Effect of Horizontal Resolution on the Representation of the Global Monsoon Annual Cycle in AGCMs

The sensitivity of the representation of the global monsoon annual cycle to horizontal resolution is compared in three AGCMs:the Met Office Unified Model-Global Atmosphere 3.0;the Meteorological Research Institute AGCM3;and the Global High Resolution AGCM from the Geophysical Fluid Dynamics Laboratory.For each model,we use two horizontal resolution configurations for the period 1998–2008.Increasing resolution consistently improves simulated precipitation and low-level circulation of the annual mean and the first two annual cycle modes,as measured by the pattern correlation coefficient and equitable threat score.Improvements in simulating the summer monsoon onset and withdrawal are region-dependent.No consistent response to resolution is found in simulating summer monsoon retreat.Regionally,increased resolution reduces the positive bias in simulated annual mean precipitation,the two annual-cycle modes over the West African monsoon and Northwestern Pacific monsoon.An overestimation of the solstitial mode and an underestimation of the equinoctial asymmetric mode of the East Asian monsoon are reduced in all high-resolution configurations.Systematic errors exist in lower-resolution models for simulating the onset and withdrawal of the summer monsoon.Higher resolution models consistently improve the early summer monsoon onset over East Asia and West Africa,but substantial differences exist in the responses over the Indian monsoon region,where biases differ across the three low-resolution AGCMs.This study demonstrates the importance of a multi-model comparison when examining the added value of resolution and the importance of model physical parameterizations for simulation of the Indian monsoon.     

How are seasonal prediction skills related to models’ performance on mean state and annual cycle?

Given observed initial conditions, how well do coupled atmosphere–ocean models predict precipitation climatology with 1-month lead forecast? And how do the models’ biases in climatology in turn affect prediction of seasonal anomalies? We address these questions based on analysis of 1-month lead retrospective predictions for 21 years of 1981–2001 made by 13 state-of-the-art coupled climate models and their multi-model ensemble (MME). The evaluation of the precipitation climatology is based on a newly designed metrics that consists of the annual mean, the solstitial mode and equinoctial asymmetric mode of the annual cycle, and the rainy season characteristics. We find that the 1-month lead seasonal prediction made by the 13-model ensemble has skills that are much higher than those in individual model ensemble predictions and approached to those in the ERA-40 and NCEP-2 reanalysis in terms of both the precipitation climatology and seasonal anomalies. We also demonstrate that the skill for individual coupled models in predicting seasonal precipitation anomalies is positively correlated with its performances on prediction of the annual mean and annual cycle of precipitation. In addition, the seasonal prediction skill for the tropical SST anomalies, which are the major predictability source of monsoon precipitation in the current coupled models, is closely link to the models’ ability in simulating the SST mean state. Correction of the inherent bias in the mean state is critical for improving the long-lead seasonal prediction. Most individual coupled models reproduce realistically the long-term annual mean precipitation and the first annual cycle (solstitial mode), but they have difficulty in capturing the second annual (equinoctial asymmetric) mode faithfully, especially over the Indian Ocean (IO) and Western North Pacific (WNP) where the seasonal cycle in SST has significant biases. The coupled models replicate the monsoon rain domains very well except in the East Asian subtropical monsoon and the tropical WNP summer monsoon regions. The models also capture the gross features of the seasonal march of the rainy season including onset and withdraw of the Asian–Australian monsoon system over four major sub-domains, but striking deficiencies in the coupled model predictions are observed over the South China Sea and WNP region, where considerable biases exist in both the amplitude and phase of the annual cycle and the summer precipitation amount and its interannual variability are underestimated.     

Different Asian Monsoon Rainfall Responses to Idealized Orography Sensitivity Experiments in the HadGEM3-GA6 and FGOALS-FAMIL Global Climate Models

