Chen, D; Cane, MA; Kaplan, A; Zebiak, SE; Huang, D

Nature [Nature]. Vol. 428, no. 6984, pp. 733-736. 15 Apr 2004.

Forecasts of El Niño climate events are routinely provided and distributed, but the limits of El Niño predictability are still the subject of debate. Some recent studies suggest that the predictability is largely limited by the effects of high-frequency atmospheric 'noise', whereas others emphasize limitations arising from the growth of initial errors in model simulations. Here we present retrospective forecasts of the interannual climate fluctuations in the tropical Pacific Ocean for the period 1857 to 2003, using a coupled ocean-atmosphere model. The model successfully predicts all prominent El Niño events within this period at lead times of up to two years. Our analysis suggests that the evolution of El Niño is controlled to a larger degree by self-sustaining internal dynamics than by stochastic forcing. Model-based prediction of El Niño therefore depends more on the initial conditions than on unpredictable atmospheric noise. We conclude that throughout the past century, El Niño has been more predictable than previously envisaged.

Clarke, AJ; Van Gorder, S

Geophysical Research Letters [Geophys. Res. Lett.]. Vol. 30, no. 7, [np]. Apr 2003.

Equatorial westerly (easterly) wind anomalies, phase-locked to the seasonal cycle, typically 'propagate' from the eastern equatorial Indian Ocean and into the western Pacific immediately before an El Niño (La Niña). A space-time integration of this Indo-Pacific signal yields an index tau that, for 11 out of 12 months of the calendar year, leads the El Niño index NINO3.4 with a correlation of 0.5 or greater for at least some lead in the range 10-15 months. Cross-validated hindcasts suggest that a linear combination of this atmospheric index and the ocean indices NINO3.4 and [h bar](t), the anomalous equatorial Pacific upper ocean heat content, is an excellent predictor of El Niño. It can predict across the nearest spring persistence barrier, but not the one after that. The present El Niño should die over the spring, leaving near neutral conditions for the rest of 2003.

Roemmich, D; Gould, WJ

Sea technology [Sea Technol.]. Vol. 44, no. 8, pp. 10-15. 2003.

Global ocean observations for climate research are a major part of the legacy of the World Climate Research Program's (WCRP) Tropical Ocean Global Atmosphere (TOGA) and World Ocean Circulation Experiment (WOCE). They remain as a major element of the WCRP's ongoing Climate Variability and Predictability (CLIVAR) project. TOGA demonstrated that an integrated observing system spanning the tropical Pacific would lead to a better understanding of El Niño /Southern Oscillation (ENSO) variability and to successful El Niño predictions. WOCE showed, with a one-time global survey, that oceans make an important contribution to the total heat budget of the climate system through heat transport, as well as heat storage. To build on the legacies of WOCE and TOGA, CLIVAR will include two classes of in situ ocean observations. Limited duration regional process studies will focus on phenomena that are poorly understood in order to improve their representation in ocean and coupled models. Sustained observations on basin-to-global scales, which are the topic of this article, should resolve the patterns of climate variability and the large-scale climate processes that the models aim to simulate.

Kang, I-S; Kug, J-S

Geophysical Research Letters [Geophys. Res. Lett.]. Vol. 27, no. 8, pp. 1167-1170. Apr 2000.

An El-Niño prediction model is developed based on an intermediate ocean model similar to the Cane and Zebiak (CZ) and a statistical atmosphere model. The present ocean model differs from CZ in the parameterization of subsurface temperature and the basic state. The predictability skill of the present model is better than that of CZ. The better performance is particularly distinctive for early stage of the prediction everywhere in the domain and in the central Pacific for all period of prediction. It is suggested that the better performance for the early stage is due to the use of SST anomalies in the initialization, and the better performance in the Central Pacific results from a better representation of subsurface temperature in the present model.

Wang, J

Proceedings of the 1998 Science Board Symposium on the Impacts of the 1997/98 El Niño event on the North Pacific Ocean and its marginal seas. no. 10, p131. PICES scientific report. Sidney BC [PICES Sci. Rep.]. Mar 1999.

The 1997/98 El Niño prediction and natural disasters over the coastal areas for China (the strongest storm surge in this century, the lighter than normal sea ice cover in the Bohai Sea etc.,) are reviewed. Methods to make the El Niño prediction more reliable are discussed. A method of forecast verification was developed. For each test parameter, the following factors were tabulated: average deviation, mean absolute error, tendency correlation, anomaly correlation, absolute correlation, skill index, and ability index. The most successful El Niño predictions were those using statistical-dynamic methods, but they still exhibited the spring predictability barrier in February to April. The primary objective of El Niño prediction is to improve the ability to predict sea surface temperature anomalies (SSTA). The analyses indicate that the correlation of SST between two or three continuous months is strong, and it shows that the regions with the strongest SST persistence are concentrated in specific areas.

Lagos, P; Pizarro, L

The Ocean Observing System for Climate: Oceanobs 99. Vol. 2, vp. 1999.

Peru has begun to implement a ten-year program for observing the eastern equatorial Pacific and continental atmosphere with real time data transmission. The ocean observing system, consisting of ATLAS type moorings and an array of coastal in situ oceanographic measurements will be used for monitoring temperature changes in the upper layer of the ocean. Ocean measurements from regularly scheduled research ships will continue and complemented with current network of standard coastal oceanographic stations that will provide surface temperature, salinity, and sea level measurements. A network of coastal and continental meteorological and hydrological stations will measure many key atmospheric parameters and transmit via satellite in real time to assess the impact of extreme weather events. Computer models for simulating and predicting weather and climate variability will be operational in year 2000.

