Collaborative Research: The Atlantic Meridional Overturning Circulation and Internal Climate Variability

合作研究：大西洋经向翻转环流和内部气候变率

• 批准号: 1558837
• 负责人: Benjamin Kirtman
• 金额: $ 35.55万
• 依托单位: University of Miami
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-08-01 至 2019-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1558837&HistoricalAwards=false
• 关键词: Collaborative; Research; Atlantic; Meridional; Overturning

The Atlantic meridional overturning circulation (AMOC) is a global ocean circulation in which colder and denser surface water sinks in the subpolar North Atlantic and flows southward at depth, reaching Antarctica and circulating through the world oceans until it eventually resurfaces and returns north from the high southern latitudes. The AMOC transports a substantial amount of heat from the Southern Hemisphere and the tropics into the North Atlantic, and variations of the AMOC are thought to be implicated in long-term variations in North Atlantic sea surface temperature (SST). The SST variations have impacts on the frequency and intensity of Atlantic hurricanes, prolonged drought in the Sahel, north-south shifts of the intertropical convergence zone (ITCZ), and rainfall changes over much of the globe including the US. This project seeks to understand the basic dynamics which drive AMOC variability as well as its effects on SST and surface climate, focusing in particular on the extent to which the slow variations of the AMOC are driven by "weather noise", the short-term and somewhat impulsive forcing of the ocean by the passage of surface weather systems. Despite its impulsive nature forcing by weather systems can induce a low-frequency ocean response due to the reddening effect of upper ocean thermal inertia. The PIs have developed a method to quantify the effect of weather noise in climate model simulations in which an ensemble of atmospheric models are coupled to a single ocean model. The surface fluxes through which the atmosphere affects the ocean are averaged over all the ensemble models before applying them to the ocean, so the ocean only feels the effects of the ensemble mean fluxes. Each atmospheric model produces its own weather systems which are unrelated to the weather generated in the other models, thus the weather noise averages out in the ensemble mean and does not affect the ocean. Here the PIs employ this strategy, which they refer to as an Interactive Ensemble (IE), using the Community Earth System Model (CESM). The IE-CESM is an updated version of the IE system they developed under previous funding (AGS-1137902/1137911), and the ensemble includes the land surface and sea ice component models as well as the atmosphere model.The PIs assess the role of weather noise in generating AMOC variability through comparisons between IE-CESM simulations and control runs using the standard CESM (in which weather noise is not removed). Additional ocean-only simulations are used to assess the role of internal ocean variability in AMOC variability. Additional simulations use a hybrid IE (HyIE) configuration, in which atmospheric forcing from the IE is used in some regions but forcing from a single atmospheric model is used in others. Weather noise is thus applied only over specified regions, and the local and remote responses of the regionally confined noise forcing can be evaluated. One hypothesis to be tested is that the AMOC responds primarily to weather noise forcing over the Labrador sea where most of the deep water formation occurs. Alternatively, remote noise forcing could affect the AMOC, which is trapped along the western boundary, through the generation of westward-propagating oceanic Rossby waves. The response of the surface climate to AMOC variability is also considered, with particular attention to the north-south shifts of the intertropical convergence zone (ITCZ) that are the expected consequence of changes in cross-equatorial transport accompanying AMOC fluctuations.As noted above AMOC variability is a matter of practical as well as scientific interest, given its association with surface climate effects such as drought and hurricane activity. In addition, the PIs are working with the CESM developers to make the IE framework available to the broader research community. The project also supports two graduate students, thereby providing for the future work force in this research area.

大西洋经向翻转环流（AMOC）是一种全球性海洋环流，其中较冷且较稠密的地表水沉入北大西洋副极地并向南深处流动，到达南极洲并在世界海洋中循环，直到最终重新浮出水面并从高纬度返回北方。南部纬度。 AMOC 将大量热量从南半球和热带地区输送到北大西洋，AMOC 的变化被认为与北大西洋海面温度 (SST) 的长期变化有关。 海温变化对大西洋飓风的频率和强度、萨赫勒地区的长期干旱、热带辐合带（ITCZ）的南北变化以及包括美国在内的全球大部分地区的降雨变化产生影响。该项目旨在了解驱动 AMOC 变化的基本动态及其对海温和地表气候的影响，特别关注 AMOC 缓慢变化在多大程度上由“天气噪声”、短期和地表天气系统的通过对海洋产生一定程度的冲击力。 尽管天气系统具有脉冲性质，但由于上层海洋热惯性的红化效应，天气系统的强迫可能会引起低频海洋响应。 PI 开发了一种方法来量化气候模型模拟中天气噪声的影响，其中将一组大气模型与单个海洋模型耦合。 大气影响海洋的表面通量在应用于海洋之前在所有集合模型上进行平均，因此海洋仅感受到集合平均通量的影响。每个大气模型都会产生自己的天气系统，这些系统与其他模型中生成的天气无关，因此天气噪声在集合平均值中平均，不会影响海洋。 在这里，PI 采用了这种策略，他们将其称为交互式集成 (IE)，并使用社区地球系统模型 (CESM)。 IE-CESM 是他们在之前的资助下开发的 IE 系统 (AGS-1137902/1137911) 的更新版本，该集合包括陆地表面和海冰成分模型以及大气模型。 PI 评估了通过比较 IE-CESM 模拟和使用标准 CESM 的控制运行（其中未消除天气噪声），了解天气噪声在生成 AMOC 变化方面的影响。 额外的仅海洋模拟用于评估内部海洋变化在 AMOC 变化中的作用。 其他模拟使用混合 IE (HyIE) 配置，其中某些区域使用 IE 的大气强迫，而其他区域则使用单一大气模型的强迫。 因此，天气噪声仅应用于指定区域，并且可以评估区域限制噪声强迫的本地和远程响应。 需要测试的一个假设是，AMOC 主要对迫使拉布拉多海上空的天气噪声做出反应，拉布拉多海是大部分深水形成的地方。另外，远程噪声强迫可能会通过产生向西传播的海洋罗斯贝波来影响被困在西部边界的AMOC。还考虑了地表气候对 AMOC 变化的响应，特别关注热带辐合带 (ITCZ) 的南北变化，这是伴随 AMOC 波动的跨赤道输送变化的预期结果。如上所述 AMOC鉴于其与干旱和飓风活动等地表气候影响的关联，变异性既具有实际意义，又具有科学意义。 此外，PI 正在与 CESM 开发人员合作，使 IE 框架可供更广泛的研究社区使用。 该项目还支持两名研究生，从而为该研究领域的未来劳动力提供支持。