The impact of the diurnal cycle on the MJO over the Maritime Continent: a modeling study assimilating TRMM rain rate into global analysis

In the present study, we use modeling experiments to investigate the impact of the diurnal cycle on the Madden-Julian Oscillation (MJO) during the Australian summer. Physical initialization and a nudging technique enable us to assimilate the observed Tropical Rainfall Measuring Mission (TRMM) rain rate and atmospheric variables from the National Centers for Environmental Prediction—National Center for Atmospheric Research Reanalysis 2 (R2) into the Florida State University Global Spectral Model (FSUGSM), resulting in a realistic simulation of the MJO. Model precipitation is also significantly improved by TRMM rain rate observation via the physical initialization. We assess the influence of the diurnal cycle on the MJO by modifying the diurnal component during the model integration. Model variables are nudged toward the daily averaged values from R2. Globally suppressing the diurnal cycle (NO_DIURNAL) exerts a strong impact on the Maritime Continent. The mean state of precipitation increases and intraseasonal variability becomes stronger over the region. It is well known that MJO weakens as it passes over the Maritime Continent. However, the MJO maintains its strength in the NO_DIURNAL experiment, and the diminution of diurnal signals during the integration does not change the propagating speed of the MJO. We suggest that diminishing the diurnal cycle in NO_DIURNAL consumes less moist static energy (MSE), which is required to trigger both diurnal and intraseasonal convection. Thus, the remaining MSE may play a major role along with larger convective instability and stronger lower level moisture convergence in intensifying the MJO over the Maritime Continent in the model simulation.     

AN ANALYSIS OF THE ASYMMETRICAL STRUCTURE OF TYPHOON AERE'S PRECIPITATION

The structural characteristics of 2004 typhoon Aere's precipitation are analyzed using the high-resolution data from the Tropical Rainfall Measuring Mission(TRMM) of the National Aeronautics Space Administration(NASA).It is found that the typhoon's characteristics vary at different stages of its development.To analyze the asymmetric causation of precipitation distribution,data from the National Center for Environmental Prediction(NCEP) reanalysis are used to calculate the vertical integral of the water vapo...     

Simulations of the Madden–Julian oscillation in four pairs of coupled and uncoupled global models

The status of the numerical reproduction of the Madden–Julian Oscillation (MJO) by current global models was assessed through diagnoses of four pairs of coupled and uncoupled simulations. Slow eastward propagation of the MJO, especially in low-level zonal wind, is realistic in all these simulations. However, the simulated MJO suffers from several common problems. The MJO signal in precipitation is generally too weak and often eroded by an unrealistic split of an equatorial maximum of precipitation into a double ITCZ structure over the western Pacific. The MJO signal in low-level zonal wind, on the other hand, is sometimes too strong over the eastern Pacific but too weak over the Indian Ocean. The observed phase relationship between precipitation and low-level zonal wind associated with the MJO in the western Pacific and their coherence in general are not reproduced by the models. The seasonal migration in latitude of MJO activity is missing in most simulations. Air–sea coupling generally strengthens the simulated eastward propagating signal, but its effects on the phase relationship and coherence between precipitation and low-level zonal wind, and on their geographic distributions, seasonal cycles, and interannual variability are inconsistent among the simulations. Such inconsistency cautions generalization of results from MJO simulations using a single model. In comparison to observations, biases in the simulated MJO appear to be related to biases in the background state of mean precipitation, low-level zonal wind, and boundary-layer moisture convergence. This study concludes that, while the realistic simulations of the eastward propagation of the MJO are encouraging, reproducing other fundamental features of the MJO by current global models remains an unmet challenge.