Recent work has shown the dominance of the Himalaya in supporting the Indian summer monsoon(ISM),perhaps by surface sensible heating along its southern slope and by mechanical blocking acting to separate moist tropical flow from drier midlatitude air.Previous studies have also shown that Indian summer rainfall is largely unaffected in sensitivity experiments that remove only the Tibetan Plateau.However,given the large biases in simulating the monsoon in CMIP5 models,such results may be model dependent.This study investigates the impact of orographic forcing from the Tibetan Plateau,Himalaya and Iranian Plateau on the ISM and East Asian summer monsoon(EASM) in the UK Met Office's Had GEM3-GA6 and China's Institute of Atmospheric Physics FGOALS-FAMIL global climate models.The models chosen feature oppositesigned biases in their simulation of the ISM rainfall and circulation climatology.The changes to ISM and EASM circulation across the sensitivity experiments are similar in both models and consistent with previous studies.However,considerable differences exist in the rainfall responses over India and China,and in the detailed aspects such as onset and retreat dates.In particular,the models show opposing changes in Indian monsoon rainfall when the Himalaya and Tibetan Plateau orography are removed.Our results show that a multi-model approach,as suggested in the forthcoming Global Monsoon Model Intercomparison Project(GMMIP) associated with CMIP6,is needed to clarify the impact of orographic forcing on the Asian monsoon and to fully understand the implications of model systematic error.     

Temperature and precipitation in the context of the annual cycle over Asia: Model evaluation and future change

Since the early or late arrival of monsoon rainfall can be devastating to agriculture and economy, the prediction of the onset of monsoon is a very important issue. The Asian monsoon is characterized by a strong annual cycle with rainy summer and dry winter. Nevertheless, most of monsoon studies have focused on the seasonal-mean of temperature and precipitation. The present study aims to evaluate a total of 27 coupled models that participated in phase 5 of the Coupled Model Intercomparison Project (CMIP5) for projection of the time evolution and the intensity of Asian monsoon on the basis of the annual cycle of temperature and precipitation. And future changes of onset, retreat, and intensity of monsoon are analyzed. Four models for good seasonal-mean (GSM) and good harmonic (GH) groups, respectively, are selected. GSM is based on the seasonal-mean of temperature and precipitation in summer and winter, and GH is based on the annual cycle of temperature and precipitation which represents a characteristic of the monsoon. To compare how well the time evolution of the monsoon is simulated in each group, the onset, retreat, and duration of Asian monsoon are examined. The highest pattern correlation coefficient (PCC) of onset, retreat, and duration between the reanalysis data and model outputs demonstrates that GH models’ MME predicts time evolution of monsoon most precisely, with PCC values of 0.80, 0.52, and 0.63, respectively. To predict future changes of the monsoon, the representative concentration pathway 4.5 (RCP 4.5) experiments for the period of 2073-2099 are compared with historical simulations for the period of 1979-2005 from CMIP5 using GH models’ MME. The Asian monsoon domain is expanded by 22.6% in the future projection. The onset date in the future is advanced over most parts of Asian monsoon region. The duration of summer Asian monsoon in the future projection will be lengthened by up to 2 pentads over the Asian monsoon region, as a result of advanced onset. The Asian monsoon intensity becomes stronger with the passage of time. This study has important implication for assessment of CMIP5 models in terms of the prediction of time evolution and intensity of Asian monsoon based on the annual cycle of temperature and precipitation.     

Variability and teleconnections of South and East Asian summer monsoons in present and future projections of CMIP5 climate models

Coupled Model Inter-comparison Project Phase 5 (CMIP5) model outputs of the South and East Asian summer monsoon variability and their tele-connections are investigated using historical simulations (1861-2005) and future projections under the RCP4.5 scenario (2006-2100). Detailed analyses are performed using nine models having better representation of the recent monsoon teleconnections for the interactive Asian monsoon sub-systems. However, these models underestimate rainfall mainly over South Asia and Korea-Japan sector, the regions of heavy rainfall, along with a bias in location of rainfall maxima. Indeed, the simulation biases, underestimations of monsoon variability and teleconnections suggest further improvements for better representation of Asian monsoon in the climate models. Interestingly, the performance of Australian Community Climate and Earth System Simulator version 1.0 (ACCESS1.0) in simulating the annual cycle, spatial pattern of rainfall and multi-decadal variations of summer monsoon rainfall over South and East Asia appears to more realistic. In spite of large spread among the CMIP5 models, historical simulations as well as future projections of summer monsoon rainfall indicate multi-decadal variability. These rainfall variations, displaying certain epochs of more rainfall over South Asia than over East Asia and vice versa, suggest an oscillatory behaviour. Teleconnections between South and East Asian monsoon rainfall also exhibit a multi-decadal variation with alternate epochs of strengthening and weakening relationship. Furthermore, large-scale circulation features such as South Asian monsoon trough and north Pacific subtropical high depict zonal oscillatory behaviour with east-west-east shifts. Periods with eastward or westward extension of the Mascarene High, intensification and expansion of the upper tropospheric South Asian High are also projected by the CMIP5 models.     

