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 ?rst two years of the integration and becomes a perennial phenomenon afterwards in the model. By way of Singular Value Decomposition (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 overestimated east-west gradient of SST in the equatorial Paci?c in the ocean spin-up process, and (2) the underestimated amount of low-level stratus over the Peruvian coast in CCM3 (the Community Climate Model, Version Three). The overestimated east-west gradient of SST brings the anomalous equatorial easterly. The anomalous easterly, a?ected by the Coriolis force in the Southern Hemisphere, turns into an anomalous westerly in a broad area south of the equator and is enhanced by atmospheric anomalous circulation due to the underestimated amount of low-level stratus over the Peruvian coast simulated by CCM3. The anomalous westerly leads to anomalous warm advection that makes the SST warm in the southeast Paci?c. The double ITCZ phenomenon in the CGCM is a result of a series of nonlocal and nonlinear adjustment processes in the coupled system, which can be traced to the uncoupled models, oceanic component, and atmospheric component. The zonal gradient of the equatorial SST is too large in the ocean component and the amount of low-level stratus over the Peruvian coast is too low in the atmosphere component.

Simulating Tropical Instability Waves in the Equatorial Eastern Pacific with a Coupled General Circulation Model

Satellite observations of SSTs have revealed the existence of unstable waves in the equatorial eastern Pacific and Atlantic oceans. These waves have a 20-40-day periodicity with westward phase speeds of 0.4-0.6 m s^-1 and wavelengths of 1000-2000 km during boreal summer and fall. They are generally called tropical instability waves （TIWs）. This study investigates TIWs simulated by a high-resolution coupled atmosphere-ocean general circulation model （AOGCM）. The horizontal resolution of the model is 120 km in the atmosphere, and 30 km longitude by 20 km latitude in the ocean. Model simulations show good agreement with the observed main features associated with TIWs. The results of energetics analysis reveal that barotropic energy conversion is responsible for providing the main energy source for TIWs by extracting energy from the meridional shear of the climatological-mean equatorial currents in the mixed layer. This deeper and northward-extended wave activity appears to gain its energy through baroclinic conversion via buoyancy work, which further contributes to the asymmetric distribution of TIWs. It is estimated that the strong cooling effect induced by equatorial upwelling is partially （-30%-40%） offset by the equatorward heat flux due to TIWs in the eastern tropical Pacific during the seasons when TIWs are active. The atmospheric mixed layer just above the sea surface responds to the waves with enhanced or reduced vertical mixing. Furthermore, the changes in turbulent mixing feed back to sea surface evaporation, favoring the westward propagation of TIWs. The atmosphere to the south of the Equator also responds to TIWs in a similar way, although TIWs are much weaker south of the Equator.

Ocean Response to a Climate Change Heat-Flux Perturbation in an Ocean Model and Its Corresponding Coupled Model

State-of-the-art coupled general circulation models(CGCMs)are used to predict ocean heat uptake(OHU)and sealevel change under global warming.However,the projections of different models vary,resulting in high uncertainty.Much of the inter-model spread is driven by responses to surface heat perturbations.This study mainly focuses on the response of the ocean to a surface heat flux perturbation F,as prescribed by the Flux-Anomaly-Forced Model Intercomparison Project(FAFMIP).The results of ocean model were compared with those of a CGCM with the same ocean component.On the global scale,the changes in global mean temperature,ocean heat content(OHC),and steric sea level(SSL)simulated in the OGCM are generally consistent with CGCM simulations.Differences in changes in ocean temperature,OHC,and SSL between the two models primarily occur in the Arctic and Atlantic Oceans(AA)and the Southern Ocean(SO)basins.In addition to the differences in surface heat flux anomalies between the two models,differences in heat exchange between basins also play an important role in the inconsistencies in ocean climate changes in the AA and SO basins.These discrepancies are largely due to both the larger initial value and the greater weakening change of the Atlantic meridional overturning circulation(AMOC)in CGCM.The greater weakening of the AMOC in the CGCM is associated with the atmosphere–ocean feedback and the lack of a restoring salinity boundary condition.Furthermore,differences in surface salinity boundary conditions between the two models contribute to discrepancies in SSL changes.