Ni, Yunqi; Zebiak, SE; Cane, MA; Marx, L; Shukla, J

Scientia atmospherica sinica/Daqi Kexue. Beijing [Sci. Atmos. Sin./Daqi Kexue]. Vol. 22, no. 3, pp. 265-273. 1998.

This study used coupled ocean-atmosphere model which consists of the COLA R15 AGCM and a simple CZ Ocean model. Fifteen experimental predictions for ENSO are performed by this hybrid coupled model. The leading correlation of predicted and observed Niño 3 exceeds 0.6 (statistical significance at level 0.01) out to a 15 month lead time. The error of predicted Niño 3 is about 0.6-0.9 before leading one and a half years. Predicted results indicate that the hybrid coupled model may possess capability of ENSO prediction for 15 months. Finally, advantages and improvement approaches of this model are discussed.

Ji, Ming; Leetmaa, A

Monthly Weather Review [Mon. Weather Rev.], vol. 125, no. 5, pp. 742-753, May 1997

In this study, the authors compare skills of forecasts of tropical Pacific sea surface temperatures from the National Centers for Environmental Prediction (NCEP) coupled general circulation model that were initiated using different sets of ocean initial conditions. These were produced with and without assimilation of observed subsurface upper-ocean temperature data from expendable bathythermographs (XBTs) and from the Tropical Ocean Global Atmosphere-Tropical Atmosphere Ocean (TOGA-TAO) buoys. These experiments show that assimilation of observed subsurface temperature data in the determining of the initial conditions, especially for summer and fall starts, results in significantly improved forecasts for the NCEP coupled model. The assimilation compensates for errors in the forcing fields and inadequate physical parameterizations in the ocean model. Furthermore, additional skill improvements, over that provided by XBT assimilation, result from assimilation of subsurface temperature data collected by the TOGA-TAO buoys. This is a consequence of the current predominance of TAO data in the tropical Pacific in recent years. Results suggest that in the presence of erroneous wind forcing and inadequate physical parameterizations in the ocean model ocean data assimilation can improve ocean initialization and thus can improve the skill of the forecasts. However, the need for assimilation can create imbalances between the mean states of the oceanic initial conditions and the coupled model. These imbalances and errors in the coupled model can be significant limiting factors to forecast skill, especially for forecasts initiated in the northern winter. These limiting factors cannot be avoided by using data assimilation and must be corrected by improving the models and the forcing fields.

Chen, D; Zebiak, SE; Busalachhi, AJ; Cane, MA

Science (Washington) [SCIENCE (WASH.)], vol. 269, no. 5231, pp. 1699-1702, 1995

A coupled ocean-atmosphere data assimilation procedure yields improved forecasts of El Niño for the 1980s compared with previous forecasting procedures. As in earlier forecasts with the same model, no oceanic data were used, and only wind information was assimilated. The improvement is attributed to the explicit consideration of air-sea interaction in the initialization. These results suggest that El Niño is more predictable than previously estimated, but that predictability may vary on decadal or longer time scales. This procedure also eliminates the well-known spring barrier to El Niño prediction, which implies that it may not be intrinsic to the real climate system.

Zhang, R-H; Zebiak, SE; Kleeman, R; Keenlyside, N

Geophysical Research Letters [Geophys. Res. Lett.]. Vol. 30, no. 19, [np]. Oct 2003.

A new intermediate coupled model (ICM) is developed and used to simulate and predict sea surface temperature (SST) variability in the tropical Pacific. The ocean component is based on an intermediate complexity model developed by Keenlyside and Kleeman [2002] that is an extension of the McCreary [1981] baroclinic modal model to include varying stratification and partial nonlinearity effects, allowing realistic simulation of the mean equatorial circulation and its variability. An empirical procedure is developed to parameterize subsurface entrainment temperature (Te) in terms of sea surface pressure (SSP) anomalies. The ocean model is then coupled to a statistical atmospheric model. The coupled system realistically produces interannual variability associated with El Niño. Hindcasts are made during the period 1980- 1997 for lead times out to 12 months. Observed SST anomalies are the only field to be incorporated into the coupled system to initialize predictions. Predicted SST anomalies from this model do not show obvious systematic biases. Another striking feature is that the model skill beats persistence at all lead times over the central equatorial Pacific.

Zavala-Garay, J*; Moore, AM; Perez, CL; Kleeman, R

Journal of Climate [J. Clim.]. Vol. 16, no. 17, pp. 2827-2842. Sep 2003.

