Sahel Seasonal to Decadal Climate Variability and Atmosphere/Surface Two-Way Interactions

萨赫勒地区从20世纪50年代的潮湿气候到70年代和80年代的干燥气候，再到90年代以来的部分恢复，这是20世纪地球上最强烈的年代际信号之一。研究表明，萨赫勒气候变率受到外部强迫的强烈影响：海表温度（SST）、陆地表面过程和气溶胶。然而，具有单一外部强迫（例如海温）的一般环流模式（GCM）研究无法重现萨赫勒气候的完整的季节、年际和年代际变率和异常（SIDVA）。在这些研究中，大多数都指定了海温、陆地和气溶胶条件，即只有单向相互作用。外部强迫的相对作用及其在萨赫勒气候SIDVA中的双向相互作用从未得到定量和全面的研究。本项目将利用大气GCM/生物物理模式/动态植被模式进行一套60年的耦合模拟，并利用大气-海洋GCM/生物物理模式/动态植被模式进行初步试验，结合双向反馈过程，探索萨赫勒气候SIDVA的成因，评估海洋和陆地的相对贡献，包括人为引起的土地利用和土地覆盖（LULC）变化。最近可用的数据将用于指定LULC变化。此外，一个区域气候模式（RCM）将使用选定年份的大气GCM模拟作为横向边界条件，用于评估萨赫勒气候SIDVA中地表-大气相互作用的区域细节。这项工作将检验rccm是否能提供额外的亚季节或年代际预测技能，以及它们是否能够提供粗糙尺度GCM预测中无法提供的补充、熟练的区域信息。这项跨学科的研究将检验一个假设，即全面了解西非季风（WAM） SIDVA及其机制需要综合调查，包括多主要外部强迫及其在气候模式中的全部反馈。来自卫星和野外测量的数据，特别是来自非洲季风多学科分析（AMMA）项目的数据以及其他来源的数据将用于模式模拟/预测的评估和规范。模型模拟的降水、土壤湿度、植被状况、海温等与萨赫勒气候SIDVA相关的主要特征和分量，以及它们之间的相互关系，将与不同来源的数据进行综合分析和比较。该项目旨在通过WAM气候模拟中关键非线性反馈过程的表现，全面了解不同的地球过程和人类活动如何在上个世纪的大部分时间里对萨赫勒气候变率和异常做出了贡献。这个项目有更广泛的影响。了解萨赫勒地区气候的季节、年际和年代际变化和异常已被国际项目认为是全球气候研究的关键组成部分。独特的气候特征、萨赫勒气候与外部强迫之间的强耦合、关于萨赫勒气候异常（尤其是干旱）原因的争议以及萨赫勒气候SIDVA对社会和经济的重大影响使该地区对科学界具有极其重要的意义。该项目将解决萨赫勒气候研究中的一些关键科学问题，并将有助于确定萨赫勒降雨变化归因于人为和自然驱动因素以及气候变化对生态系统和水资源的影响。这项研究的结果不仅将进一步加深我们对萨赫勒气候的理解，而且还将通过区域缩小尺度为社会规划提供有用的信息，这对脆弱的萨赫勒经济至关重要。它们将通过全球能源和水循环实验水文气候学小组（GEWEX GHP）数据库和非洲气象应用促进发展中心进行分发。

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).