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

Snap-Dragon将在我们对将大气变化与亚极地海洋变异性联系起来的过程、它们对该地区海洋和气候可预测性的影响，以及我们在气候模型中可以信任它们的程度的理解上产生重大变化。亚极地北大西洋横跨英国、格陵兰和加拿大，在当地和全球气候中发挥着至关重要的作用。这是一个关键区域，北大西洋上层向北流动的大部分暖水将其热量释放到大气中，并转化为冷水和稠密水，然后再次向南流动，形成所谓的大西洋翻转环流。这种环流向北携带的巨大热量释放到大气中，影响了决定西欧天气的风暴路径。翻转的环流还通过对较低纬度的海洋表面温度的影响，对非洲的降雨量和飓风统计产生深远的影响。此外，亚极区的水下沉使深海通风，将热量和碳从表面转移出去，并缓和了人为温室气体对表面温度的影响。任何不在亚极区下沉的暖水再循环或将其热量进一步向北带向北极，在它下沉并向南流动之前影响海冰条件和极地海洋生态系统。最近，亚极区北大西洋计划的翻转首次观测到北极区环流的颠覆。这些都揭示了倾覆过程中的巨大幅度变化，但也提出了关于这种变化的位置和过程，以及它可能对表层海洋条件和气候产生的影响的问题。如果我们要对季节、年际、年代际和更长的时间尺度做出有用的气候预测，在气候模型中适当地表示该地区是至关重要的。然而，当前一代模型很难代表我们所知道的在这里很重要的过程，并且与关于温水转化为稠水的位置的观察结果不一致。分歧限制了我们对模型预测的信心。我们无法正确评估模式的性能，因为我们不了解大气条件和海洋环流变化之间的所有联系。在SNAP-Dragon中，我们将把OSNAP和其他观测与能够代表小范围过程的数值模式结合起来，找出是什么导致了亚极地海洋环流的变化。一旦我们知道了哪些过程是最重要的，以及它们是如何运作的，我们就能够确定哪些气候模型是错误的，并提出改进建议。这将改善对亚极地北大西洋和北大西洋以外地区海洋和气候变化的预测。我们将调查寒冷、稠密的海水是如何进入将它们输出到南方的边界流的。我们将确定风所扮演的角色，这可能比我们过去承担的更复杂。我们将确定北极和格陵兰岛淡水出口变化对颠覆可变性的影响。为了描述和量化这些关键过程，除了使用海洋观测，我们还将执行“如果呢？”在海洋模型中进行实验，提出这样的问题：如果大西洋上空的大气急流移动或加强，亚极地海洋环流会发生什么？我们将使用天气预报中更常见的统计方法来弄清楚亚极地海洋的性质和倾覆如何与潜在的可预测的更大规模的大气环流模式相联系。我们将采用模型与观测相结合的创新方法，以确定在OSNAP观测期内北大西洋次极地演变的最佳估计。

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.