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夏季印度洋MJO活跃时间对中国长江流域降水日数的影响

利用1980—2020年中国753站逐日降水资料、NCEP/NCAR大气再分析资料以及哈得来中心的海表温度资料和实时多变量Madden-Julian振荡（ MJO）指数,研究了MJO在印度洋地区（1—3位相）活跃日数对长江流域夏季降水日数的影响。结果表明两者存在显著的统计联系,在MJO活跃日数偏多的年份,MJO相关的西北太平洋反气旋环流异常有利于向长江中下游地区输送水汽,进而导致长江流域中下游范围内降水日数的增加,且这种影响主要体现在降水等级为大雨（25 mm/d）及以上强度的日数上。进一步研究发现,MJO在印度洋活跃日数与长江中下游夏季降水日数的关系存在年代际变化,两者显著的联系仅出现在2000年之后,之前的时段两者联系则较弱。这种关系的转变可能与印度洋海表温度变率减弱的背景有关,印度洋海洋年际变率变弱导致其对于长江中下游地区的影响减弱,进而使得MJO的调控作用凸显出来。夏季季节平均的印度洋MJO活跃日数可以对长江中下游的大雨以上的降水日数产生影响,且两者的关系在大约2000年之后变得尤为显著。      

Tropical variability and stratospheric equatorial waves in the IPSLCM5 model

The atmospheric variability in the equatorial regions is analysed in the Earth System Model pre-industrial simulation done at IPSL in the framework of CMIP5. We find that the model has an interannual variability of about the right amplitude and temporal scale, when compared to the El-Niño Southern Oscillation (ENSO), but that is too confined to the western Pacific. At the intra-seasonal periods, the model variability lacks of large-scale organisation, and only produces one characteristic Madden-Julian Oscillation every 10 winters typically. At shorter time-scales and in the troposphere, the model has Rossby and Kelvin Convectively Coupled Equatorial Waves (CCEWs), but underestimates the Kelvin CCEWs signal on OLR. In the model stratosphere, a composite analysis shows that the Temperature and velocities fluctuations due to the Kelvin waves are quite realistic. In the model nevertheless, the stratospheric waves are less related to the convection than in the observations, suggesting that their forcing by the midlatitudes plays a larger role. Still in the model, the Kelvin waves are not predominantly occurring during the life cycle of the tropospheric Kelvin CCEWs, a behaviour that we find to be dominant in the observations. The composite analysis is also used to illustrate how the waves modify the zonal mean-flow, and to show that the model Kelvin waves are too weak in this respect. This illustrates how a model can have a reasonable Kelvin waves signal on the velocities and temperature, but can at the same time underestimate their amplitude to modify the mean flow. We also use this very long simulation to establish that in the model, the stratospheric equatorial waves are significantly affected by ENSO, hence supporting the idea that the ENSO can have an influence on the Quasi-Biennial Oscillation.     

Madden–Julian Oscillation simulated in BCC climate models

This study evaluates the ability of four versions BCC (Beijing Climate Center or National Climate Center) models (BCC_AGCM2.1, BCC_AGCM2.2, BCC_CSM1.1 and BCC_CSM1.1m) in simulating the MJO phenomenon using the outputs of the AMIP (Atmospheric Model Intercomparison Project) and historical runs. In general, the models can simulate some major characteristics of the MJO, such as the intensity, the periodicity, the propagation, and the temporal/spatial evolution of the MJO signals in the tropics. There are still some biases between the models and the observation/reanalysis data, such as the overestimated total intraseasonal variability, but underestimated MJO intensity, shorter significant periodicity, and excessive westward propagation. The differences in the ability of simulating the MJO between AMIP and historical experiments are also significant. Compared to the AMIP runs, the total intraseasonal variability is reduced and more realistic, however the ratio between the MJO and its westward counterpart decreases in the historical runs. This unrealistic simulation of the zonal propagation might have been associated with the greater mean precipitation over the Pacific and corresponded to the exaggeration of the South Pacific Convergence Zone structure in precipitation mean state. In contrast to the T42 versions, the improvement of model resolution demonstrate more elaborate topography, but the enhanced westward propagation signals over the Arabia Sea followed. The underestimated (overestimated) MJO variability over eastern Indian Ocean (Pacific) was assumed to be associated with the mean state. Three sets of sensitive experiments using BCC_CSM1.1m turn out to support this argument.     