Climate Simulations Based on a Different-GridNested and Coupled Model

An atmosphere-vegetation interaction model (AVIM) has been coupled with a nine-layer General Cir culation Model (GCM) of Institute of Atmospheic Physics / State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (IAP/ LASG), which is rhomboidally truncated at zonal wave number 15, to simulate global climatic mean states. AVIM is a model having inter-feedback between land surface processes and eco-physiological processes on land. As the first step to couple land with atmosphere completely, the physiological processes are fixed and only the physical part (generally named the SVAT (soil-vegetation-atmosphere-transfer scheme) model) of AVIM is nested into IAP/ LASG L9R15 GCM. The ocean part of GCM is prescribed and its monthly sea surface temperature (SST) is the climatic mean value. With respect to the low resolution of GCM, i.e., each grid cell having lon gitude 7.5?and latitude 4.5? the vegetation is given a high resolution of 1.5?by 1.5?to nest and couple the fine grid cells of land with the coarse grid cells of atmosphere. The coupling model has been integrated for 15 years and its last ten-year mean of outputs was chosen for analysis.Compared with observed data and NCEP reanalysis, the coupled model simulates the main characteris tics of global atmospheric circulation and the fields of temperature and moisture. In particular, the simu lated precipitation and surface air temperature have sound results. The work creates a solid base on coupling climate models with the biosphere.

Coupling the Common Land Model to ECHAM5 Atmospheric General Circulation Model

The ECHAM5 model is coupled with the widely used Common Land Model(CoLM). ECHAM5 is a state-of-theart atmospheric general circulation model incorporated into the integrated weather and climate model of the Chinese Academy of Meteorological Sciences(CAMS-CSM). Land surface schemes in ECHAM5 are simple and do not provide an adequate representation of the vegetation canopy and snow/frozen soil processes. Two AMIP(Atmospheric Model Intercomparison Project)-type experiments using ECHAM5 and ECHAM5-CoLM are run over 30 yr and the results are compared with reanalysis and observational data. It is found that the pattern of land surface temperature simulated by ECHAM5-CoLM is significantly improved relative to ECHAM5. Specifically, the cold bias over Eurasia is removed and the root-mean-square error is reduced in most regions. The seasonal variation in the zonal mean land surface temperature and the in situ soil temperature at 20-and 80-cm depths are both better simulated by ECHAM5-CoLM. ECHAM5-CoLM produces a more reasonable spatial pattern in the soil moisture content, whereas ECHAM5 predicts much drier soils. The seasonal cycle of soil moisture content from ECHAM5-CoLM is a better match to the observational data in six specific regions. ECHAM5-CoLM reproduces the observed spatial patterns of both sensible and latent heat fluxes. The strong positive bias in precipitation over land is reduced in ECHAM5-CoLM, especially over the southern Tibetan Plateau and middle–lower reaches of the Yangtze River during the summer monsoon rainy season.

东亚地区未来气候变化的CGCM模拟研究

该文用５个全球海气耦合模式的瞬变模拟结果分析了ＣＯ＿２加倍时东亚地区可能的气候变化，结果表明ＣＯ＿２加倍时，东亚区域平均的表面温度和表面大气温度明显增加，且增温幅度超过全球平均的增温幅度。区域平均降水增加，尤其是季风区的夏季降水增加显著，另外，还详细讨论了模拟的温度场、环流场、降水场和土壤湿度场等的变化的模式间的差异和季节差异。

