Hadley Cell and Subtropical Jet: Dynamics and Tracer Transport

哈德利环流（HCs，每个半球各一个）是一种行星翻转环流，在这种环流中，空气在赤道附近的深积云中上升，在亚热带上空向两侧下沉。下沉有利于晴朗的天空，助长了撒哈拉沙漠、喀拉哈里沙漠和美国西南部等地区炎热干燥的气候。对未来气候变化的模式模拟一致表明，在21世纪，碳含量可能会增加一个纬度。最近的研究表明，HC的膨胀已经发生，而且比模型模拟所显示的要大。HC的扩展对水资源有着明确的影响，因此是一个既具有实际意义又具有科学意义的话题。除了对热带和亚热带气候的影响外，碳碳化合物还通过输送水分、能量、角动量和化学成分在全球发挥着重要作用。热带涡旋的角动量输运是各半球热带涡旋外缘产生副热带急流的主要原因。在stj之外，还有涡旋驱动喷流（edj），它们的存在是由于中纬度南北温度对比和相关的天气模式。hc的扩张预计将导致edj和stj向极地移动，这意味着两个半球的纬度更高。但这些期望并没有完全实现。在每个半球中，HC、EDJ的扩张与副热带干燥带的极向边缘之间存在着密切的联系。另一方面，HC扩张对STJ纬度的影响最多是微弱的，这与基于HC和STJ之间的动态联系的预期相反。此外，基于发射长波辐射的卫星观测对副热带宽度的测量也没有显示出HC扩展的预期特征。本项目研究了这些差异的动力学含义，以及它们对未来气候变化的影响。这里讨论的另一个问题是hc、stj和edj在辐射和化学活性微量气体的运输和行星尺度分布中的作用。特别是，更好地了解这种传输对于了解甲烷、羟基自由基和与臭氧空洞有关的消耗臭氧物质的分布非常重要。以往的研究表明，北半球EDJ的纬度在调节示踪剂向北极的输送中起着重要作用，也有迹象表明，从北半球中纬度向北热带和南半球的输送对HC和STJ的特征很敏感。这些联系在这里通过一系列不同复杂程度的模型模拟来解决。此外，不同模式的模拟对输运的估计差异很大，而且模式也倾向于高估示踪剂在大气中停留的时间长度。这些差异及其动力学原因通过参与化学-气候模式倡议的模式的模拟来检验。化学-气候模式倡议是一项协调的国际努力，旨在产生具有可直接比较的共同协议的模拟。如上所述，由于干旱区扩大对水资源的影响，这项工作具有更广泛的影响。由于远距离示踪剂运输在确定当地空气质量方面的作用，以及了解臭氧空洞恢复的潜在障碍，因此对示踪剂运输的工作也引起了社会的兴趣。此外，该项目还通过为两名研究生提供支持和培训来促进劳动力发展。学生们被鼓励参加各种形式的外展活动，包括外展到K-12学校和约翰霍普金斯大学的成人教育课程。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 Hadley Cells (HCs, one for each hemisphere) are planetary overturning circulations in which air rises in deep cumulus clouds near the equator and subsides over the subtropics to either side. The subsidence favors clear skies and promotes the hot, dry conditions found in regions like the Sahara, the Kalahari, and the Southwestern United States. Model simulations of future climate change consistently show an expansion of the HCs, by perhaps a degree of latitude over the 21st century. Recent studies show evidence that HC expansion has already taken place and is greater than model simulations would suggest. HC expansion has clear implications for water resources and is thus a topic of practical as well as scientific interest.Beyond their impact on the climate of the tropics and subtropics the HCs play an important global role through their transports of moisture, energy, angular momentum, and chemical constituents. The angular momentum transport of the HCs is responsible for the subtropical jets (STJs) which occur at the outer edge of the HC in each hemisphere. Beyond the STJs there are eddy-driven jets (EDJs), which owe their existence to the north-south temperature contrast of the middle latitudes and the associated weather patterns. The expansion of the HCs is expected to cause the EDJs and STJs to shift poleward, meaning to higher latitudes in each hemisphere. But these expectations are not quite met. There is a robust association between the expansion of the HC, the EDJ, and the poleward edge of the subtropcal dry zone in each hemisphere. On the other hand, HC expansion has at best a weak effect on STJ latitude, contrary to expectations based on the dynamical connection between HCs and STJs. Also, measures of the width of the subtropics based on satellite observations of outgoing longwave radiation do not show the expected signature of HC expansion. The dynamical implications of these discrepancies are examined in this project, as well as their consequences for future climate change.A further issue addressed here is the role of the HCs, STJs, and EDJs in the transport and planetary-scale distribution of radiatively and chemically active trace gases. In particular, a better understanding of this transport is important for understanding the distribution of methane, the hydroxyl radical, and ozone depleting substances relevant to the ozone hole. Previous research suggests a role for the latitude of the Northern Hemisphere EDJ in regulating tracer transport into the Arctic, and there are also indications that transport from the northern mid-latitudes into the northern tropics and the Southern Hemisphere is sensitive to characteristics of the HC and the STJ. These linkages are addressed here in a suite of simulations from models at varying levels of complexity. In addition, transport estimates differ widely among simulation from different models, and models also tend to overestimate the length of time for which tracers remain in the atmosphere. These differences and their dynamical causes are examined using simulations from models participating in the Chemistry-Climate Model Initiative, a coordinated international effort to produce simulations with a common protocol that can be directly compared.The work has broader impacts due to the water resource implications of dry zone expansion, as noted above. The work on tracer transport is also of societal interest due to the role of long-range tracer transport in determining local air quality, and for understanding potential obstacles to the recovery of the ozone hole. In addition, the project promotes workforce development by providing support and training for two graduate students. The students are encouraged to participate in various forms of outreach, including outreach to K-12 schools and adult education classes at Johns Hopkins.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.