An atmosphere–ocean regional climate model for the Mediterranean area: assessment of a present climate simulation

We present an atmosphere–ocean regional climate model for the Mediterranean basin, called the PROTHEUS system, composed by the regional climate model RegCM3 as the atmospheric component and by a regional configuration of the MITgcm model as the oceanic component. The model is applied to an area encompassing the Mediterranean Sea and compared to a stand-alone version of its atmospheric component. An assessment of the model performances is done by using available observational datasets. Despite a persistent bias, the PROTHEUS system is able to capture the inter-annual variability of seasonal sea surface temperature (SST) and also the fine scale spatio-temporal evolution of observed SST anomalies, with spatial correlation as high as 0.7 during summer. The close inspection of a 10-day strong wind event during the summer of 2000 proves the capability of the PROTHEUS system to correctly describe the daily evolution of SST under strong air–sea interaction conditions. As a consequence of the model’s skill in reproducing observed SST and wind fields, we expect a reliable estimation of air–sea fluxes. The model skill in reproducing climatological land surface fields is in line with that of state of the art regional climate models.     

区域海气耦合模式对中国夏季降水的模拟

以区域气候模式RegCM3和普林斯顿海洋模式POM为基础,建立了一个区域海气耦合模式,对1963-2002年中国夏季气候进行模拟,重点分析该耦合模式对中国夏季降水的模拟性能以及降水模拟改进的可能原因.结果表明:耦合模式对中国夏季雨带分布的模拟明显优于控制试验(单独的大气模式),对长江流域以及华南降水的模拟性能改进尤为明显,同时耦合模式能够更为真实地刻画中国东部地区汛期雨带的移动.对降水的年际变化分析发现,耦合模式模拟的1963-2002年中国夏季降水年际变率与观测吻合,模拟的夏季长江流域降水与观测降水相关系数达到0.48,模拟的华南夏季降水与观测的相关系数达到0.61,而控制试验结果与观测降水的相关系数均较小.对中国东部长江流域夏季降水与近海海温的相关分析表明,用给定海温驱动的大气模式,并不能正确模拟出中国东部夏季降水与海温的关系,而耦合模式能够较好地模拟出长江流域与孟加拉湾、南海以及黑潮区海温的关系,与GISST(全球海冰和海表温度)和观测降水相关关系一致.对水汽输送通量的分析发现,控制试验模拟的水汽输送路径与NCEP/NCAR再分析资料相比差别较大,耦合模式模拟的来自海洋上的水汽输送强度和路径与NCEP/NCAR再分析资料一致,提高了耦合模式对水汽输送的模拟能力,从而改善了模式对华南以及长江流域降水的模拟.     

The seasonal variation of the upper layers of the South China Sea (SCS) circulation and the Indonesian through flow (ITF): An ocean model study

