Zonal Asymmetries in the Low-Latitude Hydrological Cycle over a Broad Range of Climates

从广义上讲，亚热带从多雨的赤道带向极地一侧延伸到西风的中纬度地区，通常以下沉和干燥为特征。 然而，亚热带地区也是世界季风系统的发源地，包括亚洲、西非、北美和南美的季风，因此，在季风季节，其特点是强降雨地区和邻近的旱地之间存在强烈的东西对比。 鉴于世界人口的很大一部分居住在亚热带地区，了解季风区和旱地之间的强烈水文对比将如何随着全球平均气温的变化而变化是有意义的。 气候模式对21世纪气候变化的预测普遍表明，这种反差将加剧，但在加剧的程度上，各模式之间在数量上的一致性很小。 虽然已经提出了几种机制来解释的加剧，一个普遍接受的理论的行为仍然是lack.This项目研究的变化，在亚热带水文对比考虑的变化，在纬向不对称的组成部分的亚热带大气环流，包括不对称的垂直质量通量，这是向上的季风区和向下的旱地。 这种方法不同于以前的工作，强调纬向平均环流（例如，扩大的Hadley细胞和副热带）和解释的能量，大气稳定性，增加水分含量。 它是基于初步的工作，使用一个简化的理想化的气候模式，海洋热通量收敛施加在一个补丁的亚热带地区，以诱导局部温暖的海面温度和降水量增加，这进一步诱导下沉和干燥的条件向西。 根据这些试验结果，提出了三个假设：1）全球平均气候变暖时对流层顶高度增加，增强了定常环流的垂直质量通量，从而增加了降水的纬向不对称性：2）全球平均气候变暖时纬向海表温度梯度减小，减弱了定常环流及其引起的水文差异; 3）假设2的机制适用于陆地，只有地表大气等效位温起着海表温度的作用，陆地地表水收支才起作用。 研究议程包括从观测和模拟中研究当今气候中副热带非对称环流的季节循环和年际变率，然后从气候模式档案中研究未来气候模拟，所有这些都是由简单的理想化模型的结果指导的。这项工作的更广泛的影响来自于世界上居住在亚热带地区的大部分人口，水文循环变化对农业、水资源和其他生活必需品的潜在影响。 发展对水文循环变化的强有力的理论和概念理解，将大大有助于提高对未来气候变化模型预测的信心，并指导未来的模型开发。 该项目还支持和培训一名博士后研究助理和一名研究生，从而为这一研究领域的未来劳动力提供支持。 该项目还支持对当地高中学生的教育和宣传。

Broadly defined, the subtropics extend from the poleward side of the rainy equatorial belt to the midlatitude region of the westerly winds, and are generally characterized by subsidence and dry conditions. Yet the subtropics are also home to the world's monsoon systems, including the Asian, West African, and North and South American monsoons, and are thus characterized during the monsoon seasons by strong east-west contrasts between regions of heavy rainfall and neighboring drylands. Given the substantial portion of the world's population inhabiting the subtropics, it is of interest to understand how the strong hydrologic contrast between monsoon regions and drylands will change in response to changes in global mean temperature. Climate model projections of 21st century climate change generally suggest that such contrasts will intensify, but there is little quantitative agreement from one model to another on the magnitude of the intensification. And while several mechanisms have been proposed to account for the intensification, a generally accepted theory for the behavior is still lacking.This project examines changes in the subtropical hydrological contrasts by considering changes in the zonally asymmetric component of the subtropical atmospheric circulation, including the asymmetric vertical mass flux which is upward over monsoon regions and downward over the drylands. This approach differs from previously work which emphasizes the zonal-mean circulation (e.g. the widening of the Hadley cell and the subtropical belts) and explanations in terms of energetics, atmospheric stability, and increased moisture content. It is based on preliminary work using a simpled idealized climate model in which ocean heat flux covergence is imposed over a patch of the subtropics to induce locally warmer sea surface temperatures and enhanced precipitation, which further induces subsidence and dry conditions to the west. Based on the results of these experiments, three hypotheses are posed: 1) The increase in tropopause height that occurs as global mean climate warms enhances the vertical mass flux in stationary circulations, thereby increasing zoanl asymmetries in precipiation; 2) zonal sea surface temperature gradients decreases as global mean climate warms, weakening stationary circulations and the hydrological contrasts they induce; and 3) the mechanism in hypothesis 2 applies over land, only with surface air equivalent potential temperature playing the role of sea surface temperature, so that the land surface water budget plays a role. The research agenda consists of an examination of the seasonal cycle and interannual variability of subtropical asymmetric circulations in present-day climate from observations and simulations, followed by an examination of future climate simulations from an archive of climate models, all guided by the results of the simple idealized model.The broader impacts of the work come from the substantial portion of the world's population inhabiting the subtropics and the potential impacts of hydrological cycle change on agriculture, water resources, and other necessities of life. The development of a robust theoretical and conceptual understanding of hydrological cycle change would have significant benefit for improving confidence in model projections of future climate change and for guiding future model development. The project also supports and trains a postdoctoral research associate and a graduate student, thereby providing for the future workforce in this research area. The project also supports education and outreach to local high school students.