The West African Monsoon (WAM) is characterized by strong decadal and multi-decadal variability and the impacts can be catastrophic for the local populations. One of the factors put forward to explain this variability...The West African Monsoon (WAM) is characterized by strong decadal and multi-decadal variability and the impacts can be catastrophic for the local populations. One of the factors put forward to explain this variability involves the role of atmospheric dynamics, linked in particular to the Saharan Heat Low (SHL). This article addresses this question by comparing the sets of preindustrial control and historical simulation data from climate models carried out in the framework of the CMIP5 project and observations data over the 20<sup>th</sup> century. Through multivariate statistical analyses, it was established that decadal modes of ocean variability and decadal variability of Saharan atmospheric dynamics significantly influence decadal variability of monsoon precipitation. These results also suggest the existence of external anthropogenic forcing, which is superimposed on the decadal natural variability inducing an intensification of the signal in the historical simulations compared to preindustrial control simulations. We have also shown that decadal rainfall variability in the Sahel, once the influence of oceanic modes has been eliminated, appears to be driven mainly by the activity of the Arabian Heat Low (AHL) in the central Sahel, and by the structure of the meridional temperature gradient over the inter-tropical Atlantic in the western Sahel.展开更多
Along the littoral shelf of northern coast of the Gulf of Guinea (GG), a minor dry season of the rainfall regime is concomitantly observed with the occurrence of a major coastal upwelling in July-August-September (JAS...Along the littoral shelf of northern coast of the Gulf of Guinea (GG), a minor dry season of the rainfall regime is concomitantly observed with the occurrence of a major coastal upwelling in July-August-September (JAS). It was then supposed that this upwelling drives that minor dry season. But no previous studies have tried to understand this minor dry season and, this study is the first focusing on this question. The investigations undertaken to explain this dry season on the Ivorian littoral shelf with the ERA-Interim data from the European Centre for Medium Range Weather Forecasts over the 1980-2016 period have shown that the minor dry season is driven by the Northward migration of the Inter Tropical Convergence Zone (ITCZ) during this period and, enhanced by the occurrence of the major coastal upwelling of the northern GG at the same time. These two phenomena interact as follow: i) the ITCZ is located in JAS far in the north cutting off convective processes along the coast, ii) the air on the coastal region is poor in humidity, iii) the air temperature on the bordering region of the GG is cooled by the coastal upwelling to value less than 26°C and not favorable for providing convection.展开更多
Two simulations of five years (2003-2007) were conducted with the Regional Climate models RegCM4, one coupled with Land surface models BATS and the other with CLM4.5 over West Africa, where simulated air temperature a...Two simulations of five years (2003-2007) were conducted with the Regional Climate models RegCM4, one coupled with Land surface models BATS and the other with CLM4.5 over West Africa, where simulated air temperature and precipitation were analyzed. The purpose of this study is to assess the performance of RegCM4 coupled with the new CLM4.5 Land</span><span style="font-family:""> </span><span style="font-family:Verdana;">surface scheme and the standard one named BATS in order to find the best configuration of RegCM4 over West African. This study could improve our understanding of the sensitivity of land surface model in West Africa climate simulation, and provide relevant information to RegCM4 users. The results show fairly realistic restitution of West Africa’s climatology and indicate correlations of 0.60 to 0.82 between the simulated fields (BATS and CLM4.5) for precipitation. The substitution of BATS surface scheme by CLM4.5 in the model configuration, leads mainly to an improvement of precipitation over the Atlantic Ocean, however, the impact is not sufficiently noticeable over the continent. While the CLM4.5 experiment restores the seasonal cycles and spatial distribution, the biases increase for precipitation and temperature. Positive biases already existing with BATS are amplified over some sub-regions. This study concludes that temporal localization (seasonal effect), spatial distribution (grid points) and magnitude of precipitation and temperature (bias) are not simultaneously improved by CLM4.5. The introduction of the new land surface scheme CLM4.5, therefore, leads to a performance of the same order as that of BATS, albeit with a more detailed formulation.展开更多