The upper layer, wind-driven circulation of the South China Sea (SCS), its through-flow (SCSTF) and the Indonesian through flow (ITF) are simulated using a high resolution model, FVCOM (finite volume coastal ocean model) in a regional domain comprising the Maritime Continent. The regional model is embedded in the MIT global ocean general circulation model (ogcm) which provides surface forcing and boundary conditions of all the oceanographic variables at the lateral open boundaries in the Pacific and Indian oceans. A five decade long simulation is available from the MITgcm and we choose to investigate and compare the climatologies of two decades, 1960–1969 and 1990–1999.The seasonal variability of the wind-driven circulation produced by the monsoon system is realistically simulated. In the SCS the dominant driving force is the monsoon wind and the surface circulation reverses accordingly, with a net cyclonic tendency in winter and anticyclonic in summer. The SCS circulation in the 90s is weaker than in the 60s because of the weaker monsoon system in the 90s. In the upper 50 m the interaction between the SCSTF and ITF is very important. The southward ITF can be blocked by the SCSTF at the Makassar Strait during winter. In summer, part of the ITF feeds the SCSTF flowing into the SCS through the Karimata Strait. Differently from the SCS, the ITF is primarily controlled by the sea level difference between the western Pacific and eastern Indian Ocean. The ITF flow, consistently southwestward below the surface layer, is stronger in the 90s.The volume transports for winter, summer and yearly are estimated from the simulation through all the interocean straits. On the annual average, there is a ∼5.6 Sv of western Pacific water entering the SCS through the Luzon Strait and ∼1.4 Sv exiting through the Karimata Strait into the Java Sea. Also, ∼2 Sv of SCS water enters the Sulu Sea through the Mindoro Strait, while ∼2.9 Sv flow southwards through the Sibutu Strait merging into the ITF. The ITF inflow occurs through the Makassar Strait (up to ∼62%) and the Lifamatola Strait (∼38%). The annual average volume transport of the ITF inflow from the simulation is ∼15 Sv in the 60s and ∼16.6 Sv in the 90s, very close to the long term observations. The ITF outflow through the Lombok, Ombai and Timor straits is ∼16.8 Sv in the 60s and 18.9 Sv in the 90s, with the outflow greater by 1.7 Sv and 2.3 Sv respectively. The transport estimates of the simulation at all the straits are in rather good agreement with the observational estimates.We analyze the thermal structure of the domain in the 60s and 90s and assess the simulated temperature patterns against the SODA reanalysis product, with special focus on the shallow region of the SCS. The SODA dataset clearly shows that the yearly averaged temperatures of the 90s are overall warmer than those of the 60s in the surface, intermediate and some of the deep layers and the decadal differences (90s − 60s) indicate that the overall warming of the SCS interior is a local effect. In the simulation the warm trend from the 60s to the 90s in well reproduced in the surface layer. In particular, the simulated temperature profiles at two shallow sites at midway in the SCSTF agree rather well with the SODA profiles. However, the warming trend in the intermediate (deep) layers is not reproduced in the simulation. We find that this deficiency is mostly due to a deficiency in the initial temperature fields provide by the MITgcm.     

区域海气耦合模式对华北夏季大气水汽输送模拟结果的检验及其与单独气候模式的比较分析

将区域海气耦合模式RegCM3-POM和区域气候模式RegCM3 40年(1963-2002年)的模拟结果与NCEP/NCAR再分析资料进行对比,检验区域海气耦合模式对中国华北地区夏季大气水汽含量和水汽输送特征的模拟能力,比较耦合模式与单独区域气候模式的差异.结果表明,区域海气耦合模式RegCM3-POM的模拟性能相对于单独区域气候模式RegCM3,大气水汽输送特征的模拟能力有了较大的改进.分析显示两种模式都能够较好地再现东哑地区气候平均夏季大气水汽储量浅红和水汽输送的空间分布特征,而耦合模式对大气水汽输送的模拟更为合理.在对流层中低层更接近观测;耦合模式对中国华北地区夏季平均大气水汽输送通量在垂直方向卜的分布型及水平4个边界水汽输送收支的模拟,相对于单独大气模式有了一定的改进;耦合模式对伴随华北地区夏季早涝的大气水汽异常输送也具有较好的模拟能力,其模拟的水汽输送异常的来源与观测基本一致,尤其是在20°N以北地区,耦合模式结果相对于单独区域气候模式有了很大的改进.但同时耦合模式在低纬度海洋上对气候平均夏季大气水汽含量模拟的偏差比区域气候模式显著;与观测相比,耦合模式对来自孟加拉湾地区的大气水汽输送模拟偏弱,而对西太平洋副热带高压西侧水汽输送模拟偏强,与华北夏季旱涝相联系的水汽输送异常的模拟在低纬度海洋上也存在明显偏差.     

