A Coupled General Circulation Model for the Tropical Pacific Ocean and Global Atmosphere

On the basis of Zeng’s theoretical design, a coupled general circulation model (CGCM) is developed with its characteristics different from other CGCMs such as the unified vertical coordinates and subtraction of the standard stratification for both atmosphere and ocean, available energy consideration, and so on. The oceanic component is a free surface tropical Pacific Ocean GCM between 30oN and 30oS with horizontal grid spacing of 1o in latitude and 2o in longitude, and with 14 vertical layers. The atmospheric component it a global GCM with low-resolution of 4o in latitude and 5o in longitude, and two layers or equal man in the vertical between the surface and 200 hPa. The atmospheric GCM includes comprehensive physical processes. The coupled model is subjected to seasonally-varying cycle. Several coupling experiments, ranging from straight forward coupling without flux correction to one with flux correction, and to so-called predictor-corrector monthly coupling (PCMC), are conducted to show the existence and final controlling of the climate drift in the coupled system. After removing the climate drift with the PCMC scheme, the coupled model is integrated for more than twenty years. The results show reasonable simulations of the annual mean and its seasonal cycle of the atmospheric and oceanic circulation. The model also produces the coherent interannual variations of the climate system, manifesting the observed El Ni?o / Southern Oscillation (ENSO).     

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.     

A Linear Diagnosis of the Coupled Extratropical Ocean–Atmosphere System in the GFDL GCM

Summary:Diagnosing a coupled system with linear inverse modelling (LIM) can provide insight into the nature and strength of the coupling. This technique is applied to the cold season output of the GFDL GCM, forced by observed tropical Pacific SSTs and including a slab mixed layer ocean model elsewhere. It is found that extratropical SST anomalies act to enhance atmospheric thermal variability and diminish barotropic variability over the east Pacific in these GCM runs, in agreement with other theoretical and modelling studies. North-west Atlantic barotropic variability is also enhanced. However, all these feedbacks are very weak. LIM results also suggest that North Pacific extratropical SST anomalies in this model would rapidly decay without atmospheric forcing induced by tropical SST anomalies.     

Versions g1.0 and g1.1 of the LASGIAP Flexible Global Ocean–Atmosphere–Land System Model

The latest two versions of the IAP Flexible Global Ocean-Atmosphere-Land System (FGOALS) model- versions g1.0 and g1.1, are described in this study. Both two versions are fully coupled GCMs without any flux correction, major changes for g1.1 mainly lie in four aspects: (1) advection schemes for tracer in the ocean component model; (2) zonal filter scheme in high latitudes in the ocean component model; (3) coupling scheme for fresh water flux in high latitudes; and (4) an improved algorithm of airsea turbule...     

A ＂Dressed＂ Ensemble Kalman Filter Using the Hybrid Coordinate Ocean Model in the Pacific

The computational cost required by the Ensemble Kalman Filter (EnKF) is much larger than that of some simpler assimilation schemes, such as Optimal Interpolation (OI) or three-dimension variational assimilation (3DVAR). Ensemble optimal interpolation (EnOI), a crudely simplified implementation of EnKF, is sometimes used as a substitute in some oceanic applications and requires much less computational time than EnKF. In this paper, to compromise between computational cost and dynamic covariance, we use the idea of ``dressing' a small size dynamical ensemble with a larger number of static ensembles in order to form an approximate dynamic covariance. The term ``dressing' means that a dynamical ensemble seed from model runs is perturbed by adding the anomalies of some static ensembles. This dressing EnKF (DrEnKF for short) scheme is tested in assimilation of real altimetry data in the Pacific using the HYbrid Coordinate Ocean Model (HYCOM) over a four-year period. Ten dynamical ensemble seeds are each dressed by 10 static ensemble members selected from a 100-member static ensemble. Results are compared to two EnKF assimilation runs that use 10 and 100 dynamical ensemble members. Both temperature and salinity fields from the DrEnKF and the EnKF are compared to observations from Argo floats and an OI SST dataset. The results show that the DrEnKF and the 100-member EnKF yield similar root mean square errors (RMSE) at every model level. Error covariance matrices from the DrEnKF and the 100-member EnKF are also compared and show good agreement.     

