Testing a Role of Vertical Mixing in Abrupt Climate Change

垂直混合在气候突变中的作用测试海洋经向翻转环流（MOC）由于其伴随的极向热通量而被视为气候系统的一个重要组成部分。一种流行的理论将地质历史的快速气候变化归因于北大西洋北部冰川融水输入引起的MOC变化。然而，这一理论目前受到了基于理想化模式的研究的挑战，这些研究表明MOC对地表强迫的响应在质量上取决于模式中垂直混合的表征：如果混合用固定的表观扩散率表示，则MOC随地表赤道-极密度对比的降低而降低（通常的结果），而如果扩散率取决于垂直密度分层（可能是一个更合理的假设），MOC随密度对比的降低而增加。在这里，我们将确定在一个现实的全球海洋模型中，是否有类似的结果适用于环流对表面变新鲜的随时间变化的响应。为了实现我们的目标，我们将用海洋环流模式进行一组数值实验，该模式依赖于不同的垂直混合假设，并将其结果与从沉积物中获得的古海洋学数据进行比较。我们的工作计划包括三个主要任务。首先，该模型将被建立为具有真实水深测量的全球海洋。其次，将生成一系列模型平衡状态，假设垂直扩散系数恒定或与稳定性相关。第三，这些状态将受到地表淡水通量异常的干扰。比较关于垂直混合的不同假设得到的结果将为我们的假设提供检验。将模式结果与古海洋学数据进行比较，将使我们能够评估这些数据在多大程度上有助于解决物理海洋学中一个悬而未决的问题，即与MOC中垂直混合的作用有关。

Testing a Role of Vertical Mixing in Abrupt Climate ChangeOlivier MarchalThe meridional overturning circulation (MOC) of the ocean is viewed as an important component of the climate system owing to its attendant poleward heat flux. A popular theory ascribes rapid climate changes of the geological past to variations in the MOC caused by glacial meltwater input to the northern North Atlantic. The theory, however, is currently challenged by studies based on idealized models, showing that the response of the MOC to surface forcing depends qualitatively on the representation of vertical mixing in the models: if the mixing is represented with a fixed, apparent diffusivity, the MOC decreases with decreased equator-to-pole density contrast at the surface (the usual result), whereas if the diffusivity depends on vertical density stratification (perhaps a more plausible assumption), the MOC increases with decreased density contrast. Here we will determine whether a similar result holds for the time-dependent response of the circulation to surface freshening in a realistic model of the global ocean. To meet our goal we will conduct a set of numerical experiments with an ocean general circulation model, which rely on different assumptions about vertical mixing, and compare their results with paleoceanographic data obtained from sediments. Our work plan includes three major tasks.First, the model will be set up to represent the global ocean with realistic bathymetry. Second, a series of model equilibrium states assuming constant or stability-dependent vertical diffusivity will be generated. Third, these states will be perturbed by applying an anomaly in the surface freshwater flux. Comparing results obtained from different assumptions about vertical mixing will provide a test of our hypothesis. Comparing model results with paleoceanographic data will enable us to assess the extent to which these data could contribute to an outstanding question in physical oceanography, related to the role of vertical mixing in the MOC.