Parameter sensitivity of a coupled atmosphere-ocean model

 The sensitivity of a coupled model to the oceanic vertical diffusion coefficient κ _v is examined. This is compared to the sensitivity of an ocean-only model forced by mixed boundary conditions (BC). The atmospheric component of the coupled model is a moist energy balance model. The ocean component is a 12-level geostrophic model, defined on a midlatitude β-plane. Atmosphere and ocean are coupled through the fluxes of heat and moisture at their interface. The coupled model contains a number of feedback processes which are not represented in the ocean-only model. This results in a temperature and salinity response to κ _v which is stronger in the coupled model than in the ocean-only model. On the other hand, there is a weaker response in oceanic processes such as meridional heat transport, deep-water formation at high latitudes, etc. Ocean-only sensitivity experiments were also performed with modified BCs, which parametrise the feedback processes included in the coupled model. These are the modified thermal BC of Rahmstorf and Willebrand and a modified freshwater BC proposed in the present study. Large-scale features of the response in oceanic surface fields are well represented with modified BCs. However, the sensitivity of the deep ocean temperature is only partly captured due to local differences in the surface response. The scaling behavior of the zonal overturning stream function was found to depend on the surface BCs. In contrast to this, the meridional overturning stream function basically scales with κ^0.5 _v in all sensitivity experiments. Differences in the heat transport response among the experiments are thus primarily related to differences in the temperature response. Received: 28 February 1997/Accepted: 12 September 1997     

Modelling Indonesian rainfall with a coupled regional model

Long-term high-resolution coupled climate model simulations using the Max Planck Institute Regional Climate Model and the Max Planck Institute Ocean Model have been performed with boundary forcings from two reanalyses: firstly from the European Centre for Medium-Range Weather Forecasts, and secondly from the joint reanalysis of the National Centers for Environmental Prediction and the National Center for Atmospheric Research. This study employs a special coupling setup using a regional atmospheric model and a global ocean model. The latter model applies a special conformal grid from a bipolar orthogonal spherical coordinate system, which allows irregular positions of the poles and focuses on the detail over the Maritime Continent. The coupled model was able to simulate stable and realistic rainfall variabilities without flux correction and at two different ocean resolutions. The coupled system is integrated for a period between 1979 and 1993 and the results are then compared to those from uncoupled runs and from observation. The results show improved performance after coupling: a remarkable reduction of overestimated rainfall over the sea for the atmospheric model and of warm SST biases for the ocean model. There is no significant change in rainfall variability at higher ocean model resolution, but the ocean circulation shows less transport variability within the Makassar Strait in comparison to observations. This paper has not been published or considered by any other journal in any language.     

Dependence of Indian monsoon rainfall on moisture fluxes across the Arabian Sea and the impact of coupled model sea surface temperature biases

The Arabian Sea is an important moisture source for Indian monsoon rainfall. The skill of climate models in simulating the monsoon and its variability varies widely, while Arabian Sea cold sea surface temperature (SST) biases are common in coupled models and may therefore influence the monsoon and its sensitivity to climate change. We examine the relationship between monsoon rainfall, moisture fluxes and Arabian Sea SST in observations and climate model simulations. Observational analysis shows strong monsoons depend on moisture fluxes across the Arabian Sea, however detecting consistent signals with contemporaneous summer SST anomalies is complicated in the observed system by air/sea coupling and large-scale induced variability such as the El Ni?o-Southern Oscillation feeding back onto the monsoon through development of the Somali Jet. Comparison of HadGEM3 coupled and atmosphere-only configurations suggests coupled model cold SST biases significantly reduce monsoon rainfall. Idealised atmosphere-only experiments show that the weakened monsoon can be mainly attributed to systematic Arabian Sea cold SST biases during summer and their impact on the monsoon-moisture relationship. The impact of large cold SST biases on atmospheric moisture content over the Arabian Sea, and also the subsequent reduced latent heat release over India, dominates over any enhancement in the land-sea temperature gradient and results in changes to the mean state. We hypothesize that a cold base state will result in underestimation of the impact of larger projected Arabian Sea SST changes in future climate, suggesting that Arabian Sea biases should be a clear target for model development.     

