SNAP-DRAGON: Subpolar North Atlantic Processes - Dynamics and pRedictability of vAriability in Gyre and OverturNing

SNAP-DRAGON将使我们对将大气变化与亚极地海洋变率联系起来的过程、它们对该地区海洋和气候可预测性的影响，以及我们对它们在气候模式中的代表性的信任程度的理解发生阶级性变化。位于英国、格陵兰岛和加拿大之间的亚极地北大西洋，对当地和全球气候都起着至关重要的作用。这是北大西洋上层向北流动的大部分温水向大气释放热量并转化为寒冷、稠密的水的关键区域，然后再次向南流动，形成所谓的大西洋翻转环流。这种环流向北携带并释放到大气中的大量热量影响了决定西欧天气的风暴路径。翻转环流还通过对低纬度海面温度的影响对非洲降雨和飓风统计数据产生深远影响。此外，亚极地地区的海水下沉为深海通风，将热量和碳从地表转移出去，并减缓了人为温室气体对地表温度的影响。在亚极地地区没有下沉的任何温水都会再循环或将其热量向北带到北极，在它下沉并向南流动之前影响海冰状况和极地海洋生态系统。最近，北大西洋次极翻转方案（opsnap, www.o-snap.org）首次观测到了北大西洋次极翻转环流。这些研究揭示了翻覆过程的大幅度变化，但也提出了一些问题，如引起这种变化的位置和过程，以及它对海洋表面条件和气候的可能影响。如果我们要在季节、年际、年代际和更长的时间尺度上作出有用的气候预测，在气候模式中适当地表示该地区是必不可少的。然而，目前这一代的模型很难表现出我们所知道的重要过程，并且与对温水转变为致密水的位置的观察结果不一致。这些分歧限制了我们对模型预测的信心。我们不能正确地评估模式的性能，因为我们不了解大气条件和海洋环流变异性之间的所有联系。在SNAP-DRAGON项目中，我们将把ocsnap和其他观测结果与能够代表小尺度过程的数值模式结合起来，找出导致亚极地海洋环流变化的原因。一旦我们知道哪些过程是最重要的，以及它们是如何工作的，我们就能够确定哪些气候模型出错了，并提出改进建议。这将改进对北大西洋次极地及其他地区海洋和气候变化的预测。我们将研究寒冷稠密的海水是如何进入向南输出的边界流的。我们将确定风所扮演的角色，这可能比我们过去所假设的要复杂得多。我们将确定对北极和格陵兰岛淡水输出变化的颠覆性的影响。为了描述和量化这些关键过程，除了使用海洋观测外，我们还将在海洋模型中进行“如果”实验，提出诸如：如果大西洋上空的大气急流转移或加强，亚极地海洋环流会发生什么？我们将使用在天气预报中更常见的统计方法来弄清楚亚极地海洋特性和倾覆如何与潜在可预测的大尺度大气环流模式联系起来。我们将采用创新的方法，将模式与观测相结合，以确定对ocsnap观测期间北大西洋次极区演变的最佳估计。

SNAP-DRAGON will produce a step change in our understanding of the processes that link atmospheric changes to subpolar ocean variability, their implications for ocean and climate predictability in this region, and the degree to which we can trust their representation in climate models.The subpolar North Atlantic Ocean, stretching between the UK, Greenland and Canada, plays a crucial role in local and global climate. This is the critical region where much of the warm water flowing northward in the upper North Atlantic releases its heat to the atmosphere and is converted to cold, dense water, before flowing southward again at depth in what is known as the Atlantic overturning circulation. The huge amount of heat this circulation carries northward and releases to the atmosphere impacts the storm track that determines the weather over western Europe. The overturning circulation also has profound implications for African rainfall and hurricane statistics via its effect on sea surface temperatures at lower latitudes. In addition, the sinking of water in the subpolar region ventilates the deep ocean, transferring heat and carbon away from the surface and moderating the impact of anthropogenic greenhouse gases on surface temperature. Any warm water which does not sink in the subpolar region recirculates or carries its heat further north towards the Arctic, influencing sea-ice conditions and polar marine ecosystems before it too sinks and flows south.Recently, the first ever observations of the overturning circulation in the subpolar North Atlantic have been made by the Overturning in the Subpolar North Atlantic Programme (OSNAP, www.o-snap.org). These have revealed large amplitude variations in the overturning, but raised questions about the locations and processes that give rise to this variability, and its likely impact on surface ocean conditions and climate.Representing this region properly in climate models is essential if we are to make useful climate predictions on seasonal, interannual, decadal and longer timescales. However, the current generation of models struggle to represent the processes we know to be important here, and disagree with the observations on the locations in which warm water is transformed into dense water. The disagreements limit our confidence in model predictions. We cannot assess model performance properly because we do not understand all the links between atmospheric conditions and ocean circulation variability.In SNAP-DRAGON we will combine OSNAP and other observations with numerical models that can represent small-scale processes to work out what causes variations in subpolar ocean circulation. Once we know which processes are most important and how they work, we will be able to establish what climate models are getting wrong, and suggest improvements. This will improve predictions of ocean and climate variability in the subpolar North Atlantic and beyond.We will investigate how cold, dense waters find their way into the boundary currents that export them to the south. We will establish the role that winds play, which is likely more complicated than we have assumed in the past. And we will determine the impact on overturning variability of changes in freshwater export from the Arctic and Greenland. To characterize and quantify these key processes, in addition to using ocean observations, we will perform "What if?" experiments in ocean models, asking questions such as: what happens to the subpolar ocean circulation if the atmospheric jet stream over the Atlantic shifts or strengthens? We will use statistical methods more common in weather forecasting to figure out how subpolar ocean properties and overturning connect to potentially predictable larger-scale atmospheric circulation patterns. And we will employ innovative ways of combining models with observations to determine a best estimate of the evolution of the subpolar North Atlantic over the OSNAP observation period.