The MJO in an AGCM with three different cumulus parameterization schemes

The Madden–Julian oscillation (MJO) is simulated using an AGCM with three different cumulus parameterization schemes: a moist convective adjustment (MCA) scheme, the Zhang–McFarlane (ZM) mass-flux scheme, and the Tiedtke scheme. Results show that the simulated MJO is highly dependent on the cumulus parameterization used. Among the three cumulus parameterizations, only the MCA scheme produces MJO features similar to observations, including the reasonable spatial distribution, intraseasonal time scales and eastward propagation. Meanwhile, the amplitude is too large and the eastward propagation speed too fast than observations and the relationship between precipitation and low-level wind anomaly is unrealistic with enhanced convection occurring within easterly anomalies instead of westerly anomalies as in observations. The over-dependence of precipitation on boundary convergence produced by the MCA scheme is presumably responsible for this unrealistic phase relation in the simulation. The other two schemes produce very poor simulations of the MJO: spectral power of westward propagation is larger than that of eastward propagation in zonal wind and precipitation, indicating a westward propagation of the intraseasonal variability.The mean state and vertical profile of diabatic heating are perhaps responsible for the differences in these simulations. The MCA scheme produces relatively realistic climate background. When either ZM or Tiedtke scheme is used, the observed extension of westerly winds from the western Pacific to the dateline is missing and precipitation over the equatorial region and SPCZ is dramatically underestimated. In addition, diabatic heating produced by both ZM and Tiedtke schemes are very weak and nearly uniform with height. The heating profile produced by the MCA scheme has a middle-heavy structure with much larger magnitude than those produced by the other two schemes. In addition, a very unrealistic boundary layer heating maximum produced by the MCA scheme induces too strong surface convergence, which perhaps contributes to the too strong intraseasonal variability in the simulation.     

Evaluation of subseasonal impacts of the MJO/BSISO in the East Asian extended summer

The Madden–Julian Oscillation (MJO)/Boreal Summer Intraseasonal Oscillation (BSISO) has been considered an important climate mode of variability on subseasonal timescales for East Asian summer. However, it is unclear how well the MJO/BSISO indices would serve as guidance for subseasonal forecasts. Using a probabilistic forecast model determined through multiple linear regression (MLR) with MJO, ENSO, and long-term trend as predictors, we examine lagged impacts of each predictor on East Asia extended summer (May–October) climate from 1982 to 2015. The forecast skills of surface air temperature (T2m) contributed by each predictor is evaluated for lead times out to five weeks. We also provide a systematic evaluation of three commonly used, real-time MJO/BSISO indices in the context of lagged temperature impacts over East Asia. It is found that the influence of the trend provides substantial summertime skill over broad regions of East Asia on subseasonal timescales. In contrast, the MJO influence shows regional as well as phase dependence outside the tropical band of the main action centers of the MJO convective anomalies. All three MJO/BSISO indices generate forecasts that yield high skill scores for week 1 forecasts. For some initial phases of the MJO/BSISO, skill reemerges over some regions for lead times of 3–5 weeks. This emergence indicates the existence of windows of opportunity for skillful subseasonal forecasts over East Asia in summer. We also explore the dynamics that contribute to the elevated skills at long lead times over Tibet and Taiwan–Philippine regions following the initial state of phases 7 and 5, respectively. The elevated skill is rooted in a wave train forced by the MJO convective heating over the Arabian Sea and feedbacks between MJO convection and SSTs in Taiwan–Philippine region. Two out of the three commonly used MJO/BSISO indices tend to identify MJO events that evolve consistently in time, allowing them to serve as reliable predictors for subseasonal forecasts for up to 5 weeks.