Sensitivity of ENSO characteristics to a new interactive flux correction scheme in a coupled GCM

A fast coupled global climate model (CGCM) is used to study the sensitivity of El Ni?o Southern Oscillation (ENSO) characteristics to a new interactive flux correction scheme. With no flux correction applied our CGCM reveals typical bias in the background state: for instance, the cold tongue in the tropical east Pacific becomes too cold, thus degrading atmospheric sensitivity to variations of sea surface temperature (SST). Sufficient atmospheric sensitivity is essential to ENSO. Our adjustment scheme aims to sustain atmospheric sensitivity by counteracting the SST drift in the model. With reduced bias in the forcing of the atmosphere, the CGCM displays ENSO-type variability that otherwise is absent. The adjustment approach employs a one-way anomaly coupling from the ocean to the atmosphere: heat fluxes seen by the ocean are based on full SST, while heat fluxes seen by the atmosphere are based on anomalies of SST. The latter requires knowledge of the model??s climatological SST field, which is accumulated interactively in the spin-up phase (??training??). Applying the flux correction already during the training period (by utilizing the evolving SST climatology) is necessary for efficiently reducing the bias. The combination of corrected fluxes seen by the atmosphere and uncorrected fluxes seen by the ocean implies a restoring mechanism that counteracts the bias and allows for long stable integrations in our CGCM. A suite of sensitivity runs with varying training periods is utilized to study the effect of different levels of bias in the background state on important ENSO properties. Increased duration of training amplifies the coupled sensitivity in our model and leads to stronger amplitudes and longer periods of the Nino3.4 index, increased emphasis of warm events that is reflected in enhanced skewness, and more pronounced teleconnections in the Pacific. Furthermore, with longer training durations we observe a mode switch of ENSO in our model that closely resembles the observed mode switch related to the mid-1970s ??climate shift??.     

Three inflow pathways of the Indonesian throughflow as seen from the simple ocean data assimilation

The SODA product is used to investigate three Indonesian throughflow (ITF) branches: the flow through the Makassar Strait; through the South China Sea; and through the eastern Indonesian basins. The results reveal strong interannual variation in the Makassar Strait and the eastern Indonesian basins throughflow. Inspection of vertically integrated dynamic height (0–1000 db), a proxy of transport function, suggests that this interannual variation can be traced to the New Guinea Coastal Current, indicative of a strong influence of the South Pacific. The vertically integrated dynamic height along the south Java coast is related to variation in the North Pacific and in particular near the east coast of Mindanao Island, whereas the vertically integrated dynamic height along the coast of West Australia is related to variation in the South Pacific, and in particular near the coast of New Guinea. The integrated dynamic height difference between the Java and New Guinea coast appears to be a good proxy of ITF transport on the interannual time scale. Regression analysis shows a phase dependence of the three ITF pathways on the Nino3.4 index. Decoupling of current anomalies between the surface and subsurface layers is identified in the developing and mature phase of El Nino, reflecting different effects of local and remote forcing through oceanic pathways at the Makassar Strait and eastern Indonesian basins.     

Indian Ocean SST Biases in a Flexible Regional Ocean Atmosphere Land System (FROALS) Model

The authors examine the Indian Ocean sea surface temperature(SST) biases simulated by a Flexible Regional Ocean Atmosphere Land System(FROALS) model.The regional coupled model exhibits pronounced cold SST biases in a large portion of the Indian Ocean warm pool.Negative biases in the net surface heat fluxes are evident in the model,leading to the cold biases of the SST.Further analysis indicates that the negative biases in the net surface heat fluxes are mainly contributed by the biases of sensible heat and latent heat flux.Near-surface meteorological variables that could contribute to the SST biases are also examined.It is found that the biases of sensible heat and latent heat flux are caused by the colder and dryer near-surface air in the model.     

