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

大西洋子午线翻转环流(AMOC)是一种全球海洋环流，在这种环流中，更冷、更密集的表层水在北大西洋次极地下沉，向南深流，到达南极洲，在世界海洋中循环，直到最终重新浮出水面，从南半球高纬度返回北方。AMOC将大量热量从南半球和热带输送到北大西洋，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)的南北移动以及包括美国在内的全球大部分地区的降雨量变化都有影响。该项目试图了解驱动大气温度变化的基本动力及其对海温和地表气候的影响，特别侧重于大气温度的缓慢变化在多大程度上是由“天气噪声”驱动的，“天气噪声”是地面天气系统通过时对海洋的短期和某种程度的脉冲强迫。尽管天气系统的强迫具有冲动性，但由于上层海洋热惯性的变红效应，它可以引起低频率的海洋反应。PI已经开发了一种方法来量化气候模型模拟中天气噪声的影响，在该方法中，将一组大气模型耦合到一个单一的海洋模型。在将大气影响海洋的表面通量应用于海洋之前，对所有集合模式进行平均，因此海洋只感受到集合平均通量的影响。每个大气模式产生自己的天气系统，与其他模式产生的天气无关，因此天气噪声平均在集合平均中，不影响海洋。在这里，PI使用这种策略，他们将其称为交互式集合(IE)，使用社区地球系统模型(CESM)。IE-CESM是他们在以前的资助(AGS-1137902/1137911)下开发的IE系统的更新版，集合包括陆面和海冰成分模式以及大气模式。PI通过IE-CESM模拟和使用标准CESM(其中没有去除天气噪声)的控制运行的比较来评估天气噪声在产生大气有机化合物变率中的作用。额外的海洋模拟被用来评估内部海洋变率在AMOC变率中的作用。其他模拟使用混合IE(HyIE)配置，其中在某些区域使用来自IE的大气强迫，而在其他区域使用来自单一大气模式的强迫。因此，天气噪声仅应用于指定的区域，并且可以评估区域受限噪声强迫的本地和远程响应。一个需要检验的假设是，AMOC主要对拉布拉多海上空的天气噪音做出反应，拉布拉多海是大多数深水形成的地方。或者，远程噪声强迫可能会通过产生向西传播的海洋Rossby波来影响被困在西部边界的AMOC。此外，还考虑了地面气候对AMOC变化的响应，特别注意热带辐合带(ITCZ)的南北变化，这是伴随AMOC波动的越赤道输送变化的预期结果。如上所述，鉴于其与干旱和飓风活动等地面气候影响的关联，AMOC变化是一个具有实际意义和科学意义的问题。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。该项目还资助了两名研究生，从而为这一研究领域的未来工作提供了资金。

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.