A Hybrid Coupled Ocean–Atmosphere Model and Its Simulation of ENSO and Atmospheric Responses

A new hybrid coupled model(HCM) is presented in this study, which consists of an intermediate tropical Pacific Ocean model and a global atmospheric general circulation model. The ocean component is the intermediate ocean model(IOM)of the intermediate coupled model(ICM) used at the Institute of Oceanology, Chinese Academy of Sciences(IOCAS). The atmospheric component is ECHAM5, the fifth version of the Max Planck Institute for Meteorology atmospheric general circulation model. The HCM integrates its atmospheric and oceanic components by using an anomaly coupling strategy. A100-year simulation has been made with the HCM and its simulation skills are evaluated, including the interannual variability of SST over the tropical Pacific and the ENSO-related responses of the global atmosphere. The model shows irregular occurrence of ENSO events with a spectral range between two and five years. The amplitude and lifetime of ENSO events and the annual phase-locking of SST anomalies are also reproduced realistically. Despite the slightly stronger variance of SST anomalies over the central Pacific than observed in the HCM, the patterns of atmospheric anomalies related to ENSO,such as sea level pressure, temperature and precipitation, are in broad agreement with observations. Therefore, this model can not only simulate the ENSO variability, but also reproduce the global atmospheric variability associated with ENSO, thereby providing a useful modeling tool for ENSO studies. Further model applications of ENSO modulations by ocean–atmosphere processes, and of ENSO-related climate prediction, are also discussed.     

Modulation of El Nino-Southern Oscillation by Freshwater Flux and Salinity Variability in the Tropical Pacific

The El Nin o-Southern Oscillation (ENSO) is modulated by many factors; most previous studies have emphasized the roles of wind stress and heat flux in the tropical Pacific. Freshwater flux (FWF) is another environmental forcing to the ocean; its effect and the related ocean salinity variability in the ENSO region have been of increased interest recently. Currently, accurate quantifications of the FWF roles in the climate remain challenging; the related observations and coupled ocean-atmosphere modeling involve large elements of uncertainty. In this study, we utilized satellite-based data to represent FWF-induced feedback in the tropical Pacific climate system; we then incorporated these data into a hybrid coupled ocean-atmosphere model (HCM) to quantify its effects on ENSO. A new mechanism was revealed by which interannual FWF forcing modulates ENSO in a significant way. As a direct forcing, FWF exerts a significant influence on the ocean through sea surface salinity (SSS) and buoyancy flux (Q B ) in the western-central tropical Pacific. The SSS perturbations directly induced by ENSO-related interannual FWF variability affect the stability and mixing in the upper ocean. At the same time, the ENSO-induced FWF has a compensating effect on heat flux, acting to reduce interannual Q B variability during ENSO cycles. These FWF-induced processes in the ocean tend to modulate the vertical mixing and entrainment in the upper ocean, enhancing cooling during La Nin a and enhancing warming during El Nin o, respectively. The interannual FWF forcing-induced positive feedback acts to enhance ENSO amplitude and lengthen its time scales in the tropical Pacific coupled climate system.     

Effects of interannual salinity variability on the barrier layer in the western-central equatorial Pacific: A diagnostic analysis from Argo

ABSTRACT In this paper, interannual variations in the barrier layer thickness （BLT） are analyzed using Argo three-dimensional temperature and salinity data, with a locus on the effects of interannually varying salinity on the evolution of the El Nifio Southern Oscillation （ENSO）. The interannually varying BLT exhibits a zonal seesaw pattern across the equatorial Pacific during ENSO cycles. This phenomenon has been attributed to two different physical processes. During E1 Nifio （La Nifia）, the barrier layer （BL） is anomalously thin （thick） west of about 160°E, and thick （thin） to the east. In the western equatorial Pacific （the western part： 130°-160°E）, interannual variations of the BLT indicate a lead of one year relative to those of the ENSO onset. The interannual variations of the BLT can be largely attributed to the interannual temperature variability, through its dominant effect on the isothermal layer depth （ILD）. However, in the central equatorial Pacific （the eastern part： 160~E- 170~W）, interannual variations of the BL almost synchronously vary with ENSO, with a lead of about two months relative to those of the local SST. In this region, the interannual variations of the BL are significantly affected by the interannually varying salinity, mainly through its modulation effect on the mixed layer depth （MLD）. As evaluated by a onedimensional boundary layer ocean model, the BL around the dateline induced by interannual salinity anomalies can significantly affect the temperature fields in the upper ocean, indicating a positive feedback that acts to enhance ENSO.     