     

SIMULATION OF SUMMER MONSOON CLIMATE OVER EASTERN CHINA USING A REGIONAL SPECTRAL MODEL

In this paper, we evaluate the characteristics of the surface air temperature and the precipitation of summer monsoon, using the National Centers for Environmental Prediction (NCEP) Regional Spectral Model (RSM) for 20 years (1984-2003). The RSM model was designated over the eastern China with a horizontal grid spacing of approximately 30 km. The model is driven by the NCEP/NCAR reanalysis data and runs from May 21 to September 1 for each of the 20 years. The distribution and variation patterns of the 20-year summer mean surface air temperature and precipitation are reproduced by the RSM and the differences between the simulation and observation are small. However, the model overestimates the interannual variability of summer precipitation in eastern China. The correlation coefficients of the 20-year averaging summer precipitation over the whole region and the sub-domains are above 0.8. The simulated probability distributions of daily maximum and minimum temperatures are similar to the observations. Days of different precipitation intensities in the simulations are generally consistent with the observations: the simulated days of light rain, moderate rain, heavy rain and torrential rain closely resemble the observations, but the simulated maximum centers of the distribution are north of the observed ones.     

Tropical intraseasonal rainfall variability in the CFSR

While large-scale circulation fields from atmospheric reanalyses have been widely used to study the tropical intraseasonal variability, rainfall variations from the reanalyses are less focused. Because of the sparseness of in situ observations available in the tropics and strong coupling between convection and large-scale circulation, the accuracy of tropical rainfall from the reanalyses not only measures the quality of reanalysis rainfall but is also to some extent indicative of the accuracy of the circulations fields. This study analyzes tropical intraseasonal rainfall variability in the recently completed NCEP Climate Forecast System Reanalysis (CFSR) and its comparison with the widely used NCEP/NCAR reanalysis (R1) and NCEP/DOE reanalysis (R2). The R1 produces too weak rainfall variability while the R2 generates too strong westward propagation. Compared with the R1 and R2, the CFSR produces greatly improved tropical intraseasonal rainfall variability with the dominance of eastward propagation and more realistic amplitude. An analysis of the relationship between rainfall and large-scale fields using composites based on Madden-Julian Oscillation (MJO) events shows that, in all three NCEP reanalyses, the moisture convergence leading the rainfall maximum is near the surface in the western Pacific but is above 925?hPa in the eastern Indian Ocean. However, the CFSR produces the strongest large-scale convergence and the rainfall from CFSR lags the column integrated precipitable water by 1 or 2?days while R1 and R2 rainfall tends to lead the respective precipitable water. Diabatic heating related to the MJO variability in the CFSR is analyzed and compared with that derived from large-scale fields. It is found that the amplitude of CFSR-produced total heating anomalies is smaller than that of the derived. Rainfall variability from the other two recently produced reanalyses, the ECMWF Re-Analysis Interim (ERAI), and the Modern Era Retrospective-analysis for Research and Applications (MERRA), is also analyzed. It is shown that both the ERAI and MERRA generate stronger rainfall spectra than the R1 and more realistic dominance of eastward propagating variance than R2. The intraseasonal variability in the MERRA is stronger than that in the ERAI but weaker than that in the CFSR and CMORPH.     

热带大气季节内振荡对西南地区东部夏季降水的影响及其可能机制

利用1979~2013年6~8月的西南地区东部20个台站日降水量资料、逐日MJO(Madden-Julian Oscillation)指数、全球OLR(Outgoing Longwave Radiation)逐日格点资料以及NCEP/NCAR再分析日资料,采用合成分析和线性回归等方法,对夏季MJO不同位相活动影响西南地区东部夏季降水的原因及其可能机制进行了初步分析。研究表明,MJO与西南地区东部夏季降水之间存在着显著的关系,当MJO处于第4(第6)位相时,由于西太平洋副高位置偏南(偏北)、向西南地区东部的水汽输送偏多(偏少),在异常上升(下沉)气流影响下,西南地区东部夏季降水偏多(偏少)。MJO影响西南地区东部夏季降水的可能原因是:当MJO处于第4位相时,赤道东印度洋地区上空大气释放凝结潜热,其激发东北向传播的异常波动,进而影响东亚环流,使得西南地区东部出现夏季降水偏多的环流形势,西南地区东部夏季降水增多;但在第6位相时,西太平洋地区上空对流释放的凝结潜热,其激发PJ(太平洋-日本)型Rossby波列,出现不利于西南地区东部夏季降水的环流形势,西南地区东部夏季降水偏少。     