The coupling of the Common Land Model (CLM0) to a regional climate model (RegCM)

Summary We replace the existing land surface parameterization scheme, the Biosphere-Atmosphere Transfer Scheme (BATS), in a regional climate model (RegCM) with the newly developed Common Land Model (CLM0). The main improvements of CLM0 include a detailed 10-layer soil model, the distinction between soil ice and water phases, a linked photosynthesis-stomatal conductance model, a multilayer snow model, and an improved runoff parameterization. We compare the performance of CLM0 and BATS as coupled to the RegCM in a one year simulation over East Asia. We find that the RegCM/CLM0 improves the winter cold bias present in the RegCM/BATS simulation. With respect to the surface energy balance, lower CLM0 albedos allow the absorption of more solar radiation at the surface. CLM0 tends to simulate higher sensible heat and lower latent heat fluxes than its BATS counterpart. The surface water balance also changes considerably between the two land surface schemes. Compared to BATS, CLM0 precipitation is reduced overall and surface runoff is increased, thereby allowing less water to enter the soil column. Evapotranspiration is lower in CLM0 due to lower ground evaporation, which leads to a wetter surface soil in CLM0 in spite of less precipitation input. However, transpiration is greater in CLM0 than BATS, which has an overall effect of less surface storage during the summertime. Comparison with station observations indicates that CLM0 tends to improve the simulation of root zone soil water content compared to BATS. Another pronounced difference between the two schemes is that CLM0 produces lower snow amounts than BATS because of different snow models and warmer CLM0 temperatures. In this case, BATS snow cover amounts are more in line with observations. Overall, except for the snow amounts, CLM0 appears to improve the RegCM simulation of the surface energy and water budgets compared to BATS.     

Climatology and interannual variability simulated by the ARPEGE-OPA coupled model

A 10-year simulation with a coupled ocean-atmosphere general circulation model (CGCM) is presented. The model consists of the climate version of the Météo-France global forecasting model, ARPEGE, coupled to the LODYC oceanic model, OPA, by the CERFACS coupling package OASIS. The oceanic component is dynamically active over the tropical Pacific, while climatological time-dependent sea surface temperatures (SSTs) are prescribed outside of the Pacific domain. The coupled model shows little drift and exhibits a very regular seasonal cycle. The climatological mean state and seasonal cycle are well simulated by the coupled model. In particular, the oceanic surface current pattern is accurately depicted and the location and intensity of the Equatorial Undercurrent (EUC) are in good agreement with available data. The seasonal cycle of equatorial SSTs captures quite realistically the annual harmonic. Some deficiencies remain including a weak zonal equatorial SST gradient, underestimated wind stress over the Pacific equatorial band and an additional inter-tropical convergence zone (ITCZ) south of the equator in northern winter and spring. Weak interannual variability is present in the equatorial SST signal with a maximum amplitude of 0.5°C.     

Simulation of a Persistent Snow Storm over Southern China with a Regional Atmosphere-Ocean Coupled Model

A regional atmosphere-ocean coupled model,RegCM3-POM,was developed by coupling the regional climate model(RegCM3) with the Princeton Ocean Model(POM).The performance of RegCM3-POM in simulating a persistent snow storm over southern China and the impact of the Madden-Julian oscillation(MJO) on this persistent snow storm were investigated.Compared with the stand-alone RegCM3,the coupled model performed better at reproducing the spatial-temporal evolution and intensity of the precipitation episodes.The power spectral analysis indicated that the coupled model successfully captured the dominant period between 30 and 60 days in the precipitation field,leading to a notable improvement in simulating the magnitude of intraseasonal precipitation variation,and further in enhancing the intensity of the simulated precipitation.These improvements were mainly due to the well-simulated low-frequency oscillation center and its eastward propagation characteristics in each MJO phase by RegCM3-POM,which improved the simulations of MJO-related low-frequency vertical motions,water vapor transport,and the deep inversion layer that can directly influence the precipitation event and that further improved the simulated MJOprecipitation relationship.Analysis of the phase relationship between convection and SST indicated that RegCM3-POM exhibits a near-quadrature relation between the simulated convection and SST anomalies,which was consistent with the observations.However,such a near-quadrature relation was not as significant when the stand-alone RegCM3 was used.This difference indicated that the inherent coupled feedback process between the ocean and atmosphere in RegCM3-POM played an important part in reproducing the features of the MJO that accompanied the snow storm.     