In this work the role that observed intraseasonal atmospheric variability may play in controlling and maintaining ENSO variability is examined. To this end, an asymptotically stable intermediate coupled model of El Niño-Southern Oscillation (ENSO) is forced with observed estimates of stochastic forcing, which are defined to be the part of the atmospheric variability that is apparently independent of the ocean circulation. The stochastic forcing (SF) was estimated from 51 yr (1950-2000) of NCEP-NCAR reanalyses of surface winds and net surface heat flux, 32 yr (1950-81) of reconstructed sea surface temperatures (SST), and 19 yr (1982-2000) of Reynolds SST in the tropical Pacific. The deterministic component of the surface wind and heat flux anomalies that can be linearly related to SST anomalies was estimated using the singular value decomposition of the covariance between the anomaly fields, and was then removed from the atmospheric anomaly fields to recover the stochastic component of the ocean surface forcing. Principal component analysis reveals that the stochastic component has no preferred mode of variability, exhibits decorrelation times of a few days, and has a spectrum that is indistinguishable from red noise. A 19-yr stochastically forced coupled model integration qualitatively shows some similarities with the observed equatorial SST. The robustness of this result is checked by performing different sensitivity experiments. The model mostly exhibits a linear (and nonnormal) response to the low-frequency tail of SF. Using the ideas of generalized linear stability theory, the dynamically important contributions of the SF are isolated, and it is shown that most of the variability in the stochastically forced model solution is produced by stochastically induced Kelvin waves forced in the western and central Pacific. Moreover, the two most dynamically important patterns of stochastic forcing (which account for 71% of the expected variance in the model response) describe eastward propagation of the forcing similar to the MJO. The results of this study support the hypothesis that a significant fraction of ENSO variability may be due to SF, and suggest that a better understanding of the influence of SF on the ocean surface in the western/central Pacific may be required in order to understand the predictability of ENSO.

Vecchi, GA; Harrison, DE

Geophysical Research Letters [Geophys. Res. Lett.]. Vol. 30, no. 18, [np]. Sep 2003.

Every new El Niño presents an opportunity to revisit our understanding of El Niño characteristics and processes. We examine the extent to which the termination of the 2002-03 El Niño followed the scenario of Harrison and Vecchi [1999], in which (1) there is a late-year southward shift of near-equatorial westerly wind anomalies, and (2) subsequent eastern equatorial Pacific cold tongue thermocline shallowing is driven by the wind shift. There was a prominent late-year southward shift in the low-frequency west Pacific zonal wind anomalies in 2002-03. Ocean general circulation model experiments establish the shift as the primary cause of cold tongue thermocline shallowing. Reflected equatorial waves and local wind anomaly changes are much less important. Successful theories and models of El Niño should address the processes that cause the year- end southward wind shift. Interactions between anomalous El Niño conditions and the seasonal cycle of solar insolation may provide such a process.

Fedorov, AV; Harper, SL; Philander, SG; Winter, B; Wittenberg, A

Bulletin of the American Meteorological Society [Bull. Am. Meteorol. Soc.]. Vol. 84, no. 7, pp. 911-919. Jul 2003.

Nobody anticipated that El Niño would be weak and prolonged in 1992, but brief and intense in 1997/98. Why are various El Niño episodes so different, and so difficult to predict? The answer involves the important role played by random atmospheric disturbances (such as westerly wind bursts) in sustaining the weakly damped Southern Oscillation, whose complementary warm and cold phases are, respectively, El Niño and La Niña. As in the case of a damped pendulum sustained by modest blows at random times, so the predictability of El Niño is limited, not by the amplification of errors in initial conditions as in the case of weather, but mainly by atmospheric disturbances interacting with the Southern Oscillation. Given the statistics of the wind fluctuations, the probability distribution function of future sea surface temperature fluctuations in the eastern equatorial Pacific can be determined by means of an ensemble of calculations with a coupled ocean-atmosphere model. Each member of the ensemble starts from the same initial conditions and has, superimposed, a different realization of the noise. Such a prediction, made at the end of 1996, would have assigned a higher likelihood to a moderate event than to the extremely strong event that actually occurred in 1997. (The rapid succession of several westerly wind bursts in early 1997 was a relatively rare phenomenon.) In late 2001, conditions were similar to those in 1996, which suggested a relatively high probability of El Niño appearing in 2002. Whether the event will be weak or intense depends on the random disturbances that materialize during the year.

Tang, Y; Kleeman, R; Moore, AM; Weaver, A; Vialard, J

Geophysical Research Letters [Geophys. Res. Lett.]. Vol. 30, no. 13, [np]. Jul 2003.

With a three-dimensional variational (3D-Var) assimilation scheme and a hybrid coupled model, we have explored the possibility of initializing ENSO prediction models by assimilating NCEP (National Centers of Environment Prediction) reanalysis subsurface temperatures. Our results show that, compared to predictions without assimilation, the reanalysis product can effectively improve prediction of both Niño3 sea surface temperature anomalies (SSTA) at all lead times up to 12 months (in particular for lead times over 4-6 months) and of El Niño episodes. The oceanic analysis from the assimilation with the reanalysis product can be as good as those generated by directly assimilating subsurface in situ temperature observations.

Yu, J-Y; Mechoso, CR

Journal of Climate [J. Clim.]. Vol. 14, no. 10, pp. 2329-2350. May 2001.

