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

亚马逊雨林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会导致降雨量减少，可以说是世界上最大的森林“消亡”。这样的死亡可能会对亚马逊的生物多样性产生严重影响，还可能导致全球空气中二氧化碳的大幅增加。但这种回落是否会发生仍是高度不确定的，气候模型显示了对全球变暖导致亚马逊地区降雨量变化的广泛预测。该项目将通过研究亚马逊降雨可变性和相关的可用观测期间的动力机制，并确定这些机制在气候模型中被正确表示的程度，来解决未来降雨变化的问题。这项工作有三个主要目标：1)从观测上确定过去几十年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，如果是，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化还是人为排放的强迫；3)验证气候模型中控制降雨季节性和干旱严重程度的物理过程，以减少对亚马逊未来气候变化预测的不确定性。这项工作是基于这样一种假设，即过去几十年来，由于雨季更晚开始和更早结束，干旱严重程度有所增加。另一种假设是，近几十年来，由于热带东太平洋和热带北部大西洋变暖，亚马逊南部雨季开始较晚，雨季较早终止。据推测，雨季开始的延迟也是南半球副热带急流向两极转移以及农业燃烧的结果。除了了解和预测亚马逊干旱的研究的好处外，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发展。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)支持德克萨斯州农村地区优秀的少数民族高中生和气候科学本科生；3)向公众传播研究成果和普通气候科学。

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.