The recent West African Monsoon (WAM) wet season (May to October) rainfall’s interannual variability has been examined with emphasis on the rainfall zones of Guinea Coast (GC), Western Sudano Sahel (WSS) and Eastern ...The recent West African Monsoon (WAM) wet season (May to October) rainfall’s interannual variability has been examined with emphasis on the rainfall zones of Guinea Coast (GC), Western Sudano Sahel (WSS) and Eastern Sudano Sahel (ESS) in wet and dry years. Rainfall observations from Climate Research Unit (CRU) and Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP), and atmospheric circulation fields from National Center for Environmental Prediction (NCEP) were evaluated from 1979 to 2014. The objectives are to evaluate the trends across the zones and their linear relationship with the identified oceanic pulsations, as well as examine the evolution of the anomalous atmospheric circulation associated with the wet and dry years during the wet season months. The results show remarkable variability across the rainfall zones. The areal averaged rainfall anomalies show significant correlation values of -0.2 with Ocean Nino Index (ONI) only on WSS and ESS respectively, whereas with South Atlantic Ocean Dipole Index (SAODI) it shows significant correlation value of 0.3 only on GC, at 95% Confidence Level from a t-test. The analysis of trends in spatial and temporal patterns of the atmospheric circulation fields has extensively presented attributes associated with the wet seasonal rainfall anomalies in the wet and dry years. FGOALS-s2 model showed an outstanding simulation of the spatial and temporal patterns of these attributes, with the discrepancies noted, hence presenting itself as a viable tool in the prediction of seasonal rainfall extremes over West Africa.展开更多
This study assesses the direct and indirect effects of natural and anthropogenic aerosols (e.g., black carbon and sulfate) over West and Central Africa during the West African monsoon (WAM) period (June-July-Aug...This study assesses the direct and indirect effects of natural and anthropogenic aerosols (e.g., black carbon and sulfate) over West and Central Africa during the West African monsoon (WAM) period (June-July-August). We investigate the impacts of aerosols on the amount of cloudiness, the influences on the precipitation efficiency of clouds, and the associated radiative forcing (direct and indirect). Our study includes the implementation of three new formulations of auto-conversion parameterization [namely, the Beheng (BH), Tripoli and Cotton (TC) and Liu and Daum (R6) schemes] in RegCM4.4.1, besides the default model's auto-conversion scheme (Kessler). Among the new schemes, BH reduces the precipitation wet bias by more than 50% over West Africa and achieves a bias reduction of around 25% over Central Africa. Results from detailed sensitivity experiments suggest a significant path forward in terms of addressing the long-standing issue of the characteristic wet bias in RegCM. In terms of aerosol-induced radiative forcing, the impact of the various schemes is found to vary considerably (ranging from -5 to -25 W m-2).展开更多
By dint of grid information from 1948 to 2007,the summer monsoon in Afro-Asian area and the precipitation in corresponding atmosphere circulation situation during the strong and weak Afro-Asian monsoon period are stud...By dint of grid information from 1948 to 2007,the summer monsoon in Afro-Asian area and the precipitation in corresponding atmosphere circulation situation during the strong and weak Afro-Asian monsoon period are studied.The results suggest that the strong or weak Afro-Asian monsoon has pretty good corresponding relation with summer precipitation in Afro-Asian area.When summer monsoon weakens year after year,precipitation also decreases every year.展开更多
