Hadley Cell and Subtropical Jet: Dynamics and Tracer Transport

哈德利环流（Hadley Cells，每个半球一个）是行星翻转环流，其中空气在赤道附近的深积云中上升，并在副热带两侧下沉。下沉有利于晴朗的天空，并促进了撒哈拉，卡拉哈里和美国西南部等地区的炎热，干燥的条件。对未来气候变化的模式模拟一致显示，在21世纪，HC的范围可能会扩大一个纬度。最近的研究表明，HC扩张已经发生，并且比模型模拟所显示的要大。HC的扩张对水资源有明显的影响，因此是一个具有实际意义和科学意义的课题。除了对热带和亚热带气候的影响外，HC还通过其水分、能量、角动量和化学成分的输送发挥着重要的全球作用。 HC的角动量输送是每个半球HC外缘副热带急流（STJ）产生的原因。 在STJ之外还有涡驱动急流（EDJ），它们的存在归功于中纬度的南北温度对比和相关的天气模式。 HC的扩张预计会导致EDJ和STJ向极移动，这意味着每个半球的高纬度地区。但这些期望并没有完全实现。 有一个强大的HC，EDJ的扩张之间的关联，并在每个半球的亚热带干燥带的向极边缘。另一方面，HC膨胀最多有一个弱的影响STJ纬度，相反的预期，根据HC和STJs之间的动态连接。此外，基于卫星观测的长波辐射的副热带的宽度的措施没有显示出预期的签名HC扩张。 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。本文进一步讨论了HC、STJ和EDJ在辐射和化学活性痕量气体的输送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洞有关的臭氧消耗物质的分布非常重要。 以前的研究表明，北方半球EDJ的纬度在调节示踪剂输送到北极的作用，也有迹象表明，从北方中纬度到北方热带和南半球的输送是敏感的HC和STJ的特性。这些联系在这里解决了一套模拟模型在不同程度的复杂性。 此外，不同模型的模拟结果对迁移的估计相差很大，而且模型往往会高估示踪剂在大气中停留的时间。 这些差异及其动力学原因是使用参加化学-气候模式倡议的模型的模拟来研究的，该倡议是一项协调的国际努力，使用可以直接比较的共同协议进行模拟，由于干旱区扩展对水资源的影响，这项工作具有更广泛的影响，如上所述。示踪剂迁移方面的工作也具有社会意义，因为远距离示踪剂迁移在确定当地空气质量方面发挥着作用，并有助于了解臭氧洞恢复的潜在障碍。此外，该项目通过为两名研究生提供支助和培训，促进劳动力发展。 鼓励学生参与各种形式的推广活动，包括K-12学校和约翰霍普金斯的成人教育课程。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估来支持。

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.