东南风低空急流影响"21·7"郑州市极端暴雨分析

利用中国气象局上海台风研究所热带气旋(Tropical Cyclone, TC)最佳路径数据、美国环境预报中心(National Center of Environmental Prediction, NCEP)最终分析格点资料(Final Analysis, FNL)及常规观测数据等,分析了2021年7月郑州极端暴雨过程的特征机理。结果表明:(1)两个TC同时远距离输送大量水汽,东南风低空急流与强降水关系密切。当东南风急流北上到太原、邢台等站后郑州市暴雨减弱。郑州市6 h降水超过62.5 mm站点数目增减与850 hPa地转偏差数值变化趋势一致。(2) 20日白天郑州站极端降水发生前,从850 hPa经500 hPa至200 hPa风向从东南风顺转为西南风。200 hPa与850 hPa水平风的垂直切变(Vertical Wind Shear, VWS)从最大值开始减小、可降水量(Precipitable Water Vapor, PWV)明显加大、不稳定程度减小,这些都是极端暴雨发生的有利条件。极端暴雨具有明显的湿对流特点。(3)纬、经向风变率诊断表明,最有利于郑州市850 hPa东风维持的因子是地转偏差,最有利于南风加大的因子是动量平流。动量对流促使郑州市暴雨过程850 hPa南风减小,故动量上、下传递并非850 hPa南风加大原因。     

Convective signals from surface measurements at ARM Tropical Western Pacific site: Manus

Madden?CJulian Oscillation (MJO) signals have been detected using highly sampled observations from the U.S. DOE ARM Climate Research Facility located at the Tropical Western Pacific Manus site. Using downwelling shortwave radiative fluxes and derived shortwave fractional sky cover, and the statistical tools of wavelet, cross wavelet, and Fourier spectrum power, we report finding major convective signals and their phase change from surface observations spanning from 1996 to 2006. Our findings are confirmed with the satellite-gauge combined values of precipitation from the NASA Global Precipitation Climatology Project and the NOAA interpolated outgoing longwave radiation for the same location. We find that the Manus MJO signal is weakest during the strongest 1997?C1998 El Ni?o Southern Oscillation (ENSO) year. A significant 3?C5-month lead in boreal winter is identified further between Manus MJO and NOAA NINO3.4 sea surface temperature (former leads latter). A striking inverse relationship is found also between the instantaneous synoptic and intraseasonal phenomena over Manus. To further study the interaction between intraseasonal and diurnal scale variability, we composite the diurnal cycle of cloudiness for 21-MJO events that have passed over Manus. Our diurnal composite analysis of shortwave and longwave fractional sky covers indicates that during the MJO peak (strong convection), the diurnal amplitude of cloudiness is reduced substantially, while the diurnal mean cloudiness reaches the highest value and there are no significant phase changes. We argue that the increasing diurnal mean and decreasing diurnal amplitude are caused by the systematic convective cloud formation that is associated with the wet phase of the MJO, while the diurnal phase is still regulated by the well-defined solar forcing. This confirms our previous finding of the anti-phase relationship between the synoptic and intraseasonal phenomena. The detection of the MJO over the Manus site provides further opportunities in using other ground-based remote sensing instruments to investigate the vertical distributions of clouds and radiative heatings of the MJO that currently is impossible from satellite observations.     

