Collaborative Research: Forced Trends in the Tropical Pacific and Global Tropical Cyclones

赤道太平洋西部的“暖池”地区和东部的“冷舌”地区之间的温度差异是显著的：所罗门群岛的游客发现海面温度（SSTs）超过80华氏度，而加拉帕戈斯群岛的游客穿着潜水服去浮潜。自然，温度差异对热带地区的天气和气候有多种影响，其中之一是飓风（也称为台风和热带气旋）相对丰富，西太平洋比世界上任何其他地方都要丰富。对比El Nino事件和La Nina事件，可以看出海温对比的影响，El Nino事件由于冷舌区变暖而减弱海温对比，而La Nina事件则增强海温对比。厄尔尼诺年在西太平洋的tc较少，在太平洋中部和东部的tc较多，而在拉尼娜年则相反。厄尔尼诺和拉尼娜对tc和热带天气和气候的其他方面的影响引出了一个问题：温室气体引起的全球变暖会增强还是减弱赤道太平洋海温对比？证据是混合的，因为气候模式通常显示随着地球变暖而减弱，但观测显示在过去几十年里增强。这种不匹配可能是由于气候系统的内部变异，即使温室气体增加使世界变暖，气候系统也在运作。但也有可能是气候模型的偏差，特别是过于寒冷和过于广泛的冷舌偏差，导致了对温室效应的不正确模拟。该奖项下的工作使用一种称为通量调整的技术来解决冷舌偏差对模拟海温对比变化的可能影响，该技术通过修改大气和海洋之间的热量和水分交换来消除当前气候模拟中的冷舌偏差。然后利用通量调整气候模式模拟海温对比对温室气体增加的响应。这项工作是利用社区地球系统模型第2版（CESM2）进行的。类似厄尔尼诺或类似拉尼娜的海温变暖型对热带气旋的影响很难通过气候模式模拟来评估，因为气候模式的分辨率通常不足以模拟tc。这里采用的一种策略是使用高分辨率大气模式的较短模拟来模拟tc，该模式是在气候模式产生的海温变暖模式上运行的。另一种方法是使用哥伦比亚风险模型（CHAZ），这是由首席研究员（PI）开发的经验TC模型。CHAZ利用来自气候模式或观测数据集的大尺度大气条件来预测基于TC发生指数随机启动的“合成”TC的轨迹和强度。鉴于气候变暖引起的TC行为变化的后果，这项工作具有社会和科学价值。很明显，气候变暖导致最强烈的飓风变得更强、更具破坏性，因此，预测太平洋上热带气旋分布变化的努力对气候适应和减少灾害风险很有价值。通过哥伦比亚世界项目，这些私人投资机构与世界银行、再保险行业和其他相关利益相关者有着密切的联系。他们还就包括气候变化风险在内的气候变化相关主题开展广泛的媒体宣传。此外，该项目还支持一名博士后和一名研究生，从而促进该研究领域的未来劳动力。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。

The temperature contrast between the "warm pool" region of the western equatorial Pacific and the "cold tongue" in the east is remarkable: visitors to the Solomon islands find sea surface temperatures (SSTs) above 80F while visitors to the Galapagos don wetsuits to go snorkeling. Naturally the temperature contrast has a variety of effects on the weather and climate of the tropics, one of which is the relative abundance of hurricanes (also called typhoons and TCs, for tropical cyclones) which are more abundant in the western Pacific than anywhere else in the world. The influence of the SST contrast can be seen in comparisons between El Nino events, in which the contrast weakens due to warming in the cold tongue region, and La Nina events, which strengthen the contrast. El Nino years feature fewer TCs in the western Pacific and more in the central and eastern Pacific, with the opposite pattern in La Nina years.The influence of El Nino and La Nina on TCs and other aspects of tropical weather and climate begs the question: will the equatorial Pacific SST contrast be strengthened or weakened by greenhouse gas-induced global warming? The evidence is mixed since climate models generally show weakening as the earth warms but observations show strengthening over the past few decades. The mismatch could be due to the internal variability of the climate system, which operates even as greenhouse gas increases warm the world. But it is also possible that climate model biases, in particular the bias of a cold tongue which is too cold and too extensive, lead to incorrect simulations of the effects of greenhouse warming.Work under this award addresses the possible effect of the cold tongue bias on the simulated SST contrast change using a technique called flux adjustment, in which the exchange of heat and moisture between the atmosphere and ocean is modified to eliminate the cold tongue bias in present-day climate simulations. The flux-adjusted climate model is then used to simulate the response of the SST contrast to greenhouse gas increases. The work is carried out with the Community Earth System Model version 2 (CESM2).The effects of an El Nino-like or La Nina-like SST warming pattern on tropical cyclones is difficult to assess from climate model simulations because the resolution of climate models is not typically high enough to simulate TCs. One strategy pursued here is to simulate TCs using shorter simulations from a high-resolution atmosphere-only model which is run over the SST warming pattern produced by the climate model. Another is to use the Columbia HAZard model (CHAZ), an empirical TC model developed by the lead Principal Investigator (PI). CHAZ uses large-scale atmospheric conditions from the climate model or observational datasets to predict the tracks and intensities of "synthetic" TCs that are initiated at random based on a TC genesis index.The work has societal as well as scientific value given the consequences of warming-induced changes in TC behavior. It is clear that warming causes the most intense hurricanes to become stronger and more destructive, thus efforts to anticipate changes in the distribution of TCs across the Pacific are valuable for climate adaptation and disaster risk reduction. The PIs have strong ties to the World Bank, the reinsurance industry, and other relevant stakeholders through the Columbia World project. They also conduct extensive media outreach on climate change-related topics including TC risk. In addition, the project supports a postdoctoral fellow and a graduate student, thereby promoting the future 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.