Simulation of salinity variability and the related freshwater flux forcing in the tropical Pacific: An evaluation using the Beijing normal university earth system model (BNU-ESM)

The climatology and interannual variability of sea surface salinity(SSS) and freshwater flux(FWF) in the equatorial Pacific are analyzed and evaluated using simulations from the Beijing Normal University Earth System Model(BNU-ESM).The simulated annual climatology and interannual variations of SSS, FWF, mixed layer depth(MLD), and buoyancy flux agree with those observed in the equatorial Pacific. The relationships among the interannual anomaly fields simulated by BNU-ESM are analyzed to illustrate the climate feedbacks induced by FWF in the tropical Pacific. The largest interannual variations of SSS and FWF are located in the western-central equatorial Pacific. A positive FWF feedback effect on sea surface temperature(SST) in the equatorial Pacific is identified. As a response to El Ni ?no–Southern Oscillation(ENSO),the interannual variation of FWF induces ocean processes which, in turn, enhance ENSO. During El Ni ?no, a positive FWF anomaly in the western-central Pacific(an indication of increased precipitation rates) acts to enhance a negative salinity anomaly and a negative surface ocean density anomaly, leading to stable stratification in the upper ocean. Hence, the vertical mixing and entrainment of subsurface water into the mixed layer are reduced, and the associated El Ni ?no is enhanced. Related to this positive feedback, the simulated FWF bias is clearly reflected in SSS and SST simulations, with a positive FWF perturbation into the ocean corresponding to a low SSS and a small surface ocean density in the western-central equatorial Pacific warm pool.     

How Are El Nio and La Nia Events Improved in an Eddy-Resolving Ocean General Circulation Model?

The present study compares the performance of two versions of the LASG/IAP(State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics/Institute of Atmospheric Physics) Climate System Ocean Model(LICOM) in reproducing the interannual variability associated with El Nio and La Nia events in the tropical Pacific. Both versions are forced with the identical boundary conditions from observed or reanalysis data, in which one version has a finer spatial resolution of(1/10)° in the horizontal domain and 55 vertical layers, and the other version has a coarse resolution of 1° in the horizontal domain and 30 vertical layers. ENSO simulations form the two versions are compared with observations and, in particular, the improvements with regard to ENSO by the finer resolution ocean model are emphasized. As a result of the finer spatial resolution, both the vertical temperature gradient and vertical velocity are better represented in the equatorial Pacific than they are by the coarse resolution model; and thus, the corresponding vertical advections of temperature are more reasonable. Besides the mean climatology, simulated ENSO events and relevant feedbacks are much improved in the finer resolution model. A heat budget analysis suggests that both thermocline feedback and Ekman feedback are mainly responsible for the rapid increase in temperature anomalies during the developing and mature phases of ENSO events.     

Remote effects of tropical cyclone wind forcing over the western Pacific on the eastern equatorial ocean

An ocean general circulation model （OGCM） is used to demonstrate remote effects of tropical cyclone wind （TCW） forcing in the tropical Pacific. The signature of TCW forcing is explicitly extracted using a locally weighted quadratic least=squares regression （called as LOESS） method from six-hour satellite surface wind data; the extracted TCW component can then be additionally taken into account or not in ocean modeling, allowing isolation of its effects on the ocean in a clean and clear way. In this paper, seasonally varying TCW fields in year 2008 are extracted from satellite data which are prescribed as a repeated annual cycle over the western Pacific regions off the equator （poleward of 10°N/S）; two long-term OGCM experiments are performed and compared, one with the TCW forcing part included additionally and the other not. Large, persistent thermal perturbations （cooling in the mixed layer （ML） and warming in the thermocline） are induced locally in the western tropical Pacific, which are seen to spread with the mean ocean circulation pathways around the tropical basin. In particular, a remote ocean response emerges in the eastern equatorial Pacific to the prescribed off-equatorial TCW forcing, characterized by a cooling in the mixed layer and a warming in the thermocline. Heat budget analyses indicate that the vertical mixing is a dominant process responsible for the SST cooling in the eastern equatorial Pacific. Further studies are clearly needed to demonstrate the significance of these results in a coupled ocean-atmosphere modeling context.     

