Simulations of the 100-hPa South Asian High and Precipitation over East Asia with IPCC Coupled GCMs

The South Asian High （SAH） and precipitation over East Asia simulated by 11 coupled GCMs associated with the forthcoming Intergovernmental Panel on Climate Change＇s （IPCC） 4th Assessment Report are evaluated. The seasonal behavior of the SAH is presented for each model. Analyses of the results show that all models are able to reproduce the seasonal cycle of the SAH. Locations of the SAH center are also basically reproduced by these models. All models underestimate the intensity and the extension of coverage in summer. The anomalous SAH can be divided into east and west modes according to its longitudinal position in summer on the interannual timescale, and the composite anomalies of the observed precipitation for these two modes tend to have opposite signs over East Asia. However, only several coupled GCMs can simulate the relationship between rainfall and SAH similar to the observed one, which may be associated with the bias in simulation of the subtropical anticyclone over the West Pacific （SAWP） at 500 hPa. In fact, it is found that any coupled GCM, that can reproduce the reasonable summer mean state of SAWP and the southward （northward） withdrawal （extension） for the east （west） mode of SAH as compared to the observed, will also simulate similar rainfall anomaly patterns for the east and west SAH modes over East Asia. Further analysis indicates that the observed variations in the SAH, SAWP and rainfall are closely related to the sea surface temperature （SST） over the equatorial tropical Pacific. Particularly, some models cannot simulate the SAWP extending northward in the west mode and withdrawing southward in the east mode, which may be related to weak major E1 Nifio or La Nifia events. The abilities of the coupled GCMs to simulate the SAWP and ENSO events are associated partly with their ability to reproduce the observed relationship between SAH and the rainfall anomaly over East Asia.

Analysis of Decadal Climate Variability in the Tropical Pacific by Coupled GCM

This study presents the spatial and temporal structures of the decadal variability of the Pacific from an extended control run of a coupled global climate model (GCM).The GCM used was version-g2.0 of the Flexible Global Ocean Atmosphere Land System (FGOALS-g2.0) developed at LASG/IAP.The GCM FGOALS-g2.0 re-produces similar spatial-temporal structures of sea surface temperature (SST) as observed in the Pacific decadal os-cillation (PDO) with a significant period of approximately 14 years.Correspondingly,the PDO signals were closely related to the decadal change both in the upper-ocean temperature anomalies and in the atmospheric circulation.The present results suggest that warm SST anomalies along the equator relax the trade winds,causing the SSTs to warm even more in the eastern equatorial Pacific,which is a positive feedback.Meanwhile,warm SST anomalies along the equator force characteristic off-equa-torial wind stress curl anomalies,inducing much more poleward transport of heat,which is a negative feedback.The upper-ocean meridional heat transport,which is asso-ciated with the PDO phase transition,links the equatorial to the off-equatorial Pacific Ocean,acting as a major mechanism responsible for the tropical Pacific decadal variations.Therefore,the positive and negative feedbacks working together eventually result in the decadal oscilla-tion in the Pacific.

Empirical Estimates of Global Climate Sensitivity:An Assessment of Strategies Using a Coupled GCM

A control integration with the normal solar constant and one with it increased by 2.5% in the National Center for Atmospheric Research （NCAR） coupled atmosphere-ocean Climate System Model were conducted to see how well the actual realized global warming could be predicted just by analysis of the control results. This is a test, within a model context, of proposals that have been advanced to use knowledge of the present day climate to make ＂empirical＂ estimates of global climate sensitivity. The scaling of the top-of-the-atmosphere infrared flux and the planetary albedo as functions of surface temperature was inferred by examining four different temporal and geographical variations of the control simulations. Each of these inferences greatly overestimates the climate sensitivity of the model, largely because of the behavior of the cloud albedo. In each inference the control results suggest that cloudiness and albedo decrease with increasing surface temperature. However, the experiment with the increased solar constant actually has higher albedo and more cloudiness at most latitudes. The increased albedo is a strong negative feedback, and this helps account for the rather weak sensitivity of the climate in the NCAR model. To the extent that these model results apply to the real world, they suggest empirical evaluation of the scaling of global-mean radiative properties with surface temperature in the present day climate provides little useful guidance for estimates of the actual climate sensitivity to global changes.