MRI-CGCM模式对东亚夏季风的模拟评估及订正

本文利用CMAP月降水资料、NCEP再分析资料、NOAA的ERSST资料和日本气象厅海气耦合模式(MRI-CGCM)的输出结果,从东亚夏季风气候态、主模态和年际变率等方面分析了MRI-CGCM模式对东亚夏季风的预测性能,并且利用观测的东亚夏季风指数(EASMI)与模拟PC(principal component)的关系建立多元线性回归方程来订正EASMI(简称PC订正法)。结果表明:MRI-CGCM模式能够较好再现东亚夏季风降水和低层风场的气候态,但模拟的西北太平洋反气旋偏弱、偏东,使得模拟的副热带地区降水量偏小。模式较好地模拟出东亚夏季风降水第一模态(EOF1)及相应的低层风场,能够较好再现出EOF1对应El Ni?o衰减位相;模拟降水的EOF1与观测之间的空间相关系数(ACC)为0.72,且能较好地再现其对应的年际变率,其时间系数PC1与观测之间的相关系数为0.41,能模拟出观测EOF1的2 a和5 a主导周期;但模拟的我国以东梅雨锋区雨带位置偏南,这与模拟的西北太平洋反气旋位置偏南有关。模式对降水第二模态EOF2的模拟能力比EOF1明显下降,模拟EOF2与观测之间的ACC降到0.36;虽然模式能较好地再现出EOF2对应El Ni?o发展位相,但模拟的西太平洋反气旋位置偏南,使得雨带位置偏南,模拟的我国梅雨锋区雨带位于江南,与观测场上江南少雨相反。模式较好地模拟出我国东部夏季降水和气温空间异常分布和年际变化,模拟与观测夏季降水和气温的多年平均ACC分别为0.74和0.68。模式模拟我国东部、江淮流域和华南地区夏季降水多年平均PS评分分别为69、70和68分,略高于我国夏季降水业务预测多年平均评分(65分)。模拟的我国东部夏季气温与观测多年平均PS评分为74分。PC订正后EASMI与实况的相关系数由0.51提高到0.65、符号一致率由84%升到91%、标准差由0.75增大到1.4、大于1个标准差年数由6年变为12年,订正后在模拟变幅偏小和梅雨锋区雨带偏南等方面均有一定的改善,对应西太平洋反气旋位置和梅雨锋区雨带位置与实况较为吻合。

How Accurately Contemporary Models Can Predict Monsoons?

Seasonal changes exhibit climate changes, so models can predict future climate change accurately only if they can reproduce seasonal cycle accu-rately. Further, seasonal changes are much larger than the changes even in long period of centuries. Thus it is unwise to ignore large ones compared to small climate change. In this paper, we determine how accurately a suite of ten coupled general circulation models reproduce the observed seasonal cycle in rainfall of the tropics. The seasonal cycles in rainfall of global tropics are known as monsoons. We found that the models can reasonably reproduce the seasonal cycle in rainfall, thus are useful in climate prediction and simulation of global monsoons.

Development of Earth/Climate System Models in China:A Review from the Coupled Model Intercomparison Project Perspective

The development of coupled earth/climate system models in China over the past 20 years is reviewed, including a comparison with other international models that participated in the Coupled Model Intercom- parison Project （CMIP） from phase 1 （CMIP1） to phase 4 （CMIP4）. The Chinese contribution to CMIP is summarized, and the major achievements from CMIP1 to CMIP3 are listed as a reference for assessing the strengths and weaknesses of Chinese models. After a description of CMIP5 experiments, the five Chinese models that participated in CMIP5 are then introduced. Furthermore, following a review of the current status of international model development, both the challenges and opportunities for the Chinese climate modeling community are discussed. The development of high-resolution climate models, earth system mod- els, and improvements in atmospheric and oceanic general circulation models, which are core components of earth/climate system models, are highlighted. To guarantee the sustainable development of climate system models in China, the need for national-level coordination is discussed, along with a list of the main compo- nents and supporting elements identified by the US National Strategy for Advancing Climate Modeling.

全球变暖背景下热带大气季节内振荡的变化特征及数值模拟

利用欧洲中期数值预报中心的ERA40再分析逐日的200 hPa风场资料,选取1958—1977年和1980—1999年各20年,对比分析了在全球变暖背景下前后两个时段热带大气季节内振荡(ISO)的特征及其变化。研究表明:近20 a来,原来在赤道中太平洋上活跃的ISO减弱,而在中印度洋、孟加拉湾地区ISO变得活跃;全球变暖背景下,ISO的强度变化幅度加大,表明ISO更加活跃,且季节变化明显,冬、春季强,夏、秋季弱;对流层上层的纬向风能量更集中于1—3波,ISO的频率有加大的趋势。还利用中国科学院大气物理研究所LASG发展的耦合气候系统模式FGOALS-1.0g中的控制试验及其二氧化碳浓度加倍试验结果,分别对应实测资料的前后20年进行对比分析。发现模式对ISO的空间结构模拟较好,但低估了ISO的强度;时空谱分析表明模式结果中包含有更多的纬向风的高频成分,由于能量的分散,导致对ISO活动强度的低估。但通过对模式的控制试验和温室气体增加试验结果的对比分析,发现耦合模式还是较好地反映出在全球变暖背景下ISO在中印度洋、孟加拉湾地区变得活跃、频率加大等变化特征。