SEASONAL DEPENDENCE OF LOCAL AIR-SEA INTERACTION OVER THE TROPICAL WESTERN PACIFIC WARM POOL

Based on the air-sea interface heat fluxes and related meteorological variables datasets recently released by Objectively Analyzed Air-Sea Fluxes (OA Flux) Project of Woods Hole Oceanographic Institution, as well as the outgoing longwave radiation and surface wind datasets from National Oceanic and Atmospheric Administration, the seasonal dependence of local air-sea interaction over the tropical western Pacific warm pool (referred to the region (1o-6oN, 144o-154oE)) is revealed and the probable impacts of remote forcing on the air-sea interaction are examined. The results indicated the dominance of oceanic forcing with the significant impact of ENSO in March and that of atmospheric feedback without notable influence of remote forcing in June. While the interannual variability of sea surface temperature anomaly (SSTA) is larger than that of SSTA tendency when oceanic forcing is dominant, the opposite is true when atmospheric feedback is dominant. The magnitude of the oceanic forcing of the atmosphere tends to decrease in March with the occurrence of ENSO, though ENSO has little influence on the atmospheric feedback to the ocean in June. The local air-sea interaction is substantially the same before and after the removal of the effect of Indian Oceanic Dipole. The reduction of shortwave radiation fluxes into the western Pacific warm pool, due to the enhanced overlaying convection in March associated with ENSO, leads to the decline of SST tendency that will weaken the oceanic forcing of the atmosphere.     

ENSO Features,Dynamics, and Teleconnections to East Asian Climate as Simulated in CAMS-CSM

This study evaluates the performance of CAMS-CSM(the climate system model of the Chinese Academy of Meteorological Sciences) in simulating the features, dynamics, and teleconnections to East Asian climate of the El Ni?o–Southern Oscillation(ENSO). In general, fundamental features of ENSO, such as its dominant patterns and phase-locking features, are reproduced well. The two types of El Ni?o are also represented, in terms of their spatial distributions and mutual independency. However, the skewed feature is missed in the model and the simulation of ENSO is extremely strong, which is found—based on Bjerknes index assessment—to be caused by underestimation of the shortwave damping effect. Besides, the modeled ENSO exhibits a regular oscillation with a period shorter than observed. By utilizing the Wyrtki index, it is suggested that this periodicity bias results from an overly quick phase transition induced by feedback from the thermocline and zonal advection. In addition to internal dynamics of ENSO,its external precursors—such as the North Pacific Oscillation with its accompanying seasonal footprinting mechanism, and the Indian Ocean Dipole with its 1-yr lead correlation with ENSO—are reproduced well by the model. Furthermore, with respect to the impacts of ENSO on the East Asian summer monsoon, although the anomalous Philippine anticyclone is reproduced in the post-El Ni?o summer, it exhibits an eastward shift compared with observation;and as a consequence, the observed flooding of the Yangtze River basin is poorly represented, with unrealistic air–sea interaction over the South China Sea being the likely physical origin of this bias. The response of wintertime lowertropospheric circulation to ENSO is simulated well, in spite of an underestimation of temperature anomalies in central China. This study highlights the dynamic processes that are key for the simulation of ENSO, which could shed some light on improving this model in the future.     