ENSO Hindcast Experiments Using a Coupled GCM

A group of seasonal hindcast experiments are conducted using a coupled model known as the Flexible Global Ocean-Atmosphere-Land System Modelgamil1.11 (FGOALS-g1.11) developed at the State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG).Two steps are included in our ElNi o-Southern Oscillation (ENSO) hindcast experiments.The first step is to integrate the coupled GCM with the Sea Surface Temperature (SST) strongly nudged towards the observation from 1971 to 2006.The second step is to remove the SST nudging term.The authors carried out a one-year hindcast by adopting the initial values from SST nudging experiments from the first step on January 1st,April 1st,July 1st,and October 1st from 1982 to 2005.In the SST nudging experiment,the model can reproduce the observed equatorial thermocline anomalies and zonal wind stress anomalies in the Pacific,which demonstrates that the SST nudging approach can provide realistic atmospheric and oceanic initial conditions for seasonal prediction experiments.The model also demonstrates a high Anomaly Correlation Coefficient (ACC) score for SST in most of the tropical Pacific,Atlantic Ocean,and some Indian Ocean regions with a 3-month lead.Compared with the persistence ACC score,this model shows much higher ACC scores for the Ni o-3.4 index for a 9-month lead.

Potential Predictability of Sea Surface Temperature in a Coupled Ocean-Atmosphere GCM

Using the Flexible Global Ocean-Atmosphere-Land System model （FGOALS） version g1.11, a group of seasonal hindcasting experiments were carried out. In order to investigate the potential predictability of sea surface temperature （SST）, singular value decomposition （SVD） analyses were applied to extract dominant coupled modes between observed and predicated SST from the hindcasting experiments in this study. The fields discussed are sea surface temperature anomalies over the tropical Pacific basin （20~0S-20~0N, 120~0E- 80~0W）, respectively starting in four seasons from 1982 to 2005. On the basis of SVD analysis, the simulated pattern was replaced with the corresponding observed pattern to reconstruct SST anomaly fields to improve the ability of the simulation. The predictive skill, anomaly correlation coefficients （ACC）, after systematic error correction using the first five modes was regarded as potential predictability. Results showed that： 1） the statistical postprocessing approach was effective for systematic error correction; 2） model error sources mainly arose from mode 2 extracted from the SVD analysis-that is, during the transition phase of ENSO, the model encountered the spring predictability barrier; and 3） potential predictability （upper limits of predictability） could be high over most of the tropical Pacific basin, including the tropical western Pacific and an extra 10-degrees region of the mid and eastern Pacific.

Global Coupled Ocean-Atmosphere General Circulation Models in LASG/IAP

Coupled ocean-atmospheric general circulation models are the only tools to quantitatively simulate the climate system. Since the end of the 1980s, a group of scientists in the State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG), Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences (CAS), have been working to develop a global OGCM and a global coupled ocean-atmosphere general circulation modei (CGCM). Prom the original flux anomaly-coupling modei developed in the beginning of the 1990s to the latest directly-coupling modei, LASG scientists have developed four global coupled GCMs. This study summarizes the development history of these models and describes the third and fourth coupled GCMs and selected applications. Strengths and weaknesses of these models are highlighted.

Coupled Model Simulations of Climate Changes in the 20th Century and Beyond

Several scenario experiments of the IPCC 4th Assessment Report （AR4） are performed by version g1.0 of a Flexible coupled Ocean-Atmosphere-Land System Model （FGOALS） developed at the Institute of Atmospheric Physics, Chinese Academy of Sciences （IAP/CAS）, including the ＂Climate of the 20th century experiment＂, ＂CO2 1% increase per year to doubling experiment＂ and two separate IPCC greenhouse gases emission scenarios AIB and B1 experiments. To distinguish between the different impacts of natural variations and human activities on the climate change, three-member ensemble runs are performed for each scenario experiment. The coupled model simulations show： （1） from 1900 to 2000, the global mean temper- ature increases about 0.5℃ and the major increase occurs during the later half of the 20th century, which is in consistent with the observations that highlights the coupled model＇s ability to reproduce the climate changes since the industrial revolution; （2） the global mean surface air temperature increases about 1.6℃ in the CO2 doubling experiment and 1.5℃ and 2.4℃ in the A1B and B1 scenarios, respectively. The global warming is indicated by not only the changes of the surface temperature and precipitation but also the temperature increase in the deep ocean. The thermal expansion of the sea water would induce the rise of the global mean sea level. Both the control run and the 20th century climate change run are carried out again with version g1.1 of FGOALS, in which the cold biases in the high latitudes were removed. They are then compared with those from version g1.0 of FGOALS in order to distinguish the effect of the model biases on the simulation of global warming.

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 25°N） 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.