This study examines interannual variability produced by a recent version of the University of California, Los Angeles, coupled atmosphere-ocean general circulation model (CGCM). The CGCM is shown to produce ENSO-like climate variability with reasonable frequency and amplitude. A multichannel singular spectrum analysis identifies the simulated ENSO cycle and permits examination of the associated evolution of atmospheric and oceanic states. During the cycle, the evolution of upper-ocean heat content in the tropical Pacific is characterized by a zonal oscillation between the western and eastern equatorial Pacific and a meridional oscillation between the equator and 10 degree N. The zonal oscillation is related to the amplification of the cycle, and the meridional oscillation is related to the transition between phase of the cycle. It is found that the north-south ocean heat content difference always reaches a threshold near the onset of a warm/cold event. The three-dimensional evolution of ocean temperature anomalies in the tropical Pacific during the simulated ENSO cycle is characterized by four major features: 1) a build up in the subsurface of the western equatorial sector during the pre-onset stage, 2) a fast spread from the western subsurface to the eastern surface along the equator during the onset stage, 3) a zonal extension and amplification at the surface during the growth stage, and 4) a northward and downward spread during the decay stage. Ocean temperature budget analyses show that the buildup of subsurface temperature anomalies is dominated by the vertical advection process in the western sector and the meridional advection process in the central sector. The former process is associated with vertical displacements of the thermocline, which is an important element of the delayed oscillator theory. The latter process is associated with a Sverdrup imbalance between trade wind and thermocline anomalies and is emphasized as the primary charge-discharge process by the recharge oscillator theory. It is argued that both processes play key roles in producing subsurface ocean memory for the phase transitions of the ENSO cycle.

Yukimoto, S; Kitamura, Y

Journal of the Meteorological Society of Japan [J. Meteorol. Soc. Japan]. Vol. 81, no. 3, pp. 599-622. 2003.

The irregularity of el Niño is investigated with a 400-year simulation of a coupled ocean-atmosphere general circulation model. The model produces irregular el Niños with peak sea surface temperature (SST) anomalies ranging from 1 degree C to 4 degree C in the equatorial central-eastern Pacific. In the equatorial Pacific, the temporal phase relationship of the upper ocean heat content (OHC) anomaly relative to SST, and wind stress anomalies can be explained by the 'recharge oscillator' mechanism. A difference of the zonal mean OHC anomaly between the equator and the northern subtropics arises before the development of equatorial SST anomaly. It is found that a larger OHC anomaly is accumulated on the equator as a precursor of strong el Niño. The heat-budget analysis suggests that horizontal advection in the ocean interior is a major contributor to the build- up of the larger OHC anomaly during the recharge phase, which is associated with the zonal-mean wind-curl anomaly in the off-equatorial North Pacific. This also implies that the surface heating in the subtropics is a potential contributor through meridional heat transport. Besides the aspect of amplitude irregularity, the model el Niño shows irregularities in frequency and seasonal phase locking. Possible linkages between these irregularities are discussed.

Hannachi, A; Stephenson, DB; Sperber, KR

Climate Dynamics [Clim. Dyn.]. Vol. 20, no. 2-3, pp. 241-256. Jan 2003.

Robust statistical tools have been used to investigate non-normality and nonlinearity of the El Niño Southern Oscillation (ENSO) in observations and coupled model simulations. The analysis confirms previous suggestions that the observed Niño-3 sea surface temperature (SST) anomalies are positively skewed. The non-linearity is estimated using a simple nonlinear stochastic model, which relates the sea surface temperature anomalies to the observed thermocline depth anomalies in the Niño-3 region. There is evidence that saturation of SST only occurs when the thermocline is deep. The nonlinearity has also been estimated for the Niño-3 SST indices from twenty four different coupled models participating in the El Niño Simulation Intercomparison Project (ENSIP). Large differences are found between models and observations. In particular, the majority of the coupled models underestimate the nonlinearity seen in the observed Niño-3 sea surface temperature index. More than half of the models have Niño-3 SST indices that are normally distributed at 99% confidence level. Only a few models exhibit significant nonlinearity yet this tends to be rather different in character from the nonlinearity seen in the observations. Furthermore, no significant association is found between the means and the spread nor between the spread and the skewness for the different coupled model Niño-3 SST indices.

Knaff, JA; Landsea, CW

Experimental Long-Lead Forecast Bulletin [Exp. Long-Lead Forecast Bull.]. Vol. 10, no. 2, pp. 31-34. Jun 2001.

To provide a baseline of skill in seasonal ENSO forecasting, a multiple regression has been used to take best advantage of CLImatology, PERsistence and trend of initial conditions the ENSO-CLIPER. This replaces simple persistence as a skill threshold.' Skill" is then redefined as the ability to outforecast the ENSO-CLIPER - a more difficult task. This statistical prediction method is based entirely on the optimal combination of persistence, month-to-month trend of initial conditions and climatology. Multiple least squares regression is employed to test a total of fourteen possible predictors for the selection of the best predictors, based upon 1950-1994 developmental data. A range of zero to four predictors were chosen in developing twelve separate regression models, developed separately for each initial calendar month. The predictands to be forecast include the Southern Oscillation (pressure) Index (SOI) and the Niño 1+2, Niño 3, Niño 4 and Niño 3.4 SST indices for the equatorial eastern and central Pacific at lead times ranging from zero seasons (0-2 months) through seven seasons (21-23 months). Though hindcast ability is strongly seasonally dependent, substantial improvement is achieved over simple persistence wherein largest gains occur for two to seven season (6 to 23 months) lead times. The ENSO-CLIPER model thus not only offers a baseline "no-skill' forecast of ENSO variability, but a practical forecast based upon the CLIPER premise.

Roulston, Mark S; Neelin, JDavid

Geophysical Research Letters, Washington, DC. Vol. 27, no. 22, pp. 3723-3726. 2000.