Future change of global monsoon in the CMIP5

This study investigates future changes of Global Monsoon (GM) under anthropogenic global warming using 20 coupled models that participated in the phase five of Coupled Model Intercomparison Project (CMIP5) by comparing two runs: the historical run for 1850–2005 and the Representative Concentration Pathway (RCP) 4.5 run for 2006–2100. A metrics for evaluation of models’ performance on GM is designed to document performance for 1980–2005 and best four models are selected. The four best models’ multi-model ensemble (B4MME) projects the following changes in the twenty-first century under the RCP4.5 scenario. (1) Monsoon domain will not change appreciably but land monsoon domain over Asia tends to expand westward by 10.6 %. (2) The annual mean and range of GM precipitation and the percentage of local summer rainfall will all amplify at a significant level over most of the global region, both over land and over ocean. (3) There will be a more prominent northern-southern hemispheric asymmetry and eastern-western hemispheric asymmetry. (4) Northern Hemisphere (NH) monsoon onset will be advanced and withdrawal will be delayed. (5) Changes in monsoon precipitation exhibits huge differences between the NH and the Southern hemisphere (SH). The NH monsoon precipitation will increase significantly due to increase in temperature difference between the NH and SH, significant enhancement of the Hadley circulation, and atmospheric moistening, against stabilization of troposphere. There is a slight decrease of the Walker circulation but not significant against the inter-model spread. There are important differences between the CMIP 3 and CMIP5 results which are discussed in detail.     

IPCC global coupled model simulations of the South America monsoon system

This study examines the variability of the South America monsoon system (SAMS) over tropical South America (SA). The onset, end, and total rainfall during the summer monsoon are investigated using precipitation pentad estimates from the global precipitation climatology project (GPCP) 1979–2006. Likewise, the variability of SAMS characteristics is examined in ten Intergovernmental Panel on Climate Change (IPCC) global coupled climate models in the twentieth century (1981–2000) and in a future scenario of global change (A1B) (2081–2100). It is shown that most IPCC models misrepresent the inter-tropical convergence zone and therefore do not capture the actual annual cycle of precipitation over the Amazon and northwest SA. Most models can correctly represent the spatiotemporal variability of the annual cycle of precipitation in central and eastern Brazil such as the correct phase of dry and wet seasons, onset dates, duration of rainy season and total accumulated precipitation during the summer monsoon for the twentieth century runs. Nevertheless, poor representation of the total monsoonal precipitation over the Amazon and northeast Brazil is observed in a large majority of the models. Overall, MIROC3.2-hires, MIROC3.2-medres and MRI-CGCM3.2.3 show the most realistic representation of SAMS’s characteristics such as onset, duration, total monsoonal precipitation, and its interannual variability. On the other hand, ECHAM5, GFDL-CM2.0 and GFDL-CM2.1 have the least realistic representation of the same characteristics. For the A1B scenario the most coherent feature observed in the IPCC models is a reduction in precipitation over central-eastern Brazil during the summer monsoon, comparatively with the present climate. The IPCC models do not indicate statistically significant changes in SAMS onset and demise dates for the same scenario.     

