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。因此，温暖的海洋表面温度(SSTs)助长了TCS，这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TCS。这种预期有一部分是正确的：最近几十年来，最强的TC变得更加强大，这与模型模拟和理论论证是一致的。但在全球变暖的情况下，TC频率将如何变化，目前还没有令人满意的理论，基于模型的研究结果喜忧参半。Chand等人的最新研究。(2022)显示了TC数量的下降，作者将其归因于南北方向的Hadley环流和东西方向的Walker环流强度的减弱。前者的特征是深热带温暖的海温上的空气上升和较凉爽的副热带上空的下沉，而后者的特征是热带西太平洋的“暖池”上的对流和东太平洋的“冷舌”上的下沉。但其他研究表明，倾覆强度的降低应该导致TC的数量增加，而不是减少。该奖项下的研究考虑了倾覆细胞在设定TC频率和其他聚合特性方面的作用，以及由于温室变暖而导致的TC翻转变化对TC行为的影响。这项工作是使用共同体地球系统模型(CESM)的两种理想化配置进行的，这两种配置旨在分离感兴趣的机制并以其最简单的形式进行研究。第一种构型是一个“水行星”，其大气与一个动态海洋模型相耦合，但没有陆地表面。在这种情况下，没有太平洋暖池和冷舌的对应物，因此也没有类似的Walker环流，尽管低纬度和高纬度之间的SST对比仍然产生了Hadley环流。第二种是“海脊”结构，一条狭窄的大洋海脊代表了分隔热带大西洋和太平洋的美洲大陆。南北海脊形成的海洋动力形成了海脊以西较冷的表层水和东面较暖的海温之间的对比，这反过来又引发了沃克环流。这些配置通过更高分辨率(0.25度网格间距)的模拟得到增强，这些模拟能够更好地捕捉TC行为。由于计算成本的原因，高分辨率模拟使用了静止的“平板”海洋，其中间接施加了由水行星和脊形构型产生的SST对比，以避免影响从海洋到TC的热传递。鉴于TCS的巨大破坏力以及评估由于温室气体变暖而导致的TC风险变化的需要，这项工作具有社会和科学意义。这项工作还通过将新的配置添加到CESM模型层次结构而产生了科学上更广泛的影响。在本项目中开发的模型版本将通过Simple Models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，该项目为两名研究生提供支持和培训，从而为这一研究领域的未来科学队伍提供支持。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.