The response of an intermediate coupled model of the tropical Pacific to different forms of stochastic wind forcing is studied. An estimate of observed Pacific wind variance that is unrelated to Pacific sea surface temperature (SST) has a red spectrum, inconsistent with standard definitions of ``weather noise''. The reddening is likely due to SST outside the basin; we propose a definition of ``climate noise'' for such reddened variance. Effects are compared for (i) red climate noise; (ii) the corresponding white weather noise estimate; (iii) intraseasonal and interannual components of the white noise (to test frequency response); and (iv) a noise product with extra power in the 30-60 day range. Power is not effectively channeled from subannual frequencies to the frequencies associated with ENSO in this model. This suggests that ENSO impacts of the Madden-Julian oscillation are largely restricted to the low-frequency tail rather than the 30-60 day spectral peak. Interannual climate noise originating outside the tropical Pacific appears important.

Bennett, Andrew F; Chua, Boon S; Harrison, DEd; McPhaden, Michael J

Journal of Climate, Boston, MA. Vol. 13, no. 15, pp. 2770-2785. 2000.

The investigation of the consequences of trying to blend tropical Pacific observations from the Tropical Atmosphere-Ocean (TAO) array into the dynamical framework of an intermediate coupled ocean-atmosphere model is continued. In a previous study it was found that the model dynamics, the prior estimates of uncertainty in the observations, and the estimates of the errors in the dynamical equations of the model could not be reconciled with data from the 1994-95 period. The error estimates and the data forced the rejection of the model physics as being unacceptably in error. In this work, data from two periods (1995-96 and 1997-98) were used when the tropical Pacific was in states very different from the previous study. The consequences of increasing the prior error estimates were explored in an effort to find out if it is possible at least to use the intermediate model physics to assist in mapping the observations into fields in a statistically consistent way. It was found that such a result is possible for the new data periods, with larger prior error assumptions. However, examination of the behavior of the mapped fields indicates that they have no dynamical utility. The model dynamical residuals, that is, the size of the quantity that is left after evaluating all of the terms in each intermediate model equation, dominate the terms themselves. Evidently the intermediate model is not able to add insight into the processes that caused the tropical Pacific to behave as it was observed to, during these time intervals. The inverse techniques described here together with the relatively dense TAO dataset have made it possible for the unambiguous rejection of the nonlinear intermediate model dynamical system. This is the first time that data have been able to provide such a clear-cut appraisal of simplified dynamics.

Neelin, JDavid

Journal of the Atmospheric Sciences, Boston, MA. Vol. 47, no. 5, pp. 674-693. 1990.

A model is developed for tropical air-sea interaction studies, which is intermediate in complexity between the large coupled general circulation models (coupled GCMs) coming into use and the simple two-level models with which pioneering El Niño-Southern Oscillation studies were carried out. The model consists of a stripped-down tropical Pacific Ocean GCM, coupled to an atmospheric model which is sufficiently simple that steady state solutions may be found for low level flow and surface stress, given oceanic boundary conditions. This hybrid coupling of an ocean GCM to a steady atmospheric model permits examination of the nature of interannual coupled oscillations in the absence of atmospheric noise. Tests of the atmospheric model against an atmospheric GCM simulation of El Niño anomalies are presented, and the ocean model climatology is examined under several different conditions. Experiments with the coupled model exhibit a variety of behaviors within a realistic parameter range. These indicate a partial bifurcation diagram in which the coupled system undergoes a Hopf bifurcation from a stable climatology, giving rise to sustained El Niño-period oscillations. The amplitude, period and eastward extent of these oscillations increase with the strength of coupling and the El Niño-period oscillation itself becomes unstable to a higher frequency coupled mode which coexists with it and may affect predictability. The difference between these flow regimes may be relevant to results found by other investigators in coupled GCM experiments.

Sun, De-Zheng

Geophysical Research Letters, Washington, DC. Vol. 24, no. 16, pp. 2031-2034. 1997.

The very existence of El Niño--the oscillatory behavior of the tropical Pacific climate--may be due to the warmth of the tropics (relative to the coldness of the high latitudes). This is elucidated by subjecting a mathematical model for the coupled tropical ocean-atmosphere system to a varying radiative heating. The temperature of the deep ocean is kept fixed. In response to an increasing radiative heating, the coupled system first experiences a pitch-fork bifurcation that breaks the zonal symmetry imposed by the solar radiation. The resulting zonal sea surface temperature (SST) gradients increase with increases in the radiative heating. When the zonal SST gradients exceed a critical value, a Hopf bifurcation takes place which brings the system to an oscillatory state, a state that closely resembles the observed tropical Pacific climate. Further increases in the radiative heating result in increases in the magnitude of the oscillation. The results shed new light on the physics of El Niño and suggest that climate change due to anthropogenic forcing may occur through the same dynamic modes that sustain natural variability.

Feschenko, OB

Proceedings of the 1998 Science Board Symposium on the Impacts of the 1997/98 El Niño event on the North Pacific Ocean and its marginal seas. no. 10, p125. PICES scientific report. Sidney BC [PICES Sci. Rep.]. Mar 1999.

Theory suggests that the water warming in the east (the El Niño phenomenon) can arise in 2 ways: 1) under simultaneous abnormally weak development of a seasonal anticyclone and abnormally strong development of a seasonal cyclone (trade winds in the winter hemisphere become substantially weaker and the monsoons in the summer hemisphere become stronger in the Western Pacific); 2) under asynchronous seasonal alternation in Asia and Australia that lead the development of cyclones (trade winds do not reach the Western Pacific; monsoons can be seen in the northern and southern hemispheres). Opposite situations will induce the La Niña phenonmenon. The strength of development and the continuation of naturally synoptical seasons in Asia and Australia are analyzed. The input was daily atmospheric pressure in Hong Kong and Darwin from 1984 through 1987. During this period one La Niña and one El Niño were observed. Analysis confirmed the reliability of the theory. Statistical testing, with a larger database, of the stability of the discovered connection is presented.