关于亚洲夏季风爆发的动力学研究的若干近期进展

资料分析显示,与850 hPa风场相比,地面风的变化能更好地表征亚洲各季风系统的特征。基于地面风的季节性反转和降水的显著变化所构建的亚洲夏季风(ASM)爆发指数和等时线图表明:亚洲热带夏季风(TASM)在5月初首先在孟加拉湾(BOB)东南部爆发后不是向西传播,而是向东经中印半岛向东推进,于5月中到达中国南海(SCS),6月初到达热带西北太平洋。印度夏季风的表面低压系统源于近赤道阿拉伯海地区,于6月初到达印度西南部喀拉拉邦,印度夏季风随之爆发。亚洲副热带夏季风(STASM)5月初在西北太平洋日本本州东南的海区发生后向西南伸展,于6月初与南海季风降水区连接,形成东北—西南向雨带,夏季风在中国东南沿海登陆,日本的“梅雨”(Baiu)开始。6月中该雨带向北到达长江流域和韩国,江淮梅雨和韩国的“梅雨”(Changma) 开始。本文还回顾了亚洲热带夏季风爆发的动力学研究的若干近期进展。春季青藏高原和南亚海陆分布的联合强迫作用使海表温度(SST)在BOB中东部形成短暂但强盛的暖池,在高层南亚高压的抽吸作用下,常伴有季风爆发涡旋(MOV)发展,使冬季连续带状的副高脊线在孟加拉湾东部断裂,导致亚洲热带季风首先在BOB爆发。BOB东/西部有东/西风型垂直切变,利于激发/抑制对流活动,并增加/减少海洋向大气的表面感热加热,从而使得亚洲夏季风爆发的向西传播在BOB西海岸遇到屏障。季风爆发逐渐向东伸展引发南海和热带西太平洋夏季风相继爆发。季风降水释放的强大潜热使南亚高压发展西伸,纬向非对称位涡强迫显著增强;在阿拉伯半岛强烈的表面感热加热所诱发的中层阿拉伯反气旋的共同作用下,位于阿拉伯海近赤道的低压系统北移发展成为季风爆发涡旋,导致印度季风爆发。由此可见,历时约一个月的亚洲热带夏季风爆发的三个阶段(孟加拉湾、南海和印度季风爆发)是发生在特定的地理环境下受特定的动力—热力学规律驱动的接续过程。     

Seasonal forecasts for regional onset of the West African monsoon

The West African monsoon has over the years proven difficult to represent in global coupled models. The current operational seasonal forecasting system of the UK Met Office (GloSea4) has a good representation of monsoon rainfall over West Africa. It reproduces the various stages of the monsoon: a coastal phase in May and June, followed by onset of the Sahelian phase in July when rainfall maxima shift northward of 10N until September; and a secondary coastal rainfall maximum in October. We explore the dynamics of monsoon onset in GloSea4 and compare it to reanalyses. An important difference is the change in the Saharan heat low around the time of Sahelian onset. In Glosea4 the deepening heat low introduces moisture convergence across an east-west Sahelian band, whereas in the reanalyses such an east-west organisation of moisture does not occur and moisture is transported northwards to the Sahara. Lack of observations in the southern Sahara makes it difficult to verify this process in GloSea4 and also suggests that reanalyses may not be strongly constrained by station observations in an area key to Sahelian onset. Timing of monsoon onset has socio-economic importance for many countries in West Africa and we explore onset predictability in GloSea4. We use tercile categories to calculate probabilities for onset occurring before, near and after average in four different onset indicators. Glosea4 has modest skill at 2–3 months’ lead time, with ROC scores of 0.6–0.8. Similar skill is seen in hindcasts with models from the ENSEMBLES project, even in models with large rainfall biases over the Sahel. Forecast skill derives from tropical SST in June and many models capture at least the influence of the tropical Atlantic. This suggests that long-range skill for onset could be present in other seasonal forecasting systems in spite of mean rainfall biases.     

Simulations of precipitation using NRCM and comparisons with satellite observations and CAM: annual cycle

The accurate representation of rainfall in models of global climate has been a challenging task for climate modelers owing to its small space and time scales. Quantifying this variability is important for comparing simulations of atmospheric behavior with real time observations. In this regard, this paper compares both the statistical and dynamically forced aspects of precipitation variability simulated by the high-resolution (36?km) Nested Regional Climate Model (NRCM), with satellite observations from the Tropical Rainfall Measuring Mission (TRMM) 3B42 dataset and simulations from the Community Atmosphere Model (CAM) at T85 spatial resolution. Six years of rainfall rate data (2000?C2005) from within the Tropics (30°S?C30°N) have been used in the analysis and results are presented in terms of long-term mean rain rates, amplitude and phase of the annual cycle and seasonal mean maps of precipitation. Our primary focus is on characterizing the annual cycle of rainfall over four land regions of the Tropics namely, the Indian Monsoon, the Amazon, Tropical Africa and the North American monsoon. The lower tropospheric circulation patterns are analyzed in both the observations and the models to identify possible causes for biases in the simulated precipitation. The 6-year mean precipitation simulated by both models show substantial biases throughout the global Tropics with NRCM/CAM systematically underestimating/overestimating rainfall almost everywhere. The seasonal march of rainfall across the equator, following the motion of the sun, is clearly seen in the harmonic vector maps. The timing of peak rainfall (phase) produced by NRCM is in closer agreement with the observations compared to CAM. However like the long-time mean, the magnitude of seasonal mean rainfall is greatly underestimated by NRCM throughout the Tropical land mass. Some of these regional biases can be attributed to erroneous circulation and moisture surpluses/deficits in the lower troposphere in both models. Overall, the results seem to indicate that employing a higher spatial resolution (36?km) does not significantly improve simulation of precipitation. We speculate that a combination of several physics parameterizations and lack of model tuning gives rise to the observed differences between NRCM and the observations.     