Downscaling large-scale NCEP CFS to resolve fine-scale seasonal precipitation and extremes for the crop growing seasons over the southeastern United States

Seasonally predicted precipitation at a resolution of 2.5° was statistically downscaled to a fine spatial scale of ~20 km over the southeastern United States. The downscaling was conducted for spring and summer, when the fine-scale prediction of precipitation is typically very challenging in this region. We obtained the global model precipitation for downscaling from the National Center for Environmental Prediction/Climate Forecast System (NCEP/CFS) retrospective forecasts. Ten member integration data with time-lagged initial conditions centered on mid- or late February each year were used for downscaling, covering the period from 1987 to 2005. The primary techniques involved in downscaling are Cyclostationary Empirical Orthogonal Function (CSEOF) analysis, multiple regression, and stochastic time series generation. Trained with observations and CFS data, CSEOF and multiple regression facilitated the identification of the statistical relationship between coarse-scale and fine-scale climate variability, leading to improved prediction of climate at a fine resolution. Downscaled precipitation produced seasonal and annual patterns that closely resemble the fine resolution observations. Prediction of long-term variation within two decades was improved by the downscaling in terms of variance, root mean square error, and correlation. Relative to the coarsely resolved unskillful CFS forecasts, the proposed downscaling drove a significant reduction in wet biases, and correlation increased by 0.1–0.5. Categorical predictability of seasonal precipitation and extremes (frequency of heavy rainfall days), measured with the Heidke skill score (HSS), was also improved by the downscaling. For instance, domain averaged HSS for two category predictability by the downscaling are at least 0.20, while the scores by the CFS are near zero and never exceed 0.1. On the other hand, prediction of the frequency of subseasonal dry spells showed limited improvement over half of the Georgia and Alabama region.     

2007/2008年冬季中国南方低温雨雪天气期间大气准双周振荡作用分析

利用NCEP/NCAR逐日格点资料和中国气象局气象信息中心提供的727站逐日降水资料,从低频振荡角度对2007/2008年冬季中国南方低温雨雪期间的环流背景和气象要素场做了分析。分析结果表明,准双周振荡是2007/2008年冬季中国南方低温雨雪期间大气变化的重要模态。通过对低频振荡的传播分析得到：10～20 d低频波由...     

Fidelity of Regional Climate Model v4.6 in capturing seasonal and subseasonal variability of Indian summer monsoon

Climate modeling studies in the context of Indian summer monsoon (ISM) variability have usually been performed on the seasonal and interannual timescales. The present study assesses the fidelity of the Regional Climate Model (RegCM v4.6) in capturing the subseasonal active and break spells along with the seasonal mean rainfall during the ISM season. The model fields are obtained from 24 years (1982–2005) of simulation and validated against the observations and latest reanalyzed ERA5 data products. Our analysis indicates that RegCM v4.6 fairly captures the large scale features of ISM and improvement in seasonal rainfall is noted as compared to its precedent RegCM v4.4. At subseasonal timescales, though the model captures the active and break spells of ISM, the length and frequency of these events seem inconsistent as compared to the observations. Occurrences of breaks and associated circulation features are mostly consistent but the active spells are significantly misconstrued in the model. The dry air intrusion from the western region and lack of monsoon low over the mainland and Bay of Bengal seem to suppress the precipitation in the model. This subseasonal bias might persist due to systematic errors linked to the lack of ocean coupling, inefficiency of land surface and cumulus parameterization schemes in the model. Overall, RegCM v4.6 offers improvements at seasonal timescale but needs further improvements to realistically represent the subseasonal variability of ISM.     