Modeling the tropical Pacific Ocean using a regional coupled climate model

A high-resolution tropical Pacific general circulation model (GCM) coupled to a global atmospheric GCM is described in this paper. The atmosphere component is the 5°×4°global general circulation model of the Institute of Atmospheric Physics (IAP) with 9 levels in the vertical direction. The ocean component with a horizontal resolution of 0.5°, is based on a low-resolution model (2°×1°in longitude-latitude).Simulations of the ocean component are first compared with its previous version. Results show that the enhanced ocean horizontal resolution allows an improved ocean state to be simulated; this involves (1) an apparent decrease in errors in the tropical Pacific cold tongue region, which exists in many ocean models,(2) more realistic large-scale flows, and (3) an improved ability to simulate the interannual variability and a reduced root mean square error (RMSE) in a long time integration. In coupling these component models, a monthly "linear-regression" method is employed to correct the model's exchanged flux between the sea and the atmosphere. A 100-year integration conducted with the coupled GCM (CGCM) shows the effectiveness of such a method in reducing climate drift. Results from years 70 to 100 are described.The model produces a reasonably realistic annual cycle of equatorial SST. The large SSTA is confined to the eastern equatorial Pacific with little propagation. Irregular warm and cold events alternate with a broad spectrum of periods between 24 and 50 months, which is very realistic. But the simulated variability is weaker than the observed and is also asymmetric in the sense of the amplitude of the warm and cold events.     

On the ENSO Mechanisms

The El Nino-Southern Oscillation (ENSO) is an interannual phenomenon involved in the tropical Paci fic Ocean-atmosphere interactions. The oscillatory nature of ENSO requires both positive and negative ocean-atmosphere feedbacks. The positive feedback is dated back to Bjerknes＇ hypothesis in the 1960s, and different negative feedbacks have been proposed since the 1980s associated with the delayed oscillator, the western Pacific oscillator, the recharge-discharge oscillator, and the advective-reflective oscillator. The de layed oscillator assumes that wave reflection at the western boundary provides a negative feedback for the coupled system to oscillate. The western Pacific oscillator emphasizes equatorial wind in the western Pacific that provides a negative feedback for the coupled system. The recharge-discharge oscillator argues that discharge and recharge of equatorial heat content cause the coupled system to oscillate. The advective-re flective oscillator emphasizes the importance of zonal advection associated with wave reflection at both the western and eastern boundaries. All of these physics are summarized in a unified ENSO oscillator. The de layed oscillator, the western Pacific oscillator, the recharge-discharge oscillator, and the advective-reflec tive oscillator can be extracted as special cases of the unified oscillator. As suggested by this unified oscillator, all of the previous ENSO oscillator mechanisms may be operating in nature.     

The role of ocean dynamics in producing decadal climate variability in the North Pacific

 Decadal time scale climate variability in the North Pacific has implications for climate both locally and over North America. A crucial question is the degree to which this variability arises from coupled ocean/atmosphere interactions over the North Pacific that involve ocean dynamics, as opposed to either purely thermodynamic effects of the oceanic mixed layer integrating in situ the stochastic atmospheric forcing, or the teleconnected response to tropical variability. The part of the variability that is coming from local coupled ocean/atmosphere interactions involving ocean dynamics is potentially predictable by an ocean/atmosphere general circulation model (O/A GCM), and such predictions could (depending on the achievable lead time) have distinct societal benefits. This question is examined using the results of fully coupled O/A GCMs, as well as targeted numerical experiments with stand-alone ocean and atmosphere models individually. It is found that coupled ocean/atmosphere interactions that involve ocean dynamics are important to determining the strength and frequency of a decadal-time scale peak in the spectra of several oceanic variables in the Kuroshio extension region off Japan. Local stochastic atmospheric heat flux forcing, integrated by the oceanic mixed layer into a red spectrum, provides a noise background from which the signal must be extracted. Although teleconnected ENSO responses influence the North Pacific in the 2–7 years/cycle frequency band, it is shown that some decadal-time scale processes in the North Pacific proceed without ENSO. Likewise, although the effects of stochastic atmospheric forcing on ocean dynamics are discernible, a feedback path from the ocean to the atmosphere is suggested by the results. Received: 23 January 2000 / Accepted: 10 January 2001     

