic wind shear anomaly is generated in the off-equatorial western Pacific. Meanwhile, the negative precipitation anomaly near the dateline, caused by the cold SSTA in the central and eastern equatorial Pacific, can induce a pair of low-level anticyclonic Rossby gyres to the west of the negative heating region (Gill, 1980). Both the positive and negative precipitation/heating anomalies reinforce the twin anticyclonic wind stress anomaly in the off-equatorial western Pacific.

    Examination of the SSTA evolution patterns shows that a La Niña-like pattern appears during June-September(-1) in both S-group and R-group (figure not shown). This indicates that the significant difference in Fig. 4c can be primarily attributed to the magnitude, not signal, of the SSTA. Both the positive SSTA over the Maritime Continent and the negative SSTA in the central and eastern equatorial Pacific are much stronger in S-group than in R-group. It is the distinctive east-west SSTA gradient that leads to the distinctive precipitation dipole shown in Fig. 4b.

    Thus, the observational analysis reveals that the fundamental difference between the S- and R-group lies in the distinctive rainfall and SST anomaly patterns in the pre-onset stage [June-September(-1)]. A much stronger cooling (warming) in the eastern Pacific (Maritime Continent) leads to a distinctive dipole rainfall pattern, which further causes a significant difference in the off-equatorial C' url and thermocline depth anomaly. The difference in the thermocline depth anomaly further leads to the distinctive SSTA tendencies in the eastern equatorial Pacific between the two groups during the onset phase.

    Figure 4.  The difference (using S minus R) in the composite (a) wind stress anomaly (vectors; units: N m$^-2$) and wind stress curl anomaly ($C'_url$; shading; units: 10$^-8$ N m$^-3$), (b) precipitation anomaly ($P'_r$; mm d$^-1$), and (c) SSTA (units: K), for June-July($-1$) and August-September($-1$). The stippling in each panel indicates the difference between S-group and R-group exceeding the 95% confidence level using the $t$-test.

4. Summary and discussion
  • This study investigates the statistically different precursory signals between groups of super and regular El Niños. For the period 1958-2008, fourteen El Niño events are identified and classified into two groups: a super El Niño group (including the 72/73, 82/83 and 97/98 El Niño events), and a regular El Niño group (including the rest of the El Niño events). Composite results show a significantly larger SSTA tendency in S-group than in R-group during the onset phase [April-May(0)] when the SSTA is nearly zero. A mixed-layer heat budget analysis shows that the difference in the SSTA tendencies during the onset phase is primarily caused by the difference in the zonal advective feedback and the associated zonal current anomaly.

    Further diagnosis illustrates that the difference in the zonal current anomaly between S-group and R-group is primarily caused by the difference in the precursory signal of the thermocline depth anomaly (D') prior to the onset phase. As revealed by three sets of ocean reanalysis (i.e., SODA, ORA-S4 and GODAS), the difference in D' at the equator during the onset phase can be traced back to the precursory D' signals over the off-equatorial western Pacific during the preceding summer [June-September(-1)], when there is a significant difference in the curl of the wind stress anomaly field between the S- and R-group. From the perspective of difference fields (S minus R), a pair of anticyclonic anomalies appear in the off-equatorial western Pacific; and the wind stress anomalies are accompanied with positive precipitation and SST anomalies over the Maritime Continent and negative precipitation and SST anomalies over the central and eastern equatorial Pacific. Physically, it is argued that the anomalous anticyclonic gyres are the Rossby and Kevin wave responses to the anomalous dipole precipitation pattern.

    Through the current observational analysis, we point out that the statistically different signals between super and regular El Niños appear in the pre-onset phase. Thus, special attention should be paid to the wind and thermocline anomalies over off-equatorial regions. On the other hand, significant SSTA tendencies between the S- and R-group also appear in the developing phase [say, June-September(0); Fig. 1c]. However, it becomes difficult to examine its cause, because of a lack of a clean way to filter out the impact from the pre-onset phase. Additionally, the D' field for each super and regular El Niño case is shown in Figs. S1 and S2 (see the electronic supplementary material). As one can see, for all three super El Niños, there is a clearly positive D' over the off-equatorial western Pacific during the pre-onset stage. Most of the regular El Niño cases have relatively weak or negative D' over the off-equatorial western Pacific, except a few regular El Niño events (e.g., 1965/66 and 1976/77), which followed strong La Niña events. With respect to the reason for the failed materialization of super El Niño in 1965/66 and 1976/77, it may be related to the lower temperature as the starting point due to the strong La Niña in advance, and the additional "prohibiting" processes, such as the occurrence of a series of easterly wind events (Hu and Fedorov, 2016) during the El Niño developing stage (figure not shown). Thus, a positive D' prior to the onset stage may not fully guarantee a super El Niño, because there might be a number of "prohibiting" processes operating during the El Niño developing phase. It is possible to identify certain "accelerating" or "prohibiting" processes during the El Niño developing phase for each individual case. For instance, the aforementioned WWEs (e.g., Fedorov et al., 2015) or the anomalous low-level equatorward flow (Hong et al., 2014) may accelerate the growth rate of SSTAs, and the negative SSTAs over the southeastern subtropical Pacific (Min et al., 2015) or easterly wind events (Hu and Fedorov, 2016) may prohibit El Niño development by suppressing the air-sea interaction. As "accelerating" or "prohibiting" processes could be different for different El Niño events, statistically significant signals may not necessarily be found. Nevertheless, such a case study is needed in order to fully understand the causal mechanisms underpinning a super El Niño.




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