Dramatic change over the Sahel from wet conditions in the 1950s to much drier conditions in the 1970s and the 1980s and then to partial recovery since the 1990s, represents one of the strongest interdecadal signals on Earth in the twentieth century. Studies suggest that Sahel climate variability is strongly influenced by external forcings: sea surface temperature (SST), land surface processes, and aerosols. However, general circulation model (GCM) studies with a single external forcing (for example, SST) have not been able to reproduce full seasonal, interannual, and decadal variability and anomalies (SIDVA) of Sahel climate. In most of these studies, SST, land, and aerosol conditions were specified, i.e., there was only one way interaction. The relative roles of the external forcings and their two-way interactions in the Sahel climate SIDVA have never been quantitatively and comprehensively examined.
This project will conduct a set of 60-year simulations with a coupled Atmospheric GCM/biophysical model/dynamic vegetation model as well as preliminary tests with Atmospheric-Ocean GCM/ biophysical model/dynamic vegetation model, incorporating two way feedback processes, to explore the causes of Sahel climate SIDVA and to assess the relative contribution of ocean and land, including human-induced land use and land cover (LULC) change. Recently available data will be used to specify LULC change. In addition, a regional climate model (RCM) using Atmospheric GCM simulations from selected years as lateral boundary conditions will be applied to assess the regional details of surface-atmosphere interactions in the Sahel climate SIDVA. This work will test whether the RCMs will provide additional subseasonal to decadal prediction skill and if they are able to provide supplementary skillful regional information not available in coarse scale GCM predictions.
This interdisciplinary research will test the hypothesis that a comprehensive understanding of West African Monsoon (WAM) SIDVA and its mechanisms requires synthesized investigation, including multi-major external forcings and their full feedbacks in the climate model. Data from satellite and field measurements, especially from African Monsoon Multidisciplinary Analyses (AMMA) project, and other sources will be applied for evaluation and specification of model simulations/predictions. Model-simulated major features and components relevant to the Sahel climate SIDVA, such as precipitation, soil moisture, vegetation condition, SST, as well as their relationship to each other will be comprehensively analyzed and compared with data from different sources. This project intends to provide a comprehensive understanding of how different earth processes and human activities have contributed to the Sahel climate variability and anomalies over much of the last century through the representation of key non-linear feedback processes in WAM climate simulations. This project has broader impacts. Understanding Sahelian climate seasonal, interannual, and decadal variability and anomaly has been recognized as a critical component in global climate studies by international programs. The distinct climate features, the strong coupling between Sahel climate and external forcings, the controversy on the causes of the Sahel climate anomalies, especially the drought, and significant social and economic implications of Sahel climate SIDVA make this region extremely important for the scientific community. This project will address some of the key scientific issues in the study of Sahelian climate and will contribute to identification of the attribution of Sahel rainfall changes to anthropogenic and natural drivers and the effect of climate variability on ecosystems and water resources. The results from this study will not only further our understanding of the Sahelian climate, but also provide useful information through regional downscaling for social planning, which is crucial for the fragile Sahelian economy. They will be distributed through the Global Energy and Water Cycle Experiment Hydroclimatology Panel (GEWEX GHP) database and the African Centre of Meteorological Application for Development (ACMAD).
Dramatic change in West Africa from wet conditions in the 1950s to severe drought conditions in the 1970s and the 1980s and then to partial recovery since the 1990s, represents one of the strongest interdecadal signals on Earth in the Twentieth century. Studies suggest that Sahel climate variability is strongly influenced by external forcings: sea surface temperature (SST), land surface processes, and aerosols. However, general circulation model (GCM) studies with a single external forcing (for example, SST) have never been able to reproduce full West African monsoon (WAM) variability and the Sahel drought. Moreover, in most of these studies, SST, land, and aerosol conditions were specified, i.e., there was only one way interaction. The relative roles of the external forcings and their two-way interactions in the Sahel climate variability have never been quantitatively and comprehensively examined. This project helps organize the second West African Monsoon Modeling and Evaluation Project experiment (WAMME II), which is the community efforts exploring the roles of all three external forcings (SST, land, and aerosol). The WAMME II ensemble results show that SST effect starts from later winter when the precipitation began moving to the north associated with the ITCZ movement. Pacific Ocean and Indian Ocean play major roles in WAM variability, especially in the WAM withdrew (Pacific) and onset (India) stages. With the maximum possible SST and land use land cover change (LULCC) forcing in the GCM experiments, which are imposed in the WAMME II experiment, SST in the simulation produced about 55% and 65% of seasonal mean precipitation and surface temperature anomaly between the 1980s and the 1950s, respectively, and LULCC produced about 40% and 45% of anomaly, respectively. Since the model groups, which fail to produce the SST impact, tend to not submit the results. This may cause exaggerating SST effects. The most important result from this study is to solidly confirm that the land surface effect produces the compatible effect on the decadal variability as the SST. Using most recent LULCC data, we investigate the climate impact of large-scale LULCC in a GCM study. It is found that LULCC warms land surface at tropics, cool land surface at middle latitudes. Overall, it amplifies surface warming by 0.11K over global land and 0.43K over degraded area. LULCC also causes precipitation reduction globally by 0.15 mm/day over global land and 0.35 mm/day over degraded area, with strongest signals over monsoon regions. For West Africa, during the summer, the temperature warning is 1.15K and precipitation reduction is 0.71 mm/day. The IPCC report indicates the land use change (albedo only) would cause global cooling, inconsistent with our results. This is because in our study, after land degradation, evaporation is reduced, leading to warm surface temperature. The reduction of net radiation due to high surface albedo only plays a secondary role. Although biomass burning constitutes a significant source of tropospheric aerosols in Africa and is thought to modulate regional climate variability and trace gas composition, little effort has been devoted to this research topic. This project conducted the first fire impact study on the West African monsoon evolution. The satellite MODIS data is used to specify area with fire and leaf area index, vegetation fraction coverage, and associated surface albedo are changed based on the survival rate for each vegetation land cover type in the burn area. The study shows that the burning has significant impact on surface energy balance, atmospheric moisture convergence, and WAM precipitation evolution. This project has also developed the fully two-way coupled land, atmosphere, and ocean modeling system. By taking a water-carbon-energy balance approach and through model validation using observational data, the dynamic vegetation model, SSiB4/TRIFFID, in both coupled and offline mode properly produces West African seasonal, interannual, and decadal variability in both atmosphere and ecosystem. Moreover, after developing the coupled ocean/land/atmosphere model, the NCEP Climate Forecast System (CFS)/SSiB, we have extended CFS’ application from seasonal prediction to the decadal climate prediction. This project provides complemental information for the CFS development. Drought is an important societal issue. The Sahel drought was the most severe and long-lasting drought in the world during the last century. Understanding Sahelian climate variability and anomaly has been recognized as a critical component in global climate studies by international programs, such as CLIVAR, GEWEX, AMMA, etc. The results from this study provide useful information for drought prevention, mitigation of the drought impact, and planning to combat the droughts, which is crucial for the fragile Sahelian economy. The results from the WAMME-II have been released through the African Monsoon Multidisciplinary Analyses data base (ftp.bddamma.ipsl.polytechnique.fr,). In addition, the project helps organize the Climate Dynamic Special Issue "West African Climate Decadal Variability and its modeling", expected to be published in late 2015. This project has also trained two post doctors as well as two female graduate students to obtain their degrees.
