Hadley Cell and Subtropical Jet: Dynamics and Tracer Transport

哈德利环流(HCS，每半球一个)是一种行星翻转环流，在这种环流中，空气在赤道附近深厚的积云中上升，然后在亚热带两侧下沉。下沉有利于晴朗的天空，并促进撒哈拉沙漠、卡拉哈里和美国西南部等地区的炎热干燥条件。对未来气候变化的模型模拟一直表明，在21世纪，气候变化中心的扩张可能会达到一定的纬度。最近的研究表明，HC膨胀已经发生，而且比模型模拟所显示的要大。HC膨胀对水资源有明显的影响，因此是一个具有实用和科学意义的话题。除了对热带和亚热带气候的影响外，HC还通过它们的水分、能量、角动量和化学成分的输送在全球发挥着重要的作用。副热带急流(STJ)的角动量输送是产生副热带急流(STJ)的主要原因，这些急流出现在各半球HC的外缘。除了STJ之外，还有涡流驱动的急流(EDJ)，它们的存在归因于中纬度南北温差和相关的天气模式。HCS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更高纬度。但这些期望并未完全实现。在每个半球中，HC、EDJ的扩展和亚热带干旱区的极地边缘之间有很强的联系。另一方面，HC膨胀对STJ纬度的影响充其量是微弱的，这与基于HC和STJ之间的动力学联系的预期相反。此外，根据卫星观测的出射长波辐射对亚热带宽度的测量也没有显示出预期的HC膨胀的特征。这个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HCS、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层空洞相关的臭氧消耗物质的分布是重要的。以前的研究表明，北半球EDJ的纬度在调节进入北极的示踪剂输送方面发挥了作用，也有迹象表明，从北半球中纬度到北热带和南半球的示踪剂输送对HC和STJ的特征很敏感。这些联系在这里从不同复杂程度的模型中进行了一系列模拟。此外，在不同模型的模拟中，传输估计值差异很大，而且模型也往往高估示踪剂在大气中停留的时间长度。通过参与化学-气候模型倡议的模型的模拟，研究了这些差异及其动力原因。化学-气候模型倡议是一个协调的国际努力，旨在用一个可以直接比较的共同协议来产生模拟。如上所述，由于干旱地带扩张对水资源的影响，这项工作具有更广泛的影响。由于远程示踪剂运输在确定当地空气质量和了解臭氧层空洞恢复的潜在障碍方面的作用，有关示踪剂运输的工作也具有社会意义。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。鼓励学生参加各种形式的外展活动，包括到K-12学校的外展活动和约翰霍普金斯大学的成人教育课程。这一奖项反映了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。

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.