Versions g1.0 and g1.1 of the LASG/IAP Flexible Global Ocean–Atmosphere–Land System Model

The latest two versions of the IAP Flexible Global Ocean-Atmosphere-Land System (FGOALS) model- versions g1.0 and g1.1, are described in this study. Both two versions are fully coupled GCMs without any flux correction, major changes for g1.1 mainly lie in four aspects: (1) advection schemes for tracer in the ocean component model; (2) zonal filter scheme in high latitudes in the ocean component model; (3) coupling scheme for fresh water flux in high latitudes; and (4) an improved algorithm of airsea turbulent flux depending on the surface current of the ocean. As a result, the substantial cold biases in the tropical Pacific and high latitudes are improved by g1.1, especially g1.1 simulates more reasonable equatorial thermocline, poleward heat transport, zonal overturning stream function in the ocean and sea ice distribution than g1.0. Significant ENSO variability are simulated by both versions, however the ENSO behavior by g1.0 differs from the observed one in many aspects: about twice ENSO amplitude as observed, false ENSO asymmetry, only one peak period around 3 years, etc. Due to improved mean climate state by g1.1, many basic characteristics of ENSO are reproduced by g1.1, e.g., more reasonable ENSO amplitude, two peaks of power spectra for ENSO events, and positive SST skewness in the eastern Pacific as observed.     

A Non-Linear Dynamical–Statistical Model for Reconstruction of the Air–Sea Element Fields in the Tropical Pacific Ocean

Aiming at tackling the difficulty in establishing a sea surface temperature (SST) dynamical model, this study develops a non-linear dynamical–statistical model of SST fields and their correlative factors based on Genetic Algorithms (GA) and the dynamical system reconstruction idea, which greatly improves the El Niño–Southern Oscillation (ENSO) forecast model. Using Hadley SST data, sea surface wind (SSW) and sea level pressure (SLP) data from the National Centers for Environmental Prediction-National Center for Environmental Research (NCEP-NCAR), with empirical orthogonal function (EOF) time-space for reconstruction, we carry out numerical integral forecasting experiments for SST, SSW, and SLP fields. By statistical analysis of the forecasting experiments, we find that forecasts for less than 25 months perform better than longer term forecasts. Based on the model, we forecast SST, SSW, and SLP fields in September, October, and November 2014 and predict a weak La Niña event. This study explores a novel method for the complex atmosphere–ocean system.     

The SST–Wind Coupling Pattern in the East China Sea Based on a Regional Coupled Ocean–Atmosphere Model

To study the interaction between sea surface temperature (SST) and surface wind in the East China Sea (ECS), the Coupled Ocean–Atmosphere–Wave–Sediment Transport (COAWST) modelling system is used to downscale a global atmospheric reanalysis product over the study area in 2013. A singular value decomposition (SVD) method is applied to SST and surface wind speed to study their coupling relationship in the ECS. The heterogeneous correlation map indicates that the surface wind has a negative correlation with the SST, especially in the Kuroshio Current. From lead-lag correlations between the first principal component of SST and surface wind SVD (filtered using a Lanczos high-pass filter with a 90-day cut-off), a correlation of about 0.1 is found at lag ?6, and a negative correlation of about ?0.3 is also found around lag 1. The results indicate a negative feedback between SST and wind fluctuations at short time-scales. Air–sea heat ?uxes contribute little to the SST variability in the ECS section of the Kuroshio and the analysis of the mixed-layer heat budget shows that the contribution of horizontal advection is dominant in determining the intraseasonal SST signals.     

Ensemble data assimilation in a simple coupled climate model: The role of ocean-atmosphere interaction

A conceptual coupled ocean-atmosphere model was used to study coupled ensemble data assimilation schemes with a focus on the role of ocean-atmosphere interaction in the assimilation. The optimal scheme was the fully coupled data assimilation scheme that employs the coupled covariance matrix and assimilates observations in both the atmosphere and ocean. The assimilation of synoptic atmospheric variability that captures the temporal fluctuation of the weather noise was found to be critical for the estimation of not only the atmospheric, but also oceanic states. The synoptic atmosphere observation was especially important in the mid-latitude system, where oceanic variability is driven by weather noise. The assimilation of synoptic atmospheric variability in the coupled model improved the atmospheric variability in the analysis and the subsequent forecasts, reducing error in the surface forcing and, in turn, in the ocean state. Atmospheric observation was able to further improve the oceanic state estimation directly through the coupled covariance between the atmosphere and ocean states. Relative to the mid-latitude system, the tropical system was influenced more by ocean-atmosphere interaction and, thus, the assimilation of oceanic observation becomes more important for the estimation of the ocean and atmosphere.     

