Collaborative Research: Understanding the Links between Tropical Cyclones and Tropical Circulation under Climate Change through Idealized Coupled Climate Modeling

热带气旋（tc），包括大西洋和加勒比海的飓风，其破坏力来自其下方海洋的热量。暖海表温度（SSTs）为暖海表温度提供了燃料，因此表明温室气体排放导致的海洋变暖应该带来更频繁和更具破坏性的暖海表温度。这种预期的一部分是正确的：在最近几十年里，最强的tc变得更强了，这与模型模拟和理论论点相一致。但是，目前还没有令人满意的理论来解释在全球变暖的情况下，TC频率将如何变化，基于模型的研究也显示出不同的结果。Chand等人（2022）最近的研究表明，TC数量下降，作者将其归因于南北走向的Hadley细胞和东西走向的Walker细胞强度的减少。前者的特征是热带深部温暖海温上空的上升空气和较冷的亚热带上空的下沉，而后者的特征是热带西太平洋“暖池”上空的对流和东太平洋“冷舌”上空的下沉。但其他研究表明，倾覆强度的降低应该导致TCs数量的增加而不是减少。该奖项下的研究考虑了翻转细胞在设置TC频率和其他总体特性方面的作用，以及由于温室变暖导致的翻转变化对TC行为的影响。这项工作是使用社区地球系统模型（CESM）的两种理想配置进行的，这两种配置旨在分离感兴趣的机制并以最简单的形式研究它们。第一种配置是“水行星”，有大气与动态海洋模型耦合，但没有陆地表面。在这种配置中，没有太平洋暖池和冷舌的等价物，因此没有类似的Walker单体，尽管低纬度和高纬度之间的海温对比仍然产生Hadley单体。第二种是“山脊”结构，其中一个狭窄的海洋山脊代表了分隔热带大西洋和太平洋的美国大陆。由南北高压脊形成的海洋动力形成了高压脊以西较冷的地表水和东部较暖的海温之间的对比，这反过来又诱发了沃克环流。通过更高分辨率（0.25度网格间距）的模拟，可以更好地捕捉TC行为，从而增强这些配置。由于计算成本的原因，高分辨率模拟使用一个静止的“平板”海洋，其中由水行星和脊结构产生的海温对比是间接施加的，以免影响从海洋到tc的热量传递。鉴于TC的巨大破坏力以及评估温室变暖导致的TC风险变化的必要性，这项工作具有社会和科学意义。通过向CESM模型层次结构添加新的配置，这项工作还具有更广泛的科学影响。本项目开发的模型版本将通过简单模型门户网站（www.cesm.ucar.edu/models/simple）提供给研究界，并将得到文件和参考模拟的支持。此外，该项目还为两名研究生提供支持和培训，从而为该研究领域的未来科学劳动力提供支持。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Tropical cyclones (TCs), including the hurricanes of the Atlantic and Caribbean, derive their destructive power from the heat of the ocean beneath them. The fueling of TCs by warm sea surface temperatures (SSTs) thus suggests that the warming of the oceans by greenhouse gas emissions should bring more frequent and more damaging TCs. One part of this expectation holds true: the strongest TCs have become even stronger in recent decades, in agreement with model simulations and theoretical arguments. But there is no satisfactory theory for how TC frequency will change in a warming world and model-based studies have shown mixed results. The recent study of Chand et al. (2022) shows a decline in TC numbers which the authors attribute to a reduction in the strength of the the north-south oriented Hadley cell and the east-west oriented Walker cell. The former features rising air over the warm SSTs of the deep tropics and subsidence over the cooler subtropics, while the latter features convection over the "warm pool" of the tropical western Pacific and subsidence over eastern Pacific "cold tongue". But other studies suggests that reductions in overturning strength should result in an increase in the number of TCs rather than a decrease.Research under this award considers the role of the overturning cells in setting TC frequency and other aggregate properties and the consequences for TC behavior of changes in overturning due to greenhouse warming. The work is conducted using two idealized configurations of the Community Earth System Model (CESM) which are designed to isolate the mechanisms of interest and study them in their simplest form. The first configuration is an "aquaplanet", with an atmosphere coupled to a dynamic ocean model but no land surface. In this configuration there is no equivalent of the Pacific warm pool and cold tongue and hence no analog for the Walker cell, although the SST contrast between lower and higher latitudes still produces a Hadley cell. The second is a "ridge" configuration, in which a narrow oceanic ridge stands in for the American landmass that separates the tropical Atlantic and Pacific Oceans. The ocean dynamics created by the imposition of a north-south ridge creates a contrast between cooler surface water to the west of the ridge and warmer SST to the east, which in turn induces a Walker circulation. These configurations are augmented by simulations at higher resolution (0.25 degree grid spacing) which are better able to capture TC behavior. For reasons of computational cost the high-resolution simulations use a motionless "slab" ocean in which the SST contrasts generated by the aquaplanet and ridge configurations are imposed indirectly so as not to compromise heat transfer from the ocean to the TCs.The work is of societal as well as scientific interest given the tremendous destructive power of TCs and the need to assess changes in TC risk due to greenhouse warming. The work also has scientific broader impacts through the addition of new configurations to the CESM model hierarchy. The model versions developed in this project will be made available to the research community through the Simpler Models web portal (www.cesm.ucar.edu/models/simple) and will be supported with documentation and reference simulations. In addition, the project provides support and training to two graduate students, thereby providing for the future scientific workforce in this research area.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.