Using model results from the first phase of the Pliocene Model Intercomparison Project (PlioMIP) and four experiments with CAM4, the intensified African summer monsoon (ASM) in the mid-Piacenzian and corresponding...Using model results from the first phase of the Pliocene Model Intercomparison Project (PlioMIP) and four experiments with CAM4, the intensified African summer monsoon (ASM) in the mid-Piacenzian and corresponding mechanisms are analyzed. The results from PlioMIP show that the ASM intensified and summer precipitation increased in North Africa during the mid-Piacenzian, which can be explained by the increased net energy in the atmospheric column above North Africa. Further experiments with CAM4 indicated that the combined changes in the mid-Piacenzian of atmospheric CO2 concentration and SST, as well as the vegetation change, could have substantially increased the net energy in the atmospheric column over North Africa and further intensified the ASM. The experiments also demonstrated that topography change had a weak effect. Overall, the combined changes of atmospheric CO2 concentration and SST were the most important factor that brought about the intensified ASM in the mid-Piacenzian.展开更多
The response of the Asian-African summer monsoon (AASM) to the fast global warming in the 1980s is studied based on several datasets, which span a long time period of nearly 100 yr, with two special periods 1980-198...The response of the Asian-African summer monsoon (AASM) to the fast global warming in the 1980s is studied based on several datasets, which span a long time period of nearly 100 yr, with two special periods 1980-1985 and 1990-1995 being focused on. Wavelet analyses are employed to explore the interdecadal variations of the AASM. It is found that after the mid 1980s, the global annual mean surface temperature rises more significantly and extensively over most parts of the African Continent, north of the Indian Ocean, and the Eurasian Continent excluding the Tibetan Plateau. Correspondingly, the global precipitation pattern alters with in- creased rainfall seen over the Sahel and North China in 1990-1995, though it is not recovered to the level of the rainy period before the mid-1960s. Changes of monsoonal circulations between the pre- and post-1980s periods display that, after the fast global warming of the 1980s, the African summer monsoon intensifies distinctly, the Indian summer mon- soon weakens a little bit, and the East Asian summer monsoon remains almost unchanged. The summer precipitation over the Asian-African Monsoon Belt (AAMB) does not change in phase coherently with the variations of the monsoonal circulations. Wavelet analyses of the land-sea thermal contrast and precipitation over North China and the Sahel indicate that interdecadal signals are dominant and in positive phases in the 1960s, leading to an overall enhanced interdecadal variation of the AASM, although the 1960s witnesses a global cooling. In the 1980s, however, in the context of a fast global warming, interdecadal signals are in opposite phases, and they counteract with each other, leading to a weakened interdecadal variation of the AASM. After the mid-1960s, the AASM weakened remarkably, whereas after the mid-1980s, the AASM as a whole did not strengthen uniformly and synchronously, because it is found that the interannual variations of the AASM in the 1980s are stronger than those in the 1960s, and they superimposed on the展开更多
文摘The West African Monsoon (WAM) is characterized by strong decadal and multi-decadal variability and the impacts can be catastrophic for the local populations. One of the factors put forward to explain this variability involves the role of atmospheric dynamics, linked in particular to the Saharan Heat Low (SHL). This article addresses this question by comparing the sets of preindustrial control and historical simulation data from climate models carried out in the framework of the CMIP5 project and observations data over the 20<sup>th</sup> century. Through multivariate statistical analyses, it was established that decadal modes of ocean variability and decadal variability of Saharan atmospheric dynamics significantly influence decadal variability of monsoon precipitation. These results also suggest the existence of external anthropogenic forcing, which is superimposed on the decadal natural variability inducing an intensification of the signal in the historical simulations compared to preindustrial control simulations. We have also shown that decadal rainfall variability in the Sahel, once the influence of oceanic modes has been eliminated, appears to be driven mainly by the activity of the Arabian Heat Low (AHL) in the central Sahel, and by the structure of the meridional temperature gradient over the inter-tropical Atlantic in the western Sahel.