Primary reasoning behind the double ITCZ phenomenon in a coupled ocean-atmosphere general circulation model

This paper investigates the processes behind the double ITCZ phenomenon, a common problem in Coupled ocean-atmosphere General Circulation Models (CGCMs), using a CGCM-FGCM-0 (Flexible General Circulation Model, version 0). The double ITCZ mode develops rapidly during the first two years of the integration and becomes a perennial phenomenon afterwards in the model. By way of Singular ValueDecomposition (SVD) for SST, sea surface pressure, and sea surface wind, some air-sea interactions are analyzed. These interactions prompt the anomalous signals that appear at the beginning of the coupling to develop rapidly. There are two possible reasons, proved by sensitivity experiments: (1) the overestimatedeast-west gradient of SST in the equatorial Pacific in the ocean spin-up process, and (2) the underestimatedamount of low-level stratus over the Peruvian coast in CCM3 (the Community Climate Model, VersionThree). The overestimated east-west gradient of SST brings the anomalous equatorial easterly. The anomalous easterly, affected by the Coriolis force in the Southern Hemisphere, turns into an anomalouswesterly in a broad area south of the equator and is enhanced by atmospheric anomalous circulationdue to the underestimated amount of low-level stratus over the Peruvian coast simulated by CCM3. Theanomalous westerly leads to anomalous warm advection that makes the SST warm in the southeast Pacific.The double ITCZ phenomenon in the CGCM is a result of a series of nonlocal and nonlinear adjustmentprocesses in the coupled system, which can be traced to the uncoupled models, oceanic component, andatmospheric component. The zonal gradient of the equatorial SST is too large in the ocean componentand the amount of low-level stratus over the Peruvian coast is too low in the atmosphere component.     

一个可供ENSO预测的海气耦合环流模式及1997/1998 ENSO的预测

利用中国科学院大气物理研究所设计发展的具有较高分辨率的热带太平洋和全球大气耦合环流模式,设计了一个初始化方案,建立了ＥＮＳＯ预测系统,进行了系统性的预测试验。预测结果检验评估表明,该预测系统表现出较强的预报能力,赤道中东太平洋地区（Ｎｉｎｏ３和Ｎｉｎｏ３４）海表温度距平预报相关技巧高于０５２的预报可持续１８个月,该预测系统可应用到试验性的海温预测实践中。利用该系统对１９９７／１９９８年ＥＮＳＯ进行了实际预测,表明预测是成功的,预测的海温距平已提供给今年我国夏季降水预测使用,取得了良好的预测效果。     

A fast version of LASG/IAP climate system model and its 1000-year control integration

A fast version of the State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geo- physical Fluid Dynamics(LASG)/Institute of Atmospheric Physics(IAP)climate system model is briefly documented.The fast coupled model employs a low resolution version of the atmospheric component Grid Atmospheric Model of IAP/LASG(GAMIL),with the other parts of the model,namely an oceanic component LASG/IAP Climate Ocean Model(LICOM),land component Common Land Model(CLM),and sea ice component from National Center for Atmospheric Research Community Climate System Model (NCAR CCSM2),as the same as in the standard version of LASG/IAP Flexible Global Ocean Atmosphere Land System model(FGOALS g).The parameterizations of physical and dynamical processes of the at- mospheric component in the fast version are identical to the standard version,although some parameter values are different.However,by virtue of reduced horizontal resolution and increased time-step of the most time-consuming atmospheric component,it runs faster by a factor of 3 and can serve as a useful tool for long- term and large-ensemble integrations.A 1000-year control simulation of the present-day climate has been completed without flux adjustments.The final 600 years of this simulation has virtually no trends in global mean sea surface temperatures and is recommended for internal variability studies.Several aspects of the control simulation’s mean climate and variability are evaluated against the observational or reanalysis data. The strengths and weaknesses of the control simulation are evaluated.The mean atmospheric circulation is well simulated,except in high latitudes.The Asian-Australian monsoonal meridional cell shows realistic features,however,an artificial rainfall center is located to the eastern periphery of the Tibetan Plateau persists throughout the year.The mean bias of SST resembles that of the standard version,appearing as a"double ITCZ"(Inter-Tropical Convergence Zone)associated with a westward extension of the equatorial eastern Pacific cold tongue.The sea ice extent is acceptable but has a higher concentration.The strength of Atlantic meridional overturning is 27.5 Sv.Evidence from the 600-year simulation suggests a modulation of internal variability on ENSO frequency,since both regular and irregular oscillations of ENSO are found during the different time periods of the long-term simulation.     