ENSO representation in climate models: from CMIP3 to CMIP5

We analyse the ability of CMIP3 and CMIP5 coupled ocean–atmosphere general circulation models (CGCMs) to simulate the tropical Pacific mean state and El Niño-Southern Oscillation (ENSO). The CMIP5 multi-model ensemble displays an encouraging 30 % reduction of the pervasive cold bias in the western Pacific, but no quantum leap in ENSO performance compared to CMIP3. CMIP3 and CMIP5 can thus be considered as one large ensemble (CMIP3 + CMIP5) for multi-model ENSO analysis. The too large diversity in CMIP3 ENSO amplitude is however reduced by a factor of two in CMIP5 and the ENSO life cycle (location of surface temperature anomalies, seasonal phase locking) is modestly improved. Other fundamental ENSO characteristics such as central Pacific precipitation anomalies however remain poorly represented. The sea surface temperature (SST)-latent heat flux feedback is slightly improved in the CMIP5 ensemble but the wind-SST feedback is still underestimated by 20–50 % and the shortwave-SST feedbacks remain underestimated by a factor of two. The improvement in ENSO amplitudes might therefore result from error compensations. The ability of CMIP models to simulate the SST-shortwave feedback, a major source of erroneous ENSO in CGCMs, is further detailed. In observations, this feedback is strongly nonlinear because the real atmosphere switches from subsident (positive feedback) to convective (negative feedback) regimes under the effect of seasonal and interannual variations. Only one-third of CMIP3 + CMIP5 models reproduce this regime shift, with the other models remaining locked in one of the two regimes. The modelled shortwave feedback nonlinearity increases with ENSO amplitude and the amplitude of this feedback in the spring strongly relates with the models ability to simulate ENSO phase locking. In a final stage, a subset of metrics is proposed in order to synthesize the ability of each CMIP3 and CMIP5 models to simulate ENSO main characteristics and key atmospheric feedbacks.     

Analysis of the Slab Ocean El Nino atmospheric feedbacks in observed and simulated ENSO dynamics

In a recent study it was illustrated that the El Nino Southern Oscillation (ENSO) mode can exist in the absence of any ocean dynamics. This oscillating mode exists just due to the interaction between atmospheric heat fluxes and ocean heat capacity. The primary purpose of this study is to further explore these atmospheric Slab Ocean ENSO dynamics and therefore the role of positive atmospheric feedbacks in model simulations and observations. The positive solar radiation feedback to sea surface temperature (SST), due to reduced cloud cover for anomalous warm SSTs, is the main positive feedback in the Slab Ocean El Nino dynamics. The strength of this positive cloud feedback is strongly related to the strength of the equatorial cold tongue. The combination of positive latent and sensible heat fluxes to the west and negative ones to the east of positive anomalies leads to the westward propagation of the SST anomalies, which allows for oscillating behavior with a preferred period of 6–7 years. Several indications are found that parts of these dynamics are indeed observed and simulated in other atmospheric or coupled general circulation models (AGCMs or CGCMs). The CMIP3 AGCM-slab ensemble of 13 different AGCM simulations shows unstable ocean–atmosphere interactions along the equatorial Pacific related to stronger cold tongues. In observations and in the CMIP3 and CMIP5 CGCM model ensemble the strength and sign of the cloud feedback is a function of the strength of the cold tongue. In summary, this indicates that the Slab Ocean El Nino dynamics are indeed a characteristic of the equatorial Pacific climate that is only dominant or significantly contributing to the ENSO dynamics if the SST cold tongue is sufficiently strong. In the observations this is only the case during strong La Nina conditions. The presence of the Slab Ocean ENSO atmospheric feedbacks in observations and CGCM model simulations implies that the family of physical ENSO modes does have another member, which is entirely driven by atmospheric processes and does not need to have the same spatial pattern nor the same time scales as the main ENSO dynamics.     

