Collaborative Research: Non-Linearity and Feedbacks in the Atmospheric Circulation Response to Increased Carbon Dioxide (CO2)

应对气候变化的努力开始于一个简单的事实，即向大气中增加二氧化碳（CO2）会使地球仪变暖。 但是，温室效应对人类的影响是区域性的，而不是全球性的，而且还涉及温度以外的其他因素。 其中一个因素是变暖引起的大气环流变化，例如，这可能导致一些地区干旱，而另一些地区变得更容易发生洪水。环流变化对区域气候的重要性对应对气候影响的努力提出了挑战，因为变暖引起环流变化的动力机制尚未得到很好的理解，用于为决策提供信息的模型模拟并未就环流变化的程度达成一致。 另一个考虑是，环流变化的量不一定与全球温度升高的量成比例。 该项目的主要研究者（PI）先前的工作发现了几个非线性行为的例子，例如北方哈德利环流的强度（翻转环流在赤道附近上升，在北方副热带地区下沉）CO2增加一倍时，循环响应会降低，但如果CO2进一步增加到三倍时，循环响应会增加。气候模式之间环流变化差异的原因以及造成环流变化非线性的机制。 非线性的一个机制是大西洋经向翻转环流（AMOC）的迅速减弱，这发生在二氧化碳增加的特定水平上，并通过在北大西洋创造一片较冷的海面温度来引起环流变化。 在不同的气候模式中，AMOC的快速减弱发生在不同的CO2水平上，因此是环流变化中模式间传播的一个来源。 另一种机制是二氧化碳引起的平流层臭氧变化，因为二氧化碳的增加影响平流层臭氧的数量和纬度分布，而臭氧又会影响急流，因为臭氧会引起辐射加热，从而产生上层温度差异。 CO2-臭氧反馈，即CO2影响温度，而温度又影响臭氧，导致进一步的温度变化，直到最近才被认为是气候和环流变化的一个促成因素。 这项研究是通过分析几个模式相互比较项目提供的模式模拟来进行的，沿着的还有利用共同体地球系统模式和戈达德空间研究所气候模式（GISS模式E2.2）产生的新模拟。 与此相关的是，区域气候变化的评估往往假设区域气候变化将与全球气温升高成正比，因此，对环流变化的非线性研究直接关系到区域气候变化的评估。 参与者完全有能力向气候影响界传播其结果，因为他们都是以前评估的作者，包括政府间气候变化专门委员会和世界气象组织的报告。 此外，PI参与社区内的外展计划，包括巴尔的摩的不受影响项目（IP），并为两名研究生提供支持和培训。该奖项反映了NSF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。

Efforts to address climate change begin with the simple fact that adding carbon dioxide (CO2) to the atmosphere warms the globe. But the consequences of greenhouse warming that matter for people are regional rather than global and involve other factors besides temperature. One such factor is warming-induced change in atmospheric circulation, which can, for example, cause some regions to dry out while others become more flood prone. The importance of circulation change for regional climate challenges efforts to address climate impacts since the dynamical mechanisms through which warming induces circulation change are not well understood, and model simulations used to inform decision making do not show strong agreement as to how much circulation change will occur. A further consideration is that the amount of circulation change is not necessarily proportional to the amount of global temperature increase. Previous work by the Principal Investigators (PIs) of this project found several examples of nonlinear behavior, for instance the strength of the Northern Hemisphere Hadley Cell (the overturning cell with rising motion near the equator and subsidence over the Northern subtropics) decreases for a doubling of CO2 but increases if CO2 is further increased to a tripling.Work under this award seeks to understand the circulation response to CO2 increase, focusing specifically on the reasons for differences in circulation change from one climate model to another and the mechanisms responsible for nonlinearity in circulation change. One mechanism for nonlinearity is the rapid weakening of the Atlantic Meridional Overturning Circulation (AMOC), which occurs at a particular level of CO2 increase and causes circulation change by creating a patch of colder sea surface temperatures in the North Atlantic. The rapid weakening of the AMOC happens at different CO2 levels in different climate models and is thus a source of inter-model spread in circulation change. Another mechanism is CO2-induced change in stratospheric ozone, as CO2 increase affects the amount and latitudinal distribution of stratospheric ozone, which in turn can influence the jet stream because ozone causes radiative heating which produces upper-level temperature contrasts. The CO2-ozone feedback, in which CO2 influences temperature which in turn affects ozone causing further temperature change, has only recently been recognized as a contributing factor to climate and circulation change. The research is conducted through analysis of model simulations available through several Model Intercomparison Projects, along with new simulations generated using the Community Earth System Model (CESM) and the climate model of the Goddard Institute for Space Studies (GISS Model E2.2).The work is of societal as well as scientific interest given the influence of circulation change on regional climate change, as noted above. A related consideration is that assessments of regional climate change often assume that regional climate changes will be proportional to global temperature increase, thus research on the nonlinearity of circulation change has direct bearing on regional climate change assessments. The PIs are well positioned to disseminate their results to the climate impacts community as both have been authors on previous assessments including reports of the Intergovernmental Panel on Climate Change and the World Meteorological Organization. In addition, the PIs participate in outreach programs within their communities including the Baltimore Ingenuity Project (IP), and the work provides support and training to two graduate students.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.