Landsea, CW; Knaff, JA

Bulletin of the American Meteorological Society [Bull. Am. Meteorol. Soc.]. Vol. 81, no. 9, pp. 2107-2120. 2000.

The very strong 1997-98 El Niño was the first major event in which numerous forecasting groups participated in its real-time prediction. A previously developed simple statistical tool-the El Niño-Southern Oscillation Climatology and Persistence (ENSO-CLIPER) model-is utilized as a baseline for determination of skill in forecasting this event. Twelve statistical and dynamical models were available in real time for evaluation. Some of the models were able to outperform ENSO-CLIPER in predicting either the onset or the decay of the 1997-98 El Niño, but none were successful at both for a medium-range two season (6-8 months) lead time. There were no models, including ENSO-CLIPER, able to anticipate even one-half of the actual amplitude of the El Niño's peak at medium-range (6-11 months) lead. In addition, none of the models showed skill (i.e., lower root-mean-square error than ENSO-CLIPER) at the zero season (0-2 months) through the two season (6-8 months) lead times. No dynamical model and only two of the statistical models [the canonical correlation analysis (CCA) and the constructed analog (ANALOG)] outperformed ENSO-CLIPER by more than 5% of the root-mean-square error at the three season (9-11 months) and four season (12-14 months) lead time. El Niño impacts were correctly anticipated by national meteorological centers one half-year in advance, because of the tendency for El Niño events to persist into and peak during the boreal winter. Despite this, the zero to two season (0-8 month) forecasts of the El Niño event itself were no better than ENSO-CLIPER and were, in that sense, not skillful-a conclusion that remains unclear to the general meteorological and oceanographic communities.

Zhang, R*; Zebiak, SE

Journal of Climate [J. Clim.]. Vol. 17, no. 14, pp. 2794-2812. Jul 2004.

An embedding approach is developed and tested to improve El Niño-Southern Oscillation (ENSO) simulations in a hybrid coupled model (HCM), focusing on the ocean thermocline effects on sea surface temperature (SST) in the eastern equatorial Pacific. The NOAA/GFDL Modular Ocean Model (MOM 3) is coupled to a statistical atmospheric model that estimates wind stress anomalies based on a singular value decomposition (SVD) of the covariance between observed wind stress and SST anomalies. Analogous to the Cane-Zebiak (CZ) coupled model, a separate SST anomaly model is explicitly embedded into the z -coordinate ocean general circulation model (OGCM). The three components exchange predicted anomalies within the coupled system: The OGCM provides anomalies of ocean currents in the surface mixed layer and the thermocline depth, which are used to calculate SST anomalies from the embedded SST model; wind anomalies are then determined according to the statistical atmospheric model, which in turn force the OGCM. Results from uncoupled and coupled runs with and without the embedding are compared. With the standard coupling, the system exhibits similar behavior to previous HCMs, including interannual variability with a dominant quasi-biennial oscillation and a westward propagation of SST anomalies on the equator. These characteristics suggest that the horizontal advection is playing a more important role than the vertical advection in determining SST changes over the eastern equatorial Pacific. Incorporating the embedded SST anomaly model, with which the thermocline effects on SST can be enhanced in the eastern equatorial Pacific, has a significant impact on performance of the HCM. The embedded HCM exhibits more realistic SST variability and coupled behavior, characterized by 3-4-yr oscillations and a more standing SST pattern along the equator. The results support the hypothesis that current physical parameterizations in the OGCM provide insufficient thermal linkage between the thermocline and the sea surface in the eastern equatorial Pacific. It is demonstrated that the long-known deficiency of some OGCMs in their depiction of the thermocline and its interactions with SST may contribute to unrealistic coupled variability in HCMs of ENSO. The embedding approach not only provides a diagnosis for parameterization deficiencies in current OGCMs but, pending progress on this difficult problem, provides a straightforward means to bypass it and improve coupled model performance.

Yeh, S*; Jhun, J; Kang, I; Kirtman, BP

Journal of Climate [J. Clim.]. Vol. 17, no. 6, pp. 1225-1238. Mar 2004.

In this study, the characteristics of decadal ENSO variability in a long (100-yr) simulation of a hybrid coupled model (HCM) are investigated. To exclude the possibility that the decadal El Niño-Southern Oscillation (ENSO) variability is forced by midlatitude ocean variability, the atmospheric component model is coupled to an ocean model that is restricted to the tropical Pacific. The sea surface temperature anomaly (SSTA) variability from a 100-yr run of HCM compares favorably to the observations and shows fluctuations in the ENSO period and amplitude on decadal time scales. The spatial structure of the interannual ENSO variability in the HCM is similar to the observations, whereas on decadal time scales the spatial structure differs significantly from the observations suggesting the importance of extratropical oceanic processes or deficiencies in the model. The decadal mean of both the SSTA and the wind stress anomaly is too equatorially confined in the HCM compared to the observations. Simple coupled model experiments are performed to determine the source of decadal ENSO variability in the HCM. These experiments indicate that the slow time-scale variations in the mean state have little effect on the character of the ENSO variability. The decadal modulation of ENSO is primarily related to the details of atmospheric noise processes.

