Collaborative research: Quantifying Global and Regional Impacts of the Atlantic Meridional Overturning Circulation (AMOC) Slowdown in the 21st (twenty-first) Century

密度较大的液体会下沉到较轻的液体下面，就像沙拉酱中的醋会下沉到油下面一样。这种下沉效应发生在格陵兰岛和冰岛周围的北大西洋，在那里，暖流带来的温暖的地表水在亚北极气候中冷却，密度比下面的水大。冰冷的地表水下沉到很深的地方，然后缓慢地向南流动，在世界各地的海洋中循环，然后重新浮出水面，回到北大西洋。由此产生的大西洋经向翻转环流（AMOC）使整个大西洋盆地的地表水向北移动，深水向南移动。AMOC在其整个长度上向北输送热量，不仅从温暖的热带地区输送到北半球较冷的高纬度地区，而且还穿过赤道，将南半球的热量输送到北半球。对气候对温室气体浓度增加的响应的模拟一致表明，随着气候变暖，AMOC减慢，这是北大西洋北部变暖导致的地表水密度减少的结果。密度降低是水温升高的直接后果，但盐度降低也有贡献，因为冰盖融化和降水增加增加了淡水（密度较低）。此外，温暖的地表空气增加的水分含量减少了蒸发，而减少的蒸发意味着更少的含盐地表水，因为蒸发会带走淡水，留下盐。有观测证据表明AMOC减缓，但记录的时间对于确定的评估来说有些短。AMOC减缓的几个气候影响已经被宣称，包括欧洲降水的变化，北大西洋风暴进入欧洲的增加，厄尔尼诺事件的频率和强度的变化，以及热带辐合带（ITCZ）的南移，热带辐合带通常位于赤道以北的热带海洋上方。许多作者认为，AMOC的减缓是北大西洋变暖空洞（NAWH）的原因，在大多数气候变化模拟中，北大西洋格陵兰岛以南的一个区域变暖最小，甚至略有变冷。进入北大西洋北部的AMOC热输送减少是对变暖空洞的合理解释，但是在全球变暖模拟中，使用一个不允许AMOC减缓的模型来产生NAWH是可能的。该奖项支持的工作使用了一种新颖的方法来研究AMOC减缓的影响及其与NAWH的联系，使用来自四个气候模式集合的模拟。模拟使用了一种新颖的方法，即从北大西洋北部的海洋表面去除足够的盐，以防止由于温室气体引起的变暖而导致的AMOC减缓。利用社区地球系统模型（CESM）的一个版本进行的初步模拟得出，由于AMOC减缓，以及变暖空洞和其他几种气候影响导致的降雨量减少，NAWH出现。但是，单一模型的结果并不是决定性的，因为模型公式的差异会导致模型行为的实质性差异。该项目还包括对AMOC减速导致本地和远程响应的机制的详细检查。由于气候变化对人类的影响以及帮助决策者应对气候变化的需要，这项工作具有社会相关性。主要研究人员在两个博物馆，河滨博物馆和皮博迪自然历史博物馆开展外展活动，包括公开讲座和专题讨论会以及教师发展讲习班。该项目支持一名研究生和一名博士后，从而支持该研究领域的劳动力发展。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。

A denser liquid will sink below a lighter one just as vinegar sinks below oil in a salad dressing. This sinking effect happens in the North Atlantic around Greenland and Iceland, where warm surface water brought by the Gulf Stream cools in the subarctic climate and becomes denser than the water below it. The cold surface water sinks to great depth where it flows slowly southward, circulating throughout the world's oceans before resurfacing and returning to the North Atlantic. The resulting Atlantic Meridional Overturning Circulation (AMOC) moves surface water northward and deep water southward throughout the entire Atlantic basin. The AMOC transports heat northward over its whole length, not just from the warmer tropics to the colder high latitudes of the Northern Hemisphere but across the equator as well, transferring heat from the Southern Hemisphere into the Northern Hemisphere.Simulations of the response of climate to increases in greenhouse gas concentrations consistently show a slowdown of the AMOC as climate warms, a consequence of warming-induced reductions in the density of surface water in the northern North Atlantic. Density reduction is a direct consequence of warmer water temperature but reductions in saltiness also contribute, as fresh water (which is less dense) is added by melting ice sheets and increased precipitation. Also, the increased moisture content of warmer surface air reduces evaporation, and reduced evaporation means less salty surface water since evaporation removes fresh water leaving salt behind. There is observational evidence for AMOC slowdown but the period of record is somewhat short for a definitive assessment.Several climatic effects of AMOC slowdown have been claimed, including changes in precipitation over Europe, an increase in North Atlantic storms tracking into Europe, changes in the frequency and strength of El Nino events, and a southward shift in the intertropical convergence zone (ITCZ), the narrow rain band that typically sits north of the equator over the tropical oceans. Many authors have argued that the AMOC slowdown is responsible for the North Atlantic Warming Hole (NAWH), a region of minimal warming or even slight cooling found in the North Atlantic south of Greenland in most climate change simulations. A reduction in AMOC heat transport into the northern North Atlantic is a reasonable explanation for the warming hole, but it is possible to produce a NAWH in global warming simulations using a model that does not allow AMOC slowdown.Work supported through this award uses a novel methodology to examine the effects of AMOC slowdown and their connection to the NAWH using simulations from an ensemble of four climate models. The simulations use a novel methodology in which just enough salt is removed from the surface ocean in the northern North Atlantic to prevent the AMOC slowdown that would otherwise occur due to greenhouse gas-induced warming. A preliminary simulation performed with a version of the Community Earth System Model (CESM) produces the NAWH as a consequence of AMOC slowdown, along with a reduction in rainfall over the warming hole and several other climatic impacts. But results from a single model are not definitive given that differences in model formulation can lead to substantial differences in model behavior. The project also includes a detailed examination of the mechanisms through which AMOC slowdown causes local and remote responses.The work has societal relevance due to the human impacts of climate change and the need to help decision makers confronting climate change. The Principal Investigators conduct outreach activities at two museums, the Museum of Riverside and the Peabody Museum of Natural History, including public lectures and symposia as well as teacher development workshops. The project supports a graduate student and a postdoctoral fellow, thereby supporting workforce development in this research area.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.