Projected Changes in Asian Summer Monsoon in RCP Scenarios of CMIP5

Responses of the Asian Summer Monsoon(ASM) in future projections have been studied based on two core future projections of phase five of the Coupled Model Intercomparison Project(CMIP5) coordinated experiments with the IAP-coupled model FGOALS_s2(the Flexible Global Ocean-Atmosphere-Land System Model).The projected changes of the ASM in climatological mean and interannual variability were respectively reported.Both the South Asian Summer Monsoon(SASM) and the East Asian Summer Monsoon(EASM) were intensified in their climatology,featuring increased monsoon precipitation and an enhanced monsoon lower-level westerly jet flow.Accordingly,the amplitude of the annual cycle of rainfall over East Asia(EA) is enhanced,thereby indicating a more abrupt monsoon onset.After the EA monsoon onset,the EASM marched farther northward in the future scenarios than in the historical runs.In the interannual variability,the leading pattern of the EASM,defined by the first multi-variable EOF analysis over EA,explains more of the total variances in the warmest future scenario,specifically,Representative Concentration Pathway(RCP8.5).Also,the correlation coefficients analysis suggests that the relationship between the EASM interannual variations and ENSO was significantly strengthened in the future projections,which may indicate improved predictability of the EASM interannual variations.     

CMIP5 model-simulated onset,duration and intensity of the Asian summer monsoon in current and future climate

A number of significant weaknesses existed in our previous analysis of the changes in the Asian monsoon onset/retreat from coupled model intercomparison project phase 3 (CMIP3) models, including a lack of statistical significance tests, a small number of models analysed, and limited understanding of the causes of model uncertainties. Yet, the latest IPCC report acknowledges limited confidence for projected changes in monsoon onset/retreat. In this study we revisit the topic by expanding the analysis to a large number of CMIP5 models over much longer period and with more diagnoses. Daily 850 hPa wind, volumetric atmospheric precipitable water and rainfall data from 26 CMIP5 models over two sets of 50-year periods are used in this study. The overall model skill in reproducing the temporal and spatial patterns of the monsoon development is similar between CMIP3 and CMIP5 models. They are able to show distinct regional characteristics in the evolutions of Indian summer monsoon (ISM), East Asian summer monsoon (EASM) and West North Pacific summer monsoon (WNPSM). Nevertheless, the averaged onset dates vary significantly among the models. Large uncertainty exists in model-simulated changes in onset/retreat dates and the extent of uncertainty is comparable to that in CMIP3 models. Under global warming, a majority of the models tend to suggest delayed onset for the south Asian monsoon in the eastern part of tropical Indian Ocean and Indochina Peninsula and nearby region, primarily due to weakened tropical circulations and eastward shift of the Walker circulation. The earlier onset over the Arabian Sea and part of the Indian subcontinent in a number of the models are related to an enhanced southwesterly flow in the region. Weak changes in other domains are due to the offsetting results among the models, with some models showing earlier onsets but others showing delayed onsets. Different from the analysis of CMIP3 model results, this analysis highlights the importance of SST warming patterns over both the tropical Pacific and Indian Oceans in affecting the modelling results. The increased atmospheric moisture content offsets some effects of the delayed onset and results in increased rainfall intensity during the active monsoon period. The deficiencies of using rainfall alone in assessing the potential changes of the monsoon system are also shown in this study.     