Relationships between the East Asian-Western North Pacific Monsoon and ENSO Simulated by FGOALS-s2

The relationships between ENSO and the East Asian-western North Pacific monsoon simulated by the Flexible Global Ocean-Atmosphere-Land System model, Spectral Version 2 （FGOALS-s2）, a state-of-the-art coupled general circulation model （CGCM）, are evaluated. For E1 Nino developing summers, FGOALS-s2 reproduces the anomalous cyclone over the western North Pacific （WNP） and associated negative precipita- tion anomalies in situ. In the observation, the anomalous cyclone is transformed to an anomalous anticyclone over the WNP （WNPAC） during E1 Nifio mature winters. The model reproduces the WNPAC and associated positive precipitation anomalies over southeastern China during winter. However, the model fails to simu- late the asymmetry of the wintertime circulation anomalies over the WNP between E1 Nifio and La Nifia. The simulated anomalous cyclone over the WNP （WNPC） associated with La Nifia is generally symmetric about the WNPAC associated with E1 Nifio, rather than shifted westward as that in the observation. The discrepancy can partially explain why simulated La Nifia events decay much faster than observed. In the observation, the WNPAC maintains throughout the E1 Nifio decaying summer under the combined effects of local forcing of the WNP cold sea surface temperature anomaly （SSTA） and remote forcing from basin- wide warming in the tropical Indian Ocean. FGOALS-s2 captures the two mechanisms and reproduces the WNPAC throughout the summer. However, owing to biases in the mean state, the precipitation anomalies over East Asia, especially those of the Meiyu rain belt, are much weaker than that in the observation.

人类活动影响与三大洋海表水温的变化及数值模拟

本文利用实测的全球气温、海表水温资料以及全球海气耦合气候模式的控制试验与CO2加倍试验模拟结果资料来探讨人类活动对三大洋海温的可能影响。结果表明:近百年来三大洋海温总体呈现上升趋势,但太平洋海温有其独立变化特点;FGOALS1.0_g耦合模式有较好地模拟三大洋海温的能力;模式模拟结果证实了人类活动影响(CO2浓度增加)将会使全球气候变暖;但模式模拟结果没有证实由于CO2浓度增加会导致三大洋海表水温明显的增高趋势。

An Assessment of ENSO Stability in CAMS Climate System Model Simulations

We present an overview of the El Ni?o–Southern Oscillation(ENSO) stability simulation using the Chinese Academy of Meteorological Sciences climate system model(CAMS-CSM). The ENSO stability was quantified based on the Bjerknes(BJ) stability index. Generally speaking, CAMS-CSM has the capacity of reasonably representing the BJ index and ENSO-related air–sea feedback processes. The major simulation biases exist in the underestimated thermodynamic damping and thermocline feedbacks. Further diagnostic analysis reveals that the underestimated thermodynamic feedback is due to the underestimation of the shortwave radiation feedback, which arises from the cold bias in mean sea surface temperature(SST) over central–eastern equatorial Pacific(CEEP). The underestimated thermocline feedback is attributed to the weakened mean upwelling and weakened wind–SST feedback(μ_a) in the model simulation compared to observation. We found that the weakened μ_a is also due to the cold mean SST over the CEEP.The study highlights the essential role of reasonably representing the climatological mean state in ENSO simulations.

EVALUATION OF CGCM AND SIMULATION OF REGIONAL CLIMATE CHANGE IN EAST ASIA

In this paper,experiment results about East Asia climate from five CGCMs are described.The ability of the models to simulate present climate and the simulated response to increased carbon dioxide are both covered.The results indicate that all models show substantial changes in climate when carbon dioxide concentrations are doubled.In particular,the strong surface warming at high latitudes in winter and the significant increase of summer precipitation in the monsoon area are produced by all models.Regional evaluation results show that these five CGCMs are particularly good in simulating spatial distribution of present climate.The main characteristics of the seasonal mean H500,SAT, MSLP field can be simulated by most CGCMs.But there are significant systematic errors in SAT, MSLP,HS00 fields in most models.On the whole,DKRZ OPYC is the best in simulating the present climate in East Asia.