Galanti, E*; Tziperman, E; Harrison, M; Rosati, A; Sirkes, Z

Monthly Weather Review [Mon. Weather Rev.]. Vol. 131, no. 11, pp. 2748-2764. Nov 2003.

An experimental ENSO prediction system is presented, based on an ocean general circulation model (GCM) coupled to a statistical atmosphere and the adjoint method of 4D variational data assimilation. The adjoint method is used to initialize the coupled model, and predictions are performed for the period 1980-99. The coupled model is also initialized using two simpler assimilation techniques: forcing the ocean model with observed sea surface temperature and surface fluxes, and a 3D variational data assimilation (3DVAR) method, similar to that used by the National Centers for Environmental Prediction (NCEP) for operational ENSO prediction. The prediction skill of the coupled model initialized by the three assimilation methods is then analyzed and compared. The effect of the assimilation period used in the adjoint method is studied by using 3-, 6-, and 9-month assimilation periods. Finally, the possibility of assimilating only the anomalies with respect to observed climatology in order to circumvent systematic model biases is examined. member of t is found that the adjoint method does seem to have the potential for improving over simpler assimilation schemes. The improved skill is mainly at prediction intervals of more than 6 months, where the coupled model dynamics start to influence the model solution. At shorter prediction time intervals, the initialization using the forced ocean model or the 3DVAR may result in a better prediction skill. The assimilation of anomalies did not have a substantial effect on the prediction skill of the coupled model. This seems to indicate that in this model the climatology bias, which is compensated for by the anomaly assimilation, is less significant for the predictive skill than the bias in the model variability, which cannot be eliminated using the anomaly assimilation. Changing the optimization period from 6 to 3 to 9 months showed that the period of 6 months seems to be a near-optimal choice for this model.

Syu, HH; Neelin, JD

Climate Dynamics [Clim. Dyn.]. Vol. 16, no. 1, pp. 19-34. 3 Jan 2000.

A hybrid coupled model (HCM) for the tropical Pacific ocean-atmosphere system is used to test the effects of physical parametrizations on ENSO simulation. The HCM consists of the Geophysical Fluid Dynamics Laboratory ocean general circulation model coupled to an empirical atmospheric model based on the covariance matrix of observed SST and wind stress anomaly fields. In this two-part work, part I describes the effects of ocean vertical mixing schemes and atmospheric spin-up time on ENSO period. Part II addresses ENSO prediction using the HCM and examines the impact of initialization schemes. The standard version of the HCM exhibits spatial and temporal evolution that compare well to observations, with irregular cycles that tend to exhibit 3- and 4-year frequency-locking behavior. Effects in the vertical mixing parametrization that produce stronger mixing in the surface layer give a longer inherent ENSO period, suggesting model treatment of vertical mixing is crucial to the ENSO problem. Although the atmospheric spin-up time scale is short compared to ENSO time scales, it also has a significant effect in lengthening the ENSO period. This suggests that atmospheric time scales may not be truly negligible in quantitative ENSO theory. Overall, the form and evolution mechanism of the ENSO cycle is robust, even though the period is affected by these physical parametrizations.

Syu, HH; Neelin, JD

Climate Dynamics [Clim. Dyn.]. Vol. 16, no. 1, pp. 35-48. 3 Jan 2000.

A hybrid coupled model (HCM) for the tropical Pacific ocean-atmosphere system is employed for ENSO prediction. The HCM consists of the Geophysical Fluid Dynamics Laboratory ocean general circulation model and an empirical atmospheric model. In hindcast experiments, a correlation skill competitive to other prediction models is obtained, so we use this system to examine the effects of several initialization schemes on ENSO prediction. Initialization with wind stress data and initialization with wind stress reconstructed from SST using the atmospheric model give comparable skill levels. In re-estimating the atmospheric model in order to prevent hindcast-period wind information from entering through empirical atmospheric model, we note some sensitivity to the estimation data set, but this is considered to have limited impact for ENSO prediction purposes. Examination of subsurface heat content anomalies in these cases and a case forced only by the difference between observed and reconstructed winds suggests that at the current level of prediction skill, the crucial wind components for initialization are those associated with the slow ENSO mode, rather than with atmospheric internal variability. A "piggyback suboptimal data assimilation is tested in which the Climate Prediction Center data assimilation product from a related ocean model is used to correct the ocean initial thermal field. This yields improved skill, suggesting that not all ENSO prediction systems need to invest in costly data assimilation efforts, provided the prediction and assimilation models are sufficiently close.

Syu, H

Dissertation Abstracts International Part B: Science and Engineering [Diss. Abst. Int. Pt. B - Sci. & Eng.]. Vol. 58, no. 8, p. 4283. Feb 1998.