Comparison of the Structure and Evolution of Intraseasonal Oscillations Before and After Onset of the Asian Summer Monsoon

High-resolution satellite-derived data and NCEP-NCAR reanalysis data are used to investigate intraseasonal oscillations （ISO） over the tropical Indian Ocean.A composite evolution of the ISO life cycle is constructed,including the initiation,development,and propagation of rainfall anomalies over the tropical Indian Ocean.The characteristics of ISO over the tropical Indian Ocean are profoundly different before and after the onset of the Indian summer monsoon.Positive precipitation anomalies before monsoon onset appear one phase earlier than those after monsoon onset.Before monsoon onset,precipitation anomalies associated with ISO first initiate in the western tropical Indian Ocean and then propagate eastward along the equator.After monsoon onset,convective anomalies propagate northward over the Indian summer monsoon region after an initial eastward propagation over the equatorial Indian Ocean.Surface wind convergence and air-sea interaction play critical roles in initiating each new cycle of ISO convection.     

Cloud radiative forcing in Asian monsoon region simulated by IPCC AR4 AMIP models

This study examines cloud radiative forcing (CRF) in the Asian monsoon region (0^o--50^oN,60^o--150^oE) simulated by Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) AMIP models. During boreal winter, no model realistically reproduces the larger long-wave cloud radiative forcing (LWCF) over the Tibet Plateau (TP) and only a couple of models reasonably capture the larger short-wave CRF (SWCF) to the east of the TP. During boreal summer, there are larger biases for central location and intensity of simulated CRF in active convective regions. The CRF biases are closely related to the rainfall biases in the models. Quantitative analysis further indicates that the correlation between simulated CRF and observations are not high, and that the biases and diversity in SWCF are larger than that in LWCF. The annual cycle of simulated CRF over East Asia (0^o--50^oN, 100^o--145^oE) is also examined. Though many models capture the basic annual cycle in tropics, strong LWCF and SWCF to the east of the TP beginning in early spring are underestimated by most models. As a whole, GFDL-CM2.1, MPI-ECHAM5, UKMO-HadGAM1, and MIROC3.2 (medres) perform well for CRF simulation in the Asian monsoon region, and the multi-model ensemble (MME) has improved results over the individual simulations. It is suggested that strengthening the physical parameterizations involved over the TP, and improving cumulus convection processes and model experiment design are crucial to CRF simulation in the Asian monsoon region.     

Long-term variability in the date of monsoon onset over western India

The date of onset of the southwest monsoon in western India is critical for farmers as it influences the timing of crop plantation and the duration of the summer rainy season. Identifying long-term variability in the date of monsoon onset is difficult, however, as onset dates derived from the reanalysis of instrumental rainfall data are only available for the region from 1879. This study uses documentary evidence and newly uncovered instrumental data to reconstruct annual monsoon onset dates for western India for the period 1781–1878, extending the existing record by 97 years. The mean date of monsoon onset over the Mumbai (Bombay) area during the reconstruction period was 10 June with a standard deviation of 6.9 days. This is similar to the mean and standard deviation of the date of monsoon onset derived from instrumental data for the twentieth century. The earliest identified onset date was 23 May (in 1802 and 1839) and the latest 22 June (in 1825). The longer-term perspective provided by this study suggests that the climatic regime that governs monsoon advance over western India did not change substantially from 1781 to 1955. Monsoon onset over Mumbai has occurred at a generally later date since this time. Our results indicate that this change is unprecedented during the last 230 years. Following a discussion of the results, the nature of the relationship between the date of monsoon onset and the El Niño-Southern Oscillation is discussed. This relationship is shown to have been stable since 1781.     

Uncertainties in the regional climate models simulations of South-Asian summer monsoon and climate change

The uncertainties in the regional climate models (RCMs) are evaluated by analyzing the driving global data of ERA40 reanalysis and ECHAM5 general circulation models, and the downscaled data of two RCMs (RegCM4 and PRECIS) over South-Asia for the present day simulation (1971–2000) of South-Asian summer monsoon. The differences between the observational datasets over South-Asia are also analyzed. The spatial and the quantitative analysis over the selected climatic regions of South-Asia for the mean climate and the inter-annual variability of temperature, precipitation and circulation show that the RCMs have systematic biases which are independent from different driving datasets and seems to come from the physics parameterization of the RCMs. The spatial gradients and topographically-induced structure of climate are generally captured and simulated values are within a few degrees of the observed values. The biases in the RCMs are not consistent with the biases in the driving fields and the models show similar spatial patterns after downscaling different global datasets. The annual cycle of temperature and rainfall is well simulated by the RCMs, however the RCMs are not able to capture the inter-annual variability. ECHAM5 is also downscaled for the future (2071–2100) climate under A1B emission scenario. The climate change signal is consistent between ECHAM5 and RCMs. There is warming over all the regions of South-Asia associated with increasing greenhouse gas concentrations and the increase in summer mean surface air temperature by the end of the century ranges from 2.5 to 5 °C, with maximum warming over north western parts of the domain and 30 % increase in rainfall over north eastern India, Bangladesh and Myanmar.     

