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: 1558821
• 负责人: Martha Buckley
• 金额: $ 45.11万
• 依托单位: George Mason University
• 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 2020-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1558821&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.

大西洋纬向翻转环流（英语：Atlantic Deconial Overlapping Circulation，缩写为AMOC）是一种全球性的海洋环流，其中温度较低、密度较高的表层水在北大西洋次极地下沉，并在深处向南流动，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南半球高纬度向北返回。 AMOC将大量的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表温度（SST）的长期变化有关。 SST的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）的南北移动以及包括美国在内的地球仪大部分地区的降雨量变化都有影响。该项目旨在了解驱动AMOC变化的基本动力学及其对SST和地表气候的影响，特别关注AMOC的缓慢变化在多大程度上是由“天气噪音”、短期和有点冲动的“天气噪音”驱动的。地表天气系统通过对海洋的强迫。 尽管天气系统的强迫具有脉冲性质，但由于上层海洋热惯性的红化效应，它可以引起低频海洋响应。 研究人员开发了一种方法来量化气候模型模拟中天气噪声的影响，其中大气模型的集合耦合到单个海洋模型。 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，因此海洋只感受到集合平均通量的影响。每个大气模型都产生自己的天气系统，这些天气系统与其他模型中产生的天气无关，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，他们称之为交互式环境（IE），使用社区地球系统模型（CESM）。 IE-CESM是他们在以前的资助下开发的IE系统的更新版本（AGS-1137902/1137911），PIs通过IE-CESM模拟和使用标准CESM的控制运行之间的比较来评估天气噪声在产生AMOC变率中的作用（其中天气噪声未被去除）。 额外的海洋模拟用于评估内部海洋变化在AMOC变化中的作用。 额外的模拟使用混合IE（HyIE）配置，其中大气强迫从IE中使用在某些地区，但强迫从一个单一的大气模式在其他地区。 因此，天气噪声只适用于指定的区域，和本地和远程响应的区域限制的噪声强迫可以进行评估。 有待检验的一个假设是，AMOC主要响应于拉布拉多海上空的天气噪声，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影响AMOC，这是被困沿着西部边界，通过产生向西传播的海洋Rossby波。还考虑了地面气候对AMOC变率的响应，特别注意热带辐合带（ITCZ）的南北移动，这是伴随着AMOC波动的越赤道输送变化的预期结果。如上所述，AMOC变率是一个具有实际意义和科学意义的问题，因为它与干旱和飓风活动等地面气候效应有关。 此外，PI正在与CESM开发人员合作，使IE框架可用于更广泛的研究社区。 该项目还支持两名研究生，从而为这一研究领域的未来劳动力提供支持。