ENSO in a hybrid coupled model. Part I: sensitivity to physical parametrizations

 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. Received: 17 April 1998 / Accepted: 22 July 1999     

Roles of Indian and Pacific Ocean air–sea coupling in tropical atmospheric variability

Sea surface temperature (SST) variations include negative feedbacks from the atmosphere, whereas SST anomalies are specified in stand-alone atmospheric general circulation simulations. Is the SST forced response the same as the coupled response? In this study, the importance of air–sea coupling in the Indian and Pacific Oceans for tropical atmospheric variability is investigated through numerical experiments with a coupled atmosphere-ocean general circulation model. The local and remote impacts of the Indian and Pacific Ocean coupling are obtained by comparing a coupled simulation with an experiment in which the SST forcing from the coupled simulation is specified in either the Indian or the Pacific Ocean. It is found that the Indian Ocean coupling is critical for atmospheric variability over the Pacific Ocean. Without the Indian Ocean coupling, the rainfall and SST variations are completely different throughout most of the Pacific Ocean basin. Without the Pacific Ocean coupling, part of the rainfall and SST variations in the Indian Ocean are reproduced in the forced run. In regions of large mean rainfall where the atmospheric negative feedback is strong, such as the North Indian Ocean and the western North Pacific in boreal summer, the atmospheric variability is significantly enhanced when air–sea coupling is replaced by specified SST forcing. This enhancement is due to the lack of the negative feedback in the forced SST simulation. In these regions, erroneous atmospheric anomalies could be induced by specified SST anomalies derived from the coupled model. The ENSO variability is reduced by about 20% when the Indian Ocean air–sea coupling is replaced by specified SST forcing. This change is attributed to the interfering roles of the Indian Ocean SST and Indian monsoon in western and central equatorial Pacific surface wind variations.     

The role of atmosphere feedbacks during ENSO in the CMIP3 models. Part II: using AMIP runs to understand the heat flux feedback mechanisms

Several studies using ocean?Catmosphere general circulation models (GCMs) suggest that the atmospheric component plays a dominant role in the modelled El Ni?o-Southern Oscillation (ENSO). To help elucidate these findings, the two main atmosphere feedbacks relevant to ENSO, the Bjerknes positive feedback (??) and the heat flux negative feedback (??), are here analysed in nine AMIP runs of the CMIP3 multimodel dataset. We find that these models generally have improved feedbacks compared to the coupled runs which were analysed in part I of this study. The Bjerknes feedback,???, is increased in most AMIP runs compared to the coupled run counterparts, and exhibits both positive and negative biases with respect to ERA40. As in the coupled runs, the shortwave and latent heat flux feedbacks are the two dominant components of ?? in the AMIP runs. We investigate the mechanisms behind these two important feedbacks, in particular focusing on the strong 1997?C1998 El Ni?o. Biases in the shortwave flux feedback, ?? _SW, are the main source of model uncertainty in ??. Most models do not successfully represent the negative ??_SW in the East Pacific, primarily due to an overly strong low-cloud positive feedback in the far eastern Pacific. Biases in the cloud response to dynamical changes dominate the modelled ?? _SW biases, though errors in the large-scale circulation response to sea surface temperature (SST) forcing also play a role. Analysis of the cloud radiative forcing in the East Pacific reveals model biases in low cloud amount and optical thickness which may affect ?? _SW. We further show that the negative latent heat flux feedback, ?? _LH, exhibits less diversity than ?? _SW and is primarily driven by variations in the near-surface specific humidity difference. However, biases in both the near-surface wind speed and humidity response to SST forcing can explain the inter-model ??_LH differences.     

ENSO in a hybrid coupled model. Part II: prediction with piggyback data assimilation

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. Received: 17 April 1998 / Accepted: 22 July 1999     

Modes of climate variability as simulated by a coupled general circulation model. Part I: ENSO-like climate variability and its low-frequency modulation

The El Niño-Southern Oscillation (ENSO) is investigated in a multicentury integration conducted with the coupled general circulation model (CGCM) ECHAM3/LSG. The quasiperiodic interannual oscillations of the simulated equatorial Pacific climate system are due to subsurface temperature anomaly propagation and a positive atmosphere-ocean feedback. The gravest internal wave modes contribute to the generation of these anomalies. The simulated ENSO has a characteristic period of 5–8 years. Due to the coarse resolution of the ocean model the ENSO amplitude is underestimated by a factor of three as compared to observations. The model ENSO is associated with the typical atmospheric teleconnection patterns. Using wavelet statistics two characteristic interdecadal modulations of the ENSO variance are identified. The origins of a 22 and 35?y ENSO modulation as well as the characteristic ENSO response to greenhouse warming simulated by our model are discussed.