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.     

NCEP/NCAR再分析资料所揭示的全球季风降水变化

大气模式是研究气候变化的重要工具,当前的大气模式在模拟季风降水时均存在较大偏差,目前尚不清楚该偏差是来自模式环流场还是模式物理过程.再分析资料由于同化了各类观测和卫星资料,其大气环流近似可被视作是“真实”的.再分析资料中的降水场是在基本真实的环流场强迫下,由当前最先进的数值预报模式计算输出的.因此,再分析资料的降水场能...     

Simulation of the annual and diurnal cycles of rainfall over South Africa by a regional climate model

The capability of a current state-of-the-art regional climate model for simulating the diurnal and annual cycles of rainfall over a complex subtropical region is documented here. Hourly rainfall is simulated over Southern Africa for 1998–2006 by the non-hydrostatic model weather research and forecasting (WRF), and compared to a network of 103 stations covering South Africa. We used five simulations, four of which consist of different parameterizations for atmospheric convection at a 0.5 × 0.5° resolution, performed to test the physic-dependency of the results. The fifth experiment uses explicit convection over tropical South Africa at a 1/30° resolution. WRF simulates realistic mean rainfall fields, albeit wet biases over tropical Africa. The model mean biases are strongly modulated by the convective scheme used for the simulations. The annual cycle of rainfall is well simulated over South Africa, mostly influenced by tropical summer rainfall except in the Western Cape region experiencing winter rainfall. The diurnal cycle shows a timing bias, with atmospheric convection occurring too early in the afternoon, and causing too abundant rainfall. This result, particularly true in summer over the northeastern part of the country, is weakly physic-dependent. Cloud-resolving simulations do not clearly reduce the diurnal cycle biases. In the end, the rainfall overestimations appear to be mostly imputable to the afternoon hours of the austral summer rainy season, i.e., the periods during which convective activity is intense over the region.     

The Asian summer monsoon: an intercomparison of CMIP5 vs. CMIP3 simulations of the late 20th century

The boreal summer Asian monsoon has been evaluated in 25 Coupled Model Intercomparison Project-5 (CMIP5) and 22 CMIP3 GCM simulations of the late twentieth Century. Diagnostics and skill metrics have been calculated to assess the time-mean, climatological annual cycle, interannual variability, and intraseasonal variability. Progress has been made in modeling these aspects of the monsoon, though there is no single model that best represents all of these aspects of the monsoon. The CMIP5 multi-model mean (MMM) is more skillful than the CMIP3 MMM for all diagnostics in terms of the skill of simulating pattern correlations with respect to observations. Additionally, for rainfall/convection the MMM outperforms the individual models for the time mean, the interannual variability of the East Asian monsoon, and intraseasonal variability. The pattern correlation of the time (pentad) of monsoon peak and withdrawal is better simulated than that of monsoon onset. The onset of the monsoon over India is typically too late in the models. The extension of the monsoon over eastern China, Korea, and Japan is underestimated, while it is overestimated over the subtropical western/central Pacific Ocean. The anti-correlation between anomalies of all-India rainfall and Niño3.4 sea surface temperature is overly strong in CMIP3 and typically too weak in CMIP5. For both the ENSO-monsoon teleconnection and the East Asian zonal wind-rainfall teleconnection, the MMM interannual rainfall anomalies are weak compared to observations. Though simulation of intraseasonal variability remains problematic, several models show improved skill at representing the northward propagation of convection and the development of the tilted band of convection that extends from India to the equatorial west Pacific. The MMM also well represents the space–time evolution of intraseasonal outgoing longwave radiation anomalies. Caution is necessary when using GPCP and CMAP rainfall to validate (1) the time-mean rainfall, as there are systematic differences over ocean and land between these two data sets, and (2) the timing of monsoon withdrawal over India, where the smooth southward progression seen in India Meteorological Department data is better realized in CMAP data compared to GPCP data.