Isolating mesoscale coupled ocean–atmosphere interactions in the Kuroshio Extension region

The Kuroshio Extension region is characterized by energetic oceanic mesoscale and frontal variability that alters the air–sea fluxes that can influence large-scale climate variability in the North Pacific. We investigate this mesoscale air-sea coupling using a regional eddy-resolving coupled ocean–atmosphere (OA) model that downscales the observed large-scale climate variability from 2001 to 2007. The model simulates many aspects of the observed seasonal cycle of OA coupling strength for both momentum and turbulent heat fluxes. We introduce a new modeling approach to study the scale-dependence of two well-known mechanisms for the surface wind response to mesoscale sea surface temperatures (SSTs), namely, the ‘vertical mixing mechanism’ (VMM) and the ‘pressure adjustment mechanism’ (PAM). We compare the fully coupled model to the same model with an online, 2-D spatial smoother applied to remove the mesoscale SST field felt by the atmosphere. Both VMM and PAM are found to be active during the strong wintertime peak seen in the coupling strength in both the model and observations. For VMM, large-scale SST gradients surprisingly generate coupling between downwind SST gradient and wind stress divergence that is often stronger than the coupling on the mesoscale, indicating their joint importance in OA interaction in this region. In contrast, VMM coupling between crosswind SST gradient and wind stress curl occurs only on the mesoscale, and not over large-scale SST gradients, indicating the essential role of the ocean mesocale. For PAM, the model results indicate that coupling between the Laplacian of sea level pressure and surface wind convergence occurs for both mesoscale and large-scale processes, but inclusion of the mesoscale roughly doubles the coupling strength. Coupling between latent heat flux and SST is found to be significant throughout the entire seasonal cycle in both fully coupled mode and large-scale coupled mode, with peak coupling during winter months. The atmospheric response to the oceanic mesoscale SST is also studied by comparing the fully coupled run to an uncoupled atmospheric model forced with smoothed SST prescribed from the coupled run. Precipitation anomalies are found to be forced by surface wind convergence patterns that are driven by mesoscale SST gradients, indicating the importance of the ocean forcing the atmosphere at this scale.     

Impacts of Indian Ocean SST biases on the Indian Monsoon: as simulated in a global coupled model

In this study, the impact of the ocean–atmosphere coupling on the atmospheric mean state over the Indian Ocean and the Indian Summer Monsoon (ISM) is examined in the framework of the SINTEX-F2 coupled model through forced and coupled control simulations and several sensitivity coupled experiments. During boreal winter and spring, most of the Indian Ocean biases are common in forced and coupled simulations, suggesting that the errors originate from the atmospheric model, especially a dry islands bias in the Maritime Continent. During boreal summer, the air-sea coupling decreases the ISM rainfall over South India and the monsoon strength to realistic amplitude, but at the expense of important degradations of the rainfall and Sea Surface Temperature (SST) mean states in the Indian Ocean. Strong SST biases of opposite sign are observed over the western (WIO) and eastern (EIO) tropical Indian Ocean. Rainfall amounts over the ocean (land) are systematically higher (lower) in the northern hemisphere and the south equatorial Indian Ocean rainfall band is missing in the control coupled simulation. During boreal fall, positive dipole-like errors emerge in the mean state of the coupled model, with warm and wet (cold and dry) biases in the WIO (EIO), suggesting again a significant impact of the SST errors. The exact contributions and the distinct roles of these SST errors in the seasonal mean atmospheric state of the coupled model have been further assessed with two sensitivity coupled experiments, in which the SST biases are replaced by observed climatology either in the WIO (warm bias) or EIO (cold bias). The correction of the WIO warm bias leads to a global decrease of rainfall in the monsoon region, which confirms that the WIO is an important source of moisture for the ISM. On the other hand, the correction of the EIO cold bias leads to a global improvement of precipitation and circulation mean state during summer and fall. Nevertheless, all these improvements due to SST corrections seem drastically limited by the atmosphere intrinsic biases, including prominently the unimodal oceanic position of the ITCZ (Inter Tropical Convergence Zone) during summer and the enhanced westward wind stress along the equator during fall.     