Bimodality of the South Asia High Simulated by Coupled Models

The observed South Asia High （SAH） center is characterized by two distinctive equilibrium modes during boreal midsummer, namely the center of SAH is located between 82.5°-92.5°E for the Tibetan Plateau mode and between 55°-65°E for the Iranian Plateau mode respectively. The present study describes the ability of 15 coupled general circulation models （CGCM） used in the Intergovernmental Panel on Climate Change＇s （IPCC） 4th Assessment Report to reproduce the observed bimodality of the SAH. These models reveal a wide range of skill in simulating this bimodality. Nearly half of the models reproduced the bimodality, while the other half of the models did not simulate well these two modes whereas usually preferring one mode, The models that reproduced the bimodality of the SAH present similar horizontal and vertical circulations as those features from the NCEP reanalysis data. The results from these models identify the warm characteristics of the SAH and indicate that these two modes have different dynamic and thermodynamic properties. Different characteristics of the simulated sensible heat and latent heat related to precipitation partly contribute to the difference in the simulations of the SAH bimodality. The majority of these models that prefer to simulate the Tibetan Plateau mode produce a small sensible heat flux difference between the Iranian Plateau and the Tibetan Plateau, and also generally simulate a very strong false precipitation center over the east of the Tibetan Plateau, which indicates strong latent release and thereby contributes to the preference of the SAH center on the Tibetan Plateau. Whereas, the models that reproduce the bimodality of the SAH tend to simulate large precipitation over the southern Himalayas and no obviously false precipitation is produced over the east of the Tibetan Plateau. In addition, the model＇s resolution may also have important impacts on the simulations of precipitation.

IMPACTS OF THE TROPICAL PACIFIC COUPLED PROCESS ON THE INTERANNUAL VARIABILITY IN THE INDIAN OCEAN

The basic features of climatology and interannual variations of tropical Pacific and Indian Oceans were analyzed using a coupled general circulation model （CGCM）, which was constituted with an intermediate 2.5-layer ocean model and atmosphere model ECHAM4. The CGCM well captures the spatial and temporal structure of the Pacific E1 Nifio-Southern Oscillation （ENSO） and the variability features in the tropical Indian Ocean. The influence of Pacific air-sea coupled process on the Indian Ocean variability was investigated carefully by conducting numerical experiments. Results show that the occurrence frequency of positive/negative Indian Ocean Dipole （IOD） event will decrease/increase with the presence/absence of the coupled process in the Pacific Ocean. Further analysis demonstrated that the air-sea coupled process in the Pacific Ocean affects the IOD variability mainly by influencing the zonal gradient ofthermocline via modulating the background sea surface wind.

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.

Annual Cycle and Interannual Variability in the Tropical Pacific as Simulated by Three Versions of FGOALS

The seasonal cycle and interannual variability in the tropical oceans simulated by three versions of the Flexible Ocean-Atmosphere-Land System （FGOALS） model （FGOALS-gl.0, FGOALS-g2 and FGOALS- s2）, which have participated in phases 3 and 5 of the Coupled Model Intercomparison Project （CMIP3 and CMIP5）, are presented in this paper. The seasonal cycle of SST in the tropical Pacific is realistically reproduced by FGOALS-g2 and FGOALS- s2, while it is poorly simulated in FGOALS-gl.0. Three feedback mechanisms responsible for the SST annual cycle in the eastern Pacific are evaluated. The ocean-atmosphere dynamic feedback, which is successfully re- produced by both FGOALS-g2 and FGOALS-s2, plays a key role in determining the SST annual cycle, while the overestimated stratus cloud-SST feedback amplifies the annual cycle in FGOALS-s2. Because of the seri- ous warm bias existing in FGOALS-gl.0, the ocean-atmosphere dynamic feedback is greatly underestimated in FGOALS-gl.0, in which the SST annual cycle is mainly driven by surface solar radiation. FGOALS-gl.0 simulates much stronger ENSO events than observed, whereas FGOALS-g2 and FGOALS- s2 successfully simulate the observed ENSO amplitude and period and positive asymmetry, but with less strength. Further ENSO feedback analyses suggest that surface solar radiation feedback is principally re- sponsible for the overestimated ENSO amplitude in FGOALS-gl.0. Both FGOALS-gl.0 and FGOALS-s2 can simulate two different types of E1 Nifio events -- with maximum SST anomalies in the eastern Pacific （EP） or in the central Pacific （CP） -- but FGOALS-g2 is only able to simulate EP E1 Nifio, because the negative cloud shortwave forcing feedback by FGOALS-g2 is much stronger than observed in the central Pacific.