文摘Along the littoral shelf of northern coast of the Gulf of Guinea (GG), a minor dry season of the rainfall regime is concomitantly observed with the occurrence of a major coastal upwelling in July-August-September (JAS). It was then supposed that this upwelling drives that minor dry season. But no previous studies have tried to understand this minor dry season and, this study is the first focusing on this question. The investigations undertaken to explain this dry season on the Ivorian littoral shelf with the ERA-Interim data from the European Centre for Medium Range Weather Forecasts over the 1980-2016 period have shown that the minor dry season is driven by the Northward migration of the Inter Tropical Convergence Zone (ITCZ) during this period and, enhanced by the occurrence of the major coastal upwelling of the northern GG at the same time. These two phenomena interact as follow: i) the ITCZ is located in JAS far in the north cutting off convective processes along the coast, ii) the air on the coastal region is poor in humidity, iii) the air temperature on the bordering region of the GG is cooled by the coastal upwelling to value less than 26°C and not favorable for providing convection.
文摘Two simulations of five years (2003-2007) were conducted with the Regional Climate models RegCM4, one coupled with Land surface models BATS and the other with CLM4.5 over West Africa, where simulated air temperature and precipitation were analyzed. The purpose of this study is to assess the performance of RegCM4 coupled with the new CLM4.5 Land</span><span style="font-family:""> </span><span style="font-family:Verdana;">surface scheme and the standard one named BATS in order to find the best configuration of RegCM4 over West African. This study could improve our understanding of the sensitivity of land surface model in West Africa climate simulation, and provide relevant information to RegCM4 users. The results show fairly realistic restitution of West Africa’s climatology and indicate correlations of 0.60 to 0.82 between the simulated fields (BATS and CLM4.5) for precipitation. The substitution of BATS surface scheme by CLM4.5 in the model configuration, leads mainly to an improvement of precipitation over the Atlantic Ocean, however, the impact is not sufficiently noticeable over the continent. While the CLM4.5 experiment restores the seasonal cycles and spatial distribution, the biases increase for precipitation and temperature. Positive biases already existing with BATS are amplified over some sub-regions. This study concludes that temporal localization (seasonal effect), spatial distribution (grid points) and magnitude of precipitation and temperature (bias) are not simultaneously improved by CLM4.5. The introduction of the new land surface scheme CLM4.5, therefore, leads to a performance of the same order as that of BATS, albeit with a more detailed formulation.
文摘The recent West African Monsoon (WAM) wet season (May to October) rainfall’s interannual variability has been examined with emphasis on the rainfall zones of Guinea Coast (GC), Western Sudano Sahel (WSS) and Eastern Sudano Sahel (ESS) in wet and dry years. Rainfall observations from Climate Research Unit (CRU) and Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP), and atmospheric circulation fields from National Center for Environmental Prediction (NCEP) were evaluated from 1979 to 2014. The objectives are to evaluate the trends across the zones and their linear relationship with the identified oceanic pulsations, as well as examine the evolution of the anomalous atmospheric circulation associated with the wet and dry years during the wet season months. The results show remarkable variability across the rainfall zones. The areal averaged rainfall anomalies show significant correlation values of -0.2 with Ocean Nino Index (ONI) only on WSS and ESS respectively, whereas with South Atlantic Ocean Dipole Index (SAODI) it shows significant correlation value of 0.3 only on GC, at 95% Confidence Level from a t-test. The analysis of trends in spatial and temporal patterns of the atmospheric circulation fields has extensively presented attributes associated with the wet seasonal rainfall anomalies in the wet and dry years. FGOALS-s2 model showed an outstanding simulation of the spatial and temporal patterns of these attributes, with the discrepancies noted, hence presenting itself as a viable tool in the prediction of seasonal rainfall extremes over West Africa.