Testing a four-dimensional variational data assimilation method using an improved intermediate coupled model for ENSO analysis and prediction

A four-dimensional variational (4D-Var) data assimilation method is implemented in an improved intermediate coupled model (ICM) of the tropical Pacific. A twin experiment is designed to evaluate the impact of the 4D-Var data assimilation algorithm on ENSO analysis and prediction based on the ICM. The model error is assumed to arise only from the parameter uncertainty. The “observation” of the SST anomaly, which is sampled from a “truth” model simulation that takes default parameter values and has Gaussian noise added, is directly assimilated into the assimilation model with its parameters set erroneously. Results show that 4D-Var effectively reduces the error of ENSO analysis and therefore improves the prediction skill of ENSO events compared with the non-assimilation case. These results provide a promising way for the ICM to achieve better real-time ENSO prediction.     

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.     

A new framework for isolating individual feedback processes in coupled general circulation climate models. Part I: formulation

This paper proposes a coupled atmosphere–surface climate feedback–response analysis method (CFRAM) as a new framework for estimating climate feedbacks in coupled general circulation models with a full set of physical parameterization packages. The formulation of the CFRAM is based on the energy balance in an atmosphere–surface column. In the CFRAM, the isolation of partial temperature changes due to an external forcing or an individual feedback is achieved by solving the linearized infrared radiation transfer model subject to individual energy flux perturbations (external or due to feedbacks). The partial temperature changes are addable and their sum is equal to the (total) temperature change (in the linear sense). The decomposition of feedbacks is based on the thermodynamic and dynamical processes that directly affect individual energy flux terms. Therefore, not only those feedbacks that directly affect the TOA radiative fluxes, such as water vapor, clouds, and ice-albedo feedbacks, but also those feedbacks that do not directly affect the TOA radiation, such as evaporation, convections, and convergence of horizontal sensible and latent heat fluxes, are explicitly included in the CFRAM. In the CFRAM, the feedback gain matrices measure the strength of individual feedbacks. The feedback gain matrices can be estimated from the energy flux perturbations inferred from individual parameterization packages and dynamical modules. The inter-model spread of a feedback gain matrix would help us to detect the origins of the uncertainty of future climate projections in climate model simulations.     

The Role of the Kuroshio in the Winter North Pacific Ocean-Atmosphere Interaction： Comparison of a Coupled Model and Observations

A comparative study between the output of the Flexible Global Climate Model Version 1.0 (FGCM- 1.0) and the observations is performed. At 500 hPa, the geopotential height of FGCM is similar to the observations, but in the North Pacific the model gives lower values, and the differences are most significant over the northern boundary of the Pacific. In a net heat flux comparison, the spatial patterns of the two are similar in winter, but more heat loss appears to the east of Japan in FGCM than in COADS. On the interannual timescale, strong (weak) Kuroshio transports to the east of Taiwan lead the increasing (decreasing) net heat flux, which is centered over the Kuroshio Extension region, by 1–2 months, with low (high) pressure anomaly responses appearing at 500 hPa over the North Pacific (north of 25N) in winter. The northward heat transport of the Kuroshio is one of the important heat sources to support the warming of the atmosphere by the ocean and the formation of the low pressure anomaly at 500 hPa over the North Pacific in winter.