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

广义上讲，亚热带从多雨的赤道带的极地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所在地，包括亚洲、西非、北美和南美季风，因此，在季风季节，亚热带地区的特点是，暴雨地区和邻近的旱地之间存在强烈的东西差异。考虑到世界上很大一部分人口居住在亚热带地区，了解季风区和旱地之间强烈的水文对比将如何随着全球平均温度的变化而变化是很有意义的。气候模式对21世纪气候变化的预估总体上表明，这种差异将会加剧，但不同模式之间对这种加剧程度的定量一致很少。虽然已经提出了几种机制来解释这种加剧，但仍然缺乏一个普遍接受的行为理论。本项目通过考虑副热带大气环流的纬向不对称分量的变化，包括不对称的垂直质量通量（在季风区向上，在旱地向下），来研究副热带水文对比的变化。这种方法不同于先前强调纬向平均环流（例如哈德利环流和副热带带的扩大）的工作，并从能量学、大气稳定性和水分含量增加等方面进行解释。它是基于使用一种简单的理想化气候模式的初步工作，在这种模式中，海洋热通量辐合作用于一块亚热带地区，导致局部海面温度升高和降水增加，进而导致下沉和西部干燥状况。基于这些实验结果，提出了三个假设：1)全球平均气候变暖导致对流层顶高度增加，增加了静止环流中的垂直质量通量，从而增加了降水的区域不对称性；2)随着全球平均气候变暖，纬向海表温度梯度减小，静止环流及其引起的水文对比减弱；3)假设2的机制适用于陆地，只有地表空气等效位温扮演海表温度的角色，从而使陆地表面水收支发挥作用。研究议程包括从观测和模拟中研究当今气候中副热带不对称环流的季节周期和年际变率，然后从气候模式档案中研究未来气候模拟，所有这些都以简单理想模式的结果为指导。这项工作的更广泛影响来自世界上大部分居住在亚热带地区的人口，以及水循环变化对农业、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，对于提高对未来气候变化模式预估的信心和指导未来模式的发展具有重大的好处。该项目还支持和培训一名博士后研究助理和一名研究生，从而为该研究领域的未来劳动力提供支持。该项目还支持当地高中生的教育和外联活动。

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.