A hybrid coupled model (HCM) for the tropical Pacific ocean-atmosphere system is employed for ENSO simulation and prediction. The ocean component is the fully nonlinear Geophysical Fluid Dynamics Laboratory ocean general circulation model. The atmospheric component is an empirical model that specifies wind stress from ocean model sea surface temperatures (SST), derived from singular value decomposition of the covariance between observed surface wind stress and SST fluctuations. A coupled seasonal cycle approach is used to model the coupled feedbacks, mainly the momentum feedbacks, in the seasonal cycle on the same basis as the interannual variability. The result indicates that the momentum feedbacks in coupled processes have significant effects in the ocean- atmosphere system in producing the seasonal cycle in the equatorial Pacific. With a more conventional coupling approach, the impacts of ocean vertical mixing schemes and atmospheric spin-up time on ENSO period are investigated. With a surface-layer parameterization that gives stronger vertical mixing, a longer inherent ENSO period is obtained. Longer atmospheric spin-up time scale further lengthens the inherent ENSO period. The standard version of the HCM exhibits 3- and 4-year frequency-locking behavior and spatial and temporal evolution that compare well to observations. The interactions between the inherent ENSO variability and the seasonal cycle can lead to frequency-locking and transition to chaos, which has been suggested to be the source of ENSO irregularity. A considerable parameter range is explored and only mild chaotic behavior is found. Error growth due to sensitive dependence on initial conditions is found to be small on a time scale of several years. This suggests that chaotic behavior is not a serious limitation to ENSO prediction, compared to other factors, such as weather noise. Our model results exhibit a scattered phase-locking behavior, due to the alternation between different integer-year intervals attempting to match the preferred season. This behavior is also found in observations, which suggests that the ENSO phase locking takes into account the integrated effects from past events and how much the preferred period can be stretched. In HCM hindcast experiments, a correlation skill competitive to other prediction models is obtained, using results of the Climate Prediction Center data assimilation product from a related ocean model. The hindcast results confirm the importance of ocean subsurface structure and show the crucial wind components are those associated with the slow ENSO mode.

Kirtman, BP; Zebiak, SE

Monthly Weather Review [MON. WEATHER REV.]. Vol. 125, no. 10, pp. 2620-2641. Oct 1997.

A hybrid coupled model (HCM) consisting of a tropical Pacific Ocean and global atmosphere is presented. The ocean component is a linear reduced gravity model of the upper ocean in the tropical Pacific. The atmospheric component is a triangular 30 horizontal resolution global spectral general circulation model with 18 unevenly spaced levels in the vertical. In coupling these component models, an anomaly coupling strategy is employed. A 40-yr simulation was made with HCM and the variability in the tropical Pacific was compared to the observed variability. The HCM produces irregular ENSO events with a broad spectrum of periods between 12 and 48 months. On longer timescales, approximately 48 months, the simulated variability was weaker than the observed and on shorter timescales (approximately 24 months) the simulated variability was too strong. The simulated variability is asymmetric in the sense that the amplitude of the warm events is realistic, but there are no significant cold events. An ensemble of 60 hindcast predictions was made with the HCM and the skill was compared to other prediction systems. In forecasting sea surface temperature anomalies in the eastern Pacific, the HCM is comparable to the other prediction systems for lead times up to 10 months. The anomaly correlation coefficient for the eastern Pacific SSTA remains above 0.6 for lead times of up to 11 months. Consistent with the 40-yr simulation, hindcasts of cold events have little skill, particularly when compared to hindcasts of warm events. Specific hindcasts also demonstrate that the HCM also has difficulty predicting the transition from warm conditions to normal or cold conditions.

Ni, Yunqi; Zebiak, SE; Cane, MA; Marx, L; Shukla, J

Scientia atmospherica sinica/Daqi Kexue. Beijing [Sci. Atmos. Sin./Daqi Kexue]. Vol. 22, no. 3, pp. 265-273. 1998.

This study used coupled ocean-atmosphere model which consists of the COLA R15 AGCM and a simple CZ Ocean model. Fifteen experimental predictions for ENSO are performed by this hybrid coupled model. The leading correlation of predicted and observed Niño 3 exceeds 0.6 (statistical significance at level 0.01) out to a 15 month lead time. The error of predicted Niño 3 is about 0.6-0.9 before leading one and a half years. Predicted results indicate that the hybrid coupled model may possess capability of ENSO prediction for 15 months. Finally, advantages and improvement approaches of this model are discussed.

Eckert, C; Latif, M

Journal of Climate [J. CLIM.]. Vol. 10, no. 7, pp. 1488-1504. Jul 1997.

The El Niño-Southern Oscillation (ENSO) phenomenon is modeled as a stochastically driven dynamical system. This was accomplished by adding to a Hybrid Coupled Model (HCM) of the tropical Pacific ocean-atmosphere system a stochastic wind stress anomaly field that was derived from observations. The model exhibits irregular interannual fluctuations, whose space-time characteristics resemble those of the observed interannual climate variability in this region. To investigate the predictability of the model, the authors performed ensemble integrations with different realizations of the stochastic wind stress forcing. The ensembles were initialized at various phases of the model's ENSO cycle simulated in a 120-yr integration with a particular noise realization. The numerical experiments indicate that the ENSO predictability is severely limited by the stochastic wind stress forcing. Linear stochastic processes were fitted to the restart ensembles in a reduced state space. A predictability measure based on a comparison of the stationary and the time-dependent probability distributions of the fitted linear models reveals an ENSO predictability limit of considerably less than an average cycle length.

Barnett, TP; Latif, M; Graham, N; Fluegel, M; Pazan, S; White, W

J. CLIMATE, vol. 6, no. 8, pp. 1545-1566, 1993

A hybrid coupled model (HCM) of the tropical ocean-atmosphere system is described. The ocean component is a fully nonlinear ocean general circulation model (OGCM). The atmospheric element is a statistical model that specifies wind stress from ocean-model sea surface temperatures (SST). The coupled model demonstrates a chaotic behavior during extended integration that is related to slow changes in the background mean state of the ocean. The HCM also reproduces many of the observed variations in the tropical Pacific ocean-atmosphere system.