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

广义上，亚热带从多雨的赤道带的极地一侧延伸到西风的中纬度地区，通常以下沉和干燥条件为特征。然而，亚热带也是世界季风系统的发源地，包括亚洲、西非、北美和南美季风，因此在季风季节，强降雨地区和邻近的旱地之间存在着强烈的东西对比。鉴于世界上有很大一部分人口居住在亚热带地区，有兴趣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温度的变化而变化。气候模型对21世纪气候变化的预测通常表明，这种对比将会加剧，但对于加剧的程度，不同的模型之间几乎没有数量上的一致。虽然已经提出了几种机制来解释这种增强，但对于这种行为的普遍接受的理论仍然缺乏。这个项目通过考虑副热带大气环流的纬向不对称分量的变化来研究副热带水文对比的变化，包括在季风区向上和在旱地向下的不对称垂直质量通量。这种方法不同于以前的工作，以前的工作强调纬向平均环流(例如，Hadley环流和副热带带的加宽)，并从能量学、大气稳定性和增加水分的角度进行解释。它的基础是使用简化的理想化气候模式的初步工作，在该模式中，海洋热通量辐合被施加在亚热带的一小块土地上，以引起当地较暖的海洋表面温度和降水增加，从而进一步导致向西的下沉和干燥条件。根据这些实验的结果，提出了三个假设：1)随着全球平均气候变暖，对流层顶高度增加，从而加强了静止环流中的垂直质量通量，从而增加了降水中的带状不对称性；2)随着全球平均气候变暖，纬向海表温度梯度减小，削弱了静止环流及其引起的水文对比；3)假设2中的机制适用于陆地，只有当地面空气等效位温起到海表面温度的作用时，陆地表层水收支才能发挥作用。研究议程包括通过观测和模拟研究当今气候中副热带不对称环流的季节循环和年际变化，然后研究未来气候模型档案中的气候模拟，所有这些都以简单理想化模型的结果为指导。这项工作的更广泛影响来自世界上居住在亚热带的相当一部分人口以及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，并指导未来模型的发展。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生的教育和推广。

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.