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始于一个简单的事实，即向大气中添加二氧化碳(CO2)会使全球变暖。但对人类来说至关重要的温室气体变暖的后果是区域性的，而不是全球性的，涉及到温度以外的其他因素。其中一个因素是气候变暖导致的大气循环变化，例如，这可能导致一些地区变得干旱，而另一些地区则变得更容易发生洪水。环流变化对区域气候的重要性挑战了应对气候影响的努力，因为气候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也没有显示出关于环流变化将发生多少的强烈共识。进一步考虑的是，环流变化量并不一定与全球气温升高的量成正比。该项目的首席调查员(PI)之前的工作发现了几个非线性行为的例子，例如北半球哈德利环流(翻转的环流在赤道附近上升，并在北亚热带上空下沉)的强度随着二氧化碳的加倍而减少，但如果二氧化碳进一步增加到三倍，则会增加。该奖项的工作旨在了解环流对二氧化碳增加的响应，特别关注不同气候模式环流变化差异的原因，以及环流变化非线性的机制。非线性的一个机制是大西洋子午线翻转环流(AMOC)的迅速减弱，这种环流在特定的二氧化碳增加水平上发生，并通过在北大西洋产生一片较冷的海面温度而导致环流变化。AMOC的迅速减弱发生在不同气候模式中的不同二氧化碳水平，因此是环流变化中模式间传播的来源。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了平流层臭氧的数量和纬度分布，而平流层臭氧又会影响急流，因为臭氧会产生辐射加热，从而产生高层温度对比。二氧化碳-臭氧反馈，即二氧化碳影响温度，进而影响臭氧，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比较项目提供的模式模拟以及利用共同体地球系统模式(CESM)和戈达德空间研究所的气候模式(GISS模式E2.2)产生的新模拟进行的。一个相关的考虑是，区域气候变化评估往往假设区域气候变化将与全球气温上升成正比，因此对环流变化非线性的研究直接影响到区域气候变化评估。由于这两个机构都是政府间气候变化专门委员会和世界气象组织报告等以往评估的起草人，因此它们能够很好地向气候影响界传播其结果。此外，PIs还参与了社区内的外展项目，包括巴尔的摩创意项目(IP)，这项工作为两名研究生提供了支持和培训。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。

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.