Dynamics of Large-scale Monsoons and their Response to Climate Changes

季风是地球大气大尺度环流的显著特征，维持着热带和亚热带地区人口密集、快速增长和社会脆弱的地区。该项目旨在提高对季风所涉及的基本过程的理解，并调节季风对气候变化的反应。该项目的前提是，季风可以被视为热带对流带及其相关的大尺度翻转环流的季节性迁移，并且这种对流驱动的翻转的基本动力学可以从角动量和能量平衡约束的角度来理解。在这个前提下，研究首先在简化的“水行星”模拟中检查季节性迁移对流带的行为，在这种模拟中，地球表面完全被海洋覆盖，没有陆地-海洋对比。在这些模式模拟之后，对理想大陆进行进一步的模拟，最后将这些模拟结果与气候模式比较项目第5版（CMIP5）的实际气候模拟结果进行比较。需要解决的具体问题包括季风的相对突然爆发，模式模拟中发现的季风爆发随着全球平均温度升高而延迟，以及全球平均温度与翻转环流强度之间的关系。该项目的目标是：1)调查季风在各种气候条件下的变化，以便将过去、现在和未来的变化置于一个广泛的背景下；2)确定在复杂性逐渐增加的模拟中季风是如何变化的；从没有陆地块的水行星模拟到具有理想大陆几何形状的模拟；3)利用从理想化模拟中产生的概念理解来评估最先进的环流模型，并更好地约束未来的预测。由于季风对世界人口稠密地区的农业和水资源的重要性，这项工作具有更广泛的影响。了解季风如何随着气候变化而变化既是气候科学的一个优先事项，也是一个主要挑战，目前的工作可能有助于提高我们解释和评估气候模式对季风变化的预测的能力，并改进用于这种预测的模式。此外，该项目将支持和培训一名研究生，从而为该领域的下一代研究人员提供支持，并将接待三名本科生进行暑期研究。

Monsoons are prominent features of the large-scale circulation of the Earth's atmosphere, sustaining densely populated, rapidly growing and socially vulnerable regions in the tropics and subtropics. This project seeks to improve understanding of the essential processes that are involved in monsoons and that mediate their response to climate change. The premise of the project is that monsoons can be viewed as seasonal migrations of zones of tropical convection and their associated large-scale overturning circulations, and that the fundamental dynamics of such convectively-driven overturning can be understood in terms of angular momentum and energy balance constraints. Under this premise the research begins by examining the behavior of seasonally migrating convection zones in simplified "aquaplanet" simulations in which the earth's surface is entirely covered by oceans and there are no land-sea contrasts. These model simulations are followed by further simulations with idealized continents, and finally results from these simulations are compared with realistic climate simulations from the Climate Model Intercomparison Project version 5 (CMIP5). Specific issues to be addressed include the relatively sudden onset of the monsoon, the delay of monsoon onset with increasing global mean temperature found in model simulations, and the relationship between global mean temperature and the strength of the overturning circulation. Goals of the project are to 1) investigate how monsoons change in a wide range of climates, in order to place past, present and future changes in a broad context; 2) determine how monsoons change in simulations in which complexity is progressively increased; starting from aquaplanet simulations with no land-masses to simulations with idealized continental geometries; and 3) use the conceptual understanding emerging from the idealized simulations to evaluate state-of-the-art general circulation models, and better constrain future projections.The work has broader impacts due to the importance of monsoons for agriculture and water resources in densely populated regions of the world. Understanding how monsoons change with changing climate is both a priority and a major challenge in climate science, and the present work may contribute both by improving our ability to interpret and evaluate projections of monsoon change from climate models, and in improving the models used for such projections. In addition, the project will support and train a graduate student, thereby providing for the next generation of researchers in this area, and three undergraduate students will be hosted for summer research.