大西洋垂直环流的全球稳定性：在二氧化碳强迫下的边缘状态、长期暂态和边界危机
Global stability of the Atlantic overturning circulation: Edge state, long transients and boundary crisis under CO$_2$ forcing
AI总结：
研究通过中间复杂度气候模型探讨不同大气二氧化碳浓度下大西洋垂直环流稳定性，揭示边缘状态和边界危机对气候系统稳定性的影响。
摘要：
大西洋垂直环流（AMOC）是一个关键的海洋环流系统，可能转变为弱状态。尽管与之相关的气候影响严重，但评估AMOC在全球变暖下的响应及其接近可能临界阈值仍困难。为了理解未来地球系统稳定性，需要超越传统早期预警方法的局部稳定性分析的全球动态观点。使用中间复杂度气候模型，我们探讨了不同大气二氧化碳浓度下AMOC的稳定性景观。我们明确计算了边缘状态（或Melancholia状态），即分离模型中强和弱AMOC吸引子的混沌鞍点。尽管不稳定，边缘状态可以主导数百年气候，支持由大西洋大气-冰-海洋相互作用驱动的数十年AMOC振荡。在预计不久将增加的二氧化碳水平下，我们揭示了一个边界危机，其中当前AMOC吸引子通过与边缘状态碰撞消失。在危机超量下，由于“鬼状态”导致的长期混沌暂态使在时间变化强迫下的集合轨迹发散。基于动力系统理论，我们的结果为在中间强迫情景下地球系统模型中观察到的大集合方差和明显“随机分支”提供了解释。
英文摘要：
The Atlantic Meridional Overturning Circulation (AMOC), a crucial ocean current system, could transition to a weak state. Despite severe associated climate impacts, assessing the AMOC's response under global warming and its proximity to possible critical thresholds remains difficult. To understand future Earth system stability, a global dynamical view is needed beyond the local stability analysis underlying classical early-warning methods. Using an intermediate-complexity climate model, we explore the stability landscape of the AMOC for different atmospheric CO$_2$ concentrations. We explicitly compute the edge state (or Melancholia state), a chaotic saddle on the basin boundary separating the strong and weak AMOC attractors found in the model. While being unstable, the edge state can govern the transient climate for centuries, supporting centennial AMOC oscillations driven by atmosphere-ice-ocean interactions in the North Atlantic. At increased CO$_2$ levels projected for the near future, we reveal a boundary crisis where the current AMOC attractor disappears by colliding with the edge state. Under crisis overshoot, long chaotic transients due to "ghost states" lead to diverging ensemble trajectories under time-varying forcing. Rooted in dynamical systems theory, our results offer an explanation of large ensemble variance and apparent "stochastic bifurcations" observed in earth system models under intermediate forcing scenarios.
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