MJO对2018年华北夏季降水的影响

本文基于华北夏季降水资料和热带大气季节内振荡（Madden–Julian Oscillation，简称MJO）指数、NCEP/NCAR（美国国家环境预报中心/美国国家大气研究中心）再分析环流资料，采用多种统计方法分析MJO与2018年华北夏季降水的关系及影响机制。结果表明:（1）MJO与华北夏季降水有密切的联系。虽然MJO不能移到较高纬度直接影响华北夏季降水，但MJO对流区的气旋会在其北侧激发出反气旋环流，这对“气旋—反气旋对”在缓慢东移过程中，处于较高纬度的反气旋会直接影响华北夏季降水。即MJO会间接影响华北夏季降水，表现为当夏季MJO进入5、6位相时，华北地区夏季会出现明显降水过程，但降水强弱与MJO振幅大小有关。（2）影响机制方面。在850 hPa，伴随MJO的“气旋—反气旋对”的东移，它会造成华北夏季偏南风水汽输送加强（对应RMM1）或东南风水汽输送加强（对应RMM2），从而有利于降水过程发生。在500 hPa层，MJO通过中层扰动向中高纬的传播，诱导副热带高压移到朝鲜半岛附近并加强，对西来高空槽形成阻挡作用，有利于华北地区产生上升运动，从而有利于华北夏季降水过程发生。（3）可以用MJO制作华北夏季延伸期降水过程预报。     

广西冬季降水的低频特征及其与MJO的联系

应用广西壮族自治区国家气象站降水,NCEP/NCAR逐日再分析资料,NOAA逐日向外长波辐射(OLR)等逐日资料,NOAA-CPC热带大气季节内振荡(MJO)指数等,使用经验正交函数分解方法分析了广西冬季降水的气候特征;用功率谱、带通滤波、相关分析和滞后线性回归等方法,以及定义MJO相关降水事件,研究了广西冬季降水异常偏多年的降水低频特征及其与MJO的联系。(1)广西冬季降水特征以全区一致型分布为主;冬季降水异常偏多年份的逐日降水具有14~26 d的低频周期。(2) MJO强对流在赤道印度洋东部发展并东传到西太平洋热带地区时,广西可出现冬季持续强降水。(3)当MJO异常对流在印度洋东部热带地区产生,中南半岛地区到华南地区上空为异常低频偏南和偏西南气流,有利于降水形成;当印度洋东部热带地区为MJO对流抑制区,华南地区上空为异常低频偏东气流控制,不利于降水产生。(4)华南地区上空大气环流的异常是由热带印度洋地区的MJO对流激发的Rossby波列造成。      

The sensitivity of African easterly waves to eastern tropical Atlantic sea-surface temperatures

The results of two regional atmospheric model simulations are compared to assess the influence of the eastern tropical Atlantic sea-surface temperature maximum on local precipitation, transient easterly waves and the West African summer monsoon. Both model simulations were initialized with reanalysis 2 data (US National Center for Environmental Prediction and Department of Energy) on 15 May 2006 and extended through 6 October 2006, forced by synchronous reanalysis 2 lateral boundary conditions introduced four times daily. One simulation uses 2006 reanalysis 2 sea-surface temperatures, also updated four times daily, while the second simulation considers ocean forcing absent the sea-surface temperature maximum, achieved here by subtracting 3°K at every ocean grid point between 0° and 15°N during the entire simulation. The simulation with 2006 sea-surface temperature forcing produces a realistic distribution of June?CSeptember mean precipitation and realistic westward propagating swaths of maximum rainfall, based on validation against Tropical Rainfall Measuring Mission (TRMM) estimates. The simulation without the sea-surface temperature maximum produces only 57% of the control June?CSeptember total precipitation over the eastern tropical Atlantic and about 83% of the Sahel precipitation. The simulation with warmer ocean temperatures generates generally stronger circulation, which in turn enhances precipitation by increasing moisture convergence. Some local precipitation enhancement is also attributed to lower vertical thermal stability above the warm water. The study shows that the eastern tropical Atlantic sea-surface temperature maximum enhances the strength of transient easterly waves and broadens the spatial extent of associated precipitation. However, large-scale circulation and its interaction with the African continent, and not sea-surface temperatures, control the timing and trajectories of the waves.