Changes of Rainfall Seasonality and Drought Severity over Amazonia and Their Connections to Global Climate Change

亚马逊雨林的光合作用约占地球陆地光合作用的15%，有人担心，人为的全球变暖可能导致降雨量减少，并使这片可以说是世界上最大的森林“枯死”。这种死亡可能会对亚马逊的生物多样性产生严重影响，也可能导致全球空气中二氧化碳的大幅增加。但是这种枯死是否会发生是高度不确定的，气候模型对全球变暖引起的亚马逊地区降雨变化的预测范围很广。该项目将通过在现有观测期间检查亚马逊地区的降雨变异性和相关动力机制，并确定这些机制在气候模式中正确表示的程度，来解决未来降雨变化的问题。这项工作有三个主要目标：1)在过去几十年里，通过观测确定亚马逊地区的降雨季节性和干旱严重程度是否发生了显著变化，如果发生了变化，是什么机制导致了这些变化；2)确定观测到的降雨变化主要是由于自然气候变率还是人为排放的强迫；3)验证气候模式中控制降雨季节性和干旱严重程度的物理过程，以减少未来亚马逊地区气候变化预估的不确定性。这项工作是基于这样一个假设，即在过去几十年里，由于雨季来得更晚、结束得更早，干旱的严重程度有所增加。另一个进一步的假设是，近几十年来，由于热带东太平洋和热带北大西洋的变暖，亚马逊南部的雨季开始得更晚，雨季结束得更早。据推测，由于南半球副热带急流向北移动和农业燃烧，雨季的延迟也会发生。除了研究对了解和预测亚马逊地区干旱的好处外，该项目还将支持美国和亚马逊地区国家的教育和科学教育的进步。目前的工作将1)通过培训博士生，特别是来自亚马逊国家的女性，并与该地区新成立的气候项目合作，支持亚马逊国家的气候研究和教育项目；2)支持来自德克萨斯州农村的优秀少数民族高中生和本科生学习气候科学；3)向公众传播研究成果和一般气候科学。

The rain forest of the Amazon accounts for roughly 15 percent of the Earth's terrestrial photosynthesis, and there is concern that anthropogenic global warming could lead to reduced rainfall and a "die-back" of what is arguably the world's greatest forest. Such a die-back could have serious implications for the biodiversity of the Amazon, and it could also lead to a substantial increase in global airborne carbon dioxide. But whether or not such die-back will occur is highly uncertain, and climate models show a wide range of projections for global warming-induced rainfall change over the Amazon. This project will address the question of future rainfall change by examing Amazon rainfall variability and associated dynamical mechanisms over the period of available observations, and determining the extent to which these mechanisms are correctly represented in climate models. The work has three main objectives: 1) to determine observationally whether or not rainfall seasonality and drought severity have changed significantly over the Amazon during the past few decades and, if so, what mechanisms have caused these changes; 2) to determine whether the observed rainfall changes are mainly attributable to natural climate variability or to forcing by anthropogenic emissions; 3) to validate the physical processes that control rainfall seasonality and drought severity in climate models, in order to reduce uncertainty in projections of future climate change in the Amazon. The work is based on the hypothesis that drought severity has increased over the past few decades due to more late onsets and early endings of the rainy season. A further hypothesis is that wet season onsets have been occurring later and wet season terminations earlier in recent decades over the southern Amazon due to warming of the tropical esastern Pacific and northern tropical Atlantic. Delays of the wet season onset are also hypothesized to occur as the result of a polward shift of the Southern Hemisphere subtropical jets, and because of agricultural burning.Aside from the benefits of the research for understanding and anticipating drought in the Amazon, the project will support education and the advancement of science education in the United States and in countries of the Amazon region. The present work will 1) support climate research and education programs in Amazonian countries by training PhD students, especially women from Amazonian countries, and collaborating with newly formed climate programs in that region; 2) support outstanding minority high school students from rural Texas and undergraduate students in climate science; 3) communicate research results and general climate science to the public.