文摘This study assesses the direct and indirect effects of natural and anthropogenic aerosols (e.g., black carbon and sulfate) over West and Central Africa during the West African monsoon (WAM) period (June-July-August). We investigate the impacts of aerosols on the amount of cloudiness, the influences on the precipitation efficiency of clouds, and the associated radiative forcing (direct and indirect). Our study includes the implementation of three new formulations of auto-conversion parameterization [namely, the Beheng (BH), Tripoli and Cotton (TC) and Liu and Daum (R6) schemes] in RegCM4.4.1, besides the default model's auto-conversion scheme (Kessler). Among the new schemes, BH reduces the precipitation wet bias by more than 50% over West Africa and achieves a bias reduction of around 25% over Central Africa. Results from detailed sensitivity experiments suggest a significant path forward in terms of addressing the long-standing issue of the characteristic wet bias in RegCM. In terms of aerosol-induced radiative forcing, the impact of the various schemes is found to vary considerably (ranging from -5 to -25 W m-2).
文摘By dint of grid information from 1948 to 2007,the summer monsoon in Afro-Asian area and the precipitation in corresponding atmosphere circulation situation during the strong and weak Afro-Asian monsoon period are studied.The results suggest that the strong or weak Afro-Asian monsoon has pretty good corresponding relation with summer precipitation in Afro-Asian area.When summer monsoon weakens year after year,precipitation also decreases every year.
基金supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB03020602)the National Natural Science Foundation of China (Grant Nos. 41175072, 41305073, 41402158 and 41472160)
文摘Using model results from the first phase of the Pliocene Model Intercomparison Project (PlioMIP) and four experiments with CAM4, the intensified African summer monsoon (ASM) in the mid-Piacenzian and corresponding mechanisms are analyzed. The results from PlioMIP show that the ASM intensified and summer precipitation increased in North Africa during the mid-Piacenzian, which can be explained by the increased net energy in the atmospheric column above North Africa. Further experiments with CAM4 indicated that the combined changes in the mid-Piacenzian of atmospheric CO2 concentration and SST, as well as the vegetation change, could have substantially increased the net energy in the atmospheric column over North Africa and further intensified the ASM. The experiments also demonstrated that topography change had a weak effect. Overall, the combined changes of atmospheric CO2 concentration and SST were the most important factor that brought about the intensified ASM in the mid-Piacenzian.
基金the Research Fund of the Laboratory for Climate Studies,China Meteorological Administration under Grant No.LCS-2006-01the Ministry of Science and Technology Project under No.2007BAC03A01Special Fund on Climate Change of China Meteorological Administration under Grant No.CCSF2007-2
文摘The response of the Asian-African summer monsoon (AASM) to the fast global warming in the 1980s is studied based on several datasets, which span a long time period of nearly 100 yr, with two special periods 1980-1985 and 1990-1995 being focused on. Wavelet analyses are employed to explore the interdecadal variations of the AASM. It is found that after the mid 1980s, the global annual mean surface temperature rises more significantly and extensively over most parts of the African Continent, north of the Indian Ocean, and the Eurasian Continent excluding the Tibetan Plateau. Correspondingly, the global precipitation pattern alters with in- creased rainfall seen over the Sahel and North China in 1990-1995, though it is not recovered to the level of the rainy period before the mid-1960s. Changes of monsoonal circulations between the pre- and post-1980s periods display that, after the fast global warming of the 1980s, the African summer monsoon intensifies distinctly, the Indian summer mon- soon weakens a little bit, and the East Asian summer monsoon remains almost unchanged. The summer precipitation over the Asian-African Monsoon Belt (AAMB) does not change in phase coherently with the variations of the monsoonal circulations. Wavelet analyses of the land-sea thermal contrast and precipitation over North China and the Sahel indicate that interdecadal signals are dominant and in positive phases in the 1960s, leading to an overall enhanced interdecadal variation of the AASM, although the 1960s witnesses a global cooling. In the 1980s, however, in the context of a fast global warming, interdecadal signals are in opposite phases, and they counteract with each other, leading to a weakened interdecadal variation of the AASM. After the mid-1960s, the AASM weakened remarkably, whereas after the mid-1980s, the AASM as a whole did not strengthen uniformly and synchronously, because it is found that the interannual variations of the AASM in the 1980s are stronger than those in the 1960s, and they superimposed on the