ENSO cycle in a coupled ocean-atmosphere model and its negative feedback mechanism

Summary A coupled ocean-atmosphere anomaly model has been developed for simulating ENSO cycle and its mechanism-study in this paper. After a long model run, the coupled model is successful in demonstrating ENSO-like irregular interannual variability and corresponding horizontal spatial structures. Based on the simulated results, the dynamics and the thermodynamics of the model ENSO cycle have been investigated, and in particular the negative feedback mechanisms that act to oppose instability of air-sea interaction, inducing termination of warm and cold events, have been examined. A detailed analysis of the oceanic wave dynamical properties and heat budget of the SST changes in a representative cycle suggest that the negative feedback mechanism to check the unstable growth of a warm event obviously differs from that of a cold event. The mechanism that induces decay and termination of a cold event is closely related to the negative, delayed feedback effect produced by the oceanic dynamical wave reflection at the western boundary. However, independent of the wave reflection effect, the negative feedback mechanism by which the coupled system returns from a warm event is associated with a slowly eastward-propagating coupling mode. Accompanied with the strong unstable development of the equatorial positive SST anomaly, the anomalous upwelling of cold water generated off the equator and the nonlinear anomalous meridional advection generated in the equator west of instability area jointly restrain the instability and finally plunge the system from a mature warm phase into a weak cold phase. A comparison between the results from the present model and the previous works is also discussed in this paper.With 16 Figures     

Effects of crop growth and development on regional climate: a case study over East Asian monsoon area

In this study, the CERES phenological growth and development functions were implemented into the regional climate model, RegCM3 to give a model denoted as RegCM3_CERES. This model was used to represent interactions between regional climate and crop growth processes. The effects of crop growth and development processes on regional climate were then studied based on two 20-year simulations over the East Asian monsoon area conducted using the original regional climate model RegCM3, and the coupled RegCM3_CERES model. The numerical experiments revealed that incorporating the crop growth and development processes into the regional climate model reduced the root mean squared error of the simulated precipitation by 2.2–10.7% over north China, and the simulated temperature by 5.5–30.9% over the monsoon region in eastern China. Comparison of the simulated results obtained using RegCM3_CERES and RegCM3 showed that the most significant changes associated with crop modeling were the changes in leaf area index which in turn modify the aspects of surface energy and water partitions and lead to moderate changes in surface temperature and, to some extent, rainfall. Further analysis revealed that a robust representation of seasonal changes in plant growth and developmental processes in the regional climate model changed the surface heat and moisture fluxes by modifying the vegetation characteristics, and that these differences in simulated surface fluxes resulted in different structures of the boundary layer and ultimately affected the convection. The variations in leaf area index and fractional vegetation cover changed the distribution of evapotranspiration and heat fluxes, which could potentially lead to anomalies in geopotential height, and consequently influenced the overlying atmospheric circulation. These changes would result in redistribution of the water and energy through advection. Nevertheless, there are significant uncertainties in modeling how monsoon dynamics responds to crop modeling and more research is needed.     

