深部解耦振荡起源的详细分析
A detailed analysis of the origin of deep-decoupling oscillations
AI总结：
本文通过引入非瞬时平滑切换函数改进Welander模型，利用动力系统方法区分了四种深部解耦振荡类型，并揭示了淡水注入对垂直混合振荡转换的影响，为解释海洋-气候变率提供了理论依据。
摘要：
大西洋经向翻转环流强度的变率在很大程度上受北大西洋深水形成的影响。在许多海洋模型中，发现了所谓的深部解耦振荡，其时间尺度取决于对流垂直混合过程的特征。迄今为止，其确切起源及对混合表示的敏感性仍不清楚。为了研究这个问题，我们重新审视了一个概念性的Welander模型，用于描述表层和深水两个垂直堆叠盒子中温度和盐度的演化，它们通过扩散和/或对流调整相互作用。已知该模型表现出几种类型的深部解耦振荡，当假设它们之间的切换是瞬时的，弱扩散混合阶段与强对流混合阶段交替出现。我们提出了一项关于Welander模型中振荡的综合研究，其中混合阶段之间的切换是非瞬时的，由平滑切换函数描述。动力系统方法使我们能够根据扩散混合与对流混合的阶段区分出四种类型的振荡，并确定它们在相关参数平面中存在的区域。对于非瞬时切换，特征性的深部解耦振荡仍然存在，但所需的切换速度要比维持振荡行为所需的快得多。此外，我们展示了逐渐的淡水注入如何导致不同垂直混合振荡之间的转换。值得注意的是，对流混合阶段变得更短甚至消失，导致深水形成大幅减少的长期阶段。这些结果对于解释模型和（代理）观测中的海洋-气候变率具有重要意义。
英文摘要：
The variability of the strength of the Atlantic Meridional Overturning Circulation is influenced substantially by the formation of deep water in the North Atlantic. In many ocean models, so-called deep-decoupling oscillations have been found, whose timescale depends on the characteristics of convective vertical mixing processes. Their precise origin and sensitivity to the representation of mixing have remained unclear so far. To study this problem, we revisit a conceptual Welander model for the evolution of temperature and salinity in two vertically stacked boxes for surface and deep water, which interact through diffusion and/or convective adjustment. The model is known to exhibit several types of deep-decoupling oscillations, with phases of weak diffusive mixing interspersed with strong convective mixing, when the switching between them is assumed to be instantaneous. We present a comprehensive study of oscillations in Welander's model with non-instantaneous switching between mixing phases, as described by a smooth switching function. A dynamical systems approach allows us to distinguish four types of oscillations, in terms of their phases of diffusive versus convective mixing, and to identify the regions in the relevant parameter plane where they exist. The characteristic deep-decoupling oscillations still exist for non-instantaneous switching, but they require switching that is considerably faster than needed for sustaining oscillatory behaviour. Furthermore, we demonstrate how a gradual freshwater influx can lead to transitions between different vertical mixing oscillations. Notably, the convective mixing phase becomes shorter and even disappears, resulting in long periods of much reduced deep water formation. The results are relevant for the interpretation of ocean-climate variability in models and (proxy) observations.
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