Development of a Downscaling Method in China Regional Summer Precipitation Prediction

A downscaling method taking into account of precipitation regionalization is developed and used in the regional summer precipitation prediction （RSPP） in China. The downscaling is realized by utilizing the optimal subset regression based on the hindcast data of the Coupled Ocean-Atmosphere General Climate Model of National Climate Center （CGCM/NCC）, the historical reanalysis data, and the observations. The data are detrended in order to remove the influence of the interannual variations on the selection of predictors for the RSPP. Optimal predictors are selected through calculation of anomaly correlation coefficients （ACCs） twice to ensure that the high-skill areas of the CGCM/NCC are also those of observations, with the ACC value reaching the 0.05 significant level. One-year out cross-validation and independent sample tests indicate that the downscaling method is applicable in the prediction of summer precipitation anomaly across most of China/vith high and stable accuracy, and is much better than the direct CGCM/NCC prediction. The predictors used in the downscaling method for the RSPP are independent and have strong physical meanings, thus leading to the improvements in the prediction of regional precipitation anomalies.

Characteristics and Numerical Simulation of the Tropical Intraseasonal Oscillations under Global Warming

Using the ECMWF reanalysis daily 200-hPa wind data during the two 20-yr periods from 1958 to 1977 and from 1980 to 1999, the characteristics and changes of Intraseasonal Oscillations （ISO） in the two periods associated with global warming are analyzed and compared in this study. It is found that during the last 20 years, the ISO has weakened in the central equatorial Pacific Ocean, but becomes more active in the central Indian Ocean and the Bay of Bengal; under the background of the global warming, increase in the amplitude of ISO intensity suggests that the ISO has become more active than before, with an obvious seasonal cycle, i.e., strong during winter and spring, but weak during summer and autumn; the energy of the upper tropospheric zonal winds has more concentrated in wave numbers 1-3, and the frequency of ISO tended to increase. Comparison between the results of control experiment and CO2 increase （1% per year） experiment of FGOALS-1.0g （developed at LASG） with the first and second 20-yr observations, is also performed, respectively. The comparative results show that the spatial structure of the ISO was well reproduced, but the strength of ISO was underestimated. On the basis of space-time spectral analysis, it is found that the simulated ISO contains too much high frequency waves, leading to the underestiniation of ISO intensity due to the dispersion of ISO energy. However, FGOALS-1.0g captured the salient features of ISO under the global warming background by two contrast experiments, such as the vitality and frequency-increasing of ISO in the central Indian Ocean and the Bay of Bengal.

Response of ocean climate to different heat-flux perturbations over the North Atlantic in FAFMIP

The diversity of surface-flux perturbations,especially for heat-flux perturbations,notably leads to uncertainties surrounding the responses of ocean climate under global warming scenarios projected by climate/Earth system models.However,when imposing heat-flux perturbations on the models,strong feedback persists between the atmosphere and the ocean,resulting in nearly doubled heat-flux perturbation over the North Atlantic(NA).Herein,quantitative evaluation of the influences of magnitude change of heat-flux perturbations over the NA on the changes in the Atlantic Meridional Overturning Circulation(AMOC),ocean heat uptake(OHU)and dynamic sea level(DSL)has been conducted by analysis of eight coupled model responses to the heat-flux perturbation experiments in the Flux-Anomaly-Forced Model Inter-comparison Project.It has been demonstrated that the magnitude of the AMOC change is extremely sensitive to the magnitude change of imposed NA heat-flux perturbation,and the weakening amplitude of the AMOC is nearly halved as the imposed heat-flux perturbation F is halved over the NA.The most remarkable responses of both DSL and OHU to the magnitude changes of NA heat-flux perturbation have been primarily found in the Atlantic and Arctic basins,especially for the NA region.Both the added ocean heat uptake(OHUa)and redistributed ocean heat uptake(OHUr)play key roles in OHU changes among the various NA heat-flux perturbation experiments.The magnitude change of NA-mean OHUa is almost linearly related to the imposed NA heat-flux perturbation,while the magnitude change of NA-mean OHUr,which is primarily caused by AMOC change and redistributed heat flux,is not proportional to the imposed NA heat-flux perturbation.There is a nearly linear relationship between the magnitude of AMOC change and the OHUr in tropical regions,including the regions in the low-latitude South Atlantic,the tropical Pacific Ocean and the Indian Ocean.