Simulation of China Summer Precipitation Using a Regional Air-Sea Coupled Model

A regional air-sea coupled model based on the regional climate model(RegCM3)and the regional oceanic model POM(Princeton Ocean Model)is developed and a series of experiments are performed to verify the ability of the coupled model in simulating the summer precipitation over China from 1963 to 2002.The results show that the space correlation coefficients between the GISST(Global Ice and Sea Surface Temperature)data and the simulated SST by RegCM3-POM exceed 0.9.Compared with the uncoupled experiments,the coupled model RegCM3-POM has a better performance in simulating the mean summer(June to August)precipitation over China,and the distribution of the rainband in the coupled model is more accurate.The improvement of the rainfall simulation is significant over the Yangtze River Valley and in South China.The rainbelt intraseasoaal evolution over eastern China in summer indicates that the simulation ability of RegCM3-POM is improved in comparison with the uncoupled model.The interannual summer rainfall variation over eastern China simulated by RegCM3-POM is in accordance with observation,while the spatial pattern of the interannual summer rainfall variation in the uncoupled model is inaccurate.The simulated correlation coefficient between the summer rainfall in the uncoupled model RegCM3 and observation is0.30 over the Yangtze River Valley and 0.29 in South China.The coefficient between the rainfall in the coupled RegCM3-POM and observation is 0.48 over the Yangtze River Valley and 0.61 in South China.The RegCM3-POM has successfully simulated the correlation coefficients between summer rainfall in the Yangtze River Valley and SST anomalies of the Bay of Bengal,South China Sea,and the Kuroshio area,whereas the uncoupled model RegCM3 fails to reproduce this relationship.The study further shows that the monsoon circulation and the path of the moisture transport flux simulated by RegCM3-POM are in good agreement with the NCEP/NCAR data.     

Influence of positive and negative Indian Ocean Dipoles on ENSO via the Indonesian Throughflow: Results from sensitivity experiments

The role of the Indonesian Throughflow(ITF) in the influence of the Indian Ocean Dipole(IOD) on ENSO is investigated using version 2 of the Parallel Ocean Program(POP2) ocean general circulation model. We demonstrate the results through sensitivity experiments on both positive and negative IOD events from observations and coupled general circulation model simulations. By shutting down the atmospheric bridge while maintaining the tropical oceanic channel, the IOD forcing is shown to influence the ENSO event in the following year, and the role of the ITF is emphasized. During positive IOD events,negative sea surface height anomalies(SSHAs) occur in the eastern Indian Ocean, indicating the existence of upwelling.These upwelling anomalies pass through the Indonesian seas and enter the western tropical Pacific, resulting in cold anomalies there. These cold temperature anomalies further propagate to the eastern equatorial Pacific, and ultimately induce a La Nia-like mode in the following year. In contrast, during negative IOD events, positive SSHAs are established in the eastern Indian Ocean, leading to downwelling anomalies that can also propagate into the subsurface of the western Pacific Ocean and travel further eastward. These downwelling anomalies induce negative ITF transport anomalies, and an El Nio-like mode in the tropical eastern Pacific Ocean that persists into the following year. The effects of negative and positive IOD events on ENSO via the ITF are symmetric. Finally, we also estimate the contribution of IOD forcing in explaining the Pacific variability associated with ENSO via ITF.     

Simulation of the Indian monsoon using the RegCM3–ROMS regional coupled model

A regional coupled atmosphere–ocean model was developed to study the role of air–sea interactions in the simulation of the Indian summer monsoon. The coupled model includes the regional climate model (RegCM3) as atmospheric component and the regional ocean modeling system (ROMS) as oceanic component. The two-way coupled model system exchanges sea surface temperature (SST) from the ocean to the atmospheric model and surface wind stress and energy fluxes from the atmosphere to the ocean model. The coupled model is run for four years 1997, 1998, 2002 and 2003 and the results are compared with observations and atmosphere-only model runs employing Reynolds SSTs as lower boundary condition. It is found that the coupled model captures the main features of the Indian monsoon and simulates a substantially more realistic spatial and temporal distribution of monsoon rainfall compared to the uncoupled atmosphere-only model. The intraseasonal oscillations are also better simulated in the coupled model compared to the atmosphere-only model. These improvements are due to a better representation of the feedbacks between the SST and convection and highlight the importance of air–sea coupling in the simulation of the Indian monsoon.